RNA polymerases catalyse the DNA dependent polymerisation of RNA. Prokaryotes contain a single RNA polymerase compared to three in eukaryotes (not including mitochondrial and chloroplast polymerases). This domain, domain 5, represents the discontinuous cleft domain that is required to form the central cleft or channel where the DNA is bound [<cite idref="PUB00002975"/>, <cite idref="PUB00008731"/>].