<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 (not including mitochondrial and chloroplast polymerases). This entry describes a structural region consisting of an OB-fold beta barrel that is found in RNA polymerase II Rpb2 [<cite idref="PUB00008731"/>, <cite idref="PUB00034483"/>].</p>