The prizes consist of two first prizes (an Art Prize and a Science Prize) and the Vinci of Excellence for scientific work of the highest international level (these scientists reached the final stage of the selection process).

See Leonardo 30, No. 3, 191--194 for further information about the LVMH Science for Art Prize.

Papers presented here are "Principles of the Architecture and Morphogenesis of Biological Assemblies" by Aaron Klug; "Genetics of Animal Design" by Peter Lawrence; "Interlocking Rings and Knots at the Molecular Level" by Jean-Pierre Sauvage; "Dendrimers and Dendrons: Controlled Macromolecular Structure According to Dendritic Branching Rules and Principles" by Donald A. Tomalia; "Skin Colors and Patterns in Fish" by Ryozo Fujii; "How To Prepare a Cornucopia of New Substances" by Arpád Furka; "Unraveling the Mysteries of Flower Development" by Elliot Meyerowitz; "DNA Dendrimers" by Thor W. Nilsen.

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Many simple viruses are spherical in shape (Fig. 1), with an outer shell made of protein units enclosing the RNA or DNA inside. One might have expected the shell to be built according to mathematical symmetry (point-group symmetry) in which all units would be identically situated, making the same (equivalent) interactions with their neighbors. We discovered, however, that a number of viruses, including the polio virus, did in fact have icosahedral symmetry, yet we found that there were more sub-units than allowed by perfect mathematical symmetry---namely, 60. My colleague Donald Caspar and I showed that the design of these viruses could be explained in terms of a generalization of icosahedral symmetry that allows identical units to be related to each other in a quasi-equivalent way with a small measure of internal flexibility. We enumerated all the possible designs, which have similarities to the geodesic domes designed by the architect R. Buckminster Fuller. However, whereas Fuller's domes have to be assembled following a fairly elaborate code, the design of the virus shell allows it to build itself. To this date, all virus shells studied have one of the predicted symmetries. I think that it would have delighted Plato to know that fundamental forms lay beneath the variety of appearances.

The Structure of Chromatin, the Material of which Chromosomes Are Made

The DNA of a chromosome would be several centimeters long if laid out straight, but it has to be packaged into a compact form with a reduction in length by a factor of about 10,000. We discovered that this was accomplished by a hierarchy of orders of helical folding of the DNA double helix. The first of these is the nucleosome, in which a short length of the double helix of DNA is wrapped in a super-helix around a spool of histone proteins. The next order of folding is a solenoid (or contact helix) in which nucleosomes containing successive lengths of DNA are assembled into a helical filament of about 300 A in diameter. This process is mediated by another histone protein. The next degree of coiling is much less precise and involves the formation of loops of this filament, which are in turn organized into a chromosome. The principle behind this scheme is that although the DNA is highly folded it can be made readily accessible at the right time for the expression of genes. The folded structures can be readily unfolded, making the DNA accessible to regulatory proteins that can switch on gene actions.

Transcription Factors

As just described, the supply of regulatory proteins (transcription factors) ensures that a gene can be switched on at the right place and at the right time. The selective expression of any one gene is accomplished through the interaction of a combination of such regulatory proteins. In recent years, a number of classes of such regulatory proteins have been discovered. In interacting with DNA, almost all regulatory proteins make use of the two-fold symmetry of the DNA double helix so that the protein binds as a two-fold symmetric dimer. However, I discovered a unique class, the "zinc finger" family, which has a quite different design. Such proteins are built of small, multiply repeated domains ("fingers")---all have the same structural framework but each is chemically distinct through variations in certain key amino acids. They are thus constructed on a modular principle; a multi-fingered protein can specifically grip a length of DNA helix whose sequence it has effectively read. Very recently we used this design to create new proteins that recognize a desired part of a gene; we have thus succeeded in switching off a deleterious (cancer) gene in a cell line [2]. This work therefore encompasses a technological application of a concept derived from a living form.

References

1. I recently wrote an article entitled "Macromolecular Order in Biology," Phil. Trans. R. Soc. Lond. A 348 (1994) pp. 167--178, which summarizes my work of 30 years and covers one of the themes of the LVMH Science for Art Prize.

2. Yen Choo, Isidro Sánchez-García and Aaron Klug, Nature 372 (1994) pp. 642--645.

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Peter Lawrence, MRC Laboratory Molecular Biology, Hills Road, Cambridge CB2 2QH, United Kingdom. E-mail: pal@mrc-lmb.cam.ac.uk.

The work reported in this Abstract was awarded a 1996 LVMH Vinci of Excellence Prize.

I study how animals are designed, investigating questions of interest to me, such as: How is the shape of animal determined? Why is a giraffe so different from a whale? How is an animal's size determined? Why is a mature blowfly many times the size of a fruitfly? How does the leopard get its spots? What arranges the colors on a butterfly's wing?

All of these questions are difficult to approach directly. Since the answers likely involve genes and how they work, it is important to choose to study an animal with well-known genetics that can be bred easily in the laboratory. Over the last few years it has been gradually dawning on scientists that the basic principles of design are largely common to all animals, and therefore it follows that it does not matter too much which beast is studied to decipher those principles. This conclusion is a surprise to many, especially to those who would like to believe that humans---and if not humans alone, then at least humans and mammals---are very special and different from other animals. The idea that humans and all animals have a fundamental commonality is pleasing to me. I am told that this conviction is particularly strong in the Hindu religion.

The principles of development are not easily predicted by scientists who think empirically. Instead, experiments are where most insights into biology come from---that is, insights do not often come from theoretical biology, where scientists sit down and imagine how, logically, they would design and build a hippopotamus or a sea horse.

When scientists looked at the insect eye ( Over the last 25 years, a school of Drosophila melanogaster scientists, particularly Antonio Garcia-Bellido and his colleagues such as Gines Morata and myself [4], have built a picture of the developing animal that is interesting, if counterintuitive. Experiments tell us that the body is built piecemeal from modules ("compartments") in the embryo, with each compartment consisting of a group of cells. A compartment is directed by a special designer gene or genes within it that are switched on and tell it which piece of the body to make. The parts made are surprising, and the lines that separate one part from another may not be visible on the body. (For example, the wing is not made in one piece, but in two halves; the line between the two halves was unsuspected and undetected for many decades.) These genes usually remain "on" during development to remind the cells what to do as the embryo grows. It is as if all the instructions on making the entire animal are written in an enormous book, and these special genes tell each group of cells which pages in the book to use. Research on vertebrate animals has shown that we are also partly built in modules and that the same designer genes are used. Flies and people have a great deal in common!

References and Notes

1. Peter A. Lawrence, The Making of a Fly (Oxford, U.K.: Blackwell Science, 1992).

2. Seymour Benzer, "Genetic Dissection of Behavior," Scientific American 229 (1973) pp. 24--37.

3. Lewis Wolpert, The Triumph of the Embryo (Oxford, U.K.: Oxford Univ. Press, 1991).

4. Drosophila melanogaster is the common fruitfly. Antonio Garcia-Bellido, Peter A. Lawrence and Gines Morata, "Genetic Dissection of Behavior" Scientific American 241 (1979) p. 102.

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Jean-Pierre Sauvage, Université Louis Pasteur, Faculte de Chimie, 4, rue Blaise Pascal, 67070 Strasbourg-Cedex, France. E-mail: sauvage@chimie.u-strasbg-fr.

The work reported in this Abstract was awarded a 1996 LVMH Vinci of Excellence Prize.

The synthetic chemist is usually highly interested in the aesthetics of the molecule he or she wants to assemble. In fact, the search for aesthetically attractive molecules has been a goal since the very origin of chemistry.

The aesthetic aspect of any object is usually connected to its shape in Euclidean geometry. However, another interesting facet of beauty rests in the topological properties of the object. The latter can be deformed as much as desired, provided edges and lines that constitute the object are not cleaved. Among the most topologically fascinating objects, interlaced design and knots (such as the archetypical trefoil knot) occupy a special position. Representing continuity and eternity in early religious symbolism, they illuminate the art of the most ancient civilizations. Developed by Egyptians, Persians and Greeks, among others, this virtually universal art reached its zenith in the Celtic culture. The magnificent illuminations consisting of extremely complex interlaced designs and knots in the Book of Kells give evidence of the fascination that braids, wreaths and knots exert on human beings. In this famous manuscript, the work of Irish monks during the eighth century, geometrical figures were converted into marvelous representations. Later, superb interlaced designs were created by Albrecht Dürer and Leonardo da Vinci.

In relation to the graphic arts, mathematical topology and biology, the creation of knots at the molecular level (using the tools of synthetic chemists) is an especially challenging objective. Despite many early difficulties, our group's results over the course of the last few years has opened the door to the preparation of knots constructed around transition-metal ion templates.

Our strategy is shown in Fig. 3. Toward the goal of preparing a trefoil knot, we extended the synthetic concept that had already proved successful in the synthesis of catenanes (i.e. interlocking ring molecular systems, Fig. 3, top). In the strategy toward a trefoil knot, we exploited the three-dimensional (3D) template effect induced by two transition metals. As shown in Fig. 3 (bottom), two molecular threads A can be interlaced on two transition metal centers (black dot), leading to a double helix B. After cyclization to C and demetalation, a knotted system D is obtained.

I will not describe here in detail the experimental work done along the strategy of Fig. 3 (bottom) for synthesizing a molecular knot. Simply, several years of research were necessary (performed mostly by C. Dietrich-Buchecker and myself) in order to achieve the preparation of the first chemical trefoil knot using copper atoms as templates (black circles in Fig. 3) and relatively complicated molecular threads interlaced by coordination to the two copper centers. The chemical structure and the X-ray structure (crystallography) of the molecular knot actually prepared are represented in Fig. 4 (left and right, respectively).

In conclusion, the main contribution of our research team to the field of chemical topology has been to introduce new synthetic concepts based on the template effect of transition metals and allowing the intertwining of molecular strings at will before incorporating them into target molecular systems. Although classical organic chemists had developed elegant synthetic procedures in the past for making interlocking rings (catenanes), the syntheses used prior to our work were tedious and low yielding. In fact, the research domain of catenanes was slowly vanishing. After we proposed efficient templated synthetic strategies (1983--1984), the research area underwent a spectacular revival, as has been testified by the many research laboratories who have joined the field in recent years and by the numerous publications on catenanes and related systems appearing regularly in scientific journals.

Our scientific contribution to topology at the molecular level culminated with the first synthesis of a chemical knot (a molecular trefoil knot, 1989) and its full characterization by X-ray crystallography (1990). Interestingly, chemical knots had been envisaged for almost 30 years before actually being made---long before biologists had demonstrated that DNA-based knots (and other interlaced or interlocking structures) are very frequent in nature.

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In the early 1970s I was fascinated by the remarkable control of 3D macroscopic space that was possible in the dendritic branching patterns of trees. These branching patterns were observed to amplify predictably as a function of their annual growth stages. I reasoned that if I could assemble small molecular building blocks (abiotic monomers) in compressed iterative growth stages (minutes/hours) in the laboratory according to these branching patterns, it should be possible to synthesize molecular trees. This was successfully accomplished in the spring of 1979 and involved the covalent bonding of various feedstock monomers to form concentric layers of geometrically progressive branches upon branches called "generations" (G) (Fig. 5). The branches are amplified by a stepwise divergent growth process from a single molecular information source, called the initiator core. A single-trunked array is referred to as a dendron (Greek work for tree), whereas multi-trunked arrays are called dendrimers. These amplifications follow precise, mathematically driven dendritic rules/principles and, as such, lead to structure-controlled macromolecular growth that rivals that observed in biological systems. This abiotic strategy involves radial transcription/translation with amplification from the initiator core by means of chemical bond formation. In essence, a 3D spatial transfer of the core size, core shape, bonding angles and stoichiometry information occurs through the covalent connectivity of the branches to the outer surface (molecular leaves). In this fashion, each generation (G) becomes the abiotic template (codon) that directs the ordering and polymerization of feedstock monomers on the dendrimer surface to produce the subsequent generation. Either the presence or absence of these amplified branching patterns in space around the initiator core defines the ultimate shape and size of the dendrimer. These events may be visualized as mimicking the transfer of molecular information that occurs from DNA to messenger-RNA and ultimately to the incipient polypeptide being formed in a biotic ribosome. As is true for the biotic strategy, these sequence steps (G) are iterative and chronological; however, the dendritic products involve a radial 3D formation of structure rather than the linear motif found in biological systems. This radial amplification of branches produces well-defined 3D dendritic structures with architectural features that clearly mimic---yet are differentiated from---tertiary protein structures. The opportunity to design dendritic shapes, sizes, concave/convex clefts and protrusions that mimic protein sizes, shapes and surfaces is virtually endless with this abiotic strategy.

Thus far, dendrons/dendrimers have been used successfully in a myriad of applications, including synthetic vaccines, gene expression vectors, pharmacological/agricultural chemical delivery systems, catalysis, medical diagnostics, advanced coatings, chemical/biological sensors and opto-electronic devices.

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Fig. 1. Aaron Klug. Three-dimensional image reconstructions from electron micrographs of some spherical viruses. Alongside are shown the underlying icosahedral surface lattices, with the five-fold and six-fold vertexes marked. (a) Human wart virus (about 550 A in diameter), (b) Turnip yellow mosaic virus (about 300 A in diameter), (c) Tomato bushy stunt virus. Viruses (b) and (c) belong to the same class, T=3, which has 180 units organized around 12 strict five-fold axes and 20 local six-fold axes. However, the units are clustered at the surface quite differently in the two cases: in (b) they are grouped into penamers and hexamers around the five-fold and six-fold positions to form 32 morphological units, whereas in (c) they are clustered into 90 dimers about two-fold positions.

Fig. 2. Peter Lawrence. Section through the center of a Drosophila melanogaster's compound eye, showing the arrangement of photoreceptor cells (dark dots). The insect's eye contains many identical units packed together, each a mini-eye with its own lens and retina. The Drosophila's eye cells are patterned in groups of seven, with each group acting as a single eye. In the middle of the eye, there is an equator; the pattern of the eye is mirror-symmetrical on either side of the equator (Photo: Peter A. Lawrence, The Making of a Fly [Oxford, U.K.: Blackwell Science, 1992]).

Fig. 3. Jean-Pierre Sauvage. Template synthesis of catenanes (one metal, represented by a black circle) or a trefoil knot (two metal centers). Each thread is a molecular fragment, incorporating various chemical functions not represented here. For instance, in the bottom line, the two threads of A can react with two metals (copper atoms) to generate a double-stranded helical molecule (B). In this process, the two metals gather and interlace the two strings of A in such a way that a double helix is formed. From the precursor B, the trefoil knot C is obtained by specific chemical reactions, their overall effect being that of producing a knotted ring ("cyclization" process). The two metals utilized to construct and maintain the molecular edifice B in the appropriate geometry during this procedure are still present in the closed molecule C. They can be removed by appropriate chemical reagents to afford the trefoil knot D. It is interesting to note that the cyclic strand of C is identical to D.

Fig. 4. Jean-Pierre Sauvage. Shown here is the molecule represented as C in Fig. 3. The chemical formula is indicated on the left. The real shape of the molecule was determined by X-ray diffraction and is represented on the right: one can recognize the two copper atoms (white circles in the median part of the drawing) and the general analogy between both geometries (chemical formula and real shape).

Interestingly, the length of the trefoil knot represented here is one-thousandth of a micron. This gives an idea of the scale at which molecular chemists construct their edifices. In a normal preparation of the knot, one obtains about 70 milligrams of the molecule (equivalent to the size of a peppercorn). This is not much, but such a sample contains 20 billions of billions of individual knots. Again, this shows how small the molecular world is.

Fig. 5. Donald A. Tomalia. Molecular building blocks (abiotic monomers) are assembled in compressed iterative growth stages in the laboratory. A non-biological strategy for transferring molecular information from a seed core to abiotic feedstock monomer units to produce dendrons and dendrimers with macromolecular structure control of size, shape, surface chemistry, topology and flexibility---critical macromolecular design parameters.

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Ryozo Fujii, Department of Biomolecular Science, Faculty of Science, Toho University, Miyama, Funabashi, Chiba 274, Japan. E-mail: .fujii@biomol.sci.toho-u.ac.jp

The work reported in this Abstract was awarded the 1996 LVMH Vinci of Excellence Prize.

We appreciate the beautiful colors and patterns displayed by many animal species. Among them, teleostean fishes constitute an extraordinarily wonderful group; we can enjoy, in addition to their "static" colors and patterns, spectacular changes to them. Such integumentary colors are dependent on the presence of pigment cells in the skin, namely, the chromatophores [1]. These include the melanophores (black or brown), xanthophores (ocher or yellow), erythrophores (red), leucophores (whitish), and the iridophores (metallic or iridescent). (I have been working for a long time to put the classification of chromatophores to rights, and my system is now widely accepted [2]). In some species of fish, we recently discovered blue chromatophores, naming them "cyanophores" [3]. Thus, six kinds of chromatophores are now known in poikilothermic vertebrates.

The colors in these fish are generated by the absorption, reflection and/or the scattering of light of certain wavelengths by the pigment and the microstructures inside the cells. Usually containing a single kind of pigmentary material, each chromatophore is a small entity. When differently colored chromatophores are distributed in the skin, the resulting color looks like a mixture of different colors. By making good use of the divisionistic effects that chromatophores allow, the fish can exhibit a number of intermediate hues almost at will [4].

In common dendritic chromatophores, the aggregation of pigmentary organelles into the cell body, or their dispersion into the cellular processes, results in the fading or the increased coloring of the skin, respectively [5]. Containing stacks of thin, flat guanine crystals, by contrast, most iridophores are non-dendritic. By means of multilayer thin-film interference phenomenon occurring in the stacks, the cells can reflect light efficiently. Among these cells, the motile iridophores possessed by many coral-reef fish---and by many beautiful aquarium fishes such as the neon tetra---are especially interesting. We found that they reflect light within limited spectral ranges through interference phenomena of the non-ideal type [6]. Very bright, fluorescent-like hues are thus realized. In addition, in response to certain nervous or hormonal signals, the spacing between adjacent platelets changes concurrently, resulting in a shift of the spectral reflectance peak. In other words, continuous changes of hue---such as violet to yellow via blue and green---can take place [7].

The motile activities of chromatophores (Fig. 1) are dependent on the intracellular presence of motor-proteins---namely, tubulin, dynein and kinesin---and are regulated by the endocrine and/or the nervous systems [8]. We have clarified that the rapid aggregation of pigment granules, the chromatosomes, is aroused by the sympathetic division of the autonomic nervous system, via alpha adrenoceptors possessed by the chromatophores [9]. We have also shown that, among endocrines, melanophore-stimulating hormone (MSH), melanin-concentrating hormone (MCH) and epinephrine take leading parts in the control of chromatophores. (MSH disperses pigmentary organelles, while MCH and melatonin normally aggregate them in chromatophores. MSH disperses pigmentary organelles [10], while MCH [11] and melatonin [12] normally aggregate them in chromatophores [13].) In many species, such as pencilfish, the pineal hormone melatonin may function in the circadian changes of color patterns [14]. We also found that endothelins aggregate chromatosomes in melanophores, erythrophores and xanthophores, and disperse them in leucophores. Our conclusion was that endothelins are involved in the formation of patterns [15]. Our recent studies further indicated that nitric oxide (NO), a substance recently identified as the endothelium-derived relaxing factor (EDRF) in mammalian vascular systems, may also be involved in the subtle control of skin coloration.

It may be astonishing to hear that so many factors are involved in the control of chromatophores in fish. Indeed, we are not aware of other effector cells (beyond those discussed here) that are regulated by so many kinds of physiological cues. The mechanisms of color manifestation as well as those controlling coloration in fish provide much useful information for humans.

References

1. R. Fujii, "Cytophysiology of Fish Chromatophores," International Review of Cytology 143 (1993) pp. 192--255.

2. R. Fujii, "Chromatophores and Pigments," in W.S. Hoar and D.J. Randall, eds., Fish Physiology, Vol. 3 (New York: Academic Press, 1969); see also R. Fujii, "Coloration and Chromatophores," in D.H. Evans, ed., The Physiology of Fishes (Boca Raton, FL: CRC Press, 1993).

3. M. Goda and R. Fujii, "Blue Chromatophores in Two Species of Callionymyd Fish," Zoological Science 11 (1995) pp. 527--535.

4. Fujii [1].

5. Fujii, "Coloration and Chromatophores" [2].

6. H. Kasukawa, N. Oshima and R. Fujii, "Mechanism of Light Reflection in Blue Damselfish Iridophore," Zoological Science 4 (1987) pp. 243--257.

7. See Kasukawa et al. [6]; see also Fujii, "Coloration and Chromatophores" [2].

8. See Fujii, "Coloration and Chromatophores" [2].

9. R. Fujii and Y. Miyashita, "Receptor Mechanisms in Fish Chromatophores---I. Alpha Nature of Adrenoceptors Mediating Melanosome Aggregation in Guppy Melanophores," Comparative Biochemistry and Physiology 51C (1975) pp. 171--178.

10. R. Fujii and Y. Miyashita, "Receptor Mechanisms in Fish Chromatophores---V. MSH Disperses Melanosomes in Both Dermal and Epidermal Melanophores of Catfish (Parasilurus asotus)," Comparative Biochemistry and Physiology 71C (1982) pp. 1--6.

11. N. Oshima, H. Kasukawa, R. Fujii, B.C. Wilkes, V.J. Hruby and M.E. Hadley, "Action of Melanin-Concentrating Hormone (MCH) on Teleost Chromatophores," General and Comparative Endocrinology 64 (1986) pp. 381--388.

12. R. Fujii and Y. Miyashita, "Receptor Mechanisms in Fish Chromatophores---IV. Effect of Melatonin and Related Substances on Dermal and Epidermal Melanophores of the Siluroid, Parasilurus asotus," Comparative Biochemistry and Physiology 59C (1978) pp. 59--63.

13. Y. Miyashita and R. Fujii, "Receptor Mechanisms in Fish Chromatophores---II. Evidence for Beta Adrenoceptors Mediating Melanosome Dispersion in Guppy Melanophores," Comparative Biochemistry and Physiology 51C (1975) pp. 179--187.

14. H. Nishi and R. Fujii, "Novel Receptors for Melatonin that Mediate Pigment Dispersion Are Present in Some Melanophors of the Pencilfish (Nannostomus)," Comparative Biochemistry and Physiology 103C (1992) pp. 363--368.

15. R. Fujii, Y. Tanaka and H. Hayashi, "Endothelin-1 Causes Aggregation of Pigment in Teleostean Melanophores," Zoological Science 10 (1993) pp. 763--772.

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Arpád Furka, Advanced ChemTech, Inc., 5609 Fern Valley Road, Louisville, KY 40228, U.S.A. E-mail: afurka@kunagota.win.net.

The work reported in this Abstract was awarded the 1996 LVMH Vinci of Excellence Prize.

What should one do if asked to list all English words composed of five letters? Nowadays, it would be a good idea to let a computer generate all possible combinations deducible from the 26 letters of the alphabet (11,881,376 five-letter strings), then select the meaningful words and discard the nonsense combinations.

Peptides are composed of 20 kinds of amino acids, combined in the same way that English words are from the characters of the alphabet. Depending on the sequence of their constituent amino acids, peptides may show biological effects and, for this reason, they are considered to be potentially useful drug candidates. Many peptides have been prepared by peptide chemists and then tested for their biological activities. Instead, how about preparing all possible combinations of amino acids, then testing them for useful properties? I began to think about this possibility about 15 years ago. Considering the conventional synthetic methods of that time, however, realization seemed absolutely impossible. Preparation of the 3.2 million possible pentapeptides (peptides containing five amino acids), for example, would have taken thousands of years, even for the most experienced peptide chemist. A very simple idea helped, however, and the impossible suddenly became possible.

The new synthetic method developed and realized with my colleagues at Ežtvžs Lorand University, Budapest, is based on the Nobel Prize-winning "solid phase procedure" of R.B. Merrifield [1]. According to his method, the first amino acid is attached to a solid support (tiny polymer beads, about 10 million in 1 gram), then further amino acids are coupled sequentially to it, and finally, the peptide is cleaved from the support. In my method [2], the following three simple operations are repeatedly used:

1. The solid support is divided into 20 equal portions

2. A different amino acid is coupled to each portion

3. The portions are mixed.

These operations are repeated until the desired length of a peptide chain is achieved. The procedure is demonstrated in a simplified form in Fig. 2. If the 20 natural amino acids are used in five consecutive cycles of the above three operations, all the possible 3.2 million pentapeptides are formed in nearly equal molar quantities. We carried out this synthesis manually, without the help of today's automatic synthesizers, and still were able to finish it in 5 days. Although the end product is a mixture of peptides, good methods are available for identification of the biologically important component.

My method, along with a few others, brought about radical improvements in the efficiency of pharmaceutical research and established a fast-growing new branch of science: combinatorial chemistry.

References

1. R.B. Merrifield, "Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide," Journal of the American Chemical Society 85, 2149--2154 (1963).

2. A. Furka, F. Sebestyén, M. Asgedom and G. Dibó, "Cornucopia of Peptides by Synthesis," in Abstracts of the 14th International Congress of Biochemistry Prague, Czechoslovakia 5 (1988) p. 47.

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Elliot Meyerowitz, Division of Biology 156-29, California Institute of Technology, Pasadena, CA 91125, U.S.A. E-mail: meyerowitze@starbase1.caltech.edu.

The work reported in this Abstract was awarded the 1996 LVMH Science for Art Prize.

I am a biologist; my colleagues and I use the methods of genetic analysis and molecular biology to try to understand how a complex living organism develops from a fertilized egg. A fertilized egg is a single cell, while a mature plant or animal contains many millions of cells of different types.

During development, the single egg cell divides, and its descendants divide, to provide huge numbers of cells in mature form. Somehow, each of the cells that forms must learn its position in the organism---either by receiving some type of positional cues from its neighbors or by having some global positional coordinates. The cells must know their positions because they eventually form specialized cell types with a specific position in the mature organism. For example, cornea cells form only in eyes, indeed, only in specific parts of eyes. We would like to know how each cell learns its position and then acts on its positional knowledge to form the appropriate cell type. We use flowers to solve this problem, because plant development is in many ways simpler than animal development, and because flowers are so highly (and attractively) organized in their patterns of organ numbers, organ positions and organ types.

We knew before starting our work that there are genes (genes are each a few thousand base pairs of deoxyribonucleic acid [DNA], and each contains information for the construction of a single type of protein) that represent critical instructions for pattern formation in flowers. Over more than a millennium, flower breeders (the records go back to ninth-century China) have purposely collected genetic variants that have altered floral patterns. Most of our cultivated flowers are descendants of such mutants---for example, double flowers such as 'Pink Perfection' camellias, which have petals in place of their male and female sex organs. We have reproduced the genetic work of our ancestors, choosing a small plant well-suited for laboratory work because it grows rapidly and requires little space---a member of the mustard family named Arabidopsis thaliana. By using mutagenic treatments, we have made many mutant lines that have normal floral organs in inappropriate places, such as petals in the positions usually occupied by stamens, or carpels (fruit parts) in the positions usually filled by sepals. Each mutation represents the loss of action of a gene whose protein product plays a role in informing cells of their positions; when the gene product is absent, the cells miscalculate their positions and form a normal organ, but in an unusual place.

By careful study of normal and mutant flowers, we have developed a theory for how individual cells learn their positions in developing flowers. In brief, the theory postulates that there are three classes of genes that act to specify position in the early developing flower. We call the classes A, B and C; the theory has thus become known as the ABC model. The basic postulates are that A function genes are active in the floral periphery (coincident with the cells that will later form sepals and petals); the B function genes are active in an overlapping region coincident with the cells that will form petals and stamens; and the C function genes are active in the center of the developing flower, where the stamens and ovary will later form. Cells are instructed of their positions by their ABC genes---for example, if a cell has both A and B genes active, it knows its relative radial position in the flower and knows to participate in the formation of a petal, while another cell, more central in the developing flower, may have only C genes active and thus be instructed to form part of the floral ovary. We have tested this theory by combining different mutations in single plants and by using molecularly cloned copies of the genes to alter positional information in transgenic plants. An example of this is shown in Fig. 3, where function B has been activated throughout the entire flower, with the result that peripheral cells have both A and B genes active, rather than only A genes, and thus have made petals where sepals would normally be found. Central cells in this plant have B and C active, rather than only C, and thus have formed stamens.

The theory explains in detail how floral cells know their positions and, thus, to some degree how it is that our ancestors created the floral and crop varieties that we prize and depend upon today. It also shows how we and our descendants may carry on the work of the past, breeding new flowers for aesthetic appreciation and new crops to serve fundamental human needs. Indeed, because so much of human evolution has depended on interactions with flowering plants (which are the source of most of our food), an understanding of the development of floral patterns may be a beginning to an understanding of the evolution of our own visual abilities and thus of the origin and nature of the aesthetic appreciation of flowers and floral patterns in the visual arts.

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Thor W. Nilsen, Poly Probe, Inc., 15 Bala Avenue, P.O. Box 2675, Bala Cynwyd, PA 19004, U.S.A. E-mail: 75240.34@compuserve.com. WWW site: http://www.polyprobe.com.

The work reported in this Abstract was awarded the 1996 LVMH Vinci of Excellence Prize.

Dendritic molecules are highly branched arborescent structures that have found applications in such products as chemical reagents, lubricants and contrast media for magnetic resonance. A new class of these molecules is comprised of dendrimers constructed entirely from unique nucleic acid monomers designed such that sequential hybridization adds successive layers of monomer, resulting in a geometric expansion of both the molecule's mass and free single-stranded ends (Fig. 4).

My contribution to DNA dendrimers began in Spring 1986. The publication of the Polymerase Chain Reaction (PCR) in December 1985 [1] had dramatically changed my thoughts regarding the detection of nucleic acids. Until that time, nucleic acid blot assay was the standard method for detecting nucleic acids. PCR assay opened my mind to the possibilities of alternative detection methods. I immediately recognized the power of the PCR lay in the geometric expansion of target molecules---however, this is also the source of one of its greatest drawbacks: the difficulty of quantifying the initial number of target molecules in a sample.

I began working on alternatives to PCR. A key component of any alternative would have to be superior quantitation ability---the new method should be capable of single-target molecule detection. I had known for some time of signal amplification methods for blot assay based on building an aggregate of signal molecules. The spark for the idea of DNA dendrimers was an image in my mind of removing the aggregate from a membrane and assembling it in free space. Once the concept of macromolecular assembly independent of a surface came to me, I rapidly proceeded to the concept of the DNA dendrimer. Interestingly, both the PCR assay and the DNA dendrimers are based on geometric expansion: PCR amplifies the target; dendrimers amplify the signal.

It was one thing to conceptualize DNA dendrimers and quite another to actually build them. I began by designing the sequences. I wanted the sequences to have an absolute minimum of non-specific homology---in essence, a Euler sequence. In this case, the Euler sequence was to utilize each possible 5-mer (AAAAA, AAAAC, AAAAG, AAAAT, etc.) only once in a double-stranded molecule. I converted each of the bases to the numbers 1, 2, 3, 4, assembled a grid of all the 5-mers (a base 4 count from 11111 to 44444). I then added two other elements to the plane of the spreadsheet: (1) a matrix for calculating the available choices extending three bases into the future and (2) an "enzyme" consisting of a set of instructions for adding the next base to the growing solution and then moving one column over to reiterate the process. The program did not create the ultimate sequence---that is, one with 512 bases---but it did produce a sequence that was 506 bases long. That 506-base sequence served as the starting material for the design of the single strands for dendrimer assembly.

The next step was the incorporation of the Euler sequence into seven monomer strands. I devised a cloning strategy based on the hybridization of complementary oligonucleotides. The process then moved from the theoretical to the laboratory. I synthesized and assembled 52 oligonucleotides into seven double-stranded inserts, which I cloned, sequenced and ultimately multimerized (tandemly repeated within plasmid). The inserts (designed sequences) were cloned such that restriction (cutting) with two enzymes yields a (+) strand eight bases shorter than the (-) strand. Large-scale plasmid preparations were completed, the plasmid restricted and the strands separated via preparative gel electrophoresis.

At this point I had the seven strands required for dendrimer assembly. One remaining hiccup was the discovery of strand exchange. Hybridization of the strands produced the desired dendrimer monomers---yet when hybridized, the structure fell apart into small groups of DNA molecules. After some experimentation, I determined that by hybridizing and subsequently cross-linking the DNA molecules, strand exchange was effectively eliminated and with that, stable and useful DNA dendrimers could be built.

DNA dendrimers have been shown to provide signal amplification of at least 60 fold in many blot formats, including Northern blots, Southern blots and dot blots [2]. Current research is underway to amplify protein detection in Western blots. Since individual fluorescently labeled four-layer DNA dendrimers are easily counted, research is being directed toward furthering the applications of dendritic nucleic acid molecules in a flowing stream---a "flow fluorescence" assay---that my colleagues and I expect to be applicable to the measurement of viral burden in people suffering from hepatitis, HIV infection and other diseases [3].

References and Notes

1. R.K. Saiki, S. Scharf, F. Faloona, K.B. Mullis, G.T. Horn, H.A. Erlich and N. Arnheim, "Enzymatic Amplification of Beta-Globin Genomic Sequences and Restriction Site Analysis for Diagnosis of Sickle Cell Anemia," Science 230, No. 4732 (December 1985) pp. 1350--1354.

2. Nucleic acid blots began with Dr. Southern (hence the name). DNA is electrophoresed, transferred (blotted) onto a membrane and "probed." DNA blots are Southern blots; RNA blots are Northern blots. A dot blot refers to a direct application of sample to membrane.

3. For more information, see http://www.polyprobe.com.

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Fig. 1. Ryozo Fujii. Diagram showing the system for control of motile activities of common chromatophores in teleosts. (ACh = acetylcholine; ACh-r = ACh receptor; AS-r = adenosine receptor; ATP = adenosine triphosphate; Ca²+; = calcium ions; cAMP = cyclic adenosine monophosphate; cGMP = cyclic guanosine monophosphate; Epi = epinephrine; ET = endothelin; ET-r = endothelin receptor; Glu = glutamate; Glu-r = glutamate receptor; IP3D>, = inositol triphosphate; MCH = melanin-concentrating hormone; MCH-r = MCH receptor; MIH = MSH-release inhibiting hormone; MSH = melanophore-stimulating hormone; MSH-r = MSH receptor; MT = melatonin; MT-r = melatonin receptor; NANC fiber = non-adrenergic, non-cholinergic nerve fiber; NE = norepinephrine; NO = nitric oxide; -r = -adrenergic receptor; -r = -adrenergic receptor.)

Fig. 2. Arpád Furka. Schematic representation of the portioning-mixing synthesis. P denotes the solid polymer; the white, gray and black discs represent three different amino acids. The divergent, parallel and convergent arrows show portioning, coupling and mixing, respectively. As can be seen in this illustration, the nine resultant dimers comprise all possible combinations of the white, gray and black discs.

Fig. 3. Elliot Meyerowitz. (left) A typical flower of Arabidopsis thaliana, greatly enlarged (in life it is only 2--3 mm across). The flower has four whorls of organs: at the periphery (barely visible) are four sepals, inside of which are four petals, six stamens and a central ovary consisting of two fused carpels. (right) An Arabidopsis thaliana flower from a transgenic plant in which the B organ identity function has been activated everywhere in the flower. The result is four petals in the positions normally occupied by sepals, and stamens in the positions where carpels are normally found, but no change in the second-whorl petals or third-whorl stamens. The flower thus has eight petals (in two whorls of four) and extra stamens, but no ovary.

Fig. 4. Thor W. Nilsen. Dendritic molecules: (top left) Initiator. (top, right) One-layer dendrimer. (bottom) Two-layer dendrimer). A new class of dendritic molecules has been created from unique nucleic acid monomers. Each monomer is a heterodimer of two single-stranded nucleic acid oligomers possessing a central double-stranded waist and four single-stranded arms. Several different monomers have been constructed such that, when assembled in the appropriate order, they will form mostly hollow spheres having multiple single-stranded DNA arms available for binding at the molecular surface. The molecular structure grows exponentially as each sequential layer is added. Molecules with two layers have 36 free ends, molecules with three layers have 108 free ends, molecules with four layers have 324 free ends, etc.