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

The genes coding for the heat-labile enterotoxin LT produced by Escherichia coli have been cloned into the plasmid pBR313. Using DNA derived from the resulting chimeric plasmid, we determined the nucleotide sequence of two regions of the gene coding for the enzymatically active A subunit of LT. 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. Our results show that the two 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.