DNA-dependent RNA polymerase catalyzes the transcription of DNA into RNA using the four ribonucleoside triphosphates as substrates. RNA polymerase I is essentially used to transcribe ribosomal DNA units. a ribonucleoside 5'-triphosphate + RNA(n) = diphosphate + RNA(n+1) Each class of RNA polymerase is assembled from 9 to 14 different polypeptides. This subunit is the largest component of RNA polymerase I. 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.