<p>The catalytic domain of several polynucleotidyl transferases share a similar structure, consisting of a 3-layer alpha/beta/alpha fold that contains mixed beta sheets, suggesting that they share a similar mechanism of catalysis. Polynucleotidyl transferases containing this domain include ribonuclease H class I (RNase HI) and class II (RNase HII), HIV RNase (reverse transcriptase domain), retroviral integrase (catalytic domain), Mu transposase (core domain), transposase inhibitor Tn5 (containing additional all-alpha subdomains), DnaQ-like 3'-5' exonucleases (exonuclease domains), RuvC resolvase, and mitochondrial resolvase ydc2 (catalytic domain) [<cite idref="PUB00016296"/>, <cite idref="PUB00016297"/>, <cite idref="PUB00016298"/>].</p>