MISMATCH CORRECTION ACTS AS A BARRIER TO HOMEOLOGOUS RECOMBINATION INSACCHAROMYCES-CEREVISIAE

A homeologous mitotic recombination assay was used to test the role of Saccharomyces cerevisiae mismatch repair genes PMS1, MSH2 and MSH3 on recombination fidelity. A homeologous gene pair consisting of S. cere visiae SPTI5 and its S. pombe homolog were present as a direct repeat on chromosome V, with the exogenous S. pombe sequences inserted either upstream or downstream of the endogenous S. cerevisiae gene. Each gen e carried a different inactivating mutation, rendering the starting st rain Spt15(-). Recombinants that regenerated SPT15 function were score d after nonselective growth of the cells. In strains wild type for mis match repair, homeologous recombination was depressed 150- to 180-fold relative to homologous controls, indicating that recombination betwee n diverged sequences is inhibited. In one orientation of the homeologo us gene pair, msh2 or msh3 mutations resulted in 17- and 9.6-fold elev ations in recombination and the msh2 msh3 double mutant exhibited an 4 3-fold increase, implying that each MSH gene can function independentl y in trans to prevent homeologous recombination. Homologous recombinat ion was not significantly affected by the msh mutations. In the other orientation, only msh2 strains were elevated (12-fold) for homeologous recombination. A mutation in MSH3 did not affect the rate of recombin ation in this orientation. Surprisingly, a pms1 deletion mutant did no t exhibit elevated homeologous recombination.