© 2005 AAAS

The paradoxical involvement of RNA-mediated gene silencing in the maintenance of some DNA silencing is bridged in Arabidopsis by an RNA polymerase that acts as a liaison between both pathways, according to UK researchers. Alan Herr, from the John Innes Centre, Norwich, and colleagues found that mutants for RNA polymerase IV (Pol IV, also called RPD1), part of a new clade of polymerases in plants, were defective in both RNA and DNA silencing.1 "The finding of a new silencing-specific RNA polymerase is a surprising twist in the evolution of RNA polymerases," Herr writes in an E-mail.

RNA silencing occurs through the multiprotein RNA-induced silencing complex that cleaves double-stranded RNA, producing short interfering RNAs (siRNA), which then amplify the cycle. Conversely, DNA silencing occurs through chromatin-mediated mechanisms that include DNA methylation and histone modifications to form heterochromatic regions. In the Pol IV mutant, both siRNA formation and...

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