Abstract
In the yeast Saccharomyces cerevisiae and most other eukaryotes, mitotic recombination is important for the repair of double-stranded DNA breaks (DSBs). Mitotic recombination between homologous chromosomes can result in loss of heterozygosity (LOH). In this study, LOH events induced by ultraviolet (UV) light are mapped throughout the genome to a resolution of about 1 kb using single-nucleotide polymorphism (SNP) microarrays. UV doses that have little effect on the viability of diploid cells stimulate crossovers more than 1000-fold in wild-type cells. In addition, UV stimulates recombination in G1-synchronized cells about 10-fold more efficiently than in G2-synchronized cells. Importantly, at high doses of UV, most conversion events reflect the repair of two sister chromatids that are broken at approximately the same position whereas at low doses, most conversion events reflect the repair of a single broken chromatid. Genome-wide mapping of about 380 unselected crossovers, break-induced replication (BIR) events, and gene conversions shows that UV-induced recombination events occur throughout the genome without pronounced hotspots, although the ribosomal RNA gene cluster has a significantly lower frequency of crossovers.
Highlights
Recombination occurs in both meiotic and mitotic cells
In order to determine different types of mitotic recombination and to determine whether the conversion events are of the 3:1 or 4:0 configuration, we used a method of identifying recombination events that allows the recovery of both daughter cells with the recombinant chromosomes
Since the induction of recombination by UV is assumed to reflect the location of processed or unprocessed pyrimidine dimers and since most (68%) of the dimers formed involve TT [33], we examined the frequencies of AA/TT dinucleotides in unselected conversion events (0.218), selected conversion tracts on chromosome V in cells irradiated with 1 J/m2 (0.217) or 15 J/m2 (0.221)
Summary
Recombination occurs in both meiotic and mitotic cells. Mitotic recombination events in S. cerevisiae are about 105-fold less frequent than meiotic exchanges [2], homologous recombination (HR) is important for the repair of double-stranded DNA breaks (DSBs) that occur spontaneously or that are induced by DNA damage. Yeast strains that lack HR grow more slowly than wild-type strains, and are sensitive to DNA damaging agents [3]. In HR events in diploid cells, the broken chromosome is repaired utilizing an intact sister chromatid or homolog as a template. Most organisms have a pathway termed ‘‘nonhomologous end-joining’’ (NHEJ) in which the broken ends are rejoined by a mechanism that does not require sequence homology. In diploid cells of S. cerevisiae, HR is much more important than NHEJ for repair of DNA damage [4]. We will first discuss pathways of HR, followed by a description of UV-induced DNA damage, and the recombinogenic effects of this damage
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