Construction and Characterization of Versatile Cloning Vectors for Efficient Delivery of Native Foreign Proteins to the Periplasm ofEscherichia coli

Induction of the wild type cholera toxin operon (ctxAB) from multicopy clones inEscherichia coliinhibited growth and resulted in low yields of cholera toxin (CT). We found that production of wild type CT or its B subunit (CT-B) as a periplasmic protein was toxic forE. coli,but by replacing the native signal sequences of both CT-A and CT-B with the signal sequence from the B subunit ofE. coliheat-labile enterotoxin LTIIb we succeeded for the first time in producing CT holotoxin in high yield inE. coli.Based on these findings, we designed and constructed versatile cloning vectors that use the LTIIb-B signal sequence to direct recombinant native proteins with high efficiency to the periplasm ofE. coli.We confirmed the usefulness of these vectors by producing two other secreted recombinant proteins. First, usingphoAfromE. coli,we demonstrated that alkaline phosphatase activity was 17-fold greater when the LTIIb-B signal sequence was used than when the native leader for alkaline phosphatase was used. Second, using thepspAgene that encodes pneumococcal surface protein A fromStreptococcus pneumoniae,we produced a 299-residue amino-terminal fragment of PspA inE. coliin large amounts as a soluble periplasmic protein and showed that it was immunoreactive in Western blots with antibodies against native PspA. The vectors described here will be useful for further studies on structure–function relationships and vaccine development with CT and PspA, and they should be valuable as general tools for delivery of other secretion-competent recombinant proteins to the periplasm inE. coli.