A model system reveals how different linker histones direct chromatin to liquid-like or fibre-like condensed states.

Abstract

In all organisms, metre-length DNA is packaged to a micrometre scale through charge neutralisation by basic polymers. In eukaryotes, the main condensing polymers are the histone proteins and the resulting protein/DNA complex is called chromatin. Chromatin organisation is hierarchical: the basic unit is the nucleosome, arrays of which form “beads on a string” that make up a 10 nm fibre. The second stage of condensation is achieved through binding of a different set of histones, the linker histones. This stage is more enigmatic and many proposals have been made for the structure that results, from a regular 30 nm fibre to a more heterogeneous, dynamic and liquid-like assembly with the 10 nm fibre as the basic unit. It is likely that both states are relevant: the liquid-like condensate allowing transcription, replication and repair, processes that would be shut down on adoption of a fibre-like state. The growing appreciation of dynamics in chromatin packaging has paralleled developments by us and others in our “bottom-up” understanding of the linker histone proteins themselves. We have developed an in vitro model system of linker histone and linker DNA, which although very minimal, displays surprisingly complex behaviour, and is sufficient to model the known states of H1-condensed chromatin: disordered complexes (“open” chromatin), dense liquid-like assemblies (dynamic condensates) and higher-order structures (organised 30 nm fibres). A crucial advantage of such a simple model is that it allows the study of the disordered state at high resolution by NMR, CD and scattering methods. Moreover, it allows capture of the thermodynamics underpinning the transitions between states through calorimetry. With these methods we can rationalise the distinct condensing properties of linker histone variants across species that are encoded by their amino acid sequence.