<p>In eukaryotes, there are three different forms of DNA-dependent RNA polymerases (<db_xref db="EC" dbkey="2.7.7.6"/>) transcribing different sets of genes. Each class of RNA polymerase is an assemblage of ten to twelve different polypeptides. In archaebacteria, there is generally a single form of RNA polymerase which also consists of an oligomeric assemblage of 10 to 13 polypeptides. A component of 14 to 18 kDa shared by all three forms of eukaryotic RNA polymerases and which has been sequenced in budding yeast (gene RPB6 or RPO26), in <taxon tax_id="4896">Schizosaccharomyces pombe</taxon> (Fission yeast) (gene rpb6 or rpo15), in human and in <taxon tax_id="10497">African swine fever virus</taxon> (ASFV) is evolutionary related to the archaebacterial subunit K (gene rpoK). The archaebacterial protein is colinear with the C-terminal part of the eukaryotic subunit. </p><p>The structures of the omega subunit and RBP6, and the structures of the omega/beta' and RPB6/RPB1 interfaces, suggest a molecular mechanism for the function of omega and RPB6 in promoting RNAP assembly and/or stability. The conserved regions of omega and RPB6 form a compact structural domain that interacts simultaneously with conserved regions of the largest RNAP subunit and with the C-terminal tail following a conserved region of the largest RNAP subunit. The second half of the conserved region of omega and RPB6 forms an arc that projects away from the remainder of the structural domain and wraps over and around the C-terminal tail of the largest RNAP subunit, clamping it in a crevice, and threading the C-terminal tail of the largest RNAP subunit through the narrow gap between omega and RPB6 [<cite idref="PUB00007936"/>].</p>