Scientists today generally agree that DNA is the result of life on Earth,
rather than its origin. But many molecular biologists are embracing the
intruiging possibility, and strong evidence that the first life on
Earth involved chemical multitasking by another key life molecule,
ribonucleic acid or RNA.
Molecular subunits of RNA have been found in alien carbonaceous
chondrite meteorites, or could have formed through chemical reactions in
the early Earth's oceans or primordial atmosphere leading to the
exciting new theory that RNA is our earliest molecular ancestor.
Many biologists still view RNA as a messenger to shutlle inforamtion
from DNA to the cell's protein manufacturing centers, ribosomes. Like
DNA, RNA is a string of four different kinds of nucleotide building
blocks, except that RNA has a single chain rather than DNA's iconic
double helix that uses a different sugar in its molecular architecture,
and substitutes uracil instead of thymine. However, unlike DNA, RNA is
capable of carrying both genetic information and getting metabolic work
done making it the "origin of life" molecule capabable of both reproducing itself and carrying the code to guide the needed copying.
Some bacterial cells can swim, morph into new forms and even become
dangerously virulent - all without initial involvement of DNA. In 2009, Yale University
researchers discovered how bacteria accomplish this amazing feat - and
in doing so provided a glimpse of what the earliest forms of life on
Earth may have looked like.
To initiate many important functions, bacteria sometimes depend
entirely upon ancient forms of RNA, once viewed simply as the chemical
intermediary between DNA's instruction manual and the creation of
proteins, said Ronald Breaker, the Henry Ford II Professor of Molecular, Cellular and Developmental Biology at Yale and senior author of the study.
Proteins carry out almost all of life's cellular functions today, but
many scientists like Breaker believe this was not always the case and
have found many examples in which RNA plays a surprisingly large role in
regulating cellular activity. The Yale study illustrates that - in
bacteria, at least - proteins are not always necessary to spur a host of
fundamental cellular changes, a process Breaker believes was common on
Earth some 4 billion years ago, well before DNA existed.
"How could RNA trigger changes in ancient cells without all the
proteins present in modern cells? Well, in this case, no proteins, no
problem," said Breaker, who is also a Howard Hughes Medical Institute investigator.
Breaker's lab solved a decades-old mystery by describing how tiny circular RNA molecules called cyclic di-GMP
are able to turn genes on and off. This process determines whether the
bacterium swims or stays stationary, and whether it remains solitary or
joins with other bacteria to form organic masses called biofilms. For
example, in Vibrio cholerae,
the bacterium that causes cholera, cyclic di-GMP turns off production
of a protein the bacterium needs to attach to human intestines.
The tiny RNA molecule, comprised of only two nucleotides, activates a
larger RNA structure called a riboswitch. Breaker's lab discovered
riboswitches in bacteria six years ago and has since shown that they can
regulate a surprising amount of biological activity. Riboswitches,
located within single strands of messenger RNA
that transmit a copy of DNA's genetic instructions, can independently
"decide'' which genes in the cell to activate, an ability once thought
to rest exclusively with proteins.
Breaker had chemically created riboswitches in his own lab and -
given their efficiency at regulating gene expressions - predicted such
RNA structures would be found in nature. Since 2002, almost 20 classes
of riboswitches, including the one described in today's paper, have been
discovered, mostly hidden in non-gene-coding regions on DNA.
"We predicted that there would be an ancient 'RNA city' out there in the jungle, and we went out and found it,'' Breaker said.
Bacterial use of RNA to trigger major changes without the involvement
of proteins resolves one of the questions about the origin of life: If
proteins are needed to carry out life's functions and DNA is needed to
make proteins, how did DNA arise?
The answer is what Breaker and other researchers call the RNA World.
They believe that billions of years ago, single strands of nucleotides
that comprise RNA were the first forms of life and carried out some of
the complicated cellular functions now done by proteins. The
riboswitches are highly conserved in bacteria, illustrating their
importance and ancient ancestry, Breaker said.
Understanding how these RNA mechanisms work could lead to medical
treatments as well, Breaker noted. For instance, a molecule that mimics
cyclic di-GMP could be used to disable or disarm bacterial infections
such as cholera, he said.
Image Credits: Universitat Pampeu Fabra (RNA) and wallpapers.free-review.net (meteorites)
Source: The Daily Galaxy via Yale University and The Stardust Revolution, by Jacob Berkowitz
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