Sequence homologies between A subunits of Escherichia coli and Vibrio cholerae enterotoxins.

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Abstract
The genes coding for the heat-labile enterotoxin (LT) produced by E. coli were cloned into the plasmid pBR313. Using DNA derived from the resulting chimeric plasmid, the nucleotide sequence of 2 regions of the gene coding for the enzymatically active A subunit of LT was determined. Translation of the nucleotide sequence gives the primary structure of the NH2-terminal and COOH-terminal regions of the LT A subunit. This permits direct comparison of the LT A subunit with the A subunit of cholera toxin. The 2 toxins possess homologous sequences, of varying degrees, in both regions of their primary structure. The order of the component A1 and A2 polypeptides is A1-A2. The nucleotide sequence predicts the existence of a signal sequence of 18 amino acids at the NH2-terminus of the A subunit.