RNA Polymerase II

RNA polymerase II (RNA pol II) is the enzyme in eukaryotes responsible for synthesizing messenger RNA (mRNA). A crucial part of RNA pol II is its carboxy-terminal domain (CTD), which is made up of heptapeptide repeats. These repeats are essential for the polymerase's activity.

RNA polymerases I, II, and III are large and complex proteins, consisting of 14, 12, and 17 subunits, respectively. Five of these subunits are shared, and a further two are found in RNA polymerases I and III, but not in RNA polymerase II. The two largest subunits of each enzyme are unique, but share substantial homology with each other and also with bacterial RNA polymerases; they fold together to provide the catalytic site of the enzyme. Besides these, RNA polymerases I, II, and III each have an additional set of unique subunits. Although most of the subunits are required for cell viability, their precise roles remain poorly characterized.