The motility defect caused by absence of the transcriptional regulator LdtR in Sinorhizobium meliloti is restored by mutations in the motility genes motA and motS

The flagellar motor is a powerful macromolecular machine used to propel bacteria through various environments. Flagellar motility of the alpha-proteobacterium Sinorhizobium meliloti is nearly abolished in the absence of the transcriptional regulator LdtR, which is involved in peptidoglycan remodeling. We report that LdtR does not regulate motility gene transcription. Remarkably, the motility defects of the DldtR mutant can be restored by secondary mutations in the motility gene motA or a previously uncharacterized gene in the flagellar regulon, which we named motS. MotS is not essential for S. meliloti motility and may serve an accessory role in flagellar motor function. Structural modeling predicts that MotS is comprised of an N-terminal transmembrane segment, a long-disordered region, and a conserved β-sandwich domain. The C-terminus of MotS is localized in the periplasm. Genetics-based substitution of MotA with a MotAG12S variant protein also restored the ΔldtR motility defect. The MotAG12S variant causes a local polarity shift at the periphery of the MotAB stator units. We propose that MotS may be required for optimal alignment of stators in wild-type flagellar motors but becomes detrimental in cells with altered peptidoglycan. Similarly, the polarity shift in the stator units composed of MotB/MotAG12S might stabilize its interaction with altered peptidoglycan. To investigate the differential gene expression profiles of Sinorhizobium meliloti ΔldtR cells compared to wild-type under motility-promoting conditions, we grew three replicates of each strain (six samples total) in Bromfield overlay plates to early log phase and harvested RNA from equivalent amounts of bacteria. RNA-seq analysis was performed for both strains (three replicates) and the data used to perform comparative gene expression analysis between the two strains.