Bacterial flagella differ in their number and spatial arrangement. In many species, the MinD-type ATPase FlhG (also YlxH/FleN) is central to the numerical control of bacterial flagella, and its deletion in polarly flagellated bacteria typically leads to hyperflagellation. The molecular mechanism underlying this numerical control, however, remains enigmatic. Using the model species Shewanella putrefaciens, we show that FlhG links assembly of the flagellar C ring with the action of the master transcriptional regulator FlrA (named FleQ in other species). While FlrA and the flagellar C-ring protein FliM have an overlapping binding site on FlhG, their binding depends on the ATP-dependent dimerization state of FlhG. FliM interacts with FlhG independent of nucleotide binding, while FlrA exclusively interacts with the ATP-dependent FlhG dimer and stimulates FlhG ATPase activity. Our in vivo analysis of FlhG partner switching between FliM and FlrA reveals its mechanism in the numerical restriction of flagella, in which the transcriptional activity of FlrA is down-regulated through a negative feedback loop. Our study demonstrates another level of regulatory complexity underlying the spationumerical regulation of flagellar biogenesis and implies that flagellar assembly transcriptionally regulates the production of more initial building blocks.

细菌鞭毛的数量和空间排列不同。在许多物种中，MinD型ATPase FlhG（也称为YlxH / FleN）对于细菌鞭毛的数值控制至关重要，并且其在极鞭毛细菌中的缺失通常会导致超鞭毛。然而，这种数控的分子机理仍然是个谜。使用模型物种Shewanella putrefaciens，我们显示FlhG将鞭毛C环的装配与主转录调节子FlrA（在其他物种中称为FleQ）的作用联系起来。尽管FlrA和鞭毛C环蛋白FliM在FlhG上具有重叠的结合位点，但它们的结合取决于FlhG的ATP依赖性二聚化状态。FliM与FlhG相互作用独立于核苷酸结合，而FlrA仅与ATP依赖的FlhG二聚体相互作用并刺激FlhG ATPase活性。我们对FliM和FlrA之间的FlhG伴侣切换的体内分析揭示了鞭毛数量限制的机制，其中通过负反馈回路下调了FlrA的转录活性。