CONDITIONAL RESISTANCE TO THYMINELESS DEATH PREDOMINANTLY SELECTS DNASYNTHESIS-DEFICIENT MUTANTS OF MAMMALIAN-CELLS

Temperature-sensitive growth mutants of the mouse mammary carcinoma ce ll line FM3A were isolated by selecting survivors of thymidylate starv ation for a limited time at the restrictive temperature (39.5-degrees- C). Nineteen lines of independent isolates were established and all we re found to be deficient in DNA synthesis. Cell-cell hybridization wit h authentic mutant lines of FM3A demonstrated that the mutants fell in to three complementation groups, which were deficient in DNA polymeras e a or ubiquitin-activating enzyme E1 or both.