Cell-specific Translation of S-Adenosylmethionine Decarboxylase mRNA
REGULATION BY THE 5' TRANSCRIPT LEADER
John R. Hill and David R. Morris
The mRNA encoding S-adenosylmethionine decarboxylase (AdoMetDC) has an
unusual distribution in polysomes from cells of T lymphocyte origin. It
associates predominantly with monosomes and small polysomes with none located
in the pre-ribosomal or ribonucleoprotein pool. In sharp contrast, it associates
broadly with larger polysomes in several nonlymphoid cell lines, including
fibroblasts and the adrenal carcinoma line, Y1. The AdoMetDC 5'-transcript
leader (5'-TL) is highly conserved between human and bovine mRNAs. It has
a length of about 330 nucleotides and contains a 21-nucleotide upstream
open reading frame (uORF) approximately 14 nucleotides downstream of the
cap site. The AdoMetDC 5'-TL, when used to replace the 5'-TL of the human
growth hormone gene in a chimeric expression construct, causes a suppressed
polysomal distribution of the chimeric mRNA identical to that of the endogenous
AdoMetDC mRNA in the T cell line, Jurkat. In contrast, mRNA encoded by
the same chimeric construct, when expressed in Yl cells, mimics the broad
polysomal distribution of the endogenous AdoMetDC mRNA. Mutations that
remove the uORF, or prevent its initiation, abolish the translational suppression
in T cells, establishing that the uORF is a negative element that modulates
the cell-specific polysomal distribution of AdoMetDC mRNA.