| The heterogeneity of DNA repair can be attributed to different genetic influences. Defects in transcription-coupled DNA repair (TCR), a pathway of nucleotide excision repair (NER) of ultraviolet light (UV)-induced DNA damage, are either due to deficiencies in transcription and/or repair. Using the yeast Saccharomyces cerevisiae as the model system, three sets of genes that affect transcription were examined for their influence on TCR or NER. First, the target of rapamycin (Tor) signaling pathway and treatment with the drug rapamycin regulate transcription. Rapamycin decreases NER independent of the rapamycin-sensitive Tor signaling pathway. Treatment of cells with rapamycin results in rapamycin forming a complex with Fpr1, which then inhibits Tor activity. Rapamycin treatment also competitively displaces proteins that interact with Fpr1, such as the Fpr1 associated protein Fap1. Fap1 is a yeast homolog of a human transcriptional repressor. Thus, in the presence of rapamycin, Fap1 appears to be an important inhibitor of transcription, and subsequently an inhibitor of NER. Secondly, transcription is influenced by chromatin-mediated repression or changes in expression patterns that are mediated by the silent information regulator (Sir) complexes. Mutations in sir3 affect TCR in a manner that seems not to be associated with chromatin-mediated silencing. In a sir3 mutant, changes in the expression of and/or interaction with DNA repair factors, such as Rad26 or Rad7, can influence the rates of strand-specific DNA repair. Thirdly, the heterologous expression of the Arabidopsis thaliana AtXPB1 plant gene in a yeast mutant defective in the homologous RAD25 gene, contributes in defining its ability to participate in the highly conserved mechanism of NER. Rad25 is a helicase subunit of the general transcription and repair factor TFIIH. AtXPB1 partially rescues NER, but not TCR, in a yeast rad25 mutant. Thus, AtXPB1 appears to interact with yeast NER machinery, but is unable to compensate for complete repair in a yeast rad25 mutant. My approach permitted me to study whether defects in TCR can be attributed to an absence of repair or transcription. This novel perspective has exposed a new area of research, namely the influence of signal transduction pathways on NER. |
