Search

Members of the RecA family of strand exchange proteins carry out the central reaction in homologous recombination. These proteins are DNA-dependent ATPases, although their ATPase activity is not required for the key functions of homology search and strand exchange. We review the literature on the role of the intrinsic ATPase activity of strand exchange proteins. We also discuss the role of ATP-hydrolysis-dependent motor proteins that serve as strand exchange accessory factors, with an emphasis on the eukaryotic Rad54 family of double strand DNA-specific translocases. The energy from ATP allows recombination events to progress from the strand exchange stage to subsequent stages. ATP hydrolysis also functions to corrects DNA binding errors, including particularly detrimental binding to double strand DNA.

The conserved Rad2/XPG family 5′–3′ exonuclease, exonuclease 1 (Exo1), plays many roles in DNA metabolism including during resolution of DNA double-strand breaks via homologous recombination. Prior studies provided evidence that the end resection activity of Exo1 is downregulated in yeast and mammals by Cdk1/2 family cyclin-dependent and checkpoint kinases, including budding yeast kinase Rad53 which functions in mitotic cells. Here, we provide evidence that the master meiotic kinase Mek1 , a paralog of Rad53 , limits 5′–3′ single-strand resection at the sites of programmed meiotic DNA breaks. Mutational analysis suggests that the mechanism of Exo1 suppression by Mek1 differs from that of Rad53. [ABSTRACT FROM AUTHOR]

The RAD54 family DNA translocases have several biochemical activities. One activity, demonstrated previously for the budding yeast translocases, is ATPase-dependent disruption of RAD51-dsDNA binding. This activity is thought to promote dissociation of RAD51 from heteroduplex DNA following strand exchange during homologous recombination. In addition, previous experiments in budding yeast have shown that the same activity of Rad54 removes Rad51 from undamaged sites on chromosomes; mutants lacking Rad54 accumulate nonrepair-associated complexes that can block growth and lead to chromosome loss. Here, we show that human RAD54 also promotes the dissociation of RAD51 from dsDNA and not ssDNA. We also show that translocase depletion in tumor cell lines leads to the accumulation of RAD51 on chromosomes, forming complexes that are not associated with markers of DNA damage. We further show that combined depletion of RAD54L and RAD54B and/or artificial induction of RAD51 overexpression blocks replication and promotes chromosome segregation defects. These results support a model in which RAD54L and RAD54B counteract genome-destabilizing effects of direct binding of RAD51 to dsDNA in human tumor cells. Thus, in addition to having genome-stabilizing DNA repair activity, human RAD51 has genome-destabilizing activity when expressed at high levels, as is the case in many human tumors.

Two RecA-like recombinases, Rad51 and Dmcl, function together during double-strand break (DSB)-mediated meiotic recombination to promote homologous strand invasion in the budding yeast Saccharomyces cerevisiae. Two partially redundant proteins, Rad54 and Tid1/Rdh54, act as recombinase accessory factors. Here, tetrad analysis shows that mutants lacking Tid1 form four-viable-spore tetrads with levels of interhomolog crossover (CO) and noncrossover recombination similar to, or slightly greater than, those in wild type. Importantly, tid1 mutants show a marked defect in crossover interference, a mechanism that distributes crossover events nonrandomly along chromosomes during meiosis. Previous work showed that dmc1[DELTA] mutants are strongly defective in strand invasion and meiotic progression and that these defects can be partially suppressed by increasing the copy number of RAD54. Tetrad analysis is used to show that meiotic recombination in RAD54-suppressed dmc1[DELTA] cells is similar to that in tid1; the frequency of COs and gene conversions is near normal, but crossover interference is defective. These results support the proposal that crossover interference acts at the strand invasion stage of recombination.

Results demonstrate that in Saccharaomyces cerevisiae and Caenorhabditis elegans, the phosphorylation of histones plays a key role in the proper segregation of chromosomes in mitotic cell division. Data further indicate that aurora kinase and its genetically related phosphatase are involved in the phosphorylation process.

Rad52, the Saccharomyces cerevisiae recombination protein, and RPA, the single-strand DNA-binding protein, combine into cytologically detectable subnuclear complexes in recombination in meiosis. Double-strand breaks (DSBs) are not adequate for the appearance of Rad51 foci, the detectable subnuclear complexes. They promote formation of RPA, Rad52 and Rad51 foci.

Mitotic DNA-damage checkpoint genes RAD17, RAD24 and MEC1 are components of checkpoint control that ensures the completion of meiotic recombination prior to the first meiotic division. Single mutants mec1 and rad24 undergo mitotic division before the completion of recombination. In both meiosis and mitosis, Rad 17 and Rad24 have to associate with ssDNA to promote an arresting mechanism. However, meiotic recombination control mechanism is different from the mitotic checkpoint control.

Immunostaining of the yeast homologs of the Escherichia coli RecA protein, Dmc1 and Rad51, indicates their presence in nearly 64 sites within spread meiotic nuclei and their colocalization indicates their synergistic activity during recombination. Immunostaining for the Zip1 protein indicates the formation of dmc1 complexes in rad51 mutants and rad51 complexes in dcm1 mutants prior to synapsis. The dissociation of Dmc1 complexes is facilitated by Zip1, as revealed by an analysis of zip1 mutants.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2006.11.029 Byline: Douglas K. Bishop (1)(2) Abstract: In this issue, reveal that different meiotic recombination mechanisms predominate in fission yeast and budding yeast. Budding yeast usually form crossover recombinants through double Holliday junctions, whereas fission yeast unexpectedly appear to form crossover recombinants through single junctions. Author Affiliation: (1) Department of Radiation and Cellular Oncology, University of Chicago, 920 E. 58.sup.th Street, Chicago, IL 60637, USA (2) Department of Molecular Genetics and Cell Biology, University of Chicago, 920 E. 58.sup.th Street, Chicago, IL 60637, USA

The number and distribution of meiotic crossovers (COs) are highly regulated, reflecting the requirement for COs during the first round of meiotic chromosome segregation. CO control includes CO assurance and CO interference, which promote at least one CO per chromosome bivalent and evenly-spaced COs, respectively. Previous studies revealed a role for the DNA damage response (DDR) clamp and the clamp loader in CO formation by promoting interfering COs and interhomolog recombination, and also by suppressing ectopic recombination. In this study, we use classical tetrad analysis of Saccharomyces cerevisiae to show that a mutant defective in RAD24, which encodes the DDR clamp loader (RAD17 in other organisms), displayed reduced CO frequencies on two shorter chromosomes (III and V), but not on a long chromosome (chromosome VII). The residual COs in the rad24 mutant do not show interference. In contrast to rad24, mutants defective in the ATR kinase homolog Mec1, including a mec1 null and a mec1 kinase-dead mutant, show slight or few defects in CO frequency. On the other hand, mec1 COs show defects in interference, similar to the rad24 mutant. Our results support a model in which the DDR clamp and clamp-loader proteins promote interfering COs by recruiting pro-CO Zip, Mer, and Msh proteins to recombination sites, while the Mec1 kinase regulates CO distribution by a distinct mechanism. Moreover, CO formation and its control are implemented in a chromosome-specific manner, which may reflect a role for chromosome size in regulation. [ABSTRACT FROM AUTHOR]

Tid1/Rdh54, a member of the Swi2/Snf2 family of DNA translocates, has been shown to stimulate Dmc1-dependent recombination. A demonstration that Dmc1 could associate with chromatin in the absence of DNA double-strand breaks (DSBs) and Tid1 suppresses this association is presented.

The discovery of a novel meiosis-specific protein, Hed1, which appears to inhibit Rad51 when Dmc1 is absent, is reported. Two-hybrid experiments show that Hed1 and Rad51 can interact directly and immuno-staining shows that subnuclear foci formed by Hed1 and Rad51 colocalize, implying that Hed1's influence on Rad51 activity is direct.

Studies conducted in budding yeast shows that crossovers (CO) and noncrossovers (NCO) designation occurs before stable strand exchange. The results throw new light on the regulation of meiotic recombination and its relationship to chromosome synapsis.