An atypical RNA polymerase involved in RNA silencing shares small subunits with RNA polymerase II.

Genetic evidence indicates that plant-specific homologs of DNA-dependent RNA polymerase (Pol) II large subunits form Pol IV and Pol V complexes involved in small interfering RNA production and RNA-directed DNA methylation. Here we describe evidence that Pol V contains subunits shared with Pol II, but that RNA polymerase II subunit (RPB)-4 is missing from Pol V and that RPB5 is present as a Pol V-specific isomer, RPB5b. Pol V also has other proteins that are not present in Pol II, consistent with a role of this complex as an effector of silencing.