The controversial 30 nm chromatin fibre.

DNA is packed as chromatin on several levels in the eukaryotic nucleus. Dissection of chromatin with nucleases produces three stable substructures: the nucleosome core particle, the chromatosome and the 30 nm fibre. Whilst the first two allow transcription, the 30 nm fibre is taken to be the first level of transcriptionally dormant chromatin and it has an important functional role in cell differentiation and epigenetic regulation. Its structure has been a subject of continuing discussion since native fibres cannot readily be crystallized. This problem has recently been addressed by reconstitution of fibres on repeats of DNA sequences having nucleosome-positioning properties and two different structures were reported. The reconstitution results and their interpretations are compared in this review with experimental data from native chromatin and it is shown that the results of Robinson et al. conform well with the known structural features of native fibres and are a good first step towards understanding the structure of the fibre.