[Cloning and expression of the cholera toxin B subunit gene]

The CT-B gene-expressing plasmid pXB1 and modified pXB2 were constructed using recombinant DNA techniques. These resulted in high CT-B gene expression in E. coli (280-350 ng/ml), with CT-B accounting for 71.5% of total extracellular protein. The results of CT-B expression kinetics showed that the CT-B product levels reached a peak after 24 h in culture. Certain concentrations (100 micrograms/ml) of MgCl2, L-histidine and asparagine can increase the CT-B product level by 10%-25%. The binding of the expressed CT-B product to GM1 receptor on free or live cell membranes was identified by GM1-ELISA, CHO cytotoxicity test, indirect immunofluorescence, and the rabbit ileal loop test. No toxic response to cells or rabbits was detected.