Mutant pso8-1 of Saccharomyces cerevisiae, sensitive to photoactivated psoralens, UV radiation, and chemical mutagens, contains a rad6 missense mutant allele.

A novel mutant isolate of Saccharomyces cerevisiae, sensitive to photoactivated mono- and bi-functional psoralens, to UV at 254 nm (UVC), and to nitrosoguanidine, was found to complement the photoactivated psoralen-sensitivity phenotype conferred by the pso1- pso7 mutations and was therefore named pso8-1. A constructed pso8-1 rad4-4 double mutant was super-sensitive to UVC, thus indicating a synergistic interaction of the two mutant alleles. Molecular cloning via complementation of the pso8 mutant's sensitivity phenotype and genetic studies revealed that pso8 is allelic to RAD6. While a pso8-1 mutant had low mutagen-induced mutability, homoallelic diploids showed nearly wild-type sporulation. Sequence analysis of the mutant allele showed pso8-1 to contain a novel, hitherto undescribed T-->C transition in nucleotide position 191, leading to a substitution by leucine of a highly conserved proline at position 64, Rad6-[P64L], which may have severe consequences for the tertiary structure (and hence binding to Rad18p) of the mutant protein.