A Double-Strand Break Repair Component Is Essential for S Phase Completion in Fission Yeast Cell Cycling

Fission yeast rad22 +, a homologue of budding yeast RAD52, encodes a double-strand break repair component, which is dispensable for proliferation. We, however, have recently obtained a cell division cycle mutant with a temperature-sensitive allele of rad22 +, designated rad22-H6, which resulted from a point mutation in the conserved coding sequence leading to one amino acid alteration. We have subsequently isolatedrad22 + and its novel homologuerti1 + as multicopy suppressors of this mutant. rti1 + suppresses all the defects of cells lacking rad22 +. Mating type switch-inactive heterothallic cells lacking eitherrad22 + or rti1 +are viable, but those lacking both genes are inviable and arrest proliferation with a cell division cycle phenotype. At the nonpermissive temperature, a synchronous culture ofrad22-H6 cells performs DNA synthesis without delay and arrests with chromosomes seemingly intact and replication completed and with a high level of tyrosine-phosphorylated Cdc2. However,rad22-H6 cells show a typical S phase arrest phenotype if combined with the rad1-1 checkpoint mutation.rad22 + genetically interacts withrad11 +, which encodes the large subunit of replication protein A. Deletion ofrad22 +/rti1 + or the presence of rad22-H6 mutation decreases the restriction temperature of rad11-A1 cells by 4–6°C and leads to cell cycle arrest with chromosomes incompletely replicated. Thus, in fission yeast a double-strand break repair component is required for a certain step of chromosome replication unlinked to repair, partly via interacting with replication protein A.