The nucleotides in the 70-170 region upstream from the initiator AUG have been shown to be important in the strong stimulation of ornithine decarboxylase (ODC) synthesis by low spermidine concentrations and in the inhibition of ODC synthesis at high spermidine concentrations [Ito, K., et al. (1990) J. Biol. Chem. 265, 13036-13041]. In this region, a GC-rich sequence as well as a small open reading frame (MGQASQATVL) existed. In order to clarify which of these was of greater importance for the spermidine regulation of ODC synthesis, the synthesis was performed with various ODC mRNAs, possessing different sizes and nucleotide sequences in the 5'-untranslated region. The results show that a GC-rich sequence, but not a small potential leader peptide, plays an important role in the spermidine regulation of ODC synthesis.