AMPLIFICATION AND EXPRESSION OF RECOMBINANT GENES IN SERUM-INDEPENDENT CHINESE-HAMSTER OVARY CELLS

CHO SSF3 cells grow as a suspension culture in unmodified commercial m edium with only low-molecular weight ingredients. Continuous serum-fre e culture unexpectedly induced expression of a low dihydrofolate reduc tase activity in the originally dhfr(-); CHO cells. Nevertheless, it w as possible with methotrexate to induce amplification of a gene coding for the hybrid plasminogen activator K2tu-PA cotransfected with a dhf r gene, Expression of K2tu-PA expression was proportionally increased to that of dhfr, which was measured with fluorescent methotrexate. Bec ause no serum proteases were present, secreted K2tu-PA was not convert ed to the enzymatically active form, but was exclusively recovered in proenzyme form.