Scientists Find Solution to a Molecular Paradox

Researchers at the University of Washington in St. Louis have made a major discovery explaining a mechanism by which plant cells silence potentially harmful genes. Craig Pikaard and his colleagues focused their attention on a type of RNA polymerase (Pol) exclusive to plants. RNA polymerases, the enzymes responsible for making RNA from DNA templates, are key players in determining which genes get switched on and which get left off. In 2005, Pikaard and his team discovered two RNA polymerases found only in plants: Pol IV and V. Since the discovery, the scientists have been on a hunt to figure out what these enzymes are making.

Using the plant model Arabidopsis, the scientists discovered that Pol V transcribes non-coding or "junk DNA" sequences. Biologists have long been baffled by this alleged "junk DNA". They don’t code for any protein, yet they are continuously being transcribed.

Pol V was found to make non-coding RNAs that the scientists think bind with the small interfering RNAs (siRNAs) generated by Pol IV, acting as a scaffold for these gene silencers. What were previously thought of as junk DNA prove to be functional regions of the genome, since transcription of these regions is necessary to keep potentially harmful genes turned off. The scientists noted that the functions of Pol IV and V provide a solution to a paradox of epigenetic control: the need for transcription in order to transcriptionally silence the same region.

Read more at http://news-info.wustl.edu/tips/page/normal/12932.html The abstract of the paper published by Cell is available at http://dx.doi.org/10.1016/j.cell.2008.09.035