Molecular and Cell Biological Analysis of SwrB in Bacillus subtilis.

Swarming motility is flagellum-mediated movement over a solid surface, and Bacillus subtilis cells require an increase in flagellar density to swarm. SwrB is a protein of unknown function required for swarming that is necessary to increase the number of flagellar hooks but not basal bodies. Previous work suggested that SwrB activates flagellar type III secretion, but the mechanism by which it might perform this function is unknown. Here, we show that SwrB likely acts substoichiometrically as it localizes as puncta at the membrane in numbers fewer than those of flagellar basal bodies. Moreover, the action of SwrB is likely transient as puncta of SwrB were not dependent on the presence of the basal bodies and rarely colocalized with flagellar hooks. Random mutagenesis of the SwrB sequence found that a histidine within the transmembrane segment was conditionally required for activity and punctate localization. Finally, three hydrophobic residues that precede a cytoplasmic domain of poor conservation abolished SwrB activity when mutated and caused aberrant migration during electrophoresis. Our data are consistent with a model in which SwrB interacts with the flagellum, changes conformation to activate type III secretion, and departs.