Genetics of cholera toxin

Cholera is caused by the toxin secreted by Vibrio cholerae 01. Cholera toxin (CT) is a protein consisting of A and B subunits. The former contributes to intracellular toxicity whereas the B subunit is required for binding of CT to eukaryotic cell surface receptor. The structural genes encoding A and B subunits are designated as ctxA and ctxB respectively. These genes are located on the chromosome forming an operon in which ctxA precedes ctxB. The ctxAB have been cloned and sequenced. Classical strains contain two full copies of unlinked ctxAB. Most el tors have single copy. However, in some strains there are two copies which are arranged in tandem. The tandem duplication and amplification of ctxAB is controlled by a transposable element like DNA sequence called RS1. A number of genes have been identified which regulate the expression of ctx operon. V. cholerae seems to elaborate more than one toxin which are different from the one encoded by ctxAB genes.