In bacteria, which protein complex recognizes and initiates repair in MMR?

• A. MutL
• B. DNA polymerase III
• C. MutS (recognizes mismatch) followed by MutL and MutH (in certain bacteria) to direct repair
• D. MutH

Answer: (C)

Mismatch repair in bacteria starts when MutS recognizes a base-pair mismatch that slipped through after DNA replication. Once MutS has detected the error, it recruits MutL, which serves as a connector and coordinator. In many bacteria, MutL then engages MutH, an endonuclease that nicks the newly replicated strand near a nearby GATC site. This nick provides a starting point for exonucleases to remove the error-containing segment, after which DNA polymerase III fills the gap and DNA ligase seals the final nick. Some bacteria lack MutH and use MutL with alternative nucleases, but the damage recognition step is still MutS recognizing the mismatch and initiating the downstream repair by recruiting MutL (and MutH when present). That’s why describing the pathway as MutS recognizing the mismatch followed by MutL and MutH directing repair best captures how bacterial MMR works.