RNA polymerase

Eukaryotes have multiple types of nuclear RNAP, each responsible for synthesis of a distinct subset of RNA. All are structurally and mechanistically related to each other and to bacterial RNAP:

• RNA polymerase I synthesizes a pre-rRNA 45S (35S in yeast), which matures into 28S, 18S and 5.8S rRNAs which will form the major RNA sections of the ribosome.^[6]
• RNA polymerase II synthesizes precursors of mRNAs and most snRNA and microRNAs.^[7] This is the most studied type, and, due to the high level of control required over transcription, a range of transcription factors are required for its binding to promoters.
• RNA polymerase III synthesizes tRNAs, rRNA 5S and other small RNAs found in the nucleus and cytosol.^[8]
• RNA polymerase IV synthesizes siRNA in plants.^[9]
• RNA polymerase V synthesizes RNAs involved in siRNA-directed heterochromatin formation in plants.^[10]