Organization, expression, and function of Caulobacter crescentus genes needed for assembly and function of the flagellar hook.

This paper reports on the organization, expression, and function of the divergently transcribed flbG and flaN operons in the hook gene cluster of Caulobacter crescentus. The transcription initiation site of flbG was determined previously, and in this work the transcription map was completed by locating the 3' end of the mRNA using nuclease S1 protection assays. A previous genetic study had suggested that the flbG operon is comprised of four genes; however, the nucleotide sequence revealed three tandemly arranged ORFs that correspond to 5'-flbG, flbH, and flgE. FlbG is similar to FliK proteins which are required for termination of hook synthesis, FlbH is similar to FlgD proteins which are essential scaffolding proteins that cap the hook during its assembly, and FlgE corresponds to the hook structural protein. The divergently transcribed flaN gene codes for a hook associated protein I homolog based on its inferred amino acid sequence similarity to FlgK proteins. Based on the amino acid sequence similarities and phenotypes of mutants, flbG, flbH, and flaN have been renamed fliK, flgD, and flgK, FlgD, FlgE, and FlgK proteins, with apparent molecular masses of 23, 68, and 41 kDa, respectively, were expressed from plasmids in a cell-free coupled transcription-translation system, and a protein corresponding to FliK was identified as part of a 190-kDa FliK-LacZ fusion protein. We present evidence showing that, in addition to its role in termination of hook synthesis, FliK is also required for initiation of hook assembly.