Bacterial chemotaxis requires bidirectional flagellar rotation at different rates. Rotation is driven by a flagellar motor, which is a supercomplex containing multiple rings. Architectural uncertainty regarding the cytoplasmic C-ring, or ‘switch’, limits our understanding of how the motor transmits torque and direction to the flagellar rod. Here we report cryogenic electron microscopy structures for Salmonella enterica serovar typhimurium inner membrane MS-ring and C-ring in a counterclockwise pose (4.0 Å) and isolated C-ring in a clockwise pose alone (4.6 Å) and bound to a regulator (5.9 Å). Conformational differences between rotational poses include a 180° shift in FliF/FliG domains that rotates the outward-facing MotA/B binding site to inward facing. The regulator has specificity for the clockwise pose by bridging elements unique to this conformation. We used these structures to propose how the switch reverses rotation and transmits torque to the flagellum, which advances the understanding of bacterial chemotaxis and bidirectional motor rotation.

细菌趋化性需要鞭毛以不同的速率双向旋转。旋转由鞭毛马达驱动，鞭毛马达是包含多个环的超级复合体。关于细胞质 C 形环或“开关”的结构不确定性限制了我们对电机如何将扭矩和方向传递到鞭毛杆的理解。在这里，我们报告了肠沙门氏菌鼠伤寒沙门氏菌内膜 MS 环和 C 环呈逆时针姿势 (4.0 Å) 以及单独的 C 环呈顺时针姿势 (4.6 Å) 并与调节器结合 (5.9 A）。旋转姿势之间的构象差异包括 FliF/FliG 结构域的 180° 移位，将向外的 MotA/B 结合位点旋转为向内。该调节器通过该构象特有的桥接元件对顺时针姿势具有特异性。我们利用这些结构提出了开关如何反转旋转并将扭矩传递到鞭毛，这增进了对细菌趋化性和双向电机旋转的理解。