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Na+ -translocating NADH:quinone oxidoreductase (Na+ -NQR) catalyzes electron transfer from NADH to ubiquinone in the bacterial respiratory chain, coupled with Na+ translocation across the membrane. Na+ -NQR maturation involves covalent attachment of flavin mononucleotide (FMN) residues, catalyzed by flavin transferase encoded by thenqr -associatedapbE gene. Analysis of complete bacterial genomes has revealed another putative gene (duf539 , here renamednqrM ) that usually follows theapbE gene and is present only in Na+ -NQR-containing bacteria. Expression of theVibrio harveyi nqr operon alone or with the associatedapbE gene inEscherichia coli , which lacks its own Na+ -NQR, resulted in an enzyme incapable of Na+ -dependent NADH or reduced nicotinamide hypoxanthine dinucleotide (dNADH) oxidation. However, fully functional Na+ -NQR was restored when these genes were coexpressed with theV. harveyi nqrM gene. Furthermore,nqrM lesions inKlebsiella pneumoniae andV. harveyi prevented production of functional Na+ -NQR, which could be recovered by annqrM -containing plasmid. The Na+ -NQR complex isolated from thenqrM -deficient strain ofV. harveyi lacks several subunits, indicating thatnqrM is necessary for Na+ -NQR assembly. The protein product of thenqrM gene, NqrM, contains a single putative transmembrane α-helix and four conserved Cys residues. Mutating one of these residues (Cys33 inV. harveyi NqrM) to Ser completely prevented Na+ -NQR maturation, whereas mutating any other Cys residue only decreased the yield of the mature protein. These findings identify NqrM as the second specific maturation factor of Na+ -NQR in proteobacteria, which is presumably involved in the delivery of Fe to form the (Cys)4 [Fe] center between subunits NqrD and NqrE.IMPORTANCE Na+ -translocating NADH:quinone oxidoreductase complex (Na+ -NQR) is a unique primary Na+ pump believed to enhance the vitality of many bacteria, including important pathogens such asVibrio cholerae ,Vibrio parahaemolyticus ,Haemophilus influenzae ,Neisseria gonorrhoeae ,Pasteurella multocida ,Porphyromonas gingivalis ,Enterobacter aerogenes , andYersinia pestis . Production of Na+ -NQR in bacteria requires Na+ -NQR-specific maturation factors. We earlier identified one such factor (ApbE) that covalently attaches flavin residues to Na+ -NQR. Here we identify the other protein factor, designated NqrM, and show that NqrM and ApbE suffice to produce functional Na+ -NQR from theVibrio harveyi nqr operon. NqrM may be involved in Fe delivery to a unique Cys4 [Fe] center during Na+ -NQR assembly. Besides highlighting Na+ -NQR biogenesis, these findings suggest a novel drug target to combat Na+ -NQR-containing bacteria.

NqrM (DUF539) Protein Is Required for Maturation of Bacterial Na + -Translocating NADH:Quinone Oxidoreductase

NqrM (DUF539) Protein Is Required for Maturation of Bacterial Na ⁺ -Translocating NADH:Quinone Oxidoreductase