RESUMEN
Recombination plasmid pMM085 possessed both immunogens heat-labile enterotoxin(LT) and fimbriae antigen K88 of enterotoxigenic Escherichia coli (ETEC). Althouth vaccine strain MM-3 carrying pMM085 had good effect to protect piglets against diarrhea due to ETEC infections,it was not ideal live vaccine for pMM085 bringing chloramphenicol resistance gene (cat). To solve the problem,the host-plasmid balanced lethal system was introduced which including the replacement of cat gene by asd gene and transformation the new plasmid to the strain X6097 which asd gene was knocked out in its chromosome. Considering pMM085 was a big plasmid (23kb) and traditional genetic manipulations was not easy to carry on,?-Red recombination system was adopt in this work to realize the replacement of cat gene by asd gene. The results indicated that ?-Red recombination system was convenient and efficient to reconstruct big plasmid.