Initiation of UV mutagenesis in Saccharomyces cerevisiae

Abstract

STUDIES of UV mutagenesis in RAD⁺ and rad 1-1 (excision defective) strains of yeast have shown that the majority of forward mutations induced in RAD⁺ occur before the first cell division following UV exposure, whereas in rad 1-1 the first mutations only appear after the cell has divided and DNA replication has presumably occurred^1,2. These data have been interpreted as showing that error-prone repair in both RAD⁺ and rad 1-1 is initiated by the production of a single-stranded gap opposite a pyrimidine dimer in the complementary DNA strand. However, although this structure may be the same in both strains, it probably arises in entirely different ways. In RAD⁺ it can, in theory, be formed before DNA synthesis occurs provided two dimers are induced in complementary DNA strands sufficiently close together for excision of one to uncover the other (compare refs 3 and 4). In rad 1-1, which lacks excision, this cannot occur and, by analogy with E. coli, it is believed that only when dimers pass through DNA synthesis and daughter strand gaps are formed⁵, can error-prone repair be initiated. If these ideas are correct two predictions can be made concerning the behaviour of reverting auxotrophs in UV experiments with RAD⁺ and rad 1-1. Here we test these two predictions. The results support the dimer/gap model.