Bulky DNA lesions such as those generated by UV-irradiation block the progress of RNA polymerase II and therefore pose a particular threat to genome integrity and cell survival. Multiple mechanisms have evolved to minimise their detrimental effect.
Transcription-coupled nucleotide excision repair (TC-NER) ensures that the transcribed strand of an active gene is repaired much faster than the non-transcribed strand and the genome in general. Elucidating the mechanism of TC-NER is an important part of understanding the mechanisms of DNA repair.
Research in several laboratories has shown that Cockayne syndrome B (CSB) protein is recruited to damage-stalled RNAPII and is instrumental in recruiting basal nucleotide excision repair (NER) factors, as well as proteins required for subsequent repair-dependent DNA synthesis.
Without CSB, no TC-NER complex is assembled, and the CSA protein is not recruited either. CSA is dispensable for both CSB recruitment and TC-NER complex assembly, but is still absolutely required for TC-NER to take place. CSA is a component of a cullin-based ubiquitin ligase complex, but the functional importance of this activity in TC-NER is not known.
Interestingly, however, we have found that CSB contains a ubiquitin-binding domain (UBD), which is dispensable for repair complex assembly, but essential for activation of the DNA incision. With the discovery that CSB contain a functionally important UBD, it is an obvious possibility that CSA/Cullin-mediated ubiquitylation of a factor in the repair complex is recognized by CSB via its UBD, and that this in turn regulates the repair reaction.
It is a major goal for our research into transcription-coupled repair to understand the biochemical function of the CSA-cullin complex and CSB translocase in human cells.