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The possibility that a certain molecular alteration in the constituents of DNA by UV can be directly related to the mutational event has haunted many investigators (see Freese 1963 and Setlow 1964, for reviews). It has recently been possible to correlate the lethal action of UV with thymine dimer (TT) production in some systems (Setlow and Setlow 1962, 1963), but it is not yet possible to make a definite correlation for mutagenesis. In the meantime, clear evidence for TT was provided in the case of streptomycin resistance in E, coli by Witkin et al. (1963) that mutational events were photoreversible in normal strains but not in strains that lack photoreactivating enzyme. However, this was not the case for the mutation represented by reversions to prototrophy, indicating there are lesions other than TT that contribute to mutation (Kaplan 1963; Witkin 1963). In the previously reported experiments (Ito et al. 1964), a more physical analysis for the induced forward mutation, adenine independent to adenine requiring, was attempted, penetrating into the molecular level, by combining UV action spectrum for the mutation induction with photoreversion experiments. The shape of the action spectrum generally resembled that of the absorption spectrum of thymine in the experiments. It was also 0 found that the total cross section of the mutation at 2700A was phenomenologically separable into two components, photoreversible (major) and non-photoreversible (minor), indicating bases other than thymine may also be involved in this particular mutation. In this paper are presented further studies on the two components of the cross sections extended to several other wavelengths. The results may provide the complementary bases to the biochemical approaches for the interpretation of mutagenesis by UV.

FURTHER STUDIES ON THE UV-INDUCED PHOTOREVERSIBLE AND NON-PHOTOREVERSIBLE MUTATION THROUGH THE ACTION SPECTRA IN YEAST CELLS