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Chalcone synthase (CHS; EC 2.3.1.74), the first committed enzyme of the multibranched pathway of flavonoid/isoflavonoid biosynthesis is encoded by a multigene family in soybean, (Glycine max L. Merrill). Our results suggest that this gene family comprises at least seven members, some of which are clustered. We have identified four chs clusters in the allo-tetraploid G. max genome and chs5, a newly characterized member of the chs gene family is present in two of them. We describe the complete nucleotide sequence of chs5, the identification of its immediate neighbors and the organization of the four hitherto identified chs clusters in the Gm genome.

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Ultraviolet-B (UV-B) radiation can have a negative impact on the growth and development of plants. Plants tolerant to UV-B alleviate these effects using UV-screening pigments that reduce the penetration of UV-B into mesophyll tissue. Little is known about the relative contribution of specific phenolic compounds to the screening capacity of leaves. The D1 and D2 proteins constituting the photosystem (PS) II reaction center heterodimer are targets of UV-B radiation and can be used as an in situ sensor for UV penetration into photosynthetic tissue. Degradation of these proteins occurs under very low fluences of UV-B, and is strongly accelerated in the presence of visible light. Using the D1-D2 degradation assay, we characterized UV-B sensitivity of Arabidopsis mutants (tt4, tt5, and fah1) that are genetically altered in their composition of phenolic compounds. We found that changes in phenol metabolism result in altered rates of PSII reaction center heterodimer degradation under mixtures of photosynthetically active radiation and UV-B. A comparison of D2 degradation kinetics showed increased UV sensitivity of the Landsberg (Landsberg erecta) tt5 mutant relative to the Landsberg tt4 mutant and the Landsberg wild type. Despite a lack of flavonoid accumulation, the tt4 mutant is not particularly UV sensitive. However, the tolerance of this mutant to UV-B may reflect the increased accumulation of sinapate esters that strongly absorb in the UV range, and may thus protect the plant against environmentally relevant UV-B radiation. This sinapate-mediated protection is less obvious for the tt4 mutant of Columbia ecotype, indicating that the relative contribution of particular phenolics to the total screening capacity varies with the genetic background. The role of sinapate esters in UV screening is further substantiated by the results with the fah1 mutant where absence of most of the sinapate esters results in a significantly accelerated degradation of D2 under mixed light conditions. Because the latter mutant is not expected to be deficient in flavonoids, the relative contribution of flavonoids as protectants of PSII reaction center heterodimer against UV-B damage in Arabidopsis needs to be re-evaluated vis-a-vis screening by simple phenolics like sinapate esters.

RNA preparations containing 70-80% mouse K-chain mRNA have been prepared. The remainder consists of many RNA species, each of which represents a small fraction of the total RNA. The K-chain mRNA preparation hybridizes with mouse liver DNA with biphasic kinetics, indicating that it consists of two fractions -"unique" and "reiterated." Competition hybridization experiments show that the homology among the unique fractions from different mRNAs is the same as the homology among the amino acid sequences of the corresponding K-chains. Hence, in addition to the C-region (constantregion) sequences, (most of) the V-region (variableregion) sequences are also derived from unique germ line genes. The reiterated fractions from different K-chain mRNAs show essentially complete homology with each other. This fraction seems to consist mostly of sequences which do not code for amino-acid sequences of the secreted polypeptide chain, i.e., the "external" section of the mRNA molecule. It is concluded that the number of germ line genes is too small to account for the observed diversity of antibody molecules.One of the most fascinating problems of immunology is posed by the enormous diversity of the antibody molecules that one animal is able to synthesize. One inbred strain of mice has been shown to be capable of producing eight thousand different antibody molecules against the hapten NIP (4-hydroxy-5-iodo-3-nitrophenacetyl) (1). Another inbred mouse strain was shown to produce many different antibody molecules against DNP (dinitrophenyl), of which more than 500 crossreact strongly with TNP (trinitrophenyl) (2). The occurrence of species-specific residues in the V-regions of antibody polypeptide chains and the mendelian inheritance of rabbit a allotypes point against a germ line theory (3). Nevertheless, controversial ideas of evolutionary versus somatic generation of antibody diversity have continued to coexist (3, 4). The direct approach to solving this controversy is to count the number of antibody V-genes present in the DNA of one cell. To this end, we have analysed the kinetics of hybridization of a mouse K-chain mRNA to mouse liver DNA. For the analysis to be meaningful, three points are essential:1. The physical purity of the K-chain mRNA preparation must be known; that is, the amount and nature of contaminants must be determined.Abbreviations: Cot, moles of deoxyribonucleotide,/liter of incubation X see; SSC, standard saline-citrate solution (0.15 M sodium chloride-0.015 M sodium citrate, pH 7); 2 X SSC means that the concentration of the solution used is two times that of the standard saline-citrate solution; V-region, variable region; Cregion, constant region. 40272. Since K-chain mRNA contains both a unique and a reiterated fraction (5, 6), it is necessary to assign the V-region sequences to one of these fractions.3. The proportion of all possible V-genes which would have cross-hybridized with V-region sequences of the particular K-chain mRNA used, must be determined.We have recently reported a preliminary account ...