Nucleotide sequence analysis of the A2 and B subunits of Vibrio cholerae enterotoxin.

We have determined the sequence of the DNA encoding the A2 (gamma) and B subunits of Vibrio cholerae enterotoxin. The order of the subunits as they would be transcribed is A2-B and the termination codon of the A2 subunit overlaps the initiation codon of the B subunit by four bases. Sequence analysis revealed a region capable of coding for a 21-amino acid leader peptide located at the NH2 terminus of the B subunit. While the nucleotide sequence homology between the cholera enterotoxin subunits and the analogous sequences of the genes for Escherichia coli heat-labile enterotoxin (LT) was 72 and 77% for the A2 and B subunits, respectively, the predicted amino acid sequences of the A2 subunits were less similar. Twenty-nine of 46 (63%) amino acids of the A2 subunits and 98 of 124 (79%) amino acids of the B subunits were identical between cholera enterotoxin and LT. The predicted amino acid sequence of the enterotoxin from a V. cholerae El Tor biotype strain reported here differs from the previously published amino acid sequences of the toxin from a classical biotype of V. cholerae at seven residues. Comparison of the A2 and B amino acid sequences among El Tor and classical biotypes of V. cholerae and E. coli LT demonstrates two regions of highly conserved sequences: 12 and 22 uninterrupted amino acids are the same among the A2 and B subunits, respectively, from the three strains.