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. No visible phenotype; due to the redundancy with NRPB9B. No effect on methylation at RdDM target sites. Nrpb9a and nrpb9b double mutants are embryo lethal. Pol IV and V functions are not impaired in nrpb9a mutants and Pol II functions are complemented by NRPB9B. Belongs to the archaeal RpoM/eukaryotic RPA12/RPB9/RPC11 RNA polymerase family.