Title: Post-transcriptional control of cholera toxin and factors affecting the virulence of Vibrio cholerae
Description: This thesis examined the V. cholerae factors important in human infection and the epidemiologically important hyperinfectious phenotype. The main focus of this work centered on the expression of the major virulence factor, cholera toxin (CT), which is responsible for the copious diarrhea associated with the disease cholera. Previous research on the V. cholerae virulence cascade has focused on determining the environmental factors and the genes necessary for transcriptional regulation of ctxAB. While CT is highly expressed in vitro under virulence inducing conditions, no toxin protein is seen under noninducing conditions. It has been assumed that this difference in toxin production between the two environmental conditions is due only to changes in ctxAB transcription. However, we found through the use of RT-PCR, Northern blots and quantitative RT-PCR, that there were high levels of the ctxAB transcript after growth in both Inducing and Noninducing conditions. Despite substantial amounts of the ctxAB mRNA in both conditions, CT protein was produced at a level at least 100-fold lower in Noninducing conditions than in the Inducing conditions, while the difference in transcription between the conditions was no more than 10-fold. Even when ctxAB was transcribed equally in Inducing and Noninducing conditions, less CT protein was produced under Noninducing conditions.

Therefore, the change in the amount of CT protein made in response to Inducing conditions was greater than predicted by the change in the level of transcription, suggesting another level of regulation of CT expression at the post-transcriptional level. Potential mechanisms of CT post-transcriptional regulation were examined. CT was not specifically retained within the cell under Noninducing conditions. The stability of the ctxAB transcript, compared by Northern blot and QRT-PCR, was equal in both growth conditions. Secondary structures of the ctxAB mRNA, tested by sequence manipulation, did not affect CT protein production. We found no evidence of an intra- or extracellular protease specifically degrading CT in Noninducing conditions. Although the exact mechanisms remain unknown, we provide evidence of post-transcriptional control regulating the amount of CT protein expressed.