Mutagenesis of bleomycin‐damaged lambda phage in SOS‐deficient and repair endonuclease‐deficient escherichia coli

Previous DNA sequence analysis of bleomycin‐induced forward mutations in repackaged lambda phage has suggested SOS‐dependent replicative bypass of oxidized apyrimidinic sites as a possible mechanism of mutagenesis. In order to evaluate this hypothesis further, frequencies of mutation to a clear‐plaque phenotype were compared for bleomycin‐damaged phage grown in various repair‐deficient strains of Escherichia coli. Survival of bleomycin‐damaged phage was virtually identical in all host strains. Studies in SOS‐deficient strains indicated specific requirements for functional recA+ and umuC+ alleles in the generation of the majority of bleomycininduced mutations, as well as a less stringent requirement for induction of the SOS response by ultraviolet irradiation of the host cells. These results are expected for mutagenesis resulting from apyrimidinic sites. However, the mutation frequency for bleomycin‐damaged phage was the same whether the phage were grown in a wild‐type strain or in strains deficient in apurinic/apyrimidinic repair endonucleases; this was true even for an nth∼nfo∼xth∼ strain lacking all three major apurinic/apyrimidinic endonucleases (endonuclease III, endonuclease IV, and exonuclease III). Likewise, phage grown in an endonuclease IV‐overproducing strain showed the same mutation frequency as those grown in wild‐type cells. These data suggest that either i) bleomycin‐induced mutagenesis results from SOS‐dependent bypass of lesions other than apyrimidinic sites or ii) the number of apyrimidinic sites available for SOS processing is virtually independent of the level of apurinic/apyrimidinic endonuclease activity in the cell. It is possible that a fraction of the apyrimidinic sites induced by bleomycin either are intrinsically resistant to repair or undergo secondary reactions that render them resistant.