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 (photoreact ivation) to repair cyclobutane pyrimidine dimers (CPDs) generated by u ltraviolet light, In active genes, NER preferentially repairs the tran scribed strand (TS), In contrast, we recently showed that photolyase p referentially repairs the non-transcribed strands (NTS) of the URA3 an d HIS3 genes in minichromosomes, To test whether photoreactivation dep ends on transcription, repair of CPDs was investigated in the transcri ptionally regulated GAL10 gene in a yeast strain deficient in NER [AMY 3 (rad1 Delta)], In the active gene (cells grown in galactose), photor eactivation was fast in the NTS and slow in the TS demonstrating prefe rential repair of the NTS, In the inactive gene (cells grown in glucos e), both strands were repaired at similar rates. This suggests that RN A polymerases II blocked at CPDs inhibit accessibility of CPDs to phot olyase, In a strain in which both pathways are operational [W303-1a (R AD1)], no strand bias was observed either in the active or inactive ge ne, demonstrating that photoreactivation of the NTS compensates prefer ential repair of the TS by NER, Moreover, repair of the NTS was more q uickly in the active gene than in the repressed gene indicating that t ranscription dependent disruption of chromatin facilitates repair of a n active gene.