DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Component of RNA polymerases IV and V which mediate short-interfering RNAs (siRNA) accumulation and subsequent RNA-directed DNA methylation-dependent (RdDM) transcriptional gene silencing (TGS) of endogenous repeated sequences, including transposable elements. Component of the RNA polymerase IV and V complexes. Interacts with NRPB11, SHH1, GRP23 and NRPD1. A number of isoforms are produced. According to EST sequences. Belongs to the archaeal Rpo3/eukaryotic RPB3 RNA polymerase subunit family.