Enzyme Found to Control Gene Expression in a Novel Way—By
Breathing New Life into Old RNA
By Jason Socrates
Bardi
A recently identified enzyme from a family of enzymes that
is ubiquitous in all eukaryotic life—from yeast to humans—has
been found to play a novel role in regulating the expression
of a gene in fission yeast Schizosaccharomyces pombe.
This finding may have implications for the treatment of cancer.
The characterization of this regulatory enzyme, the result
of a collaboration between the groups of investigators Paul
Russell and John Yates at The Scripps Research Institute (TSRI),
is described in an article in the latest issue of the journal
Cell. In addition to having potential applications
in medicine, the research is important from a basic science
perspective because, unlike other enzymes of the same class,
this new enzyme, called Cid13, works in the cytoplasm.
"Normally we think of genes that are involved in expression
of mRNA operating in the nucleus," says Russell, who
led the study. "This is a new class of enzyme that can
control the activity of mRNA after it has been exported to
the cytoplasm."
When genes are expressed in eukaryotic cells, the DNA of
the genes is first transcribed into RNA "messages"
in the nucleus by RNA polymerases, which read off the gene
starting from the leading 5' end and terminating at a 3' tail.
Then, this mRNA is transported outside the cell nucleus into
the cytoplasm, but not before it undergoes splicing to remove
any non-coding regions (introns) that are present and is polyadenylated
by an enzyme called a poly(A) polymerase.
Poly(A) polymerases adds several adenines to the tail end
of each mRNA molecule. Without this tail, the mRNA will not
be translated into protein. In fact, once in the cytoplasm,
the mRNA will only be translated into protein as long as the
poly(A) tail is present. Eventually, nucleases in the cytoplasm
can remove the tail, and then that mRNA becomes degraded.
Cid13, a type of poly(A) polymerase, rejuvinates expressed
mRNA molecules that are about to be recycled by the cell by
adding a new poly(A) tail to the mRNA.
"Our enzyme steps in when the poly(A) tail has been
shortened but before the mRNA has been degraded," says
Russell. "It resynthesizes a new poly(A) tail and breathes
new life into that mRNA."
Previously it was thought that all poly(A) polymerases resided
in the nucleus of somatic cells, where they did their work
to freshly spliced mRNAs before they were transported out
of the cell to meet ribosomes, but Cid13 works in the cytoplasm.
Interestingly, unlike other poly(A) polymerases in the nuclei,
which add the tails indiscriminately, the Cid13 seems to reactivate
only one particular gene—the mRNA of an important metabolic
enzyme called ribonucleotide reductase.
Ribonucleotide reductase is a crucial enzyme because it
supplies cells with the deoxyribonucleotides they need to
build DNA. When cells begin to copy their DNA before they
divide, these essential ribonucleotide reductase enzymes are
upregulated in a variety of ways. One of these, apparently,
is recycling used ribonucleotide reductase mRNAs and activating
them before they can otherwise be degraded by the cell.
Interestingly, unlike other known poly(A) polymerases, Cid13
seems to be specific for ribonucleotide reductase mRNA, though
follow up work in Russell’s lab is aimed at determining
whether other mRNAs are targeted as well. Other follow-up
work aims at determining how the enzymes are regulated. And
given the ubiquity of this class of enzymes, they speculate
that there may be specific Cid13-like enzymes in other organisms,
including humans.
"The activity is possibly universal in [eukaryotes],"
says Shigeaki Saitoh, who is first author on the paper.
Russell and Saitoh believe that the enzyme is a cell’s
way of dealing with certain types of stress that can lead
to DNA damage, like heat shock or oxidative damage. In such
times of stress, the cell can use the Cid13 pathway to reduce
the need to make fresh, potentially damaged, mRNA. But this
type of enzyme might also be an important tool used by cancerous
cells to fight chemotherapy.
In fact, the current study stemmed from Russell and Saitoh’s
interest in understanding how cells survive treatment with
hydroxyurea. Hydroxyurea is a U.S. Food and Drug Administration-approved
cancer chemotherapy and is commonly given to AIDS and sickle-cell
anemia patients. It works by inhibiting ribonucleotide reductase
and choking off a cell’s supply of nucleotides.
Cells can become resistant to hydroxyurea by increasing
the activity of the ribonucleotide reductase enzyme, a task
which it may accomplish by using the molecule Cid13.
Saitoh found that strains with an abundance of Cid13 are
resistant to hydroxyurea. Tumor cells that are resistant to
hydroxyurea may be using a similar enzyme to increase the
amount of ribonucleotide reductase that is translated. If
this proves to be the case, then a Cid13 inhibitor in combination
with hydroxyurea might be a more potent treatment than hydroxyurea
alone.
This work was a collaboration between the laboratories of
Paul Russell and John Yates. The Yates laboratory did mass
spectroscopy on Cid13, which had already been identified but
which scientists had assumed was a DNA polymerase. The mass
spectroscopy allowed Russell and Saitoh to determine which
other proteins Cid13 interacts with, which turned out to be
proteins that bind to the poly(A) tail of mRNA. None were
involved in DNA polymerization.
"Right away, this reinforced [the idea] that the enzyme
was likely to be involved in RNA metabolism and poly (A) addition
as opposed to DNA replication," says Russell, who stresses
that the mass spectrometry technique was essential.
The article "Cid13 is a Cytoplasmic Poly(A) Polymerase
that Regulates Ribonucleotide Reductase mRNA" is authored
by Shigeaki Saitoh, Andrei Chabes, W. Hayes McDonald, Lars
Thelander, John R. Yates III, and Paul Russell, and appears
in the May 31, 2002 issue of the journal Cell.
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