Primary structure of heat-labile enterotoxin produced by Escherichia coli pathogenic for humans

Heat-labile enterotoxin of Escherichia coli pathogenic for humans (LTh) or for piglets (LTp) and Vibrio cholerae enterotoxin (CT) are structurally and functionally similar toxins. We have determined the complete nucleotide sequence of the toxA gene which encodes the subunit A of LTh (LTh A). The deduced amino acid sequence consists of 258 residues including a signal peptide of 18 residues. According to the previously completed LTh B sequence (103 residues), the predicted holotoxin (1A5B) of LTh comprises 755 residues and has Mr = 87,866. With respect to LTh A and LTh B, secondary structures, local hydrophilicity, and sites for antigenic determinants were predicted. Both codon usage and G + C content of the toxA gene and the LTh B gene (toxB) were markedly different from those observed with several E. coli chromosomal genes. Its relatively low G + C content was rather close to that of the V. cholerae chromosome. Although the toxA gene shares a common ancestor with the LTp A gene (eltA), the two genes are apparently distinguishable from each other in their sequences. Like LTh B reported previously, the predicted sequence of the catalytic fragment LTh A1 also showed more homology to that of CT A1 than did that of LTp A1. In contrast, unique sequences were found in LTh A2.