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The ability to move towards favorable environmental conditions, called chemotaxis, is common among motile bacteria. In particular aerotaxis has been extensively studied in Escherichia coli. Three putative aer gene homologs were identified in the V. cholerae genome designated VCAer-1 (VC0512) VCAer-2 (VCA0658), and VCAer-3 (VCA0988). Deletion analyses indicated that only one of them, VCAer-2, actively mediates an aerotaxis response, as assayed in soft agar plates as well as a capillary assay. Complementation studies showed that VCAer-2 is in fact responsible for guiding V. cholerae along an oxygen gradient. In addition, overexpression of the gene resulted in a marked increase of the aerotactic response in succinate soft agar plates. No observable phenotypes in mutants deleted in the V. cholerae aer-1 and aer-3 genes were detected under standard aerotaxis testing conditions. Furthermore, the V. cholerae aer-1 and aer-3 genes, even when expressed from a strong independent promoter, did not show any phenotypes. Several lines of evidence suggested differences in the mechanism of aerotactic signal transduction between V. cholerae and E. coli.
First, a key amino acid residue involved in the binding of the FAD prosthetic group in the E. coli Aer protein is not conserved in the V. cholerae VCAer-2 protein. Moreover, unlike other chemotaxis genes, the V. cholerae aer-2 gene did not complement the heterologous E. coli Aer mutant, although a weak activity of the E. coli aer gene in the V. cholerae VCAer-2 mutant was observed. In the absence of oxygen and any other chemoattractants, V. cholerae does not display any chemotactic behavior, making it tempting to speculate that the VCAer-2 protein senses oxygen directly.
As in other bacterial species, the results presented in this study indicate the presence of a secondary aerotaxis transducer in V. cholerae. Two putative V. cholerae MCP homologs with high sequence similarity to the Tsr protein, found to be a secondary mediator of aerotaxis in E. coli, were analyzed for involvement in aerotaxis. Neither gene, deleted either by itself or in combination with aer-2, seemed to be important for aerotaxis of V. cholerae. Thus, one of the many other MCPs of V. cholerae is expected to be part of the complex pathways underlying the aerotaxis signal transduction.
Although, the role of chemotaxis and particularly aerotaxis in the biology of V. cholerae, including its environmental and infectious life stages, remains to be fully understood, this study provides a solid foundation for future studies into functions of the multiple chemosensors found in this organism. The enormous complexity of the potential signals perceived, including oxygen, makes V. cholerae a particularly interesting model organism to study chemotactic behavior. |
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