Abstract
Flagella are assembled sequentially from the inside-out with morphogenetic checkpoints that enforce the temporal order of subunit addition. Here we show that flagellar basal bodies fail to proceed to hook assembly at high frequency in the absence of the monotopic protein SwrB of Bacillus subtilis. Genetic suppressor analysis indicates that SwrB activates the flagellar type III secretion export apparatus by the membrane protein FliP. Furthermore, mutants defective in the flagellar C-ring phenocopy the absence of SwrB for reduced hook frequency and C-ring defects may be bypassed either by SwrB overexpression or by a gain-of-function allele in the polymerization domain of FliG. We conclude that SwrB enhances the probability that the flagellar basal body adopts a conformation proficient for secretion to ensure that rod and hook subunits are not secreted in the absence of a suitable platform on which to polymerize.
Highlights
Some bacteria swim through liquid and swarm over surfaces by synthesizing trans-envelope nanomachines called flagella
We show that the proficiency of the export apparatus for hook subunit secretion was coupled to the conformation of the basal body as C-ring mutants phenocopied the absence of SwrB
The reduction in Hag protein was likely due to reduced transcription of the hag gene as cells mutated for either SwrB or SwrA showed lower expression from a reporter in which the hag promoter (Phag) was fused to the lacZ gene encoding β-galactosidase (Phag-lacZ) (Fig 2B, white bars), and a reduced frequency of cells expressing a reporter in which the Phag promoter was fused to green fluorescent protein (GFP) (Phag-gfp) (Fig 2C, white bars, and S1 Fig) [39,41]
Summary
Some bacteria swim through liquid and swarm over surfaces by synthesizing trans-envelope nanomachines called flagella. Flagella spontaneously self-assemble from over twenty separate proteins thought to be organized into three structural domains called the basal body, the hook and the filament [1,2,3]. The basal body is composed of a ring of a transmembrane protein.
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