A single amino acid substitution in the A subunit of Escherichia coli enterotoxin results in a loss of its toxic activity

A plasmid encoding a mutant gene of heat-labile enterotoxin (LT), produced by enterotoxigenic Escherichia coli, was induced by treatment of plasmid EWD 299 with hydroxylamine. A mutant strain of E. coli HB 101 carrying the mutant plasmid pTUH 6A produced a low toxic LT analogue (mutant LT), which was cross-reactive with anti-LT antibody. The mutant LT activity was less than 0.15 and 0.006% of the normal LT in the rabbit ileal loop test and in the rabbit skin permeability test, respectively. The amino acid composition of the mutant LT-B subunit was the same as that of the normal B subunit. Though the A₂ fragment of the mutant LT was identical to normal LT by DNA analysis, the A₁ fragment of the mutant LT differed from the normal A₁ fragment in one amino acid at position 112; namely it had lysine instead of glutamic acid from the N terminus. These data suggest that glutamic acid at position 112 from the N terminus of the A₁ fragment is important for the A subunit to express its biological activity.