RNA polymerase I

RNA polymerase I (also called Pol I) is, in eukaryotes, the enzyme that only transcribes ribosomal RNA, a type of RNA that accounts for over 50% of the total RNA synthesized in a cell.

Pol I consists of 8-14 protein subunits. rDNA transcription is confined to the nucleolus where several hundreds of copies of rRNA genes are present, arranged as tandem head-to-tail repeats. Pol I transcribes one large transcript, encoding an rDNA gene over and over again. This gene encodes the 18S, the 5.8S, and the 28S RNA molecules of the ribosome in eukaryotes. The transcripts are cleaved by snoRNA. Because of the simplicity of Pol I transcription, it is the fastest-acting polymerase.