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Cultured monkey (TC7) and mouse (3T6) cells synthesize an Escherichia coli enzyme, xanthine-guanine phosphoribosyltransferase (XGPRT; 5-phospho-a-D-ribose-1-diphosphate:xanthine phosphoribosyltransferase, EC 2.4.2.22), after transfection with DNA vectors carrying the corresponding bacterial gene, Ecogpt. In contrast to mammalian cells, which do not efficiently use xanthine for purine nucleotide synthesis, cells that produce E. coli XGPRT can, synthesize GMP from xanthine via XMP. After transfection with vector-Ecogpt DNAs, surviving cells producing XGPRT can be selectively grown with xanthine as the sole precursor for guanine nucleotide formation in a medium containing inhibitors (aminopterin and mycophenolic acid) that block de novo purine nucleotide synthesis. Cells transformed for Ecogpt arise with a frequency of 10-4 to 10-5; they appear to be genetically stable in as much as there is no discernible decrease in XGPRT formation or loss in their ability to grow in selective medium after propagation in nonselective medium. (1, 7), the recovery of transformants without a selection is impractical. Therefore, our first goal was to obtain a gene whose expression in the transduced cells would allow them to be grown selectively. That purpose has been achieved by the isolation of a gene from Escherichia coli (Ecogpt) that encodes xanthineguanine phosphoribosyltransferase (XGPRT; 5-phospho-a-Dribose-l-diphosphate:xanthine phosphoribosyltransferase, EC 2.4.2.22), a purine salvage-pathway enzyme.E. coli XGPRT and the analogous mammalian enzyme, hypoxanthine phosphoribosyltransferase (HPRT; IMP pyrophosphate phosphoribosyltransferase, EC 2.4.2.8), catalyze the conversion of hypoxanthine and guanine to IMP and GMP, respectively; the bacterial enzyme also efficiently converts xanthine to XMP (8), a reaction catalyzed only very poorly by the mammalian enzyme (9). We have previously reported that infection of cultured mammalian cells with recombinant DNAs containing the Ecogpt segment induces the synthesis of bacterial XGPRT (5). Moreover, HPRT-negative cell lines transfected with appropriate vectors containing the Ecogpt gene synthesize XGPRT and grow selectively in hypoxanthine/ aminopterin/thymidine medium (5). This finding suggests that E. coli XGPRT can provide the purine salvage function ofmammalian, HPRT After 3 days at 370C in Eagle's medium containing 5% fetal calf serum, the transfected cell monolayers were treated with Abbreviations: SV40, simian virus 40; kb, kilobase(s); XGPRT, xanthineguanine phosphoribosyltransferase; GPRT, guanine phosphoribosylb transferase; HPRT, hypoxanthine phosphoribosyltransferase; T and t, SV40 large and small tumor antigens, respectively; APRT, adenine phosphoribosyltransferase; DHFR, dihydrofolate reductase; TK, thymidine kinase.

A simple and reproducible procedure for the introduction of DNA into mammalian cells by electroporation is described. The parameters involving the cells, the DNA, and the electric field are investigated. The procedure has been applied to a broad range of animal cells. It is capable of transforming more than 1% of the viable cells to the stable expression of a selectable marker.