DNA-dependent RNA polymerases catalyze the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. Component of RNA polymerase I (Pol I) which synthesizes ribosomal RNA precursors. Besides, RNA polymerase I has intrinsic RNA cleavage activity. RPA190 and RPA135 both contribute to the polymerase catalytic activity and together form the Pol I active center. In addition, subunit RPA12 contributes a catalytic zinc ribbon that is required for RNA cleavage by Pol I. A single stranded DNA template strand of the promoter is positioned within the central active site cleft of Pol I. A bridging helix emanates from RPA190 and crosses the cleft near the catalytic site and is thought to promote translocation of Pol I by acting as a ratchet that moves the RNA-DNA hybrid through the active site by switching from straight to bent conformations at each step of nucleotide addition. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Component of the RNA polymerase I (Pol I) complex consisting of 14 subunits: RPA135, RPA190, RPC40, RPA14, RPB5, RPO26, RPA43, RPB8, RPA12, RPB10, RPC19, RPC10, RPA49 and RPA34. The complex is composed of a horseshoe-shaped core containing ten subunits (RPA135, RPA190, RPB5, RPO26, RPB8, RPB10, RPC10, RPA12, RPC19 and RPC40) where RPA135 and RPA190 form the DNA-binding cleft. Outside of the core, RPA14 and RPA43 form the stalk that mediates interactions with transcription initiation factors and newly synthesized RNA. 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. Present with 14100 molecules/cell in log phase SD medium. Belongs to the RNA polymerase beta chain family.