In a series of genetic and biochemical tests , Pikaard and his collaborators discovered that Pol IV does not share in the duties of Pol I, II or III. But when the Pol IV subunits are knocked out, the most tightly packed DNA in the nucleus becomes less condensed, small RNAs called siRNAs corresponding to highly repeated 5S rRNA genes and retrotransposons (jumping genes) are completely eliminated and DNA methylation at 5S genes and retrotransposons is lost.
Methylation is a vital process involving a chemical modification in cytosine, one of the four chemical subunits of DNA. Without proper DNA methylation, higher organisms from plants to humans have a host of developmental problems, from dwarfing in plants to tumor development in humans to certain death in mice.
Pikaard thinks that Pol IV helps make siRNAs that then direct DNA methylation to sequences matching the siRNAs.
The results were published in Cell online, Feb. 10, 2005 and will appear in the March, 2005 print version of the journal.
"Pol IV is somehow involved in maintaining the integrity of the Arabidopsis genome, principally in keeping the silent DNA silent," Pikaard said. "Plants can get by without Pol IV, whereas they can't do without the other three. We don't see anything obviously like Pol IV in any other genome, but it's possible it might have been overlooked."
While Pikaard and his collaborators have indirect evidence that Pol IV is a distinct RNA polymerase, they still have many aspects of Pol IV to unravel.
"We know what happen
Source:Washington University in St. Louis