Secretion Relieves Translational Co-repression by a Specialized Flagellin Paralog.

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.