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We describe here a new member of the LysR family of transcriptional regulators, AphB, which is required for activation of theVibrio cholerae ToxR virulence cascade. AphB activates the transcription of thetcpPH operon in response to environmental stimuli, and this process requires cooperation with a second protein, AphA. The expression of neitheraphA oraphB is strongly regulated by environmental stimuli, raising the possibility that the activities of the proteins themselves may be influenced under various conditions. Strains of the El Tor biotype ofV. cholerae typically exhibit lower expression of ToxR-regulated virulence genes in vitro than classical strains and require specialized culture conditions (AKI medium) to induce high-level expression. We show here that expression ofaphB from thetac promoter in El Tor biotype strains dramatically increases virulence gene expression to levels similar to those observed in classical strains under all growth conditions examined. These results suggest that AphB plays a role in the differential regulation of virulence genes between the two disease-causing biotypes.

A Vibrio cholerae LysR Homolog, AphB, Cooperates with AphA at the tcpPH Promoter To Activate Expression of the ToxR Virulence Cascade