The roles of Rad16 and Rad26 in repairing repressed and actively transcribed genes in yeast.

Nucleotide excision repair (NER) is a conserved DNA repair mechanism capable of removing a variety of helix-distorting DNA lesions. Rad26, a member of the Swi2/Snf2 superfamily of proteins, has been shown to be involved in a specialized NER process called transcription coupled NER. Rad16, another member of the same protein superfamily, has been shown to be required for genome-wide NER. Here we show that Rad16 and Rad26 play different roles in repairing repressed and actively transcribed genes in yeast. Rad16 is partially dispensable, and Rad26 plays a significant role in repairing certain regions of the repressed GAL1-10, PHO5 and ADH2 genes, especially in the core DNA of well-positioned nucleosomes. Simultaneous elimination of Rad16 and Rad26 results in no detectable repair in these regions of the repressed genes. Transcriptional induction of the GAL1-10 genes abolishes the role of Rad26, but does not affect the role of Rad16 in repairing the nontranscribed strand of the genes. Interestingly, when the transcription activator Gal4 is eliminated from the cells, Rad16 becomes partially dispensable and Rad26 plays a significant role in repairing both strands of the GAL1-10 genes even under inducing conditions. Our results suggest that Rad16 and Rad26 play different and, to some extent, complementary roles in repairing both strands of repressed genes, although the relative contributions of the two proteins can be different from gene to gene, and from region to region of a gene. However, Rad16 is solely responsible for repairing the nontranscribed strand of actively transcribed genes.