Guardians of the genome Libraries – of books and genes – need protection.
ou may be deeply attached to the library of books stored on your iPad or Kindle, but we all carry a much more profound digital library within our cells: our genome, which comprises 23 books (chromosomes) that together contain more than 25 000 chapters (coding genes).
has come up with two analogous methods of suppressing the disruptive actions of transposons: one locks transposon DNA into an inactive state by adding chemical tags called methyl groups so that they cannot jump around, whereas the other silences any rogue transposons that might still roam the cell.
Yet the integrity of this library is under constant threat from genetic vandals called transposons. These are bits of DNA that can jump from location to location and insert themselves into meaningful paragraphs of genetic text, turning them into gibberish and rendering genes useless. Previous research has shown that a failure to keep transposon activity in check leads to sterility, so dealing with this threat is a biological priority. In two papers published back-to-back in Nature last year, three independent groups at EMBL-EBI, Monterotondo and Grenoble provided new insights into just how this is achieved.
These biological solutions for policing genomic vandals might sound simple, but their implementation is not — not surprising given the complexity of how transposons wreak genomic havoc in the first place.
Both libraries and cells have to protect their precious stores of information. In a traditional library, valuable books can be preserved in a number of ways. Some may be locked in secure cases to prevent anyone getting their hands on them in the first place; in addition, the library can hire guards to patrol the bookshelves to catch anyone who might wish to steal or deface their contents. The studies reported in Nature suggest that evolution 26 EMBL Annual Report 11·12
One method they use is a form of ‘copy and paste’ that, like normal gene expression, begins with the process of transcription, in which the DNA of a transposon is used as a template to produce a long RNA molecule. In ordinary gene expression, this RNA would contain the instructions for assembling a protein, but transposon RNAs do not serve this purpose. Instead, they are used by an enzyme called reverse transcriptase to recreate the original DNA molecule, after which it can be pasted into another part of the genome. The new experiments focused on this intricate copy-and-paste variety of transposon, and the ways in which their activity is regulated. Previous research in fruit flies had suggested a complex picture of transposon regulation. In the first step, enzymes called Piwi proteins use their endonuclease or ‘slicer’ activity to cut