<p>DNA-directed RNA polymerases <db_xref db="EC" dbkey="2.7.7.6"/> (also known as DNA-dependent RNA polymerases) are responsible for the polymerisation of ribonucleotides into a sequence complementary to the template DNA. In eukaryotes, there are three different forms of DNA-directed RNA polymerases transcribing different sets of genes. Most RNA polymerases are multimericenzymes and are composed of a variable number of subunits. The core RNA polymerase complex consists of five subunits (two alpha, one beta, one beta-prime and one omega) and is sufficient for transcription elongation and termination but is unable to initiate transcription. Transcription initiation from promoter elements requires a sixth, dissociable subunit called a sigma factor, which reversibly associates with the core RNA polymerase complex to form a holoenzyme [<cite idref="PUB00000061"/>]. The core RNA polymerase complex forms a "crab claw"-like structure with an internal channel running along the full length [<cite idref="PUB00033173"/>]. The key functional sites of the enzyme, as defined by mutational and cross-linking analysis, are located on the inner wall of this channel.</p><p>RNA synthesis follows after the attachment of RNA polymerase to a specific site, the promoter, on the template DNA strand. The RNA synthesis process continues until a termination sequence is reached. The RNA product, which is synthesised in the 5' to 3'direction, is known as the primary transcript.Eukaryotic nuclei contain three distinct types of RNA polymerases that differ in the RNA they synthesise: <ul> <li>RNA polymerase I: located in the nucleoli, synthesises precursors of most ribosomal RNAs.</li> <li>RNA polymerase II: occurs in the nucleoplasm, synthesises mRNA precursors. </li> <li>RNA polymerase III: also occurs in the nucleoplasm, synthesises the precursors of 5S ribosomal RNA, the tRNAs, and a variety of other small nuclear and cytosolic RNAs. </li> </ul>Eukaryotic cells are also known to contain separate mitochondrial and chloroplast RNA polymerases. Eukaryotic RNA polymerases, whose molecular massesvary in size from 500 to 700 kDa, contain two non-identical large (>100 kDa) subunits and an array of up to 12 different small (less than 50 kDa) subunits.</p><p>RNA polymerases (<db_xref db="EC" dbkey="2.7.7.6"/>) catalyse the DNA dependent polymerisation of RNA.Prokaryotes contain a single RNA polymerase compared to three in eukaryotes (notincluding mitochondrial and chloroplast polymerases). This domain represents thehybrid-binding domain and the wall domain [<cite idref="PUB00008731"/>]. Thehybrid-binding domain binds the nascent RNA strand/template DNA strand in thePol II transcription elongation complex. This domain contains the important structuralmotifs, switch 3 and the flap loop and binds an active site metal ion [<cite idref="PUB00008731"/>]. This domain is also involved in binding to Rpb1 and Rpb3[<cite idref="PUB00008731"/>]. Many of the bacterial members contain large insertionswithin this domain, which are known as dispensable region 2 (DRII).</p>