INVOLVEMENT OF CPXA, A SENSOR OF A 2-COMPONENT REGULATORY SYSTEM, IN THE PH-DEPENDENT REGULATION OF EXPRESSION OF SHIGELLA-SONNEI VIRF GENE

In Shigella species, IpaBCD proteins encoded on the virulence plasmid direct the entry of this bacterium into host epithelial cells. Express ion of the ipaBCD genes is under the control of several environmental conditions, such as temperature and osmolarity. Extracellular pH also controlled the expression of the genes, and this regulation occurred m ainly at the step of expression of virF, a plasmid-encoded positive re gulator of ipaBCD. The expression of virF was activated at high pH (pH 7.4) and repressed at low pH (pH 6.0). We isolated a Tn10 transposon mutant in Escherichia coli K-12 which altered this regulation at the t ranscriptional level. The Tn10 in the mutant inserted within a reading frame of the cpxA gene, whose product belongs to a family of sensor p roteins of two-component signal transduction systems. Complementation analysis showed that cpxA was involved in the pH-dependent regulation of virF gene expression. A gene homologous to cpxA was conserved in Sh igella spp. as well as in E. coli. These results may indicate that Cpx A senses directly or indirectly a change in extracellular pH and influ ences the expression of virF in E. coli and Shigella spp.