The RNA polymerase II carboxy-terminal domain: links to a bigger and better 'holoenzyme'?

The largest subunit of eukaryotic RNA polymerase II has an unusual tandemly repeated heptapeptide sequence at its carboxyl terminus. The function of this evolutionarily conserved carboxy-terminal domain is unknown; however, new evidence links it to the formation of a large multi-component RNA polymerase II complex possessing enhanced transcriptional initiation properties. The existence of a pre-assembled RNA polymerase II 'holoenzyme' in the cell calls into question the long held view of transcription initiation as an ordered multi-step process.