Resistance to 5-fluoroorotic acid and pyrimidine auxotrophy: a new bidirectional selective system for mammalian cells.

We have isolated a clone of murine erythroleukemic cells which will yield a population when a single cell is suspended in a medium composed of dialyzed sera and small molecules. We report that it is feasible--in one experiment--to screen more than 10 of these cells for growth under selective media containing 10(-4) M 5-fluoroorotic acid and 10(-4) M uridine. Cells capable of sustained growth in such media were eventually recovered. Clones of these cells, unlike clones of the parental population, required uridine for growth and contained only 0.39% as much orotate phosphoribosyltransferase, less than 0.5% as much 5-fluoroorotate phosphoribosyltransferase, and 0.013% as much orotidine-5'-monophosphate decarboxylase activity as the parental clones. The parental and variant clones had similar levels of activity for six other enzymes that participate in pyrimidine metabolism.