THE SACCHAROMYCES-CEREVISIAE RAD7 AND RAD16 GENES ARE REQUIRED FOR INDUCIBLE EXCISION OF ENDONUCLEASE-III SENSITIVE-SITES, YET ARE NOT NEEDED FOR THE REPAIR OF THESE LESIONS FOLLOWING A SINGLE UV DOSE

The RAD7 and RAD16 genes of Saccharomyces cerevisiae have roles in the repair of UV induced CPDs in nontranscribed genes [1], and in the rep air of CPDs in the nontranscribed strand of transcribed genes [2]. Pre viously, we identified an inducible component to nucleotide excision r epair (NER), which is absent in a rad16 Delta strain [3]. We have exam ined the repair of UV induced endonuclease III sensitive-sites (EIIISS ), and have shown repair of these lesions to proceed by NER but their removal from nontranscribed regions is independent of RAD7 and RAD16. Furthermore, EIIISS are repaired with equal efficiency from both trans cribed and nontranscribed genes [4]. In order to dissect the roles of RAD7 and RAD16 in the above processes we examined the repair of EIIISS in the MAT alpha and HML alpha loci, which are, respectively, transcr iptionally active and inactive in a haploid cells. These loci have ele vated levels of these lesions after UV (in genomic DNA EIIISS constitu te about 10% of total lesions, whereas CPDs are about 70% of total les ions). We have shown that excision of UV induced EIIISS is enhanced fo llowing a prior UV irradiation. No enhancement of repair was detected in either the rad7 Delta or the rad16 Delta mutant. The fact that RAD7 and RAD16 are not required for the repair of EIIISS per se yet are re quired for the enhanced excision of these lesions from MAT alpha and H ML alpha suggests two possibilities. These genes have two roles in NER , namely in the repair of CPDs from nontranscribed sequences, and in e nhancing NER itself regardless of whether these genes' products are re quired for the excision of the specific lesion being repaired. In the latter case, the induction of RAD7 and RAD16 may increase the turnover of complexes stalled in nontranscribed DNA so as to increase the avai lability of NER proteins for the repair of CPDs and EIIISS in all regi ons of the genome.