Sense and sensibility: flagellum-mediated gene regulation

Abstract

The flagellum, a rotary engine required for motility in many bacteria, plays key roles in gene expression. It has been known for some time that flagellar substructures serve as checkpoints that coordinate flagellar gene expression with assembly. Less well understood, however, are other more global effects on gene expression. For instance, the flagellum acts as a 'wetness' sensor in Salmonella typhimurium, and as a mechanosensor in other bacteria. Additionally, it has been implicated in a variety of bacterial processes, including biofilm formation, pathogenesis and symbiosis. Although for many of these processes it might be simply that motility is required, in other cases it seems that the flagellum plays an underappreciated role in regulating gene expression.

Figure 1

The bacterial flagellum and associated flagellar protein export apparatus. (a) The diagram depicts a typical cell wall with flagellum of a gram-negative bacterium. The basal body (red) consists of the flagellar protein export apparatus, motor, and membrane spanning rods and rings. The hook (blue) forms a flexible linker and the filament (green) acts as the propeller. (b) Diagram indicating the membrane-bound components (FliO, FliP, FliQ, FlhA, FlhB, FliR) and cytoplasmic components (FliH, FliI, FliJ) of the flagellar protein export apparatus. The export apparatus transports substrates across the cytoplasmic membrane into the lumen of the nascent flagellum for assembly. The scissors indicate the relative site of FlhB autocleavage between the two cytoplasmic subdomains of the protein. The arrangement of the membrane components relative to each other is not known.

Figure 2

Three model systems for flagellar gene regulation. The diagrams depict the major checkpoints in flagellar gene regulation for selected bacteria. Components of the basal body, hook and filament are depicted in red, blue and green colours, respectively. (a) In S. typhimurium, transcription of the class II flagellar genes is dependent on the major form of RNA polymerase holoenzyme (σ⁷⁰-holoenzyme) and is activated by the master regulator FlhDC. The major checkpoint for flagellar assembly is completion of the HBB complex, which is linked to secretion of anti-σ²⁸ factor, FlgM, via the flagellar protein export apparatus. Secretion of FlgM alleviates its inhibitory effect on expression of the class III (σ²⁸-dependent) genes, including those required for formation of the filament. (b) The master regulator in C. crescentus, CtrA, activates transcription of class II genes together with the major form of RNA polymerase holoenzyme (σ⁷³-holoenzyme). Class II genes encode components of an intermediate structure containing elements from both the export apparatus and the basal body, and formation of this intermediate activates the FlbD/FliX system which in turn stimulates transcription of class III (σ⁵⁴-dependent) genes. Completion of the HBB complex is required to alleviate the inhibitory effect of FlbT on translation of the class IV flagellar genes, which include the flagellin genes fljK and fljL (products of these genes shown in green). Like the class III genes, the class IV genes are dependent on σ⁵⁴ for their transcription. (c) In H. pylori and C. jejuni a master regulator has not yet been identified. Early (class II) flagellar genes are dependent on the primary sigma factor, σ⁸⁰, for their expression. These early flagellar genes are referred to as class I genes in the literature, but we designate them here as class II genes to be consistent with other bacterial systems. Completion of an export apparatus-basal body intermediate is thought to activate the FlgS/FlgR two-component system, which is required for transcription of σ⁵⁴-dependent flagellar genes that encode components of the basal body, hook and a minor flagellin. Transcription of the late (class IV) genes is dependent on σ²⁸. H. pylori FlgM inhibits transcription of σ²⁸-dependent genes, and its export is presumed but has not been demonstrated. FlgM does not appear to inhibit transcription of σ²⁸-dependent genes in C. jejuni strain 81–176 [41].

Figure 3

Mechanism for control of substrate specificity switching by the hook length control protein FliK. (i) Early in flagellar assembly the flagellar protein export apparatus is in a conformation that exports rod- and hook-type substrates. (ii) FliK is occasionally exported during hook assembly. The unfolded FliK_N domain (purple) within the lumen of the nascent HBB structure temporarily anchors its N-terminus to the hook cap (orange). If the hook has not reached its mature length of about 55 nm, FliK is secreted into the surrounding medium. (iii) When the hook reaches its mature length, the stretched conformation of FliK_N and part of FliK_C (magenta) measure the length of the rod and hook, and the T3S4 domain of FliK is able to interact with FlhB_C (blue). Interactions between FlhB_C and the T3S4 domain result in a switch to filament-type protein export. Abbreviations: OM, outer membrane; PG, peptidoglycan layer; CM, cytoplasmic membrane.