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 6, represents a mobile module of the RNA polymerase. Domain 6 forms part of the shelf module [<cite idref="PUB00002975"/>, <cite idref="PUB00008731"/>]. This family appears to be specific to the largest subunit of RNA polymerase II.