Aw. Williams et al., MUTATIONS IN FLIK AND FLHB AFFECTING FLAGELLAR HOOK AND FILAMENT ASSEMBLY IN SALMONELLA-TYPHIMURIUM, Journal of bacteriology, 178(10), 1996, pp. 2960-2970
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.