RNA Polymerase

The step-by-step elongation of the RNA chain on a DNA template is catalysed by the enzyme RNA polymerase. RNA polymerase was isolated from E.coli.

Prokaryotes

In prokaryotes a single RNA polymerase enzyme controls the synthesis of all the different types of cellular RNA.
The entire RNA polymerase enzyme (holozyme) consists of a core enzyme and a sigma factor. The α chain is present twice. The sigma factor is not very firmly attached to the core enzyme and can therefore be easily isolated.
The β subunit is required for the binding of RNA polymerase to the DNA template. The β subunit is required for binding with the sigma factor. The function of the two α subunits is not clear, but it has a possible role in promoter recognition. The function of the omega subunit, a small protein, is also not known. It does not appear to be essential for normal activity of the enzyme, since the enzyme functions normally in the absence of ω. The sigma factor (α) recognized the start signal on the DNA template and directs and binding

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 name enzymes I, II and III. According to another terminology, based on sensitivity to amanitin, RNA polymerases are divided into three classes, A, B and C.
Class A polymerases are insensitive to amanitin and are 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, and are synonymous with polymerase II. They have been purified from calf thymus, rat liver, mouse myeloma and chick liver, among other tissues.

Molecular structure. Each RNA polymerase is composed of two high molecular weight subunits of polypeptides and 4-6 smaller ones. The localization of class A polymerases in the mucleolus suggests a role in the synthesis of rRNAs. The involvement of Class C polymerases is not excluded. C polymerases are not excluded. Class B polymerases are involved in the synthesis of heteronuclear RNA (hnRNA), a precursor of messenger RNA (mRNA). Class C polymerases appear to be involved in the synthesis of 5S RNA and pre-4S RNAs which are precursors of cytoplasmic tRNAs. Thus is 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.