DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Second largest component of RNA polymerase IVa and IVb which mediate short-interfering RNAs (siRNA) accumulation and subsequent RNA-directed DNA methylation-dependent (RdDM) silencing of endogenous repeated sequences, including transposable largest subunit. Also required for full erasure of methylation elements. Required for intercellular RNA interference (RNAi) leading to systemic post-transcriptional gene silencing (Probable). a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Component of the RNA polymerase IVa and IVb (Pol IV) complexes. Belongs to the RNA polymerase beta chain family.