Escherichia coli heat-labile enterotoxin genes are flanked by repeated deoxyribonucleic acid sequences

The enterotoxin regions of the heat-labile and heat-stable enterotoxin (LT+ ST+) plasmid, pJY11, originating in a clinically isolated Escherichia coli strain, have been isolated as various-sized deoxyribonucleic acid (DNA) fragments by using cloning vehicles. The structure of the LT+ region and its neighboring DNA regions was studied by utilizing these recombinant plasmids. The LT+ region consisted of at least two genes, toxA and toxB, which could complement each other in trans. The toxA- and toxB-encoded polypeptides (LT subunits A and B, respectively) were identified by their immunological cross-reactivity with Vibrio cholerae enterotoxin subunit A or B. These tox genes and the promoter(s) were localized with respect to the restriction endonuclease cleavage map. The LT+ region was flanked by repeated DNA sequences (designated as beta). Another tox gen(s), encoding ST (designated as toxS), which was also flanked by inverted, repeated DNA sequences (designated as alpha), was located between one of the beta sequences and the LT+ region. These novel DNA structures (beta-alpha-toxS-alpha-toxA-toxB-beta) suggest the possibility that the LT+ region is on a transposon containing an ST transposon within the structure.