This protein is involved in the repair of mismatches in DNA. It is required for dam-dependent methyl-directed DNA mismatch repair. May act as a 'molecular matchmaker', a protein that promotes the formation of a stable complex between two or more DNA-binding proteins in an ATP-dependent manner without itself being part of a final effector complex (By similarity). Overexpression of mutSL partially suppresses the high spontaneous mutation frequency of a ytkD/mutM/mutY triple disruption which lacks the system required to prevent damage by oxidized guanine (8-oxo-dGTP). This suggests that MutSL also functions to repair mismatches due to oxidative stress in both growing and stationary phase cells. Cells lacking this gene have an 80-fold increased spontaneous mutation frequency. A double mutS/mutL and a triple mutS/mutL/nth disruption has a 200 to 280-fold increased spontaneous mutation frequency. Belongs to the DNA mismatch repair MutL/HexB family.