Regulation of Saccharomyces cerevisiae ornithine decarboxylase expression in response to polyamine.

The mechanism of yeast ornithine decarboxylase (ODC) regulation in response to polyamines was examined. ODC catalyzes the first step of polyamine biosynthesis, the conversion of ornithine to putrescine. ODC activity was modulated approximately 200-fold by varying the availability of polyamines. Variations in ODC activity were associated with parallel changes in the amount of active enzyme molecules. Regulation of ODC activity did not appear to be mediated by changes in the stability of the enzyme. The half-life of ODC was approximately 75 min and was unaffected by the availability of polyamines. Polyamines had no demonstrable effect on the transcriptional or translational stages of ODC expression. The amount of ODC mRNA and the number of ribosomes associated with the mRNA were constant despite the variations in ODC activity. Unlike translationally controlled genes, the regulation of ODC expression did not require either the 5'- or the 3'-untranslated regions of the ODC gene transcript. While gene dosage experiments indicated that negative trans-acting factors are involved in control of ODC expression, these implied factors did not appear to affect translation of ODC mRNA. The results are interpreted to suggest that polyamines regulate ODC expression at a post-translational step prior to assembly of the active form of the enzyme.