Biologjia Online

Plants encode subunits for a fourth RNA polymerase (Pol IV) in addition to the well known DNA-dependent RNA polymerases I, II, and III. By mutation of the two largest subunits (RPD1 and RPD2), we show that Pol IV silences certain transposons and repetitive DNA in a short interfering RNA (siRNA) pathway involving RNA-dependent RNA polymerase 2 (RDR2) and Dicer-Like 3 (DCL3). The existence of this distinct silencing polymerase may explain the paradoxical involvement of an RNA silencing pathway in maintenance of transcriptional silencing.