RecA-mediated excision repair: a novel mechanism for repairing DNA lesions at sites of arrested DNA synthesis

In Escherichia coli, bulky DNA lesions are repaired primarily by nucleotide excision repair (NER). Unrepaired lesions encountered by DNA polymerase at the replication fork create a blockage which may be relieved through RecF-dependent recombination. We have designed an assay to monitor the different mechanisms through which a DNA polymerase blocked by a single AAF lesion may be rescued by homologous double-stranded DNA sequences. Monomodified single-stranded plasmids exhibit low survival in non-SOS induced E. coli cells; we show here that the presence of a homologous sequence enhances the survival of the damaged plasmid more than 1 0-fold in a RecA-dependent way. Remarkably, in an NER proficient strain, 80% of the surviving colonies result from the UvrA-dependent repair of the AAF lesion in a mechanism absolutely requiring RecA and RecF activity, while the remaining 20% of the surviving colonies result from homologous recombination mechanisms. These results uncover a novel mechanism - RecA-mediated excision repair - in which RecA-dependent pairing of the mono-modified single-stranded template with a complementary sequence allows its repair by the UvrABC excinuclease.