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 occurred1,2. 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.