Mycobacterium smegmatis NucS-promoted DNA mismatch repair involves limited resection by a 5'-3' exonuclease and is independent of homologous recombination and NHEJ.

The MutSL mismatch repair (MMR) systems in bacteria and eukaryotes correct mismatches made at the replication fork to maintain genome stability. A novel MMR system is represented by the EndoMS/NucS endonuclease from Actinobacterium Corynebacterium glutamicum, which recognizes mismatched substrates in vitro and creates dsDNA breaks at the mismatch. In this report, a genetic analysis shows that an M. smegmatis ΔnucS strain could be complemented by expression of wild type NucS protein, but not by one deleted of its last five amino acids, a region predicted to be critical for binding to the β-clamp at the replication fork. Oligo-recombineering was then leveraged to deliver defined mismatches to a defective hygromycin resistance gene on the M. smegmatis chromosome. We find that NucS recognizes and repairs G-G, G-T, and T-T mismatches in vivo, consistent with the recognition of these same mismatches in C. glutamicum in vitro, as well as mutation accumulation studies done in M. smegmatis. Finally, an assay that employs an oligo that promotes the generation of two mismatches in close proximity on the chromosome shows that a NucS-promoted cut is processed by a 5'-3' exonuclease (or 5'-Flap endonuclease) and that NucS-promoted MMR is independent of both homologous recombination and non-homologous end-joining.