RNA polymerase

An RNA polymerase is an enzyme which synthesises (or polymerises) RNA molecules by copying the DNA (in which case we refer to DNA-dependent RNA polymerase) or by copying the RNA (in which case we refer to RNA-dependent polymerase).

Whilst prokaryotes have only one RNA polymerase, eukaryotes (including human beings) have three enzymes which can synthesise RNA. They are large enzyme complexes composed of several protein chains. They all have sites which can recognise and attach to certainDNAsequences, recruit the necessary nucleotides to extend the chain, and catalyse the formation of the nuclear bond (phosphodiester bond).

Bacterial and Archaeal RNA polymerase can synthesise all of the cell's RNA. On the other hand, the different eukaryotic RNA polymerases have quite distinct roles.

• Pol I RNA synthesises ribosomal RNA;
• Pol II RNA synthesises messenger RNA, the precursors of proteins ;
• Pol III RNA synthesises transfer RNA and 5S ribosomal RNA.

Other RNA polymerases (IV and V) are currently being studied and appear to be involved in the RNA interference mechanism.