Radiation-sensitive pyrimidine auxotrophs of Ustilago maydis. II. A study of repair mechanisms and UV recovery in pyr I.

Two mutants at the pyr I locus have been used to study the radiation sensitivity of pyrimidine auxotrophs of U. maydis. The mutant pry I-I 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 gamma-rays and nitrosoguanidine (NG) as well as to UV. A diploid heteroallelic at the pyr I 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 I-I 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.