The bacterial swarming factor SwrD belongs to a diverse family of proteins that form hexameric rings

Abstract

Prokaryotic swarming is a collective surface-associated behavior that supports rapid colonization, biofilm formation, and host interactions. Swarming requires enhanced flagellar motor (FM) torque generation under high-load conditions. In many bacteria, this adaptation is achieved through increased stator unit recruitment and coordinated transcriptional regulation of motility genes. Genetic studies have implicated several relatively poorly characterized proteins in swarming, including SwrD, a component of the fla-che operon whose loss impairs torque generation in Bacillus subtilis. Intriguingly, SwrD is conserved in spirochetes that experience high viscous loads but do not exhibit canonical swarming, suggesting a broader functional role in FM regulation. Here, we structurally and biophysically characterize SwrD from B. subtilis and the spirochete Borrelia burgdorferi. In both cases, we show that SwrD assembles into a hexameric ring-like structure featuring a highly charged, disordered peripheral tail. Whereas SwrD displays structural similarities to periplasmic lipoproteins and DNA-binding proteins, lipidation regions and DNA interaction motifs of these other proteins are not conserved in SwrD. Deletion of swrD in B. subtilis results in pronounced swarming defects and altered expression of fla-che operon transcripts. In silico interaction analyses further identify the stator protein MotA as a high-confidence SwrD interactor, but this interaction could not be validated biochemically. Conserved SwrD residues mediate subunit contacts or locate to the hexamer periphery where they may mediate functionally important interactions.