Over at her old blog, Karmen had a nice overview of Deinococcus radiodurans, a fascinating organism that's able to withstand many different extremes: genotoxic chemicals, oxidative damage, high levels of ionizing and ultraviolet radiation, dehydration, and, as the name suggests, incredibly high doses of radiation. (We're talking high--up to 5,000 Gy without breaking a sweat, while it only takes about 10 Gy to kill a human). However, despite 50 years of study, no one's really figured out just how it does it, though some clues (such as higher levels of manganese and low levels of iron) have emerged that make D. radiodurans stand out). Over at Small Things Considered, a recent paper is highlighted suggesting that these minerals protect not the DNA from damage, but instead, the proteins:
The researchers postulated that manganese ions transform damaging superoxide radicals (which can't easily cross the cell membrane) into hydrogen peroxide, which can be excreted. Indeed, resistant cells excrete peroxide following radiation exposure.
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What about all the Holliday junctions that are supposed to make the repair of double-strand breaks possible?
Yep it uses Holliday junctions too, but it's better at it than other organisms.
Following radiation exposure, Deinococcus can reassemble a broken genome that's been shattered into thousands of pieces. It only gives up and dies if its genome is broken into pieces of 10kb or smaller (while E.coli can't even handle 500kb pieces).
It uses a technique called extended synthesis-dependent strand annealing (ESDSA). Basically, the broken chunks are partially digested leaving single-stranded tails. These join up and are used as templates for building new DNA. Deinococcus makes this process easier by having (a) lots of repeated sequences and (b) at least 4-12 redundant copies of its genome at any one time. More on this mechanism in this Nature News and Views article, following this paper
The radiation resistance is apparently an evolutionary side effect. The same mechanisms evolved to allow the creature to survive extremely dry conditions in arid soil, which also break its genome apart.
This is one hell of a tough bacterium that we have in here. Woah... I am amazed...
I am going to blog this and talk about it with all of my friends... Well, the one who care about these things...
Wow, that little guy is neat.
Thanks for describing the mechanism of DNA repair Ed. I wonder, is that mechanism used by any Eukaryotes or is it a Bacteria specialty?
Matt, I think it's a variant (ESDSA) of a universal technique (SDSA), which I think we use as well.
See also review by Liza Gross:
http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1…