Expression and amplification of cloned rat liver tyrosine aminotransferase in nonhepatic cells

A full-length cDNA for the rat liver enzyme tyrosine aminotransferase has been used to construct mammalian expression vectors by recombinant DNA techniques. These vectors, which have employed either a simian virus 40 or a Rous sarcoma virus promoter, were transfected into a variety of nonhepatic mammalian cell lines in culture. Transient expression of tyrosine aminotransferase was readily observed after transfection into monkey COS cells and mouse L cells. Stable clones that express cloned tyrosine aminotransferase have been isolated from mouse L cells, hamster Wg1a fibroblasts, and Chinese hamster ovary (CHO) cells. A vector capable of expressing both tyrosine aminotransferase and dihydrofolate reductase was stimulated to undergo amplification by treatment with methotrexate in a CHO cell line deficient in the latter enzyme. Levels of tyrosine aminotransferase as much as 50-fold higher than typically seen in glucocorticoid-induced hepatoma cells were achieved in some CHO clones by this technique. The tyrosine aminotransferase produced at these highly amplified levels appeared structurally normal and had no major harmful effects on the cells.