THE FREQUENCY AND ACCURACY OF REPLICATION PAST A THYMINE-THYMINE CYCLOBUTANE DIMER ARE VERY DIFFERENT IN SACCHAROMYCES-CEREVISIAE AND ESCHERICHIA-COLI

We have compared the mutagenic properties of a T-T cyclobutane dimer i n baker's yeast, Saccharomyces cervisiae, with those in Escherichia co li by transforming each of these species with the same single-stranded shuttle vector carrying either the cis-syn or the trans-syn isomer of this UV photoproduct at a unique site. The mutagenic properties inves tigated were the frequency of replicational bypass of the photoproduct , the error rate of bypass, and the mutation spectrum. In SOS-induced E. coli, the cis-syn dimer was bypassed in approximately 16% of the ve ctor molecules, and 7.6% of the bypass products had targeted mutations . In S. cerevisiae, however, bypass occurred in about 80% of these mol ecules, and the bypass was at least 19-fold more accurate (approximate ly 0.4% targeted mutations). Each of these yeast mutations was a singl e unique event, and none were like those in E. coli, suggesting that i n fact the difference in error rate is much greater. Bypass of the tra ns-syn dimer occurred in about 17% of the vector molecules in both spe cies, but with this isomer the error rate was higher in S. cerevisiae (21 to 36% targeted mutations) than in E. coli (13%). However, the spe ctra of mutations induced by the latter photoproduct were virtually id entical in the two organisms. We conclude that bypass and error freque ncies are determined both by the structure of the photoproduct-contain ing template and by the particular replication proteins concerned but that the types of mutations induced depend predominantly on the struct ure of the template. Unlike E. coli, bypass in S. cerevisiae did not r equire UV-induced functions.