Core component of RNA polymerase I (Pol I), a DNA-dependent RNA polymerase which synthesizes ribosomal RNA precursors using the four ribonucleoside triphosphates as substrates. Can mediate Pol I proofreading of the nascent RNA transcript. Anchors into the Pol I active site to monitor transcription fidelity and cleave mis-incorporated 5′-ribonucleotides.
