Tfb5 is partially dispensable for Rad26 mediated transcription coupled nucleotide excision repair in yeast

Nucleotide excision repair (NER) is a conserved DNA repair mechanism capable of removing a variety of helix-distorting DNA lesions. A specialized NER pathway, called transcription coupled NER (TC-NER), refers to preferential repair in the transcribed strand of an actively transcribed gene. To be distinguished from TCR-NER, the genome-wide NER process is termed as global genomic NER (GG-NER). In Saccharomyces cerevisiae, GG-NER is dependent on Rad7, whereas TC-NER is mediated by Rad26, the homolog of the human Cockayne syndrome group B protein, and by Rpb9, a non-essential subunit of RNA polymerase II. Tfb5, the tenth subunit of the transcription/repair factor TFIIH, is implicated in one group of the human syndrome trichothiodystrophy. Here, we show that Tfb5 plays different roles in different NER pathways in yeast. No repair takes place in the non-transcribed strand of a gene in tfb5 cells, or in both strands of a gene in rad26 rpb9 tfb5 cells, indicating that Tfb5 is essential for GG-NER. However, residual repair occurs in the transcribed strand of a gene in tfb5 cells, suggesting that Tfb5 is important, but not absolutely required for TC-NER. Interestingly, substantial repair occurs in the transcribed strand of a gene in rad7 tfb5 and rad7 rpb9 tfb5 cells, indicating that, in the absence of GG-NER, Tfb5 is largely dispensable for Rad26 mediated TC-NER. Furthermore, we show that no repair takes place in the transcribed strand of a gene in rad7 rad26 tfb5 cells, suggesting that Tfb5 is required for Rpb9 mediated TC-NER. Taken together, our results indicate that Tfb5 is partially dispensable for Rad26 mediated TC-NER, especially in GG-NER deficient cells. However, this TFIIH subunit is required for other NER pathways.