DOUBLE MUTANTS OF SACCHAROMYCES-CEREVISIAE WITH ALTERATIONS IN GLOBALGENOME AND TRANSCRIPTION-COUPLED REPAIR

The nucleotide excision repair (NER) pathway is thought to consist of two subpathways: transcription-coupled repair, limited to the transcri bed strand of active genes, and global genome repair for nontranscribe d DNA strands, Recently we cloned the RAD26 gene, the Saccharomyces ce rvisiae homolog of human CSB/ERCC6, a gene involved in transcription-c oupled repair and the disorder Cockayne syndrome, This paper describes the analysis of yeast double mutants selectively affected in each NER subpathway. Although rad26 disruption mutants are defective in transc ription-coupled repair, they are not UV sensitive, However, double mut ants of RAD26 with the global genome repair determinants RAD7 and RAD1 6 appeared more UV sensitive than the single rad7 or rad16 mutants but not as sensitive as completely NER-deficient mutants, These findings unmask a role of RAD26 and transcription-coupled repair in UV survival , indicate that transcription-coupled repair and global genome repair are partially overlapping, and provide evidence for a residual NER mod ality in the double mutants, Analysis of dimer removal from the active RPB2 gene in the rad7/16 rad26 double mutants revealed (i) a contribu tion of the global genome repair factors Rad7p and Rad16p to repair of the transcribed Strand, confirming the partial overlap between both N ER subpathways and (ii) residual repair specifically of the transcribe d strand, To investigate the transcription dependence of this repair a ctivity, strand-specific repair of the inducible GAL7 gene was investi gated, The template stand of this gene was repaired only under induced conditions, pointing to a role for transcription in the residual repa ir in the double mutants and suggesting that transcription-coupled rep air can to some extent operate independently from Rad26p, Our findings also indicate locus heterogeneity for the dependence of transcription -coupled repair on RAD26.