How tardigrades protect their DNA to defy death

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Tardigrades may partly owe their ability
to survive outer space to having the molecular equivalent of cotton candy.

Water bears, as the creatures are also
known, can famously survive
just about anything
(SN: 7/14/17),
including being bombarded with X-rays or cosmic rays, or being doused in
hydrogen peroxide. Such radiation and chemical exposure result in production of
DNA-damaging hydroxyl radicals, molecules composed of oxygen and hydrogen.

Previous research indicated that a protein called Dsup, for damage suppressor, shields the tardigrade species Ramazzottius varieornatus from radiation. When added to human cells, the protein also protects against radiation. Now researchers have found out how.

Dsup surrounds
nucleosomes
— DNA wound around proteins called histones —  “like a fluffy cloud of cotton candy,”
molecular biologist James Kadonaga of the University of California, San Diego
in La Jolla and colleagues report October 1 in eLife. That cloud keeps hydroxyl radicals away from DNA.

tardigrade
When a tardigrade dries out, it draws in its legs and head and rolls into a ball called a tun (six tardigrades in the tun state shown in this scanning electron microscope image). The Dsup protein may help protect a tardigrade’s DNA from damage while it’s in this vulnerable state.T.C. Boothby

Another tardigrade species Hypsibius exemplaris, previously thought
to lack Dsup, has its own version of the protective protein, the researchers
discovered. Only about 26 percent of the amino acids in the two species’ Dsup
proteins are alike, but both shroud DNA against damage.

Kadonaga says the proteins probably
evolved to protect tardigrades from hydroxy radicals when the moss-dwellers are
dried
out
, a frequent occurrence (SN:
12/16/15
). Drying increases the concentration of DNA-dinging radicals in
cells. And damage can’t be repaired while the animals are dormant in their
desiccated state. Since X-rays also form hydroxy radicals, tardigrades “just
happen to be X-ray resistant,” too, he says.

Humans have similar proteins called high
mobility group nucleosome-binding proteins or HMGNs. But the researchers don’t
yet know whether the human proteins also form a similar shield against DNA-damaging
chemicals.

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