Cloning, nucleotide sequence, and hybridization studies of the type IIb heat-labile enterotoxin gene of Escherichia coli

Type IIb heat-labile enterotoxin (LT-IIb) is produced by Escherichia coli 41. Restriction fragments of total cell DNA from strain 41 were cloned into a cosmid vector, and one cosmid clone that encoded LT-IIb was identified. The genes for LT-IIb were subcloned into a variety of plasmids, expressed in minicells, sequenced, and compared with the structural genes for other members of the Vibrio cholerae-E. coli enterotoxin family. The A subunits of these toxins all have similar ADP-ribosyltransferase activity. The A genes of LT-IIa and LT-IIb exhibited 71% DNA sequence homology with each other and 55 to 57% homology with the A genes of cholera toxin (CT) and the type I enterotoxins of E. coli (LTh-I and LTp-I). The A subunits of the heat-labile enterotoxins also have limited homology with other ADP-ribosylating toxins, including pertussis toxin, diphtheria toxin, and Pseudomonas aeruginosa exotoxin A. The B subunits of LT-IIa and LT-IIb differ from each other and from type I enterotoxins in their carbohydrate-binding specificities. The B genes of LT-IIa and LT-IIb were 66% homologous, but neither had significant homology with the B genes of CT, LTh-I, and LTp-I. The A subunit genes for the type I and type II enterotoxins represent distinct branches of an evolutionary tree, and the divergence between the A subunit genes of LT-IIa and LT-IIb is greater than that between CT and LT-I. In contrast, it has not yet been possible to demonstrate an evolutionary relationship between the B subunits of type I and type II heat-labile enterotoxins. Hybridization studies with DNA from independently isolated LT-II-producing strains of E. coli also suggested that additional variants of LT-II exist.