DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Second largest core component of RNA polymerase I which synthesizes ribosomal RNA precursors. Proposed to contribute to the polymerase catalytic activity and forms the polymerase active center together with the largest subunit. Pol I is composed of mobile elements and RPA2 is part of the core element with the central large cleft and probably a clamp element that moves to open and close the cleft (By similarity). a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Component of the RNA polymerase I (Pol I) complex consisting of 14 subunits. Three distinct zinc-containing RNA polymerases are found in eukaryotic nuclei: polymerase I for the ribosomal RNA precursor, polymerase II for the mRNA precursor, and polymerase III for 5S and tRNA genes. Belongs to the RNA polymerase beta chain family.