The Role of Nucleotide Excision Repair in Restoring Replication Following UV-Induced Damage in Escherichia coli

Following low levels of UV exposure, Escherichia coli cells deficient in nucleotide excision repair (uvrA6 mutants) recover and synthesize DNA at near wild type levels, an observation that formed the basis of the post-replication recombination repair model. In this study, we characterized the DNA synthesis that occurs following UV-irradiation in the absence of nucleotide excision repair and show that although synthesis resumes at near wild type levels, less than 1% of the cells survive to form viable cells. Using twodimensional agarose gel analysis, we confirm that the replication occurring under these conditions involves extensive levels of strand exchange. However, cells undergoing this form of replication accumulate strand exchange intermediates that fail to resolve into discrete molecules, resulting in grossly filamentous, multinucleate cells. Taken together the results demonstrate that the DNA synthesis that occurs in UV-irradiated uvrA6 mutants is aberrant and suggests that post-replication repair is not an efficient mechanism to promote survival in the absence of nucleotide excision repair.