Reflecting recent work in the

In plants, transcription of selfish genetic elements such as transposons and DNA viruses is suppressed by RNA-directed DNA methylation.

This process is guided by 24-nt short-interfering RNAs
(siRNAs) whose double-stranded precursors are synthesized by
DNA-dependent NUCLEAR RNA POLYMERASE IV (Pol IV) and
RNA-DEPENDENT RNA POLYMERASE 2 (RDR2). Pol IV and RDR2 coimmunoprecipitate, and their activities are tightly coupled, yet the basis for their association is unknown.

Herein is shown that an interval near the RDR2 active site contacts the Pol IV catalytic subunit, NRPD1, the largest of Pol IV’s 12 subunits. Contacts between
the catalytic regions of the two enzymes suggests that RDR2 is
positioned to rapidly engage the free 3′ ends of Pol IV transcripts
and convert these single-stranded transcripts into double-stranded
RNAs (dsRNAs).