RNA Polymerase - RNA Polymerase in Eukaryotes

RNA Polymerase in Eukaryotes

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.
• RNA polymerase II synthesizes precursors of mRNAs and most snRNA and microRNAs. 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.
• RNA polymerase IV synthesizes siRNA in plants.
• RNA polymerase V synthesizes RNAs involved in siRNA-directed heterochromatin formation in plants.

Eukaryotic chloroplasts contain an RNAP very highly structurally and mechanistically similar to bacterial RNAP ("plastid-encoded polymerase").

Eukaryotic chloroplasts also contain a second, structurally and mechanistically unrelated, RNAP ("nucleus-encoded polymerase"; member of the "single-subunit RNAP" protein family).

Eukaryotic mitochondria contain a structurally and mechanistically unrelated RNAP (member of the "single-subunit RNAP" protein family).

Given that DNA and RNA polymerases both carry out template-dependent nucleotide polymerization, it might be expected that the two types of enzymes would be structurally related. However, x-ray crystallographic studies of both types of enzymes reveal that, other than containing a critical Mg2+ ion at the catalytic site, they are virtually unrelated to each other; indeed template-dependent nucleotide polymerizing enzymes seem to have arisen independently twice during the early evolution of cells. One lineage led to the modern DNA Polymerases and reverse transcriptases, as well as to a few single-subunit RNA polymerases from viruses. The other lineage formed all of the modern cellular RNA polymerases.