Structural basis of nucleosome transcription mediated by Chd1 and FACT

Transcription of eukaryotic protein-coding genes requires passage of RNA polymerase II (Pol II) through nucleosomes. Efficient Pol II passage through nucleosomes depends on the chromatin remodelling factor Chd11 and the histone chaperone FACT2. How Chd1 and FACT mediate Pol II passage through nucleosomes remains unclear. Here we first show that Chd1 and FACT cooperate with the elongation factors Spt4/5 and TFIIS to facilitate Pol II transcription through a nucleosome in a defined biochemical system. We then determine cryo-EM structures of transcribing Saccharomyces cerevisiae Pol II-Spt4/5-nucleosome complexes with bound Chd1 or FACT at 2.9 [A] and 3.1 [A] resolution, respectively. In the first structure, transcribing Pol II has partially unwrapped nucleosomal DNA and exposed the proximal histone H2A/H2B dimer, which is bound by the acidic N-terminal region of Spt5 (Spt5N). The inhibitory DNA-binding region of Chd1 is released3 and the Chd1 translocase adopts an activated state that is poised to pump DNA towards Pol II. In the second structure, transcribing Pol II has generated a partially unravelled nucleosome that binds FACT in a manner that excludes Chd1 and Spt5N. These results suggest a dynamic model of Pol II passage through a nucleosome. In the model, Pol II enters the nucleosome4, activates Chd1 by releasing its DNA-binding region, and thereby stimulates its own progression. Pol II progression then enables FACT binding, liberates Chd1 and Spt5N, and eventually displaces a complex of FACT with histones that is transferred to upstream DNA.