How many types of RNA polymerase are there?

Prokaryotes (bacteria and viruses) have a single type of RNA polymerase that synthesizes all subtypes of RNA.

Eukaryotes (multi-cellular organisms) have 5 different types of RNA polymerase. The 5 eukaryotic RNA polymerases differ in structure and function.

• RNA polymerase I is located in the nucleolus, a specialized nuclear substructure in the cell. RNAP I synthesizes almost all ribosomal RNAs (rRNA) transcripts. These transcripts are directly involved in the production of ribosomes. The synthesis of rRNA by transcription and the production of ribosomes occurs in the nucleolus. In yeast, the RNAP I enzyme has 13 subunits and a mass of approximately 600kDa.
• RNA polymerase II is located in the nucleus and contains transcription factors and transcriptional regulators. RNAP II is responsible for transcribing protein-coding genes into messenger RNA (mRNA). On completion of synthesis, the pre-mRNAs are released by the RNAP II enzyme within the nucleus where biochemical modifications prepare the transcripts for translation. RNAP II has 12 subunits and a mass of approximately 550 kDa.
• RNA polymerase III is located in the nucleus and is responsible for transcribing small RNAs such as transfer RNA (tRNA) and 5S rRNA. RNAP III is made up of 14 or more distinct subunits with a mass of approximately 700 kDa.
• RNA polymerase IV and RNA polymerase V are found exclusively in plants. They are located in the chloroplast (plastids) and mitochondria and are responsible for synthesizing small interfering RNA (siRNA).