AbstractThe mismatch repair system (MMR) ensures the stability of genetic information during DNA replication in almost all organisms. Mismatch repair is initiated after recognition of a non-canonical nucleotide pair by the MutS protein and the formation of a complex between MutS and MutL. Eukaryotic and most bacterial MutL homologs function as endonucleases that introduce a single-strand break in the daughter strand of the DNA, thus activating the repair process. However, many aspects of the functioning of this protein remain unknown. We studied the ATPase and DNA binding functions of the MutL protein from the pathogenic bacterium Neisseria gonorrhoeae (NgoMutL), which exhibits endonuclease activity. For the first time, the kinetic parameters of ATP hydrolysis by the full-length NgoMutL protein were determined. Its interactions with single- and double-stranded DNA fragments of various lengths were studied. NgoMutL was shown to be able to efficiently form complexes with DNA fragments that are longer than 40 nucleotides. Using modified DNA duplexes harboring a 2-pyridyldisulfide group on linkers of various lengths, we obtained NgoMutL conjugates with DNA for the first time. According to these results, the Cys residues of the wild-type protein are located at a distance of approximately 18–50 Å from the duplex. The efficiency of the affinity modification of Cys residues in NgoMutL with reactive DNAs was shown to decrease in the presence of ATP or its non-hydrolyzable analog, as well as ZnCl2, in the reaction mixture. We hypothesize that the conserved Cys residues of the C-terminal domain of NgoMutL, which are responsible for the coordination of metal ions in the active center of the protein, are involved in its interaction with DNA. This information may be useful in reconstruction of the main stages of MMR in prokaryotes that are different from γ-proteobacteria, as well as in the search for new targets for drugs against N. gonorrhoeae.