Amplification and expression of phenylalanine hydroxylase in mouse erythroleukemia cells

A cell line, CY-1, was selected in tyrosine free (tyr-) medium after fusion of mouse erythroleukemia (MEL) cells with mitomycin C-treated rat hepatoma cells. MEL cells do not express the enzyme phenylalanine hydroxylase (PH) and are unable to grow in tyr- medium, whereas the rat hepatoma cells constitutively express PH and are able to grow in tyr- medium. CY-1 cells resemble MEL cells morphologically, karyotypically, and in being inducible for hemoglobin synthesis. In contrast to MEL cells, CY-1 expresses PH and is therefore able to grow in tyr- medium. Using a rat cDNA probe for the PH gene, Southern blot analyses were carried out on DNA isolated from CY-1 and parental cells. CY-1 showed the characteristic mouse PH gene pattern but the gene copy number was amplified four- to eightfold compared to parental MEL cells.