<p>Transcription factor S-II (TFIIS) is a eukaryotic protein which induces mRNA cleavage by enhancing the intrinsic nuclease activity of RNA polymerase (Pol) II, past template-encoded pause sites. TFIIS shows DNA-binding activity only in the presence of RNA polymerase II [<cite idref="PUB00002480"/>]. It is widely distributed being found in mammals, Drosophila, yeast and in the archaebacteria <taxon tax_id="2285">Sulfolobus acidocaldarius</taxon> [<cite idref="PUB00004419"/>]. S-II proteins have a relatively conserved C-terminal region but variable N-terminal region, and some members of this family are expressed in a tissue-specific manner [<cite idref="PUB00006267"/>, <cite idref="PUB00006554"/>].</p><p> TFIIS is a modular factor that comprises an N-terminal domain I, a central domain II, and a C-terminal domain III [<cite idref="PUB00011910"/>]. The weakly conserved domain I forms a four-helix bundle and is not required for TFIIS activity. Domain II forms a three-helix bundle, and domain III adopts a zinc-ribbon fold with a thin protruding beta-hairpin. Domain II and the linker between domains II and III are required for Pol II binding, whereas domain III is essential for stimulation of RNA cleavage. TFIIS extends from the polymerase surface via a pore to the internal active site, spanning a distance of 100 Angstroms. Two essential and invariant acidic residues in a TFIIS loop complement the Pol II active site and could position a metal ion and a water molecule for hydrolytic RNA cleavage. TFIIS also induces extensive structural changes in Pol II that would realign nucleic acids in the active centre.</p><p>This domain is found in the central region of transcription elongation factor S-II and in several hypothetical proteins.</p>