Cell-specific Translational Regulation of S-Adenosylmethionine Decarboxylase
mRNA
DEPENDENCE ON TRANSLATION AND CODING CAPACITY OF THE CIS-ACTING UPSTREAM
OPEN READING FRAME
John R. Hill and David R. Morris
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.