Volume 1 1 Number 24 1983 Nucleic Acids Research Nucleotide sequence of the Escherichia coli xanthine-guanine phosphonbosyl transferase gene Dickson Pratt and Suresh Subramani Department of Biology, B-022, Bonner Hall, University of California, San Diego, La Jolla, CA 92093, USA Received 6 October 1983; Revised and Accepted 7 November 1983 ABSTRACT The Escherichia coli gene coding for the enzyme xanthine-guanine phos- phoribosyl transferase (gpt) has been widely used as a dominant selectable marker in a variety of mammalian cells. We have determined the complete nucleotide sequence of the 1057 base pair (bp) segment of DNA containing this gene. The coding sequence for the enzyme is 456 nucleotides long and can code for a 152 amino acid (16.9 Kd) polypeptide. A comparison of the amino acid sequence of the bacterial enzyme with that of the mammalian hypoxanthine- guanine phosphoribosyl transferase (hprt) reveals no significant homology between the two polypeptides. INTRODUCTION Xanthine-guanine phosphoribosyl transferase (EC 2.4.2.22) from E. coli is a purine salvage enzyme which converts either xanthine or guanine, using phos- phoribosylpyrophosphate (PRPP) as the other substrate, to XMP or GMP, respec- tively. Besides the gpt enzyme, E. coli also has adenine (1,2) and hypoxanthine guanine phosphoribosyltransferases (3) to salvage purines. The gpt and hprt enzymes are distinct and are coded for by different genes (4). Recently, the product of the gpt gene has been purified from E. coli harboring a plasmid that overproduces gpt and the enzyme has been reported to be a tri- mer with a subunit molecular weight of about 18.5 Kd(5). The gpt gene has previously been expressed from simian virus 40-pBR322 hybrid plasmid vectors (6,7) and has been extremely useful as a dominant selectable marker for mammalian cells (6). The basis for this selection resides in the unique ability of the gpt gene product to salvage xanthine under appropriate selective conditions to produce XMP. The mammalian hprt enzyme lacks tne ability to utilize xanthine. In the course of our studies using this gene to analyze recombination in mammalian cells, it became necessary to obtain sequence information from the 1.06 kb DNA segment within which the gene had been previously localized (6). We report here the complete nucleotide sequence of this 1057 bp DNA segment © I RL Press Limited, Oxford, England. 881 7