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Meiotic recombination enhances genetic diversity as well as ensures proper segregation of homologous chromosomes, requiring Spoil-initiated double-strand breaks (DSBs). DNA deaminases act on regions of single-stranded DNA and deaminate cytosine to uracil (dU). In the immunoglobulin locus, this lesion will initiate point mutations, gene conversion, and DNA recombination. To begin to delineate the effect of induced base lesions on meiosis, we analyzed the effect of expressing DNA deaminases (activation-induced deaminase, AID, and APOBEC3C) in germ cells. We show that meiotic dU:dG lesions can partially rescue a spo11[DELTA] phenotype in yeast and worm. In rec12 Schizosaccharomyces pombe, AID expression increased proper chromosome segregation, thereby enhancing spore viability, and induced low-frequency meiotic crossovers. Expression of AID in the germ cells of Caenorhabditis elegans spo-11 induced meiotic RAD-51 foci formation and chromosomal bivalency and segregation, as well as an increase in viability. RNAi experiments showed that this rescue was dependent on uracil DNA-glycosylase (Ung). Furthermore, unlike ionizing radiation-induced spo-11 rescue, AID expression did not induce large numbers of DSBs during the rescue. This suggests that the products of DNA deamination and base excision repair, such as uracil, an abasic site, or a single-stranded nick, are sufficient to initiate and alter meiotic recombination in uni- and multicellular organisms. DOI: 10.1534/genetics.109.101683 more...

Elevated mitotic recombination and cell cycle delays are two of the cellular responses to UV-induced DNA damage. Cell cycle delays in response to DNA damage are mediated via checkpoint proteins. Two distinct DNA damage checkpoints have been characterized in Schizosaccharomyces pombe: an intra-S-phase checkpoint slows replication and a [G.sub.2]/M checkpoint stops cells passing from [G.sub.2] into mitosis. In this study we have sought to determine whether UV damage-induced mitotic intrachromosomal recombination relies on damage-induced cell cycle delays. The spontaneous and UV-induced recombination phenotypes were determined for checkpoint mutants lacking the intra-S and/or the [G.sub.2]/M checkpoint. Spontaneous mitotic recombinants are thought to arise due to endogenous DNA damage and/or intrinsic stalling of replication forks. Cells lacking only the intra-S checkpoint exhibited no UV-induced increase in the frequency of recombinants above spontaneous levels. Mutants lacking the [G.sub.2]/M checkpoint exhibited a novel phenotype; following UV irradiation the recombinant frequency fell below the frequency of spontaneous recombinants. This implies that, as well as UV-induced recombinants, spontaneous recombinants are also lost in [G.sub.2]/M mutants after UV irradiation. Therefore, as well as lack of time for DNA repair, loss of spontaneous and damage-induced recombinants also contributes to cell death in UV-irradiated [G.sub.2]/M checkpoint mutants. more...

Activation of Cds1 kinase that is S-phase-specific gives definition of a subpathway of the checkpoint response in Schizosaccharomyces pombe. Cds1 kinase is necessary to slow S phase where there are of DNA-damaging agents. Cds1 is not necessary for the mitotic-arrest checkpoints; it defines an S-phase specific subpathway of the checkpoint response. That is different from what happens with the Saccharomyces cerevisiae counterpart, Rad53. It seems that an intrinsic mechanism linking S phase and mitosis may work in a way that is independent of known checkpoint proteins. more...

The Saccharomyces cerevisiae HO gene and MATa cutting site were used to introduce site-specific double-strand breaks (DSBs) within intrachromosomal recombination substrates in Schizosaccharomyces pombe. The recombination substrates consisted of nontandem direct repeats of ade6 heteroalleles. DSB induction stimulated the frequency of recombinants 2000-fold. The spectrum of DSB-induced recombinants depended on whether the DSB was introduced within one of the ade6 repeats or in intervening unique DNA. When the DSB was introduced within unique DNA, over 99.8% of the recombinants lacked the intervening DNA but retained one copy of ade6 that was wild type or either one of the heteroalleles. When the DSB was located in duplicated DNA, 77% of the recombinants were similar to the deletion types described above, but the single ade6 copy was either wild type or exclusively that of the uncut repeat. The remaining 23% of the induced recombinants were gene convertants with two copies of ade6 and the intervening sequences; the ade6 heteroallele in which the DSB was induced was the recipient of genetic information. Half-sectored colonies were isolated, analyzed and interpreted as evidence of heteroduplex DNA formation. The results are discussed in terms of current models for recombination. more...

Twenty-three mutants that were hypersensitive to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and 4-nitroquinoline-1-oxide (4-NQO) were isolated and studied for their effects on spontaneous intragenic recombination. Ten of these had no significant effects on recombination. Reactions to different concentrations of MNNG and 4-NQO were varied. Most of the nuv mutations were fully recessive and only a few were semidominant. Six nonallelic loci were identified in trans complementation analysis of seven of these mutations. more...