RNA Polymerases

Prokaryotic organisms (Eubacteria and Archaea) have only one type of RNA polymerase. Eukaryotic organisms (animals, plants, fungi, and protists) have three types, called pol I, II, and III, and each transcribes a different set of genes. Pol I synthesizes RNA for the large subunit of the ribosome (the protein-making machinery of the cell), and one piece of RNA for the small subunit. Pol II creates messenger RNAs, which provide a template for protein synthesis. Pol II also creates numerous small nuclear RNAs (snRNAs), which modify RNAs after they are formed. Pol III synthesizes transfer RNAs (tRNAs), the RNA for the small subunit of the ribosome, and other snRNAs.

The three eukaryotic polymerases can be distinguished in the laboratory by the degree to which they are inhibited by the alpha-amanitin poison from the mushroom Amanita phalloides. Pol I is completely resistant to its effects, pol III is moderately sensitive, and pol II is highly sensitive. (The reason this poison is so deadly is precisely because it inhibits RNA polymerase.)

Each eukaryotic RNA polymerase is composed of a dozen or more subunits. Some of these are small and unique to each type, but the two largest subunits are similar among all three polymerases, and similar as well to the two largest prokaryotic subunits. This is clear evidence that all of them evolved from the same original polymerase. These shared subunits are thought to form the functional core of the polymerases, while the smaller subunits may provide the gene specificity of each type.