Secretion Relieves Translational Co-repression by a Specialized Flagellin Paralog

Area of Science:

• Microbiology
• Molecular Biology
• Cell Biology

Background:

• Cellular checkpoints coordinate complex molecular processes with cell division.
• The assembly of the flagellum, a complex macromolecular machine, is tightly regulated during the Caulobacter crescentus cell cycle.

Purpose of the Study:

• To elucidate the regulatory mechanism coupling flagellin expression to cell-cycle progression and flagellar assembly in Caulobacter crescentus.
• To identify the key molecular players involved in this translational checkpoint.

Main Methods:

• Investigated the role of flagellin paralogs, secretion chaperones, and translational regulators.
• Utilized genetic and biochemical approaches to analyze protein interactions and translational control.
• Examined the timing of gene expression relative to cell-cycle events and hook-basal-body assembly.

Main Results:

• Discovered a translational checkpoint that licenses flagellin transcript expression.
• Identified the flagellin FljJ as a linchpin protein, interacting with both the FlaF chaperone and the FlbT translational regulator.
• Demonstrated that FljJ/FlbT inhibits flagellin translation until a secretion-competent hook-basal-body (HBB) is assembled and FlaF is available.
• Showed that FlaF binding to FljJ releases translational inhibition, allowing flagellin synthesis.

Conclusions:

• The FljJ/FlaF/FlbT system acts as a critical checkpoint, ensuring flagellar components are synthesized only when the cell is ready for assembly.
• This regulatory mechanism ensures the orderly assembly of the flagellum, coupling it to cell-cycle progression.
• The identified regulatory pair (FlbT/FlaF) is conserved in other alpha-proteobacteria, suggesting broad biological relevance.