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This chapter describes the composition of the Campylobacter jejuni chemotaxis system in the context of pathways found in other bacterial species and derives models of the mechanism of signal transduction in the C. jejuni chemotaxis pathway. The six C. jejuni chemotaxis signal transduction pathway genes are located in three separate regions of the genome. In two regions, the che genes are located with genes of apparently unrelated function, and the distances between open reading frames and the strand-specific grouping of open reading frames suggests an operon arrangement. The cheY gene is located adjacent to the Pgl protein glycosylation gene cluster and possibly at the start of an operon, where no other gene is likely to be involved in chemotaxis. The second region includes the genes cheV, cheA, and cheW, which are located next to one another, and the genes flanking the three che genes in the operon do not appear to be associated with chemotaxis. The third region of the C. jejuni chromosome in which che genes are located contains the genes cheR and cheB, which are likely to be cistronic and not cotranscribed with the flanking genes, rpiB and pebC. There is evidence that methylation- and demethylation- based adaptation of the chemoreceptors also occurs in C. jejuni. Clearly, this model for the most part is speculative, being largely based on the exploitation of genomic sequence data, but ongoing investigation of chemotaxis in C. jejuni is providing experimental support.
Diagrammatic overview of the E. coli chemotaxis signal transduction pathway. In E. coli, the receptor complex consists of MCP, CheW, and CheA. In the absence of chemoattractant, the CheA kinase domain is active, and after auto-phosphorylation of CheA, the phosphate is transferred to CheY. Phospho-CheY then binds to FliM on the flagellum motor. Sufficient binding of phospho-CheY to the flagellum motor leads to reversal from counterclockwise to clockwise rotation (not shown). Signal termination occurs by the action of the phosphatase CheZ on phospo-CheY. System adaptation, which resets the signaling properties of the receptor, occurs by reversible methylation by CheB and CheR; the level of methylation is controlled by phosphorylation of the response regulator domain on CheB. For further details, see text.
Model for the control of CheB activity in campylobacters. (1) CheA-mediated kinase activity is inhibited when chemoreceptors bind chemoattractants. In consequence, phosphorylation of the response regulator domain in CheV is reduced (probably involving the phosphatase CheZ; Fig. 3 ). Unphosphorylated CheV inhibits the methylesterase activity of CheB, resulting in increased methylation of receptors due to CheR. (2) In the absence of chemoattractant CheA phosphorylates the response regulator domain present in CheV. Phospho-CheV ceases to inhibit CheB methylesterase activity, leading to increased demethylation of chemoreceptors.
Model for the dephosphorylation of CheY and other chemotaxis-related response regulator domains in campylobacters. Cj0700, the proposed CheZ, dephosphorylates phospho-CheY, terminating the signal transmitted to the motor by CheY. In the absence of attractant, CheA is proposed to phosphorylate CheV, and CheZ will dephosphorylate CheV to terminate continued CheB methylesterase activity. Phosphorylation of the CheA response regulator domain CheA-RR is also proposed to be reversed by CheZ, but the regulatory role of the CheA-RR domain remains to be determined.
Model of chemotaxis signal transduction in campylobacters. Three pathways are proposed to be associated with different chemosensory complexes focused on the structure-based grouping of predicted chemoreceptors. Group A receptors, possibly arranged in large array clusters of mixed receptor specificity and positioned at the cell poles, are predicted to sense periplasmic signals. Binding of ligands to periplasmic domains of group A receptors might involve accessory periplasmic ligand-binding proteins (not shown). The specificity of Tlp1 has been shown to be aspartate. For the group B receptor CetA, a chemotactic response to redox potential changes is mediated by interaction with CetB. The group C receptors might play a role in chemotactic responsesto intracellular signals, possibly by other cytoplasmic ligand-binding proteins (not shown). Some receptors, for example Aer1, might transduce signals into other nonchemotaxis systems. Signal loss, for example decreasing binding of attractant to the receptor, is transduced to CheA in the complex activating the CheA kinase domain. Complexes may contain CheW alone or both CheW and CheV (shown left and right of group A receptors, respectively). After autophosphorylation of CheA, the phosphate is transferred to CheY, and phospho-CheY then binds to FliM on the flagellum motor. Sufficient binding of phospho-CheY to the flagellum motor leads to reversal from counterclockwise to clockwise rotation (not shown). System adaptation, which resets the signaling properties of the receptor, is presumed to occur via reversible methylation by CheB and CheR. The CheA kinase domain might also pass phosphate to CheV that in turn controls CheB activity. Signal termination by dephosphorylation of CheY is mediated by CheZ, which also may dephosphorylate CheV and the response regulator domain on CheA.
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