 It forms a promoter-binding domain (PBD) with helix bundles in phage ssRNAPs, a feature not found in mitochondrial ssRNAPs.T7 polymerase is a representative member of the single-subunit DNA-dependent RNAP (ssRNAP) family.Other members include phage T3 and SP6 RNA polymerases, the mitochondrial RNA polymerase (POLRMT), and the chloroplastic ssRNAP.