Cells re-write genetic code to defend themselves

30 November 2009
Posted by Chris Smith.

Following the genetic code faithfully was thought to be a sacrosanct cellular rule, but apparently not at times of stress, scientists have shown.

Double HelixWriting in Nature, University of Chicago researcher Tao Pan and his colleagues have found that when cells are exposed to bacteria, viruses or other toxic insults they depart from the genetic recipe written into their DNA code and begin to produce proteins enriched with an amino acid called methionine.  When the scientists compared proteins made by stressed and unstressed cells, as many as 1 in every 100 of the methionine amino acids in the protein was there inappropriately.  The occasional amino acid creeping in by mistake is to be expected but not at the observed level.

"This was happening 1000 times more than the textbook says should be there," says Pan.  Instead the scientists think that the cells are adding methionine into critical parts of proteins because it contains sulphur and so can soak up free-radicals and thereby resist oxidative damage.  This has the effect of making the protein much more robust.

So why aren't they there all the time?  Probably, say the scientists, for two reasons.  First, the metabolic cost but also, secondly, by allowing them to creep in randomly at times of stress like this, it's a way of ensuring that all proteins look different and are therefore more resistant to damage by any given mechanism because no two are exactly alike.

"This sounds chaotic and doesn't make a lot of sense according to the textbook," says Pan, "but this way the cells can always ensure that a subset of these proteins is somewhat less sensitive to the extra hits.  I think that's the most important part of this - to make every protein molecule different - and you cannot do that genetically."

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