Cell-specific Translational Regulation of S-Adenosylmethionine Decarboxylase
mRNA
INFLUENCE OF THE STRUCTURE OF THE 5' TRANSCRIPT LEADER ON REGULATION BY
THE UPSTREAM OPEN READING FRAME
Hangjun Ruan, John R. Hill, S. Fatemie-Naine and David R. Morris
A small upstream open reading frame (uORF), located 14 nucleotides from
the cap in the 5' trrnscript leader (5' TL) of the mRNA encoding S-adenosylmethionine
decarboxylase (AdoMetDC), suppresses translation of the downstream cistron
in normal T cells and T cell lines. In the present study, we examined the
structural features of the 5' TL that overcome this suppressive influence
in cells of nonlymphoid origin. Initiation at the downstream cistron in
nonlymphoid cells is by a cap-dependent mechmism that requires ribosome
scanning along the 5' TL and does not involve an internal ribosome entry
site. Extending the uORF so that it overlapped the main cistron by 101
nucleotides had no effect on translation of the downstream cistron in either
HeLa or Jurkat cells. Wlien the distance between the uORF and the cap was
extended to 47 nucleotides, using sequence previously found to be neutral,
translation of the main cistron was inhibited 5-fold in HeLa cells and
the mRNA was moved from polysomes to monosomes, a location identical to
that of the wild type mRNA in Jurhat cells. Therefore, in contrast to T
cells, initiation at the uORF seems to be relatively infrequent in non-lymphoid
cells due to its proximity to the cap, allowing efficient translation of
the downstream cistron.