Schizosaccharomyces pombe Mms1 channels repair of perturbed replication into Rhp51 independent homologous recombination

In both Schizosaccharomyces pombe and Saccharomyces cerevisiae, Mms22 and Mms1 form a complex with important functions in the response to DNA damage, loss of which leads to perturbations during replication. Furthermore, in S. cerevisiae, Mms1 has been suggested to function in concert with a Cullin-like protein, Rtt101/Cul8, a potential paralog of Cullin 4. We performed epistasis analysis between Δmms1 and mutants of pathways with known functions in genome integrity, and measured the recruitment of homologous recombination proteins to blocked replication forks and recombination frequencies. We show that, in S. pombe, the functions of Mms1 and the conserved components of the Cullin 4 ubiquitin ligase, Pcu4 and Ddb1, do not significantly overlap. Furthermore, unlike in S. cerevisiae, the function of the H3K56 acetylase Rtt109 is not essential for Mms1 function. We provide evidence that Mms1 function is particularly important when a single strand break is converted into a double strand break during replication. Genetic data connect Mms1 to a Mus81 and Rad22Rad52 dependent, but Rhp51 independent, branch of homologous recombination. This is supported by results demonstrating that Mms1 is recruited to a site-specific replication fork barrier and that, in a Δmms1 strain, Rad22Rad52 and RPA recruitment to blocked forks are reduced, whereas Rhp51 recruitment is unaffected. In addition, Mms1 appears to specifically promote chromosomal rearrangements in a recombination assay. These observations suggest that Mms1 acts to channel repair of perturbed replication into a particular sub-pathway of homologous recombination.