Regulation of ornithine decarboxylase mRNA translation by polyamines. Studies using a cell-free system and a cell line with an amplified ornithine decarboxylase gene.

The translational control of ornithine decarboxylase (ODCase) by polyamines has been studied using a cellular as well as a cell-free system. A mutant L1210 cell line, in which ODCase represents 4-5% of all soluble protein synthesized, was isolated by stepwise selection for resistance to the ODCase inhibitor 2-difluoromethylornithine (DFMO). The exceptionally high expression of ODCase in these cells was due to amplification of the ODCase gene. When the cells were grown in the absence of DFMO, dramatic increases in cellular putrescine and spermidine levels occurred. These increases were accompanied by a rapid decrease in ODCase synthesis. The change in ODCase synthesis was not associated with an alteration in the amount of ODCase mRNA, demonstrating a translational control in these cells. The effects of polyamines on ODCase mRNA translation were also studied in rabbit reticulocyte lysates using mRNA isolated from the DFMO-resistant cells. Low concentrations of spermidine stimulated synthesis of ODCase and that of total protein, when added to gel-filtered lysates. Notably, optimal stimulation of ODCase synthesis was achieved at a spermidine concentration lower than that required for an optimal rate of total protein synthesis. Higher concentrations of spermidine were inhibitory, and their effects of ODCase synthesis were stronger than on protein synthesis in general, resulting in a decrease in the fraction of protein synthesis accounted for by ODCase. The present results demonstrate that at least part of the feedback regulation of ODCase exerted by the polyamines is due to direct inhibition of ODCase mRNA translation.