Activation of phenylalanine hydroxylase expression following genomic DNA transfection of hepatoma cells.

Genomic DNA from cells producing the liver-specific enzyme phenylalanine hydroxylase (PAH) should contain, in active form, genes encoding regulators of PAH expression. We have transfected genomic DNA from PAH-producing rat hepatoma cells to PAH-deficient mouse hepatoma cells, and selected in tyrosine-deficient medium for cells producing the enzyme. The frequency of colonies obtained was similar to that for transfer of a single-copy gene. Genomic DNA from the primary transfectants permitted the isolation in tyrosine-free medium of secondary transfectants. Control experiments, using donor DNA from PAH-negative rat or mouse hepatoma cells also permitted the isolation of PAH-expressing cells, but at a frequency 10-30 times lower. The transfectants isolated in tyrosine-deficient selective medium all produced PAH mRNA. This transcript was from the previously silent mouse gene, which had not undergone amplification or gross rearrangement. Most of the transfectants contained less than 0.1% rat DNA. A search for other functions that might have been simultaneously activated was negative. It is concluded that the mouse transfectants acquired from the PAH+ rat donor some sequences whose presence permits activity of the previously silent PAH gene.