An expression vector for high-level protein synthesis in Vero cells.

We have constructed two new multi-purpose cloning vectors, pNI1 and pNI2, that carry the Escherichia coli gene Ecogpt encoding the enzyme xanthine-guanine phosphoribosyl transferase as a dominant selective marker. The Ecogpt gene is under the control of either the long-terminal-repeat promoter of mouse mammary tumor virus, pNI1, or the simian virus 40 early promoter, pNI2. Another feature of the vectors is a polylinker preceded by the human metallothionein IIA promoter. We have used pNI2 for the synthesis of the hepatitis B surface antigen (HBsAg) at a high level in monkey Vero cells. We show that gene amplification and a concomitant stable increase of HBsAg synthesis can be achieved in these cells using modified selective medium containing hypoxanthine, aminopterin and thymidine, i.e., increasing the aminopterin and decreasing the hypoxanthine concentrations.