How cellular checkpoints couple the orderly assembly of macromolecular machines with cell-cycle progression is poorly understood. The alpha-proteobacterium Caulobacter crescentus assembles a single polar flagellum during each cell cycle. We discovered that the expression of multiple flagellin transcripts is licensed by a translational checkpoint responsive to a dual input signal: a secretion-competent hook-basal-body (HBB) structure and a surge in the FlaF secretion chaperone during cytokinesis, instructed by the cell-cycle program. We find that the unorthodox FljJ flagellin, one of the six flagellin paralogs, acts as a checkpoint linchpin, binding both FlaF and the FlbT translational regulator. FljJ recruits FlbT to inhibit translation at the 5′ untranslated region in other flagellin transcripts before HBB assembly. Once FlaF is synthesized and stabilized, it directs FljJ secretion through the HBB, thereby separating FlbT from its co-activator and relieving translational inhibition. The FlbT/FlaF pair is wide spread and its functional properties are conserved in alpha-proteobacteria, including pathogens.

细胞检查点如何将大分子机器的有序组装与细胞周期进程结合在一起的了解很少。α-变形杆菌Caulobacter crescentus在每个细胞周期内组装一个极地鞭毛。我们发现，多个鞭毛蛋白转录物的表达已被翻译检查点响应，该翻译检查点响应双重输入信号：一种能分泌的钩基体（HBB）结构，以及胞质分裂期间FlaF分泌伴侣的激增，由细胞指示循环程序。我们发现非正统的FljJ鞭毛蛋白，六个鞭毛蛋白旁系同源物之一，充当检查点关键，结合FlaF和FlbT翻译调节子。FljJ募集FlbT以在HBB组装前抑制其他鞭毛蛋白转录本中5'非翻译区的翻译。一旦FlaF合成并稳定下来，它就会通过HBB引导FljJ分泌，从而将FlbT与它的共激活剂分离并减轻翻译抑制。