RNA polymerase II transcription inhibits DNA repair by photolyase in the transcribed strand of active yeast genes.

Yeast uses nucleotide excision repair (NER) and photolyase (photoreactivation) to repair cyclobutane pyrimidine dimers (CPDs) generated by ultraviolet light. In active genes, NER preferentially repairs the transcribed strand (TS). In contrast, we recently showed that photolyase preferentially repairs the non-transcribed strands (NTS) of the URA3 and HIS3 genes in minichromosomes. To test whether photoreactivation depends on transcription, repair of CPDs was investigated in the transcriptionally regulated GAL10 gene in a yeast strain deficient in NER [AMY3 (rad1Delta)]. In the active gene (cells grown in galactose), photoreactivation was fast in the NTS and slow in the TS demonstrating preferential repair of the NTS. In the inactive gene (cells grown in glucose), both strands were repaired at similar rates. This suggests that RNA polymerases II blocked at CPDs inhibit accessibility of CPDs to photolyase. In a strain in which both pathways are operational [W303-1a (RAD1)], no strand bias was observed either in the active or inactive gene, demonstrating that photoreactivation of the NTS compensates preferential repair of the TS by NER. Moreover, repair of the NTS was more quickly in the active gene than in the repressed gene indicating that transcription dependent disruption of chromatin facilitates repair of an active gene.