Decoding a Radiation-Resistant Bug

[Susan Gawarecki <loc@icx.net>]

November 19, 1999 - New York Times
Decoding a Radiation-Resistant Bug
By NICHOLAS WADE

It thrives in deserts, medical autoclaves and food irradiation chambers.

It can withstand more radiation than the Incredible Hulk. The Energy
Department has used it to help clean up perpetually boiling vats of
radioactive wastes. Its place of origin is a mystery, but some
scientists wonder how it acquired all the hardy properties required of a
space traveler. 

It is just a tiny red bacterium, but in deference to its awesome powers
scientists have named it Deinococcus radiodurans, a Greco-Latin amalgam
meaning weird, radiation-resistant, berry-shaped bug. Hoping to learn
the secret of its origins and amazing resistance to radiation,
biologists at the Institute for Genomic Research in Bethesda, Md., have
decoded the 3,284,156 letters of its genetic message. 

And what have they learned? 

"The bottom line is that it has not given us the answer as to why the
organism is so radiation resistant," said Dr. Michael J. Daly of the
Uniformed Services University of the Health Sciences in Bethesda and a
co-author of a new report about it. 

Dr. J. R. Battista, a Deinococcus expert at Louisiana State University
in Baton Rouge, who was not involved in the genome decoding, said, "The
part that is absolutely amazing to me is that there is no obvious
explanation for its ionizing radiation resistance." 

Still, the sequence of the strange bacterium's genome, described in
today's issue of Science, hints at many of the little organism's
survival stratagems. 

The $2.1 million cost of the project was paid for by the Energy
Department as part of a program for sequencing industrially important
microbes. The department hopes the microbe will clean up hot sites and
yield information about protecting people from ionizing radiation, said
Dr.  Ari Patrinos, a senior official. 

Deinococcus radiodurans was first isolated in 1956 from cans of meat
that had been sterilized, or so it was thought, with gamma radiation. 
When Deinococcus is dried, some bacteria can survive exposure to 12
million rads of radiation, Dr. Battista has found. One thousand rads
will kill a person. 

Radiation causes mutation-damage to individual DNA units-but is deadly
to cells because it can also cut both strands of the DNA double helix.
Most bacteria can repair a couple of double breaks but cannot cope with
more. Deinococcus can knit together its DNA even after the genome has
been blasted into more than 100 pieces. 

Even more surprisingly, the bacterium somehow recognizes and corrects
all the mutated DNA units. "It repairs double-strand breaks and keeps
the genome totally free of mutation; it truly is extraordinary," Dr.
Battista said. 

The genome sequencing team at the Institute for Genomic Research has now
found that Deinococcus's DNA is arranged in the form of four circles,
which between them code for 3,187 genes. The team, led by Dr.  Owen
White, has identified the role of some 2,000 of these genes by comparing
them with genes of known function logged in DNA data banks. The other
thousand genes have no match and have unknown roles. 

All bacteria have a kit of special proteins, known as enzymes, which
repair damaged DNA. Dr. White said Deinococcus seemed to have much the
same type of enzymes in its DNA repair kit, although in several cases it
possessed more copies of the enzyme genes. "Deinococcus is like a
Cadillac: it doesn't have any new features the other cars don't have, it
just happens to have all of them," he said. 

One special capability, though, is a family of proteins that evict
damaged DNA units from the cell. Eviction is a smart step because it
prevents the damaged units from being reincorporated into DNA and
reintroducing a mutation. 

Deinococcus maintains at least four copies of its genome in each cell,
and Dr. Daly believes that identical circles of DNA are stacked up like
lifesavers in a tube. When a double-strand break occurs in one circle,
the repair kit enzymes can refer to the neighboring strand's sequence
for mending the break. 

Evolution teaches that an organism will possess only the abilities that
it needs to survive. So where in the world did Deinococcus live that
required it to withstand 12 million rads of ionizing radiation? Because
natural radiation nowhere reaches the barest fraction of this level,
Dr.  Battista believes that Deinococcus evolved to withstand conditions
of extreme dryness. 

Desiccation, he said, will produce the same double breaks in DNA as
radiation, and probably mutations as well. "So radiation resistance is
just a fortuitous consequence of that evolutionary process," he said. 

The completed sequence of Deinococcus's genome confirms the suspicion
that it is quite closely related to Thermus thermophilus, a bacterium
that lives in near-scalding water. This common ancestry may have
equipped Deinococcus to live in another kind of extreme environment,
that of high aridity. 

It has been found in the granite of Antarctic mountains that have not
seen rain for thousands of years, Dr. Daly said. 

Dr. White sees Deinococcus as a scavenger, probably adapted to feed off
other bacteria. The microbe is found all over the world, yet nowhere is
it very common. Its strategy may be that of a survival specialist. Other
bacteria outgrow it but will eventually succumb to drought or
ultraviolet radiation; Deinococcus will always outlast them, and dine on
the carcasses. 

Fortunately, having no taste for people, this Incredible Hulklet of a
bacterium is entirely nonpathogenic.

-- 
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Susan L. Gawarecki, Ph.D., Executive Director
Oak Ridge Reservation Local Oversight Committee, Inc.
136 S Illinois Ave, Ste 208, Oak Ridge, TN 37830
Please note new area code:
Phone (865) 483-1333; Fax (865) 482-6572; E-mail loc@icx.net
OCTOBER INSIGHTS CAN BE FOUND AT: http://www.local-oversight.org
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