Termination of Vibrio cholerae virulence gene expression is mediated by proteolysis of the major virulence activator, ToxT

Vibrio cholerae is the causative agent of cholera, a severe diarrhoeal illness. V. cholerae produces two major virulence factors: the cholera toxin, which directly causes diarrhoea, and the toxin-coregulated pilus, which is required for intestinal colonization. Production of these virulence factors is dependent on the major virulence regulator, ToxT. Under virulence-inducing growth conditions, transcription factors ToxR and TcpP initially activate transcription of toxT. However, once ToxT has been expressed, it produces more of itself independent of ToxR and TcpP by activating transcription of the long tcpA operon, within which toxT is located. It is known that V. cholerae terminates virulence gene expression prior to escape from the host, but it is unknown how this ToxT-positive feedback loop is broken, an essential step in terminating virulence gene expression. To better understand how ToxT protein activity is regulated, we monitored ToxT accumulation and activity under virulence-inducing and -repressing growth conditions. Our results suggest that ToxT protein undergoes proteolytic degradation to terminate virulence gene expression. This directed degradation of ToxT supports a model for terminating V. cholerae virulence gene expression late in infection, with both ToxT and TcpP undergoing proteolysis prior to escape from the host.