New gene-silencing pathway found...(tly. The Space between Genes ....)

The Space between Genes

Using Arabidopsis thaliana, the "laboratory rat" of the plant world, Pikaard and his colleagues carried out a series of genetic tests to pinpoint where in the genome Pol V was getting down to business.

Following a hunch, postdoctoral scholar Andrzej Wierzbicki decided to take a closer look at the stretches of DNA that lie between genes, the so-called intergenic regions. Biologists have long been baffled by this alleged "junk DNA" because transcription seems to occur here, and just about everywhere, genes and junk alike.

"This is a hot topic in genetics right now," says Pikaard. "It looks like not just the genes are getting transcribed, but pretty much all of the DNA is getting transcribed. And why? What's the purpose of all this transcription?"

Wierzbicki's instincts paid off. Using high levels of PCR amplification, he determined that Pol V was indeed hard at work within the intergenic region. In this space between genes, Pol V makes noncoding RNA transcripts that he and Pikaard think bind with the siRNAs generated by Pol IV. By acting as a scaffold for these siRNAs, the Pol V transcripts enable silencing of adjacent, virus-derived genes such as retrotransposons (jumping genes) that can be detrimental if activated. Pikaard and colleagues were able to confirm that both Pol IV and Pol V are necessary for silencing by examining mutations that knock out, or disable, the different genes coding for the polymerases.

'Junk' no more

This research adds to a growing body of evidence suggesting that "junk DNA" is in fact a functional part of the genome, since transcription of the intergenic regions is necessary to keep potentially harmful genes turned off. In addition, Pikaard and his colleagues have resolved a paradox that has recently puzzled geneticists: the need for transcription in order to transcriptionally silence the same region. In the case of
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