User:Kinkreet/MGEv2

Overview
RNA synthesis in a cell are all mediated by RNA polymerase. RNA polymerase binds to one of two DNA strand in the duplex, and uses it as a template to make a RNA copy of the other DNA strand.

All RNA polymerases are complexes containing many subunits. Although sequence identity is not high (53% between humans and yeast, and much lower between other organisms), the structure of RNA polymerase is conserved throughout all three domains of life, with higher conservation observed at and around the active site. The structure of the archaeal RNA polymerases are very similar to the eukaryotic RNAPII. Bacterial RNAP is smaller than the archaeal or eukaryotic RNAP and has an extension in its β' subunit absent in the archaeal or eukaryotic RNAP.

Bacteria only has one type of RNAP, which must make every type of RNAs in the cell including mRNAs, rRNAs and tRNAs. There are at least three different types of RNAPs in each cell of an eukaryote; this allows different RNAPs to specialize in their function, giving more efficiency through division of labour. RNAPI transcribes the 5.7S/18S/28S rRNAs and work purely in the nucleolus, RNAPII transcribes all mRNAs and some snRNAs, RNAPIII transcribes the 5.7S rRNA, all tRNAs and other small RNAs. Mitochondria RNA polymerase and chloroplast RNA polyermase are organelle-specific RNAPs, transcribing their respective organelle-resident genomes. Owing to the evolutionary-derived symbiotic event, the mitochondria RNAP resembles viral RNA polymerases such as the bacteriophage T7 RNAP and the chloroplast RNAP resembles the bacterial RNAP.

Some higher plants also has RNAPIV and RNAPV in the nucleus; these are specialized forms of RNAPII and are non-essential, able to be knocked out with much effect. RNAPIV and RNAPV transcribes non-coding RNAs (ncRNAs) and are thus involved in gene regulation via ncRNAs. The ncRNAs are involved in gene silencing of retrotransposons, endogenous repeats of DNA and transgenes, and is thus takes part in immunity and maintaining genome stability.

In terms of complexity of composition, bacterial RNAP is unsurprisingly the simplest of all, with the core polymerase made up of only four subunits - α2ββ'ω