Roles for the Transcription Elongation Factor NusA in Both DNA Repair and Damage Tolerance Pathways in Escherichia coli

Here we report our observations that have led us to propose that the transcription elongation factor NusA promotes a novel class of transcription-coupled repair (TCR) in addition to its previously proposed role in recruiting translesion synthesis (TLS) DNA polymerases to gaps encountered during transcription. Earlier we have reported that NusA physically and genetically interacts with the TLS DNA polymerase DinB (DNA pol IV). We find that Escherichia coli nusA11(ts) mutant strains, at the permissive temperature, are highly sensitive to nitrofurazone (NFZ) and 4-nitroquinolone-1-oxide but not to ultraviolet radiation. Gene expression profiling suggests this sensitivity is unlikely to be due to an indirect effect on gene expression affecting a known DNA repair or damage tolerance pathway. We demonstrate that an N2-furfuryl-dG (N2-f-dG) lesion, a structural analog of the principal lesion generated by NFZ, blocks transcription by E. coli RNA polymerase (RNAP) when present in the transcribed strand, but not when present in the non-transcribed strand. Our genetic analysis suggests that NusA participates in a nucleotide excision repair (NER)-dependent process to promote NFZ resistance. We provide evidence that transcription plays a role in the repair of NFZ-induced lesions through the isolation of RNAP mutants that display altered ability to survive NFZ exposure. We propose that NusA participates in a novel class of TCR involved in the identification and removal of a class of lesion, such as the N2-f-dG lesion, which are accurately and efficiently bypassed by DinB in addition to recruiting DinB for TLS at gaps encountered by RNAP. Wild-type and nusA11 samples were analyzed, with 3 replicates per sample.