Polyamine regulation of S-adenosylmethionine decarboxylase synthesis through the 5'-untranslated region of its mRNA.

The effect of the 5'-untranslated region (5'-UTR) of S-adenosylmethionine decarboxylase (SAMDC) mRNA and of polyamines on the translation of SAMDC mRNA was studied in a rabbit reticulocyte cell-free system. Using synthetic SAMDC mRNAs possessing different sizes of 5'-UTR, it was shown that nucleotides in the 5'-end of 5'-UTR were responsible for polyamine inhibition at high concentrations. Existence of the 5'-end nucleotides decreased SAMDC synthesis and slightly increased the degree of polyamine stimulation of the synthesis at low concentrations. When poly(A)+ RNA from mouse SAMDC-overproducing cells was used as mRNA, the degree of polyamine inhibition at high concentrations was nearly the same, but that of polyamine stimulation at low concentrations was greater than with synthetic SAMDC mRNAs. The reason for this difference is discussed.