Function and Structure of FlaK, a Master Regulator of the Polar Flagellar Genes in Marine <i>Vibrio</i>.

Vibrio alginolyticus has a flagellum at the cell pole, and the <i>fla</i> genes, involved in its formation, are hierarchically regulated in several classes. FlaK (also called FlrA) is an ortholog of Pseudomonas aeruginosa FleQ, an AAA+ ATPase that functions as a master regulator for all later <i>fla</i> genes. In this study, we conducted mutational analysis of FlaK to examine its ATPase activity, ability to form a multimeric structure, and function in flagellation. We cloned <i>flaK</i> and confirmed that its deletion caused a nonflagellated phenotype. We substituted amino acids at the ATP binding/hydrolysis site and at the putative subunit interfaces in a multimeric structure. Mutations in these sites abolished both ATPase activity and the ability of FlaK to induce downstream flagellar gene expression. The L371E mutation, at the putative subunit interface, abolished flagellar gene expression but retained ATPase activity, suggesting that ATP hydrolysis is not sufficient for flagellar gene expression. We also found that FlhG, a negative flagellar biogenesis regulator, suppressed the ATPase activity of FlaK. The 20 FlhG C-terminal residues are critical for reducing FlaK ATPase activity. Chemical cross-linking and size exclusion chromatography revealed that FlaK mostly exists as a dimer in solution and can form multimers, independent of ATP. However, ATP induced the interaction between FlhG and FlaK to form a large complex. The <i>in vivo</i> effects of FlhG on FlaK, such as multimer formation and/or DNA binding, are important for gene regulation. <b>IMPORTANCE</b> FlaK is an NtrC-type activator of the AAA+ ATPase subfamily of σ⁵⁴-dependent promoters of flagellar genes. FlhG, a MinD-like ATPase, negatively regulates the polar flagellar number by collaborating with FlhF, an FtsY-like GTPase. We found that FlaK and FlhG interact in the presence of ATP to form a large complex. Mutational analysis revealed the importance of FlaK ATPase activity in flagellar gene expression and provided a model of the <i>Vibrio</i> molecular mechanism that regulates the flagellar number.