Nucleotide sequence of the bacterial transposon Tn1681 encoding a heat-stable (ST) toxin and its identification in enterotoxigenic Escherichia coli strains.

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Abstract
The E. coli heat-stable toxin (ST I) is encoded within a transposon (Tn1681) flanked by inverted repeats of insertion sequence 1 (IS1). By subcloning restriction fragments and by insertion mutagenesis, the gene for ST I within the transposon was located precisely. The complete nucleotide sequence of the central portion of Tn1681 (i.e., that part flanked by IS1) was determined and the coding sequence of the toxin was identified. From the nucleotide sequence, a probable amino acid sequence for ST I was deduced. The NH2-terminal portion of the amino acid sequence is extremely hydrophobic and bears a striking resemblance to the signal sequence of the fd phage minor coat protein. Using a subcloned restriction fragment containing the gene for ST I but no IS1 sequences showed that the ST toxin with activity assayable in suckling mice (ST I) is genetically distinct from the ST toxin assayable in ligated ileal loops (ST II) and that ST I can be responsible for diarrheal disease in different animals.

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