Cloning and molecular characterization of the B subunit of Escherichia coli heat-labile enterotoxin.

We have constructed a plasmid containing the gene for production of the B subunit of the heat-labile enterotoxin (LT-B) from a human isolate of Escherichia coli, strain H10407. The 0.8-kilobase gene fragment encoding synthesis of LT-B was cloned onto plasmid pBR322 after sequential digestion of the enterotoxin plasmid of strain H10407 with restriction endonucleases PstI and HindIII. LT-B was isolated by agarose affinity chromatography from cell lysates of recombinant clones expressing the B subunit. The B subunit was isolated in its oligomeric form, was structurally identical to native B subunit when examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, dissociated to monomeric B in the presence of 5 M guanidine, was immunologically identical to native B subunit in an enzyme-linked immunosorbent assay, and contained no demonstrable A subunit in any of the assays.