The flagellar motor is a powerful macromolecular machine used to propel bacteria through various environments. We determined that flagellar motility of the alpha-proteobacterium Sinorhizobium meliloti is nearly abolished in the absence of the transcriptional regulator LdtR, known to influence peptidoglycan remodeling and stress response. LdtR does not regulate motility gene transcription. Remarkably, the motility defects of the ΔldtR 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 comprised 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 MotA_G12S also restored the ΔldtR motility defect. The MotA_G12S variant protein features 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 stator units composed of MotB/MotA_G12S might stabilize its interaction with altered peptidoglycan.

中文翻译：

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苜蓿中华根瘤菌因缺乏转录调节因子 LdtR 而导致的游动缺陷可通过运动基因 motA 和 motS 的突变来恢复

鞭毛马达是一种强大的大分子机器，用于推动细菌穿过各种环境。我们确定，在缺乏转录调节因子 LdtR 的情况下，α-变形菌苜蓿中华根瘤菌的鞭毛运动几乎消失，LdtR 已知会影响肽聚糖重塑和应激反应。 LdtR 不调节运动基因转录。值得注意的是， ΔldtR突变体的运动缺陷可以通过运动基因motA或鞭毛调节子中先前未表征的基因（我们将其命名为motS ）的二次突变来恢复。 MotS 对于苜蓿中华根瘤菌的运动并不是必需的，并且可能在鞭毛运动功能中发挥辅助作用。结构模型预测 MotS 包含 N 端跨膜片段、长无序区域和保守的 β 夹心结构域。 MotS 的 C 末端位于周质中。基于遗传学的 MotA _G12S替代也恢复了 Δ ldtR运动缺陷。 MotA _G12S变体蛋白在 MotAB 定子单元外围具有局部极性转变。我们认为，MotS 可能是野生型鞭毛马达中定子最佳对齐所必需的，但在肽聚糖发生改变的细胞中却变得有害。类似地，由 MotB/MotA _G12S组成的定子单元的极性变化可能会稳定其与改变的肽聚糖的相互作用。