Riboswitch ribozyme
Gene expression controlled directly by metabolites binding to RNA
By Cathy Holding

So far, all repressor molecules that have been characterized are proteins.
But a new study in the March 18 Nature describes a novel catalytic RNA that
controls its own gene expression. These findings, say the study's authors,
point to the gene control mechanisms employed by primitive organisms and
suggest that modern cells retain some of these systems.

All metabolite-binding riboswitches so far discovered induce a structural
change in the messenger RNA that controls gene expression by straightforward
mechanisms, said Ron Breaker, associate professor in the Department of
Molecular, Cellular, and Developmental Biology at Yale and senior author on
the study. "The glmS ribozyme is the first riboswitch to be found that works
by cutting the messenger RNA," Breaker said in an E-mail to The Scientist.

"A ribozyme in the RNA actually carries out the chemical cleavage of that
RNA and thereby makes it useless," said David M.J. Lilley, director of
Cancer Research UK's Nucleic Acid Structure Research Group at the University
of Dundee. "So, one huge difference is it only becomes activated [to cut
itself] by the binding of a small molecule-in other words, this glucosamine
6 phosphate," said Lilley, who was not involved in the study.

"If it's the first member of a very large family of riboswitch ribozymes,
then that shows that ribozymes are participating in genetic control as
opposed to gene expression," said Scott K. Silverman from the Department of
Chemistry, University of Illinois at Urbana-Champaign. A riboswitch that is
a ribozyme offers one mechanism by which encoded information, in the form of
genes, could be regulated by RNA-one of the basic underlying requirements
for an RNA world.

Read the rest at The Scientist.com
http://www.biomedcentral.com/news/20040318/01

Prion folding produces strains
Yeast prion strain differences directly linked to conformation of original
infecting prion
By David Secko

In the decades since prions were discovered by Stanley Prusiner, scientists
have become comfortable with the idea that they cause disease. But how an
infectious agent made solely of protein can produce varying strains has
remained unclear.

In the March 18 Nature, two papers based on studies of prion infections of
yeast report that the conformational differences in the infecting prions
determine strain variation.

"This has been a major mystery of the whole prion field," said Jonathan
Weissman, professor at the University of California, San Francisco, and
senior author of the first paper. "For example, people are used to there
being different viral strains, because every time you get a new mutation,
you get a new strain out. But if you have an infectious protein and protein
is the only component, how could you have strain differences?" he told The
Scientist.

To answer this question, both teams used the prion form of the Sup35p
protein-in Saccharomyces cerevisiae, Sup35p is required by ribosomes to
terminate polypeptide production. Conversion of Sup35p to its prion form
produces a defective termination phase, generating [PSI +] strains that can
be readily screened by cell color and protein analysis. "We wanted to
explore whether it was really possible for the different strains to be
encoded within the conformation of this protein," Weissman said.

It was the design of a high-efficiency protocol for infecting yeast with
preformed prion particles that enabled Weissman and his colleagues to
generate different infectious prion amyloids-composed of a fragment of the
Sup35p protein (Sup-NM)-at different temperatures. The infection of yeast
with these different amyloids lead to different [PSI +] strains, inferring
that Sup-NM conformation determined prion strain variation.

http://www.biomedcentral.com/news/20040318/02

Comment:
Protein world?

Posted by
Robert Karl Stonjek.
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