<p> Prokaryotes contain a single DNA-dependent RNA polymerase (RNAP; <db_xref db="EC" dbkey="2.7.7.6"/>) that is responsible for the transcription of all genes, while eukaryotes have three classes of RNAPs (I-III) that transcribe different sets of genes. Each class of RNA polymerase is an assemblage of ten to twelve different polypeptides. Certain subunits of RNAPs, including RPB5 (POLR2E in mammals), are common to all three eukaryotic polymerases. RPB5 plays a role in the transcription activation process. Eukaryotic RPB5 has a bipartite structure consisting of a unique N-terminal region (<db_xref db="INTERPRO" dbkey="IPR005571"/>), plus a C-terminal region that is structurally homologous to the prokaryotic RPB5 homologue, subunit H (gene rpoH) [<cite idref="PUB00008456"/>, <cite idref="PUB00003393"/>, <cite idref="PUB00004780"/>, <cite idref="PUB00008455"/>].</p><p>This entry represents prokaryotic subunit H and the C-terminal domain of eukaryotic RPB5, which share a two-layer alpha/beta fold, with a core structure of beta/alpha/beta/alpha/beta(2). </p>