Significance and context
Cholera is an acute diarrheal disease caused by infection of the intestine with the Gram-negative bacterium Vibrio cholerae. Infection can occur through drinking water or eating undercooked food contaminated with V. cholerae. Although the disease is rare nowadays in industrialized countries, it is still common, for example, in the Indian subcontinent and sub-Saharan Africa. Infection by V. cholerae to cause cholera requires both the production of toxin-coregulated pilus protein (TCP) and the cholera toxin (CT). The production of both is controlled by a complex cascade of regulators. A three-component VieSAB signal transduction system has previously been suspected of being involved in the regulation of toxin production. In contrast to conventional two-component systems, the VieSAB regulatory system contains two putative response regulators, VieA and VieB, as well as the sensor histidine kinase VieS. The vieA gene is expressed in vitro, in contrast to vieB which is expressed only in vivo, and vieA is dependent on TCP-mediated infection. Tischler et al. now show that the VieSAB three-component system is required for the full expression of the ctxAB operon, which encodes the A and B subunits of cholera toxin, during both growth in vitro and infection in vivo.
Using a combination of various V. cholerae mutants affecting vieSAB, RNA protection assays of RNA from wild-type V. cholerae, and the infant mouse model of cholera for in vivo studies, the authors show that the VieSAB system is not absolutely required for initial induction of ctxA and ctxB in vivo or in vitro, nor to reach the highest level of expression, but is needed to maintain ctxAB expression in vitro. Decreased production of ToxT, a positive regulator of ctxA, may explain the lowered expression of ctxAB in a vieSAB V. cholerae mutant. Western blot analyses showed that the decreased ctxAB expression in the absence of VieSAB correlated with a decrease in CT production. Finally, the affected ctxAB expression and CT production noted in a vieSAB mutant could be rescued by the introduction of the vieSAB genes in trans, upon introduction of a vieSAB-containing plasmid.
The entire genome sequence of V. cholerae is accessible at The Institute of Genomic Research.
This study adds to our understanding of how genes required for cholera toxin production are regulated. Future research should include the identification of the environmental signals of human origin that are sensed by the VieSAB three-component system, the characterization of other V. cholerae genes that are putatively regulated by the VieSAB system, and the isolation of genes encoding other regulatory mechanisms that control toxin production.