RNA Polymerases in Eukaryotes

In eukaryotic cells multiple forms of DNA dependent RNA polymerase are responsible for synthesis of cellular RNA. These differ in structure, function and localization. According to one terminology the RNA polymerases are named enzymes I, II and III. According to another terminology, based on sensitivity to amanitin, RNA polymerases are divided into three classes, A, Band C.

Class A polymerases are insensitive to amanitin and arc localized in the nucleoli. They include enzyme AI (RNA polymerase I) and AII (IB). AI has been purified from calf thymus, mouse myeloma and rat liver.
Class B polymerases are sensitive to low concentrations of amanitin(10-9 to 10-8 M), and are synonymous with polymerase II. They have been purified from calf thymus, rat liver, mouse myeloma and chick liver, among other tissues.

Class B polymerases can be resolved into three forms, BO (IIo), BI (IIA) and BII (a+b) (IIB). The BII form can be further resolved into two isoenzymes BIIa and BIIb. Class B polymerases are nucleoplasmic in location.
Class C polymerases are sensitive to high concentrations of amanitin(10-5 to 10-4 M). and have been resolved into CI, CII, CIIIa and CIIIb forms. Mouse myeloma polymerase III has been resolved into two chromatographic forms, IIIA which appears to be nuclear in origin and IIIB which is found mainly in the cytoplasm. Polymerase IIIA is synony­mous with CIIIa and polymerase IIIB with CIIIb

Molecular structure. Each RNA polymerase is composed of two high molecular weight subunits or polypeptides and 4-6 smaller ones. Calf thymus RNA polymerases AIa, AIb, BI, BIIa and BIIb have been studied in detail.
Calf thymus RNA polymerase BII exists in two forms, BIIa and BIIb which differ only in their largest subunits (MW 180,000). In BIIa the subunit is SB2a and in BIIb it is SB2b. In class B polymerases the SB6 subunit consists of two isomers, SB6a and SB6b.BI and BII polymerases have additional SB5' subunits. SA3 does not appear to be essential for RNA synthesis on DNA templates.
The localization of class A polymerases (polymerase I) in Q),e nucleolus suggests a role in the synthesis of rRNAs. The involvement of Class C polymerases is not excluded. Class B polymerases (polymerase II) are involved hi the synthesis of heteronuclear RNA (hnRNA), a precursor of messenger RNA (mRNA).

Class C polymerases (polymerase III) appear to be involved in the synthesis of 5S RNA and pre 4S RNAs which are precursors of cytoplasmic tRNAs. Thus in eukaryotes, polymerases I, II and III are involved in the synthesis of rRNA, mRNA and tRNA, respectively. This is in contrast to the condition in prokaryotes where the synthesis of the three types of RNA is controlled by a single RNA polymerase.