Coupling flagellar gene expression to flagellar assembly in Caulobacter crescentus

Abstract

Bacterial flagellar filaments are long cell surface appendages that generate propulsion for movement. They also play key roles in surface attachment and host-bacterial interactions. The filament is made from a single protein species flagellin. Approximately 45 % of annotated flagellar systems possess multiple flagellin genes. We have investigated the ability of Caulobacter crescentus to build a flagellar filament using six flagellins: FljJ, FljK, FljL, FljM, FljN and FljO. Our analysis showed that this flagellar system exhibits extensive structural redundancy, in that one species of flagellin is sufficient to sustain motility. However, when that flagellin is FljJ cells are non motile. Distinct flagellar assembly checkpoints are utilised by bacteria in order to coordinate and couple flagellar gene expression to the assembly pathway. One of these checkpoints, hook-basal-body completion, is sensed and overcome by the secretion of a flagellar-associated secretion substrate. This mechanism results in a system switch to the export of proteins, which are needed for filament assembly. In C. crescentus the post-transcriptional regulators FlbT and FlaF have been implicated to function at this switch. As the flagellins themselves are being secreted we asked the logical question: are the flagellins involved in subunit feedback control of the regulation of filament assembly? In a bacterial two-hybrid assay, FljJ was the only flagellin able to interact with FlbT and FlaF. Furthermore, FljJ along with FlbT was found to interact with flagellin mRNA. In light of our data we a propose model for the regulation of flagellar filament assembly in C. crescentus and discuss the implications with respect to other flagellar systems