The Upstream Open Reading Frame of the mRNA Encoding S-Adenosylmethionine
Decarboxylase Is a Polyamine-responsive Translational Control Element
Hangjun Ruan, Lisa M. Shantz, Anthony E. Pegg, and David R. Morris
S-Adenosylmethionine decarboxylase (AdoMetDC) is a key enzyme in the pathway
of polyamine biosynthesis. The cellular levels of the polyamines specifically
regulate AdoMetDC translation through the 5'-leader of the mRNA, which
contains a small upstream open reading frame (uORF) 14 nucleotides from
the cap. Mutating the initiation codon of the uORF, which encodes a peptide
product with the sequence MAGDIS, abolished regulation. In addition, the
uORF is sufficient, by itself, to provide polyamine regulation when inserted
into the 5'-leader of the human growth hormone mRNA. Changing the amino
acid sequence at the carboxyl terminus of the peptide product of the uORF
abolished polyamine regulation. In contrast, altering the nucleotide sequence
of the uORF at degenerate positions, without changing the amino acid sequence
of the peptide, did not affect regulation. Extending the distance between
cap and uORF, thereby changing the rate of initiation at the initiator
AUG of the uORF, did not alter polyamine regulation. When the uORF was
extended so as to overlap, out of frame, the downstream main cistron, polyamine
regulation was abolished. We propose that polyamines do not modulate the
rate of recognition of the uORF but rather regulate interaction of the
peptide product of the uORF with its target.