Inducible nucleotide excision repair (NER) of UV-induced cyclobutane pyrimidine dimers in the cell cycle of the budding yeast Saccharomyces cerevisiae : evidence that inducible NER is confined to the G1 phase of the mitotic cell cycle
We previously reported on an inducible component of nucleotide excision repair in Saccharomyces cerevisiae that is controlled by the RAD16 gene. Here we describe a study of this event at the MATα and HMLα mating-type loci and on the transcribed (TS) and nontranscribed (NTS) strands of the RAD16 gene. Events were examined at various stages of the mitotic cycle in cells synchronised by centrifugal elutriation. Repair of cyclobutane pyrimidine dimers (CPDs) following a single UV dose does not vary significantly in different stages of the mitotic cell cycle. CPDs are removed more rapidly from the transcriptionally active MATα locus than from the silent HMLα locus, and the TS of RAD16 is repaired faster than the NTS in all stages of the cycle following a single UV irradiation. Enhanced excision of CPDs at MATα and HMLα can be induced only in the G1 and early S stages of the cell cycle. Here prior irradiation of cells with 25 J/m2 enhances the removal of CPDs following a second UV dose of 70 J/m2. The level of enhancement of repair does not differ significantly between MATα and HMLα in G1. Enhanced removal of CPDs is absent when cells receive the inducing dose in late S or G2/M. Repair of CPDs in both strands of RAD16 is similarly enhanced only if cells receive the initial irradiation in G1 and early S. The level of enhanced removal of CPDs is not significantly different in the TS and NTS of RAD16 either in asynchronous cells or in cells preirradiated in G1 and early S. It has been shown by others that UV-induced expression of RAD16 remains at high levels if cells are held in G1 by treatment with α factor. Therefore the increase in RAD16 transcript levels in G1 may be responsible for the ability to enhance NER solely in this stage of the cell cycle.
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