MUTATIONS IN FLIK AND FLHB AFFECTING FLAGELLAR HOOK AND FILAMENT ASSEMBLY IN SALMONELLA-TYPHIMURIUM

Mutations in the fliK gene of Salmonella typhimurium commonly cause fa ilure to terminate hook assembly and initiate filament assembly (polyh ook phenotype), Polyhook mutants give rise to pseudorevertants which a re still defective in hook termination but have recovered the ability to assemble filament (polyhook-filament phenotype), The polyhook mutat ions have been found to be either frameshift or nonsense, resulting in truncation of the C terminus of FliK. Intragenic suppressors of frame shift mutations were found to be ones that restored the original frame (and therefore the C-terminal sequence), but in most cases with subst antial loss of natural sequence and sometimes the introduction of arti ficial sequence; in no cases did intragenic suppression occur when sig nificant disruption remained within the C-terminal region, By use of a novel PCR protocol, in-frame deletions affecting the N-terminal and c entral regions of FliK were constructed and the resulting phenotypes w ere examined, Small deletions resulted in almost normal hook length co ntrol and almost wild-type swarming, Larger deletions resulted in loss of control of hook length and poor swarming, The largest deletions se verely affected filament assembly as well as hook length control, Extr agenic suppressors map to an unlinked gene, flhB, which encodes an int egral membrane protein (T. Hirano, S. Yamaguchi, K. Oosawa, and S.-I. Aizawa, J, Bacteriol, 176:5339-5419, 1994; Ii, Kutsukake, T, Minamino, and T, Yokoseki, J, Bacteriol. 176:7625-7629, 1994), They were either point mutations in the C-terminal cytoplasmic region of FlhB or frame shift or nonsense mutations close to the C terminus, The processes of hook and filament assembly and the roles of FliK and FlhB in these pro cesses are discussed in light of these and other available data, We su ggest that FliK measures hook length and, at the appropriate point, se nds a signal to FlhB to switch the substrate specificity of export fro m hook protein to late proteins such as flagellin.