RNA polymerase IV directs silencing of endogenous DNA

Plants encode subunits for a fourth RNA polymerase (Pot IV) in addition to the well-known DNA-dependent RNA polymerases I, II, and III. By mutation of the two largest subunits (NRPD1a and NRPD2), we show that Pot IV silences certain transposons and repetitive DNA in a short interfering RNA pathway involving RNA-dependent RNA polymerase-2 and Dicer-like 3. The existence of this distinct silencing polymerase may explain the paradoxical involvement of an RNA silencing pathway in maintenance of transcriptional silencing.