Overcoming the nucleosome barrier during transcript elongation.

RNA polymerase II (Pol II) must break the nucleosomal barrier to gain access to DNA and transcribe genes efficiently. New single-molecule techniques have elucidated many molecular details of nucleosome disassembly and what happens once Pol II encounters a nucleosome. Our review highlights mechanisms that Pol II utilizes to transcribe through nucleosomes, including the roles of chromatin remodelers, histone chaperones, post-translational modifications of histones, incorporation of histone variants into nucleosomes, and activation of the poly(ADP-ribose) polymerase (PARP) enzyme. Future studies need to assess the molecular details and the contribution of each of these mechanisms, individually and in combination, to transcription across the genome to understand how cells are able to regulate transcription in response to developmental, environmental and nutritional cues.