In vitro translation of the upstream open reading frame in the mammalian mRNA encoding S-adenosylmethionine decarboxylase.

The upstream open reading frame (uORF) in the mRNA encoding S-adenosylmethionine decarboxylase is a polyamine-responsive element that suppresses translation of the associated downstream cistron in vivo. In this paper, we provide the first direct evidence of peptide synthesis from the S-adenosylmethionine decarboxylase uORF using an in vitro translation system. We examine both the influence of cation concentration on peptide synthesis and the effect of altering the uORF sequence on peptide synthesis. Synthesis of wild type and altered peptides was similar at all concentrations of magnesium tested. In contrast, synthesis of the wild type peptide was more sensitive than that of altered peptides to elevated concentrations of the naturally occurring polyamines, spermidine and spermine, as well as several polyamine analogs. The sensitivity of in vitro synthesis to spermidine was influenced by both the amino acid sequence and the length of the peptide product of the uORF. Findings from the present study correlate with the effects of the uORF and polyamines on translation of a downstream cistron in vivo and support the hypothesis that polyamines and the structure of the nascent peptide create a rate-limiting step in uORF translation, perhaps through a ribosome stalling mechanism.