Topological constraints on the possible structures of the 30 nm chromatin fibre.

The packing of mammalian DNA into chromatin plays an important role in cell differentiation and selection of epigenetically marked genes for expression or silencing. The first level of folding, the nucleosome, is evolutionary conserved. It allows transcription, after remodeling and/or histone modifications. The second level, the transcriptionally dormant 30 nm fibre, exhibits species and tissue variations in the chromatin repeat length. Nevertheless, very similar structures of fibres have been observed in all metazoans, and therefore, have to accommodate variable linker lengths with a corresponding change of tilt of the nucleosomes, which is defined by the DNA helical periodicity. So far, none of the models for a regular fibre structure has considered this requirement nor the relationship between repeat length and orientations of nucleosomes in the fibre. Here, we present two regular structural arrangements with negatively tilted consecutive nucleosomes which can compensate for a non-integer number of bp/turn of DNA; one can accommodate a series of structures with discrete repeat lengths differing by 5 bp in the region around 200 bp and the other from around 220 to 250 bp, accommodating repeat lengths differing by 10 to 12 bp.