RNA polymerase II structure: from core to functional complexes.

New structural studies of RNA polymerase II (Pol II) complexes mark the beginning of a detailed mechanistic analysis of the eukaryotic mRNA transcription cycle. Crystallographic models of the complete Pol II, together with new biochemical and electron microscopic data, give insights into transcription initiation. The first X-ray analysis of a Pol II complex with a transcription factor, the elongation factor TFIIS, supports the idea that the polymerase has a 'tunable' active site that switches between mRNA synthesis and cleavage. The new studies also show that domains of transcription factors can enter polymerase openings, to modulate function during transcription.