Expression of human genes for adenine phosphoribosyltransferase and hypoxanthine-guanine phosphoribosyltransferase after genetic transformation of mouse cells with purified human DNA

Human DNA purified from HeLa cells and from three strains of skin fibroblasts was precipitated with calcium phosphate and added to mouse cells that were deficient in adenine phosphoribosyltransferase (APRT) and hypoxanthine phosphoribosyltransferase (HPRT). Selection for cells possessing either of the phosphoribosyltransferases was imposed by blocking de novo synthesis of purine nucleotides with azaserine in a medium supplemented with adenine and hypoxanthine. The frequency of colony formation after selection was 1.7×10^–7–3.3×10^–6. Excepting some azaserine-resistant colonies that appeared only in the first experiment and infrequent revertants expressing mouse APRT, all characterized clones expressed the human forms of APRT or HPRT according to the criteria of specific immunoprecipitation and electrophoretic mobility. The frequency of transfer of the human APRT gene was much greater than that of HPRT. Transfer efficiency was not significantly reduced when HeLa DNA was sheared to 6.5–13.5 kb size or when the donor DNA was isolated from a transferent that expressed human APRT.