In vitro mutagenesis of the LTA gene, encoding the A subunit of the Escherichia coli heat‐labile enterotoxin, has been used to obtain A subunits deficient in enzymic activity. One inactive A‐subunit mutant which contained two amino acid substitutions, was shown to associate with native B subunits to form a holotoxoid lacking toxin activity. A serine to phenylalanine mutation appears to be responsible for the loss of toxicity.

The nucleotide sequences of four variants of the Escherichia coli K88 antigen gene, K88ab1, K88ab2, K88ac, and K88ad, have been determined. The K88ab2 and K88ac sequences have not been reported previously. The K88ab1 sequence is very similar to that determined by other workers, but the K88ad sequence differs considerably from that described in a previous report. Comparison of the amino acid sequences inferred from the gene sequences revealed certain clusters of amino acid substitutions which have been correlated with areas of potential antigenicity in the mature proteins.

The heat‐labile enterotoxin genes from plasmid P307 have been cloned into pAT153. A comparison of the sequence of the LT‐A gene with that of the A subunit of cholera toxin shows extensive homology. There are a number of significant differences between the sequence of the LT‐A gene reported here and that published previously.