A conjugative plasmid with potential usefulness for vaccine strains was constructed. In the first step, a 5.9-kilobase DNA segment containing the two loci for the A and B subunits of heat-labile enterotoxin with a mutation in the gene for the A subunit was joined to the cloning vehicle pGA22, generating the nonconjugative plasmid pPMC4 with genes for resistance to tetracycline and chloramphenic...
