Interactions of the <Emphasis Type="Italic">HSM3</Emphasis> gene with genes initiating homologous recombination repair in yeast <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

It was assumed previously that the mutator phenotype of the hms3 mutant was determined by processes taking place in the D-loop. As a next step, genetic analysis was performed to study the interactions between the hsm3 mutation and mutations of the genes that control the initial steps of the D-loop formation. The mutations of the MMS4 and XRS2 genes, which initiate the double-strand break formation and subsequent repair, were shown to completely block HSM3-dependent UV-induced mutagenesis. Mutations of the RAD51, RAD52, and RAD54 genes, which are also involved in the D-loop formation, only slightly decreased the level of UV-induced mutagenesis in the hsm3 mutant. Similar results were observed for the interaction of hsm3 with the mph1 mutation, which stabilizes the D-loop. In contrast, the shu1 mutation, which destabilizes the D-loop structure, led to an extremely high level of UV-induced mutagenesis and displayed epistatic interactions with the hsm3 mutation. The results made it possible to assume that the hsm3 mutation destabilizes the D-loop, which is a key substrate of both Rad5- and Rad52-dependent postreplicative repair pathways.