A novel regulatory protein involved in motility of Vibrio cholerae.

The facultative pathogen Vibrio cholerae is the causative agent of the human intestinal disease cholera. Both motility and chemotaxis of V. cholerae have been shown to contribute to the virulence and spread of cholera. The flagellar gene operons are organized into a hierarchy composed of four classes (I to IV) based on their temporal expression patterns. Some regulatory elements involved in flagellar gene expression have been elucidated, but regulation is complex and flagellar biogenesis in V. cholerae is not completely understood. In this study, we determined that the virulence defect of a V. cholerae cheW1 deletion mutant was due to polar effects on the downstream open reading frame VC2058 (flrD). Expression of flrD in trans restored the virulence defect of the cheW1 deletion mutant, and deletion of flrD resulted in a V. cholerae strain attenuated for virulence, as determined by using the infant mouse intestinal colonization model. The flrD mutant strain exhibited decreased transcription of class III and IV flagellar genes and reduced motility. Transcription of the flrD promoter, which lies within the coding sequence of cheW1, is independent of the flagellar transcriptional activators FlrA and RpoN, which activate class II genes, indicating that flrD does not fit into any of the four flagellar gene classes. Genetic epistasis studies revealed that the two-component system FlrBC, which is required for class III and IV flagellar gene transcription, acts downstream of flrD. We hypothesize that the inner membrane protein FlrD interacts with the cytoplasmic FlrBC complex to activate class III and IV gene transcription.