Inhibition of translation of mRNAs for ornithine decarboxylase and S-adenosylmethionine decarboxylase by polyamines.

The effect of spermidine and spermine on the translation of the mRNAs for ornithine decarboxylase and S-adenosylmethionine decarboxylase was studied using a reticulocyte lysate system and specific antisera to precipitate these proteins. It was found that the synthesis of these key enzymes in the biosynthesis of polyamines was much more strongly inhibited by the addition of polyamines than was either total protein synthesis or the synthesis of albumin. Translation of the mRNA for S-adenosylmethionine decarboxylase was maximal in a lysate which had been substantially freed from polyamines by gel filtration. Addition of 80 microM spermine had no significant effect on total protein synthesis and stimulated albumin synthesis but reduced the production of S-adenosylmethionine decarboxylase by 76%. Similarly, addition of 0.8 mM spermidine reduced the synthesis of S-adenosylmethionine decarboxylase by 82% while albumin and total protein synthesis were similar to that found in the gel-filtered lysate. Translation of ornithine decarboxylase mRNA was greater in the gel-filtered lysate than in the control lysate but synthesis of ornithine decarboxylase was stimulated slightly by low concentrations of polyamines and was maximal at 0.2 mM spermidine or 20 microM spermine. Higher concentrations were strongly inhibitory with a 70% reduction occurring at 0.8 mM spermidine or 150 microM spermine. Further experiments in which both polyamines were added together confirmed that the synthesis of ornithine and S-adenosylmethionine decarboxylases were much more sensitive to inhibition by polyamines than protein synthesis as a whole. These results indicate that an important part of the regulation of polyamine biosynthesis by polyamines is due to a direct inhibitory effect of the polyamines on the translation of mRNA for these biosynthetic enzymes.