Evolution and structure of two ADP-ribosylation enterotoxins, Escherichia coli heat-labile toxin and cholera toxin.

Nucleotide sequence comparisons of the heat-labile enterotoxin (LTh) genes of E. coli pathogenic for humans with cholera toxin (CT) genes suggest that the two toxin genes have evolved from a common ancestry by a series of single base changes, while conserving the catalytic fragment A1 (ADP-ribose transferase). Based on the local hydrophilicity profiles of LTh and CT peptides, a transmembrane segment appears to be present in A1 in both toxins.