Recognition of RNA Polymerase II and Transcription Bubbles by XPG, CSB, and TFIIH: Insights for Transcription-Coupled Repair and Cockayne Syndrome

Abstract

Loss of a nonenzymatic function of XPG results in defective transcription-coupled repair (TCR), Cockayne syndrome (CS), and early death, but the molecular basis for these phenotypes is unknown. Mutation of CSB, CSA, or the TFIIH helicases XPB and XPD can also cause defective TCR and CS. We show that XPG interacts with elongating RNA polymerase II (RNAPII) in the cell and binds stalled RNAPII ternary complexes in vitro both independently and cooperatively with CSB. XPG binds transcription-sized DNA bubbles through two domains not required for incision and functionally interacts with CSB on these bubbles to stimulate its ATPase activity. Bound RNAPII blocks bubble incision by XPG, but an ATP hydrolysis-dependent process involving TFIIH creates access to the junction, allowing incision. Together, these results implicate coordinated recognition of stalled transcription by XPG and CSB in TCR initiation and suggest that TFIIH-dependent remodeling of stalled RNAPII without release may be sufficient to allow repair.

Transcription-coupled nucleotide excision repair in mammalian cells: molecular mechanisms and biological effects

“…Recent in vitro studies suggested that XPG might play an important role in the assembly of the TC-NER complex as XPG interacts with stalled RNAPIIo both independently and cooperatively with CSB [77]. Nonetheless in the context of chromatin, XPG appears to act downstream of CSB in TC-NER complex formation [41].…”

“…As mentioned above, several studies suggest that the mammalian TC-NER complex is built up without displacement of RNAPIIo [41,70,71,77]. Importantly, the stalled RNAPIIo at the lesion may not be a sufficient barrier to prevent the DNA lesion from being accessible to repair proteins [54,71].…”

“…Importantly, the stalled RNAPIIo at the lesion may not be a sufficient barrier to prevent the DNA lesion from being accessible to repair proteins [54,71]. Conformational changes of the RNAPIIo might be required to allow accessibility to repair proteins, and such changes require the CSB protein although in vitro the assembly of the NER complex occurs in the absence of CSB [70,77]. Backtracking of RNAPIIo might be one of the key mechanisms to allow repair and/or transcription restart.…”

Other Proteins Interacting with XP Proteins

“…6 This finding may suggest a DNA damage recognition mechanism in which XPG initiates the recruitment of repair factors to remodel the stalled transcription bubble to repair lesions encountered by the elongating polymerase. 66 As with XPC, XPG also interacts with proteins from the BER pathway, in this case Nth1. Nth1 is a DNA glycosylase-AP lyase involved in repairing thymine glycol lesions and other types of BER lesions.…”

XPG: Its Products and Biological Roles

“…Another study has suggested that XPG interacts directly with RNA polymerase II pointing to an earlier role of the protein in TC-NER in cooperation with the CSB protein. 46 The detailed mechanism of TC-NER and the role of XPG in this process are still not fully resolved and continue to be an active area of research. 47 …”