Characterization of functional interactions among the Escherichia coli mismatch repair proteins using a bacterial two-hybrid assay.

Vsr mediates very short patch repair in Escherichia coli, correcting T/G mismatches caused by deamination of 5-methylcytosine to thymine. MutS and MutL, part of the post-replication mismatch repair system, stimulate VSP repair. In this study, we use a bacterial two-hybrid assay to show that MutL interacts with Vsr. We also show that interaction between Vsr and MutL inhibits the ability of MutL to dimerize, to interact with MutS and MutH and to mediate a previously unknown interaction between MutS and MutH. This inhibition may explain why high levels of Vsr are mutagenic in vivo. In addition, we show that the Mut fusion proteins are repair proficient in the bacterial two-hybrid assay, making it possible to study their interactions in various genetic backgrounds, or in the presence of DNA damaging agents.