According to the World Health Organization, each year there are an estimated 3 to 5 million cases of cholera, resulting in over 100,000 deaths. Cholera is caused by Vibrio cholerae, a pathogenic bacterium that utilizes a highly regulated transcriptional cascade to produce its two major virulence genes. The studies in our lab are designed to investigate the structural and functional characteristics of these regulatory proteins in order to elucidate how virulence gene expression is regulated at the molecular level so that better strategies can be developed to prevent and cure bacterial diseases. Achieving these goals requires an understanding of how the specific regulatory proteins function at their promoters to control gene expression and, ultimately, how they are influenced by environmental stimuli.
The expression of the primary V. cholerae virulence factors, toxin-coregulated pilus and cholera toxin, occurs via a transcriptional cascade involving several activator proteins and serves as a paradigm for the regulation of bacterial virulence. AphA and AphB initiate the expression of the cascade by a novel interaction at the tcpPH promoter. AphA is a member of a new regulator family and AphB is a LysR-type activator, one of the largest transcriptional regulatory families. Once expressed, cooperation between TcpP/TcpH and the homologous transmembrane activators ToxR/ToxS activates the toxT promoter. ToxT, an AraC-type regulator, then directly activates the promoters of the primary virulence factors in a fatty acid dependent manner. Transcriptional activation at these various promoters occurs only in response to certain environmental stimuli. One such stimulus, cell density, influences the virulence cascade through the quorum sensing system regulator HapR which represses the expression of the aphA promoter. Members of our lab have solved the x-ray crystal structures of AphA, AphB, HapR, ToxT, and FadR. We are currently working to build upon this structural data, as well as our functional results, in order to continue to elucidate the detailed mechanisms required for regulation of the V. cholerae virulence genes.