RNA polymerase I

From Freepedia

RNA polymerase I (also called RNAP I and Pol I) consists of a 928-residue polypeptide. Said to be a processive enzyme because it catalyzes a series of successive nucleotide polymerization steps, typically 20 or more, transcribing the DNA template until it reaches a terminator sequence.

RNAP I has three active sites:

• a 5' → 3' polymerase site;
• a 5' → 3' exonuclease site;
• a 3' → 5' exonuclease site.

The polymerase activity is responsible for the initiation and elongation of the RNA. The 5' → 3' exonuclease activity is primarily for repair, while the 3' → 5' exonuclease activity is for proofreading.

RNAP 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.