Elucidating the correlation between the number of TTTTGAT heptamer repeats and cholera toxin promoter activity in Vibrio cholerae O1 pandemic strains

A complex regulatory cascade controls expression of the cholera toxin genes (ctxAB) in Vibrio cholerae; which eventually leads to choleragen (CT) production and secretion, resulting in rice watery diarrhoea. The cholera toxin promoter (PctxAB) contains a series of heptad repeats (5-TTTTGAT-3); which have been previously shown to play crucial role in ctxAB transcriptional regulation by recruiting the transcriptional activators ToxT, ToxR, and the nucleoid-associated protein H-NS along the ctx promoter. The numbers of these repeats vary between the two biotypes of V. cholerae O1 strains, and even among strains of the same biotype. In this study, we examined PctxAB activation of V. cholerae O1 pandemic strains to understand the significance of the distal heptad repeats in regulating ctx expression. Interestingly, we found that ctx activation may depend on the number of TTTTGAT heptad repeats within PctxAB, and we posit that the occupation of the distal repeats by H-NS could further prevent transcriptional activation of ctx genes in V. cholerae. We hypothesize that ToxT-dependent transcriptional activation may not require entire displacement of H-NS and propose a revision in the currently accepted model of ToxT dependent PctxAB transcriptional activation. IMPORTANCECT production by pathogenic V. cholerae O1 strains is regulated through the transcriptional silencing of CTX promoter by H-NS and counter repression by ToxT. The highly AT rich PctxAB is composed of the tandem repeats 5{square} TTTTGAT 3{square}; the numbers of which differ among the classical and El Tor biotypes. However, it is still not very clear whether the numbers of these repeats could be correlated with promoter activation of the ctx operon. Here we report the role of the distal repeats in ctxAB expression levels. We demonstrate that PctxAB activation changes with the number of the heptad repats within the core promoter element, thereby suggesting a model for ctx regulation in the toxigenic strains of V. cholerae.