A structural model of elongating RNA polymerase II (cyan) in the act of transcribing a gene. The DNA strands are yellow and green, respectively, while the newly synthesized RNA is red.

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Transcription strands

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.

Stepwise model for TC-NER.

Figure 1. Stepwise model for TC-NER. Note that RNAPII is likely to be present during the whole reaction and to restart transcription after repair, but it has been omitted from the cartoon in later steps for simplicity.