Molecular Mechanism of Pyrimidine Dimer Excision in Saccharomyces cerevisiae. I. Studies with Intact Cells and Cell-Free Systems
We have investigated a number of aspects of the excision of pyrimidine dimers from the DNA of wild-type and UV sensitive mutants of the yeast Saccharomyces cerevisiae. Our studies show that a number of rad mutants in the RAD3 group (rad1-2, rad1-11; rad2-2, rad2-4; rad3−l; rad4−2, rad4−3) that are defective relative to wild-type strains in pyrimidine dimer excision in vivo, are also defective in the production of single strand breaks in their DNA during post-UV incubation. The presence of UV-induced incubation-independent single-strand breaks prevents definitive conclusions regarding the role of various RAD loci in the incision process but provides evidence of a biochemical subdivision in the RAD3 group loci. Using UV irradiated DNA preincised with dimer-specific endonuclease activity from Micrococcus luteus, we have also detected enzymatic activity from extracts of wild-type yeast that catalyzes the selective excision of thymine-containing pyrmidine dimers. Normal levels of this activity are present in all mutant strains thus far examined (rad1-11; rad2-4; rad3−l; rad4−3).
Thus, in Saccharomyces cerevisiae it appears that at least four of the nine genetic loci governing pyrimidine dimer excision affect events associated with DNA incision or preincision. This situation is strikingly analagous to that observed with the numerous complementation groups in the human disease xeroderma pigmentosum.
KeywordsNucleotide Excision Repair Single Strand Break Xeroderma Pigmentosum Micrococcus Luteus Pyrimidine Dimer
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