RNA polymerase I is also called Pol I, consists of a 928-residue polypeptide. Said to be a processive enzyme because it catalyzes a series of sucessive nucleotide polymerization steps, typically 20 or more, without releasing the single-stranded DNA template. It consists of 3 acitve sites, a 5'->3' polymerase site, a 5'->3' exonuclease site, and a 3'->5' exonuclease site. The 5'->3' exonuclease site is primarily for repair while the 3'->5' exonuclase site is for proofreading. Pol I is not the most active polymerase, DNA polymerase III is, but it was the first to be discovered in 1957 by Arthur Kornberg in Escherichia coli. RNA polymerase I synthesizes most types of rRNA.