General structural features of RNAP
RNA polymerisation has been ‘invented’ at least six times during evolution, as judged by independent structural folds of RNAP active sites. However, it is noteworthy that all RNAP responsible for transcribing cellular genomes of all living organisms – without exceptions – are evolutionary conserved, which implies that they are derived from a common ancestor. This is reflected in the sequence, structure and function of the RNAP subunits, and transcription factors that regulate their activity (Table 1).