Histone octamer helical tubes suggest that an internucleosomal four-helix bundle stabilizes the chromatin fiber.

A major question in chromatin involves the exact organization of nucleosomes within the 30-nm chromatin fiber and its structural determinants of assembly. Here we investigate the structure of histone octamer helical tubes via the method of iterative helical real-space reconstruction. Accurate placement of the x-ray structure of the histone octamer within the reconstructed density yields a pseudoatomic model for the entire helix, and allows precise identification of molecular interactions between neighboring octamers. One such interaction that would not be obscured by DNA in the nucleosome consists of a twofold symmetric four-helix bundle formed between pairs of H2B-alpha3 and H2B-alphaC helices of neighboring octamers. We believe that this interface can act as an internucleosomal four-helix bundle within the context of the chromatin fiber. The potential relevance of this interface in the folding of the 30-nm chromatin fiber is discussed.