Structural basis of RNA polymerase II transcription on the histone H3-H4 octasome

The histone H3-H4 octasome is a nucleosome-like particle in which two DNA gyres are wrapped around each histone (H3-H4)₂ tetramer disk, forming a clamshell-like configuration. In the present study, we performed in vitro RNA polymerase II (RNAPII) transcription assays with the H3-H4 octasome and found that RNAPII transcribed the H3-H4 octasome more efficiently than the nucleosome. RNAPII paused at only one position, superhelical location (SHL) -4 in the H3-H4 octasome, in contrast to pausing at the SHL(-5), SHL(-2), and SHL(-1) positions in the nucleosome. Cryo-electron microscopy analysis revealed that two (H3-H4)₂ tetramer disks are retained when the RNAPII paused at the SHL(-4) position of the H3-H4 octasome. However, when RNAPII reached the SHL(-0.5) position, five base pairs before the dyad position of the H3-H4 octasome, the proximal (H3-H4)₂ tetramer was disassembled but the distal (H3-H4)₂ tetramer still remained on the DNA. Therefore, RNAPII efficiently transcribes the H3-H4 octasome by stepwise (H3-H4)₂ tetramer disassembly.