J. Biol. Chem., Vol. 261, Issue 25, 11697-11703, Sep, 1986
Isolation of a cDNA clone encoding S-adenosylmethionine decarboxylase. Expression of the gene in mitogen-activated lymphocytes
M Mach, MW White, M Neubauer, JL Degen and DR Morris
S-Adenosylmethionine decarboxylase was purified from bovine liver and
digested with endopeptidase Lys-C; the resulting peptides were
chromatographically separated. Peptides containing either methionine or
tryptophan were subjected to sequence analysis. An oligonucleotide mixture
of 48 sequences, which was 17 nucleotides in length, was synthesized based
on one of these peptide sequences. This synthetic oligonucleotide mixture
was labeled and used to screen a bovine cDNA library in phage lambda gt11.
A clone was identified which contained a 1350-nucleotide insert. This
insert contained nucleotide sequences coding for amino acid sequences of
two of the peptides that were analyzed, thus proving that this cDNA clone
codes for S- adenosylmethionine decarboxylase. A subcloned fragment from
the coding region of the cDNA was used as a probe to analyze the expression
of this gene in mitogen-activated lymphocytes. Northern blots revealed two
message species of 2.4 and 3.6 kilobases in length. Both mRNAs were
coordinately expressed and were present in polysomes. The levels of these
mRNAs increased approximately 4-fold by 9 h after activation of the cells.
The magnitude of the increase in these messages is to be compared with an
8- to 10-fold increase in the rate of synthesis of the protein. The
apparent increase in translational efficiency of this message upon
lymphocyte activation was confirmed by analyzing polysomes from these
cells. In resting lymphocytes, the average size of polysomes containing
mRNA coding for S-adenosylmethionine decarboxylase was 1.4 ribosomes per
mRNA, and this value increased to 2.7 in stimulated cells. Thus, it appears
that the increase in translational efficiency of this mRNA arises from an
elevated rate of translational initiation, leading to more ribosomes per
polysome encoding this particular message. This is not a general effect on
the expression of all proteins, since there is no change in the
translational efficiency of cytoplasmic actin upon activation of
lymphocytes.