Amplification and expression of recombinant genes in serum‐independent Chinese hamster ovary cells

CHO SSF3 cells grow as a suspension culture in unmodified commercial medium with only low-molecular weight ingredients. Continuous serum-free culture unexpectedly induced expression of a low dihydrofolate reductase activity in the originally dhfr- CHO cells. Nevertheless, it was possible with methotrexate to induce amplification of a gene coding for the hybrid plasminogen activator K2tu-PA cotransfected with a dhfr gene. Expression of K2tu-PA expression was proportionally increased to that of dhfr, which was measured with fluorescent methotrexate. Because no serum proteases were present, secreted K2tu-PA was not converted to the enzymatically active form, but was exclusively recovered in proenzyme form.