A' is the largest subunit of the archaeal RNA polymerase (RNAP). Archaeal RNAP is closely related to RNA polymerases in eukaryotes based on the subunit compositions. Archaeal RNAP is a large multi-protein complex, made up of 11 to 13 subunits, depending on the species, that are responsible for the synthesis of RNA. Structure studies suggest that RNAP complexes from different organisms share a crab-claw-shaped structure. The largest eukaryotic RNAP subunit is encoded by two separate archaeal subunits (A' and A'') which correspond to the N- and C-terminal domains of eukaryotic RNAP II Rpb1, respectively. The N-terminal domain of Rpb1 forms part of the active site and includes the head and the core of one clamp as well as the pore and funnel structures of RNAP II. Based on a structural comparison among the archaeal, bacterial and eukaryotic RNAPs the DNA binding channel and the active site are part of A' subunit which is conserved. The strong similarity between subunit A' and the N-terminal domain of Rpb1 suggests a similar functional and structural role for these two proteins.
Comment:A complex active site region of Archaeal DNA-dependent RNA polymerases equivalent to eukaryotic Pol II formed by Mg binding site, Bridge Helix, Switch and Funnel domains
Jiyao W et al.(2023). "The conserved domain database in 2023.",