RNA polymerase II (pol II) produces all mRNA in eukaryotic cells. Structures of the ten-subunit core of yeast pol II in free form [1, 2], in complex with DNA and RNA [3], and in complex with alpha-amanitin [4] have elucidated the mRNA transcription mech-anism [reviewed in 5, 6]. Based on this work, models for the com-plete 12-subunit pol II were obtained [7, 8]. We have subsequently solved the first structure of pol II in complex with a transcription factor, the elongation factor TFIIS [9, 10].
