Selection for animal cells that express the Escherichia coli gene coding for xanthine-guanine phosphoribosyltransferase.

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Abstract
Cultured monkey (TC7) and mouse (3T6) cells synthesize an E. coli enzyme, xanthine-guanine phosphoribosyltransferase (XGPRT; 5-phospho-.alpha.-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 DNA, 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 because there is no discernible decrease in XGPRT formation or loss in their ability to grow in selective medium after propagation in nonselective medium. Although several of the vector-gpt DNA can replicate in monkey and mouse cells, none of the transformants contain autonomously replicating vector-gpt DNA. The gpt transformants contain 1-5 copies of the transfecting DNA associated with, and most probably integrated into, cellular DNA sequences. In several transformants, vector-coded gene products for which there was no selection are also synthesized. Recombinant DNA containing Ecogpt as a selective marker may be useful for cotransformation of nonselectable genes.