Abstract
Two mutants at the pyr 1 locus have been used to study the radiation sensitivity of pyrimidine auxotrophs of U. maydis. The mutant pyr 1-1 has a reduced level of thymidine nucleotides, and this is a likely basis of the sensitivity. This strain is able to excise pyrimidine dimers from its DNA and is cross-sensitive to γ-rays and nitrosoguanidine (NG) as well as to UV. A diploid heteroallelic at the pyr 1 locus was UV-sensitive but not deficient in UV-induced mitotic recombination. The results suggest that the UV sensitivity may be due to the failure of a repair DNA polymerase to fill post-excision single-strand gaps in the DNA. The mutant pyr 1-1 exhibits the property of UV recovery, and this is shown to be dependent on the presence of dimers in the DNA. A mechanism for UV recovery is proposed in which a repair system, possibly involving recombination, is induced by the UV irradiation.
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