Strandedness of UV mutagenesis in E. coli is directed by transcription
We have investigated the consequences of transcription-coupled repair on UV mutagenesis throughout the genome. Wild type and mfd- cultures were irradiated, sequenced, and mutations were ascribed to presumptive dipyrimidine premutagenic damage sites and mapped. In wild-type cells, reduced transcribed strand mutations were seen particularly in highly transcribed genes, presumably due to removal of premutagenic damage by transcription-coupled repair. In mfd- cells, more transcribed strand mutations were seen, also in highly transcribed genes, consistent with the fact that in the absence of Mfd, RNA polymerase remains stalled at template strand damage and inhibits repair. Polymerase stalled at damage in mfd- cells had a greater effect on mutagenesis than TCR in wild type cells. These effects on mutation confirm and extend to the entire genome earlier work limited to individual genes. Our findings thus characterize the roles of Mfd and transcription in UV mutagenesis and the basis for the mutagenic phenotype associated with deletion of mfd.