Structural features of RNA polymerase (RNAP) are crucial to its catalytic function. RNAPs of bacteria, archaea, and eukaryotes (except mitochondrial RNAP of eukaryotes) are homologous multi-subunit complexes, sharing a high degree of similarity in the core enzyme. The structures of multi-subunit RNAP has been likened to a crab's claw, which is able to clamp and stabilize the transcription bubble. Furthermore, distinct structural features of these polymerases (trigger loop and bridge helix, Figure 10.1A right) play important roles in orchestrating the transcription elongation cycle (TEC). In contrast to multi-subunit RNAPs, the RNAP of T7 bacteriophage (T7 RNAP) is a single subunit...