Cell-specific translation of S-adenosylmethionine decarboxylase mRNA. Regulation by the 5' transcript leader

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 preribosomal 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 Y1 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.