Stalled transcription complexes promote DNA repair at a distance.

Transcription-coupled nucleotide excision repair (TCR) accelerates the removal of noncoding lesions from the template strand of active genes, and hence contributes to genome-wide variations in mutation frequency. Current models for TCR suppose that a lesion must cause RNA polymerase (RNAP) to stall if it is to be a substrate for accelerated repair. We have examined the substrate requirements for TCR using a system in which transcription stalling and damage location can be uncoupled. We show that Mfd-dependent TCR in bacteria involves the formation of a damage search complex that can detect lesions downstream of a stalled RNAP, and that the strand specificity of the accelerated repair pathway is independent of the requirement for a lesion to stall RNAP. We also show that an ops (operon polarity suppressor) transcription pause site, which causes backtracking of RNAP, can promote the repair of downstream lesions when those lesions do not themselves cause the polymerase to stall. Our findings indicate that the transcription-repair coupling factor Mfd, which is an ATP-dependent superfamily 2 helicase that binds to RNAP, continues to translocate along DNA after RNAP has been displaced until a lesion in the template strand is located. The discovery that pause sites can promote the repair of nonstalling lesions suggests that TCR pathways may play a wider role in modulating mutation frequencies in different parts of the genome than has previously been suspected.

Regulation and rate enhancement during transcription-coupled DNA repair.

Transcription-coupled DNA repair (TCR) is a subpathway of nucleotide excision repair (NER) that is triggered when RNA polymerase is stalled by DNA damage. Lesions targeted by TCR are repaired more quickly than lesions repaired by the transcription-independent "global" NER pathway, but the mechanism underlying this rate enhancement is not understood. Damage recognition during bacterial NER depends upon UvrA, which binds to the damage and loads UvrB onto the DNA. Bacterial TCR additionally requires the Mfd protein, a DNA translocase that removes the stalled transcription complexes. We have determined the properties of Mfd, UvrA, and UvrB that are required for the elevated rate of repair observed during TCR. We show that TCR and global NER differ in their requirements for damage recognition by UvrA, indicating that Mfd acts at the very earliest stage of the repair process and extending the functional similarities between TCR in bacteria and eukaryotes.