The sequentiallity of nucleosomes in the 30 nm chromatin fibre

The folding of eukaryotic DNA into the 30 nm fibre comprises the first level of transcriptionally dormant chromatin. Understanding its structure and the processes of its folding and unfolding is a prerequisite for understanding the epigenetic regulation in cell differentiation. Although the shape of the fibre and its dimensions and mass per unit length have been described, the path of the internucleosomal linker DNA and the sequentiallity of the nucleosomes in the fibre are poorly understood. In the present study, we have chemically crosslinked adjacent nucleosomes along the helix of chicken erythrocyte oligonucleosome fibres, digested the internucleosomal linker DNA and then examined the digestion products by sucrose gradient sedimentation. We found that the digestion products contain considerable amounts of mononucleosomes but less dinucleosomes, which suggests that there are end-discontinuities in the fibres. This can be explained by a nonsequential arrangement of the nucleosomes along the fibre helix.