The flk gene of Salmonella typhimurium couples flagellar P- and L-ring assembly to flagellar morphogenesis

Abstract

Flagellar biosynthesis in Salmonella typhimurium and Escherichia coli represents the best-understood system in which a large regulatory network of over 50 genes is coupled to the assembly of a large, complex structure, the flagellar organelle. The flagellum is required for motility in response to chemical attractants and repellents in a liquid environment (5, 35, 54). The flagellum is commonly divided into three parts: the basal body, the hook, and the filament. The basal body is contained within the cell envelope, and over 30 genes are required for its synthesis (reviewed in references 2 and 35). The flagellar structure is diagrammed in Fig. 1. The basal body is composed of the following components: (i) a reversible rotary motor driven by a proton pump; (ii) a rod structure that transverses the cell wall, cell membranes, and periplasm; (iii) three ring proteins that stabilize the rod structure in the inner membrane, peptidoglycan layer, and lipopolysaccharide; (iv) a switch complex that determines clockwise or counterclockwise rotation in response to the chemosensory system; and (v) a flagellum-specific type III secretion pathway. Attached to the basal body on the outside of the cell is a flexible hook structure, from which the long filament structure extends. The flagellum is hollow, and a flagellum-specific export pathway sends subunits through the hollow passage to be assembled at the tip of the growing structure (35).The flagellar genes are organized into a transcriptional hierarchy of three classes that are coupled to morphogenesis of the flagellar organelle ( Fig. 1) (25). Each class must be functional for the subsequent class to be expressed.Class 1 genes, flhD and flhC, determine whether flagella are made. Expression of the FlhD and -C proteins is dependent on the general state of the cell. In addition, these proteins are required for normal cell growth. The flhDC operon is regulated in response to environmental stimuli such as cyclic AMP levels and temperature (23, 53), heat shock (50), osmolarity and membrane biosynthesis (22,38,50,51), and the H-NS protein (4). In addition, mutants of the flhDC operon affect cell division (45). The FlhD and FlhC proteins make up a heteromultimeric complex that acts as a transcriptional activator to stimulate transcription from promoters of class 2 flagellar genes by 70 -containing RNA polymerase (32). The FlhDC complex binds a ϳ40-bp region upstream of the Ϫ35 promoter sequence and is thought to interact with the C-terminal region of the ␣ subunit of RNA polymerase to initiate transcription (32,33).Class 2 genes (Fig. 1) encode the proteins needed for the structure and assembly of the basal body-hook (BBH) structure as well as the fliA regulatory gene (25). The fliA gene encodes an alternative sigma transcription factor, 28 , necessary for transcription of most class 3 genes (41). In addition,

Coupling of Flagellar Gene Expression to Flagellar Assembly in Salmonella enterica Serovar Typhimurium and Escherichia coli

“…FlgM inhibition of 28 -dependent class 3 flagellar gene transcription was relieved in P-and L-ring assembly mutants (flgA, flgH, or flgI) by introduction of a null mutation in the flk gene (51). Strains defective in hook-basal body synthesis exhibit an increase in the intracellular levels of FlgM, presumably due to the inability of FlgM to be secreted.…”

“…The flk gene was identified as a positive regulator of the activity of FlgM at an assembly step just prior to hook-basal body completion, at the point of assembly of the P-and L-rings (51). FlgM inhibition of 28 -dependent class 3 flagellar gene transcription was relieved in P-and L-ring assembly mutants (flgA, flgH, or flgI) by introduction of a null mutation in the flk gene (51).…”

“…This suggests that the secretion apparatus possesses a physical barrier to secretion of late substrates prior to hook-basal body completion in addition to secretion signal specificity by FliK and FlhB. Null mutations in the rflH locus, called flk, were isolated as causing a defect in flgM translation in strains defective in P-and L-ring assembly (51,53). Thus, the physical barrier to secretion of FlgM by RflH (Flk) may be associated with a role in cotranslation secretion of FlgM (discussed below).…”

“…As mentioned above, the Flk protein is hypothesized to regulate flgM mRNA translation in response to completion of the P-and L-rings and the elongation of the hook outside the cell (51,53). In this way, intracellular FlgM levels may be lowered to allow some filament transcription just prior to hook completion.…”

Protein Export According to Schedule: Architecture, Assembly, and Regulation of Type III Secretion Systems from Plant- and Animal-Pathogenic Bacteria

“…As mentioned above, this interaction is probably favored at a hook length of approximately 55 nm (159,381,383,392). Prior to hook completion, binding of FliK C to FlhB C is suppressed by an additional regulatory protein, termed RflH/Flk or Fluke (to clearly distinguish it from FliK), which prevents the premature secretion of filament components and is anchored in the IM (7,265,296,385).…”