S. cerevisiae Srs2 helicase ensures normal recombination intermediate metabolism during meiosis and prevents accumulation of Rad51 aggregates

Positive and negative regulators of RAD51/DMC1 in homologous recombination and DNA replication

RAD51 recombinase plays a central role in homologous recombination (HR) by forming a nucleoprotein filament on single-stranded DNA (ssDNA) to catalyze homology search and strand exchange between the ssDNA and a homologous double-stranded DNA (dsDNA). The catalytic activity of RAD51 assembled on ssDNA is critical for the DNA-homology-mediated repair of DNA double-strand breaks in somatic and meiotic cells and restarting stalled replication forks during DNA replication. The RAD51-ssDNA complex also plays a structural role in protecting the regressed/reversed replication fork. Two types of regulators control RAD51 filament formation, stability, and dynamics, namely positive regulators, including mediators, and negative regulators, so-called remodelers. The appropriate balance of action by the two regulators assures genome stability. This review describes the roles of positive and negative RAD51 regulators in HR and DNA replication and its meiosis-specific homolog DMC1 in meiotic recombination. We also provide future study directions for a comprehensive understanding of RAD51/DMC1-mediated regulation in maintaining and inheriting genome integrity.

The Dmc1 recombinase physically interacts with and promotes the meiotic crossover functions of the Mlh1-Mlh3 endonuclease

The accurate segregation of homologous chromosomes during the Meiosis I reductional division in most sexually reproducing eukaryotes requires crossing over between homologs. In baker's yeast approximately 80 percent of meiotic crossovers result from Mlh1-Mlh3 and Exo1 acting to resolve double-Holliday junction (dHJ) intermediates in a biased manner. Little is known about how Mlh1-Mlh3 is recruited to recombination intermediates and whether it interacts with other meiotic factors prior to its role in crossover resolution. We performed a haploinsufficiency screen in baker's yeast to identify novel genetic interactors with Mlh1-Mlh3 using sensitized mlh3 alleles that disrupt the stability of the Mlh1-Mlh3 complex and confer defects in mismatch repair but do not disrupt meiotic crossing over. We identified several genetic interactions between MLH3 and DMC1, the recombinase responsible for recombination between homologous chromosomes during meiosis. We then showed that Mlh3 physically interacts with Dmc1 in vitro and at times in meiotic prophase when Dmc1 acts as a recombinase. Interestingly, restricting MLH3 expression to roughly the time of crossover resolution resulted in a mlh3 null-like phenotype for crossing over. Our data are consistent with a model in which Dmc1 nucleates a polymer of Mlh1-Mlh3 to promote crossing over.

Repeated strand invasion and extensive branch migration are hallmarks of meiotic recombination

Currently favored models for meiotic recombination posit that both noncrossover and crossover recombination are initiated by DNA double-strand breaks but form by different mechanisms: noncrossovers by synthesis-dependent strand annealing and crossovers by formation and resolution of double Holliday junctions centered around the break. This dual mechanism hypothesis predicts different hybrid DNA patterns in noncrossover and crossover recombinants. We show that these predictions are not upheld, by mapping with unprecedented resolution parental strand contributions to recombinants at a model locus. Instead, break repair in both noncrossovers and crossovers involves synthesis-dependent strand annealing, often with multiple rounds of strand invasion. Crossover-specific double Holliday junction formation occurs via processes involving branch migration as an integral feature, one that can be separated from repair of the break itself. These findings reveal meiotic recombination to be a highly dynamic process and prompt a new view of the relationship between crossover and noncrossover recombination.

Rad51 filament dynamics and its antagonistic modulators

Rad51 recombinase is the central player in homologous recombination, the faithful repair pathway for double-strand breaks and key event during meiosis. Rad51 forms nucleoprotein filaments on single-stranded DNA, exposed by a double-strand break. These filaments are responsible for homology search and strand invasion, which lead to homology-directed repair. Due to its central roles in DNA repair and genome stability, Rad51 is modulated by multiple factors and post-translational modifications. In this review, we summarize our current understanding of the dynamics of Rad51 filaments, the roles of other factors and their modes of action in modulating key stages of Rad51 filaments: formation, stability and disassembly.

Special issue on "recent advances in meiosis from DNA replication to chromosome segregation"

Meiosis is the special division that produces haploid gametes, such as sperm and eggs. It involves a complex series of events that integrate large structural changes at the chromosome scale with fine regulation of recombination events in localized regions. To evaluate the complexity of these processes, the meiosis field covers a variety of disciplines and model organisms, making it an exciting and rapidly changing area of research. The field as a whole highlights both the conserved aspects of meiosis, as well as the marked diversity of the means taken to ensure that, ultimately, gametes will contain a balanced number of chromosomes and genetic diversity will have been produced. Studying meiosis is also critically important for the improvement of our human condition as errors of meiosis are a leading cause of infertility, miscarriage, and developmental disabilities. Finally, the complex chromosome behavior of meiosis is a genetically tractable paradigm, the study of which improves our understanding of many fundamental cellular processes including DNA repair, genome stability, cancer etiology, chromatin structure, and chromosome dynamics.This special issue on meiosis contains twenty-two papers, of which five are in-depth reviews that complement and put in context the experimental data presented in the seventeen original research articles. The content of this issue illustrates the diversity of topics covered by researchers in the field, ranging from the effects of environment and external factors on the success of meiosis, the cell cycle actors that control the meiotic divisions, the mechanism of chromosome segregation, and the mechanisms that ensure proper homologous chromosome pairing, recombination, and synapsis. Multiple organisms are covered. Also evident is the fact that more and more studies use multicellular organisms as a model system, in large part due to the increased availability of tools that were previously restricted to studies in budding and fission yeasts.

Resolvases, Dissolvases, and Helicases in Homologous Recombination: Clearing the Road for Chromosome Segregation

The execution of recombinational pathways during the repair of certain DNA lesions or in the meiotic program is associated to the formation of joint molecules that physically hold chromosomes together. These structures must be disengaged prior to the onset of chromosome segregation. Failure in the resolution of these linkages can lead to chromosome breakage and nondisjunction events that can alter the normal distribution of the genomic material to the progeny. To avoid this situation, cells have developed an arsenal of molecular complexes involving helicases, resolvases, and dissolvases that recognize and eliminate chromosome links. The correct orchestration of these enzymes promotes the timely removal of chromosomal connections ensuring the efficient segregation of the genome during cell division. In this review, we focus on the role of different DNA processing enzymes that collaborate in removing the linkages generated during the activation of the homologous recombination machinery as a consequence of the appearance of DNA breaks during the mitotic and meiotic programs. We will also discuss about the temporal regulation of these factors along the cell cycle, the consequences of their loss of function, and their specific role in the removal of chromosomal links to ensure the accurate segregation of the genomic material during cell division.

Network Rewiring of Homologous Recombination Enzymes during Mitotic Proliferation and Meiosis

Homologous recombination (HR) is essential for high-fidelity DNA repair during mitotic proliferation and meiosis. Yet, context-specific modifications must tailor the recombination machinery to avoid (mitosis) or enforce (meiosis) the formation of reciprocal exchanges-crossovers-between recombining chromosomes. To obtain molecular insight into how crossover control is achieved, we affinity purified 7 DNA-processing enzymes that channel HR intermediates into crossovers or noncrossovers from vegetative cells or cells undergoing meiosis. Using mass spectrometry, we provide a global characterization of their composition and reveal mitosis- and meiosis-specific modules in the interaction networks. Functional analyses of meiosis-specific interactors of MutLγ-Exo1 identified Rtk1, Caf120, and Chd1 as regulators of crossing-over. Chd1, which transiently associates with Exo1 at the prophase-to-metaphase I transition, enables the formation of MutLγ-dependent crossovers through its conserved ability to bind and displace nucleosomes. Thus, rewiring of the HR network, coupled to chromatin remodeling, promotes context-specific control of the recombination outcome.