Schizosaccharomyces pombe Rad22A and Rad22B have similar biochemical properties and form multimeric structures

Genetic interaction of the histone chaperone hip1 + with double strand break repair genes in Schizosaccharomyces pombe

Schizosaccharomyces pombe hip1 + (human HIRA) is a histone chaperone and transcription factor involved in establishment of the centromeric chromatin and chromosome segregation, regulation of histone transcription, and cellular response to stress. We carried out a double mutant genetic screen of Δhip1 and mutations in double strand break repair pathway. We find that hip1 + functions after the MRN complex which initiates resection of blunt double strand break ends but before recruitment of the DNA damage repair machinery. Further, deletion of hip1 + partially suppresses sensitivity to DNA damaging agents of mutations in genes involved in Break Induced Replication (BIR), one mechanism of rescue of stalled or collapses replication forks ( rad51 + , cdc27 + ). Δhip1 also suppresses mutations in two checkpoint genes ( cds1 + , rad3 + ) on hydroxyurea a drug that stalls replication forks. Our results show that hip1 + forms complex interactions with the DNA double strand break repair genes and may be involved in facilitating communication between damage sensors and downstream factors.

Biochemical properties of fission yeast homologous recombination enzymes

Homologous recombination (HR) is a universal phenomenon conserved from viruses to humans. The mechanisms of HR are essentially the same in humans and simple unicellular eukaryotes like yeast. Two highly diverged yeast species, Saccharomyces cerevisiae and Schizosaccharomyces pombe, have proven exceptionally useful in understanding the fundamental mechanisms of eukaryotic HR by serving as a source for unique biological insights and also complementing each other. Here, we will review the features of S. pombe HR mechanisms in comparison to S. cerevisiae and other model organisms. Particular emphasis will be put on the biochemical characterization of HR mechanisms uncovered using S. pombe proteins.

FRET-based assays to monitor DNA binding and annealing by Rad52 recombination mediator protein

During homologous recombination and homology-directed repair of broken chromosomes, proteins that mediate and oppose recombination form dynamic complexes on damaged DNA. Quantitative analysis of these nucleoprotein assemblies requires a robust signal, which reports on the association of a recombination mediator with its substrate and on the state of substrate DNA within the complex. Eukaryotic Rad52 protein mediates recombination, repair, and restart of collapsed replication forks by facilitating replacement of ssDNA binding protein replication protein A (RPA) with Rad51 recombinase and by mediating annealing of two complementary DNA strands protected by RPA. The characteristic binding mode whereby ssDNA is wrapped around the Rad52 ring allowed us to develop robust and sensitive FRET-based assays for monitoring Rad52 interactions with protein-free DNA and ssDNA-RPA complexes. By reporting on the configuration of ssDNA dually labeled with Cy3 and Cy5 fluorescent dyes, solution-based FRET is used to analyze Rad52-RPA-DNA interactions under equilibrium binding conditions. Finally, FRET between Cy3 and Cy5 dyes incorporated into two homologous ssDNA molecules can be used to analyze interplay between Rad52-mediated DNA strand annealing and duplex DNA destabilization by RPA.

Human Rad52 binds and wraps single-stranded DNA and mediates annealing via two hRad52-ssDNA complexes

Rad52 promotes the annealing of complementary strands of DNA bound by replication protein A (RPA) during discrete repair pathways. Here, we used a fluorescence resonance energy transfer (FRET) between two fluorescent dyes incorporated into DNA substrates to probe the mechanism by which human Rad52 (hRad52) interacts with and mediates annealing of ssDNA–hRPA complexes. Human Rad52 bound ssDNA or ssDNA–hRPA complex in two, concentration-dependent modes. At low hRad52 concentrations, ssDNA was wrapped around the circumference of the protein ring, while at higher protein concentrations, ssDNA was stretched between multiple hRad52 rings. Annealing by hRad52 occurred most efficiently when each complementary DNA strand or each ssDNA–hRPA complex was bound by hRad52 in a wrapped configuration, suggesting homology search and annealing occur via two hRad52–ssDNA complexes. In contrast to the wild type protein, hRad52^RQK/AAA and hRad52^1–212 mutants with impaired ability to bind hRPA protein competed with hRPA for binding to ssDNA and failed to counteract hRPA-mediated duplex destabilization highlighting the importance of hRad52-hRPA interactions in promoting efficient DNA annealing.

Human Rad52-mediated homology search and annealing occurs by continuous interactions between overlapping nucleoprotein complexes

The Rad52 protein has critical functions in distinct pathways of the homology-directed DNA repair, one of which is to promote the annealing of complementary strands of DNA. Both yeast and human Rad52 proteins organize into ring-shaped oligomers with the predominant form being a heptamer. Despite the wealth of information obtained in previous investigations, how Rad52 mediates homology search and annealing remains unclear. Here, we developed single-molecule fluorescence resonance energy transfer approaches to probe hRad52-mediated DNA annealing events in real time. We found that annealing proceeds in successive steps involving rearrangements of the ssDNA-hRad52 complex. Moreover, after initial pairing, further search for extended homology occurs without dissociation. This search process is driven by an interaction between 2 overlapping nucleoprotein complexes. In light of these observations we propose a model for hRad52-mediated DNA annealing where ssDNA release and dsDNA zippering are coordinated through successive rearrangement of overlapping nucleoprotein complexes.