[Possible role of protein damage in the development of UV-induced isochromatid breaks]

The rate of structural chromosome mutations at the second K-mitosis was studied in the primary culture of embryonic fibroblasts from BALB mice, after treating these cells with UV-rays at various wavelength during G2-phase. The action spectrum of UV-rays for chromatid aberrations, determined at 254, 265, 280 and 302 nm, was found to be closely similar to the absorption spectra of thymidine and to the spectrum of the formation of DNA crosslinks. The action spectrum for isochromatid breaks, determined at the same wavelength, was similar to the spectra of protein absorption. Caffeine (1.37 mM) increases the rate of UV-induced chromatid breaks, when the cells were treated at the S-period of the second mitotic cycle after irradiation and had no effect on other aberration types. It is suggested that various primary damages are responsible for chromatid aberrations and isochromatid breaks. Transversal DNA-DNA crosslinks, formed as a result of thymine dimerization, underlie chromatid aberrations. Chromophor of protein nature is of importance in the origin of isochromatid breaks.