An ornithine decarboxylase-deficient mutant of Chinese hamster ovary cells

We have selected a mutant of Chinese hamster ovary cells that is severely deficient in ornithine decarboxylase activity and is auxotrophic for putrescine. The mutant we obtained (C54) has only 3% of the maximum inducible ornithine decarboxylase activity of the parental Chinese hamster ovary cells and the rate of incorporation of [3H]ornithine into acid-soluble material is correspondingly reduced. The defect in the mutant was recessive in somatic cell hybrids. The mutant requires at least 10(-5) M putrescine in the medium to maintain a normal growth rate. Spermidine and spermine can also serve as a polyamine source, and very high (millimolar) concentrations of ornithine can support a normal growth rate. In the absence of polyamine supplementation the cells stop growing after about 3 population doublings and begin to die after 5 or 6 days. The intracellular concentrations of putrescine and spermidine are depleted after 24 and 48 h, respectively. Spermine levels remain essentially constant for 4 days. Unexpectedly, the Km for ornithine of the enzyme from mutant cells was consistently somewhat lower than the Km for the enzyme from wild type cells. It is possible but not yet certain that the mutant is the result of a mutation in the structural gene coding for ornithine decarboxylase.