EXCISION REPAIR INFLUENCES THE SITE AND STRAND SPECIFICITY OF SUNLIGHT MUTAGENESIS IN YEAST

A collection of 384 mutations recovered in a tRNA gene (SUP4-o) following exposure of isogenic excision-repair-proficient (RAD1) or deficient (rad1) strains of the yeast Saccharomyces cerevisiae to sunlight was characterized by DNA sequencing. In each case, > 90% of the mutations were single base-pair substitutions with events at G . C pairs constituting most of the changes. However, more than half of these substitutions were transversions in the RAD1 strain whereas transitions predominated in the rad1 strain. Tandem double substitutions were recovered in both strains and the individual changes were exclusively G . C --> A . T transitions. The majority of single substitutions, and all tandem double changes, were at base-pairs where the pyrimidine(s) was part of a dipyrimidine sequence and the site specificities were consistent with cyclobutane dimers and/or pyrimidine (6-4) pyrimidone photoproducts contributing to sunlight mutagenesis. Yet, the data also pointed to an important role for lesions that form at G . C pairs and give rise to transversions. Analysis of the strand specificity of sunlight mutagenesis indicated that transitions or transversions at G - C pairs occurred preferentially in SUP4-o at sites where a dipyrimidine or a guanine, respectively, was on the transcribed strand. These biases required a functional excision-repair system.