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Abstract— The photochemical and chemical properties of the four dimers of thymine have been studied. The extinction coefficients, reversal cross‐sections and quantum yields for reversal are presented as a function of wavelength in the range 200–289 nm. At any wavelength, the dimers have different reversal cross‐sections but also different extinction coefficients. The quantum yields for reversal are nearly the same for all four dimers, the values ranging from 0·6 to 0·9 over the wavelength range 200–289 nm. Titration curves for the four dimers show that for each dimer, two groups are involved with two pK's at about 10·5 and 12·2. Dimers A and B are stable at alkali and acid pH's. Dimers C and D are stable from pH 1 to 3 and unstable at other pH's, with thymine the main degradation product.

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The crystal structure analysis of bis(dimethyl)thymine photodimer C, C14H20N4O4, confirms that it is a 5,6:5,6 syn stereoisomer. The cyclobutane ring is puckered, each atom lying 0.5 A out of the plane of the other three. The thymine nuclei are planar if C( 6) is omitted from the plane in each residue; the angle between these planes is 33°. The thymine residues are skewed with respect to each other, the angle of rotation being about 29°. The crystals are monoclinic, space group Cc, with cell dimensions a = 7.849, b = 15.142, and c = 13.265 Á, ß = 102.65°; Zfour dimers per cell. The structure was solved by obtaining a partial structure with phases determined by the symbolic addition procedure and the tangent formula, and the remainder with phases based on the partial structure. Photodimers of thymine were first isolated and identified by Beukers, Ijlstra, and Berends,3 as products of ultraviolet irradiation of frozen aqueous solutions of thymine and of aqueous solutions of DNA. These compounds are of considerable biological importance since there is good evidence that the formation of pyrimidine dimers is responsible for the majority of photobiological effects caused by ultraviolet irradiation of DNA; for example, it has been shown that this is the major factor in the inactivation of microorganisms by uv light.4Beukers and Berends6 suggested that the dimers were formed by linking of two thymines by a cyclobutane ring formed across their 5,6 double bonds, and Wulff and Fraenkel6 pointed out that cis linkage of the thymines to the cyclobutane ring could give rise to "our stereoisomeric dimers: 5,5:6,6 (or head-to-head) syn and anti (dimers A and B) and 5,6:5,6 (or head-to-tail) syn and anti (dimers C and D). Weinblum and Johns7 obtained four different thymine dimers, and from chemical and spectroscopic evidence tentatively assigned to these the four isomeric structures. Recently the structures of dimer A of dimethylthymine8 and of ura-cil9 and dimer D of thymine10 and of 1-methylthy-mine11 have been determined by X-ray diffraction.

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Marine sponges, e.g. Geodia cydonium, have been intensively used to investigate the biochemical and molecular biological basis of cell-cell- and cell-matrix adhesion. It has been shown that a family of galactose-specific lectins, which are present in the extracellular space of G. cydonium, is a main component involved in cell-matrix adhesion in the sponge system. In the present study it is outlined that the purified 16-kDa lectin-1 binds to a 67-kDa membrane-associated protein. This lectin-binding protein undergoes mono- and diubiquitination after incubation of dissociated sponge cells with the homologous aggregation factor (AF), a molecule involved in cell-cell adhesion. The gene coding for polyubiquitin was characterized and found to be composed of three tandem repeat building blocks. Northern analysis indicated the presence of only one type of ubiquitin-specific mRNA (1.65 kb). The level of this transcript increased by 10-fold after incubation of the dissociated cells with AF for 8 h; in contrast, lectin-1 caused only a small effect on the steady-state level of ubiquitin mRNA. These data indicate that the expression of the polyubiquitin gene is directly or indirectly regulated by the AF and suggest that ubiquitination might be a process which controls the function of the membrane-associated lectin-binding protein during matrix-cell adhesion.