Evidence that MutSβ repairs indels generated by mispair initiated template slippage.

This study examines the specificity of MutSβ dependent DNA mismatch repair (MMR) at the URA3 reporter gene in Saccharomyces cerevisiae. Addition and deletion mutations containing multiple base changes are observed at elevated rates in msh3∆ strains. This is consistent with a role for MutSβ in repairing multibase mismatches, such as those important in development of certain diseases and possibly including those made during Okazaki fragment maturation. Mutations containing single C-G base pair deletions are also observed at elevated rates in msh3∆ strains. These mutations are observed at still higher rates in msh3∆ strains with mutator variants of the three major replicases, implicating a defect in insertion/deletion repair during nuclear DNA replication. These deletions are observed at only a subset of detectable locations in URA3 and they occur at higher rates in one of two orientations of URA3 relative to the nearest replication origin. This subset of sites has a strong local DNA sequence context bias suggesting that the deletions are initiated by transient base•base mismatches which lead to template strand slippage, a type of transient initiator mutagenesis (TIM). We conclude that these mispair-initiated template slippage events require MMR via MutSβ before the next round of DNA synthesis to prevent deletion of an unpaired cytosine in the template strand.