DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Component of RNA polymerase II which synthesizes mRNA precursors and many functional non-coding RNAs. Pol II is the central component of the basal RNA polymerase II transcription machinery. It is composed of mobile elements that move relative to each other. 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. Required for RNA silencing. Component of the RNA polymerase II, IV and V complexes. Interacts with NRPD1. Shorter siliques and ovate leaves with shorter petioles, smaller trichomes, prominent leaf veins and changed cuticular wax coating. Loss of methylation at RdDM target sites. Partially redundant with NRPB9A with respect to Pol II. Nrpb9a and nrpb9b double mutants are embryo lethal. The loss of silencing in nrpb9b mutants is due to a defect in Pol V function, but not at the level of transcription. Pol IV functions are not impaired in nrpb9b mutants and Pol II functions are partially complemented by NRPB9A (PubMed:22550619). Belongs to the archaeal RpoM/eukaryotic RPA12/RPB9/RPC11 RNA polymerase family.