Figure 2.
Proposal for a folding pathway for chromatin fibres. With constant and optimal short linker lengths, the ‘10-nm' zigzag folds to a twisted crossed-linker structure corresponding to the tetranucleosome crystallized by Schalch et al (2005). In the presence of linker histone, a more coiled structure is formed that, for shorter linker lengths, compacts into a helical ribbon corresponding to the condensed structures visualized by Robinson et al (2006). For clarity, the nucleosomes in the condensed fibres are shown parallel to the superhelical axis, although in actual fibres they are substantially tilted with respect to the axis. With suboptimal or heterogeneous linker lengths, a less-compact fibre—corresponding to the classic 30-nm fibre—will be formed (top right). The orange arrows indicate the approximate direction of the long axis of the fibres.