Cell-specific translational regulation of S-adenosylmethionine decarboxylase mRNA. Dependence on translation and coding capacity of the cis-acting upstream open reading frame.

The mRNA encoding S-adenosylmethionine decarboxylase (AdoMetDC) has a 330-nucleotide 5'-transcript leader containing an open reading frame (uORF) that codes for the hexapeptide MAGDIS. The uORF restricts the intracellular distribution of AdoMetDC mRNA primarily to monosomes in normal T-lymphocytes and in T-cell lines. In contrast, non-lymphoid cells normally carry an average of seven to nine ribosomes per AdoMetDC mRNA molecule (Hill, J.R., and Morris, D. R. (1992) J. Biol. Chem. 267, 21886-21893). Several alterations abolish the negative regulatory effect of the uORF in T-cells. These include removing the site of translational initiation; weakening the context of the translational initiation site; changing the coding capacity of the fourth, fifth, and sixth codons; increasing the length of the uORF at either the 5' or 3' end; or changing the primary order of the codons. In contrast, altering the nucleic acid sequence of the uORF at degenerative positions without changing the amino acid coding capacity did not cause deregulation. The uORF does not regulate translation in the trans-configuration. Our results support a model in which translation of the uORF generates a nascent hexapeptide that interacts with its translating ribosome to suppress translation of AdoMetDC mRNA in a cell-specific manner. Structural features of the carboxyl-terminal 3 amino acids of the putative hexapeptide govern the interaction of the peptide with a component of the translation machinery.