Regulated Translation Termination at the Upstream Open Reading Frame in S-Adenosylmethionine Decarboxylase mRNA

Alexa Raney, G. Lynn Law, Gregory J. Mize and David R. Morris

The upstream open reading frame (uORF) in the mRNA encoding S-adenosylmethionine decarboxylase is a cis-acting element that confers feedback control by cellular polyamines on translation of this message. Recent studies demonstrated that elevated polyamines inhibit synthesis of the peptide encoded by the uORF by stabilizing a ribosome paused in the vicinity of the termination codon. These studies suggested that polyamines act at the termination step of uORF translation. In this paper, we demonstrate that elevated polyamines stabilize an intermediate in the termination process, the complete nascent peptide linked to the tRNA that decodes the final codon. The peptidyl-tRNA molecule is found associated with the ribosome fraction and decay of this molecule correlated with release of the paused ribosome from the message. Furthermore, the stability of this complex is influenced by the same parameters that influence regulation by the uORF in vivo, namely the concentration of polyamines and the sequence of the uORF-encoded peptide. These results suggest that the regulated step in uORF translation is after formation of the peptidyl-tRNA molecule, but before hydrolysis of the peptidyl-tRNA bond. This regulation may involve an interaction between the peptide, polyamines and a target in the translational apparatus.