<p>Eukaryotic RNA polymerase II (RNAPII) is composed of a ten-subunit core and an RPB4-RPB7 heterodimer that reversibly associates with the core [<cite idref="PUB00010734"/>]. The heterodimer both binds RNA and serves to stabilise the transcription complex. The single RNA polymerase of archaeal species is likewise composed of twelve subunits, most of which display considerable homology to the eukaryotic RNAPII subunits. The RPB4 and RPB7 homologs are called subunits F and E, respectively. While the RPB7 homologue is reasonably well conserved, the similarity between the eukaryotic RPB4 and the archaeal F subunit is barely detectable at the primary sequence level [<cite idref="PUB00007873"/>, <cite idref="PUB00010735"/>]. Like their eukaryotic counterparts, the archaeal E and F subunits have been shown to form a stable heterodimer [<cite idref="PUB00010736"/>]. Structural studies of the <taxon tax_id="2190">Methanocaldococcus jannaschii</taxon> (Methanococcus jannaschii) E-F heterodimer indicate that subunit E likely mediates binding to the nascent RNA transcript, while the F subunit serves to stabilise the conformation of the E subunit [<cite idref="PUB00010736"/>].</p>