Atomic force microscopy reveals the local ordering characteristics of nucleosomal chain from cell

Atomic force microscopy was used to investigate the local ordering characte ristics of hypotonic spread HeLa nucleosomal chains. Nucleosomal chains wer e diluted in tridistilled water and dropped on freshly cleaved mica, rinsed with ultrapure water and air dried. We have found that nucleosomal chains show varied structures in vitro: beads-on- a-string, irregular arrangement, open rings, closed rings. Neither the hydrolysation of trypsin nor the dig estion of micrococcal nuclease or nuclease is necessary for observation of 'beads-on-a-string'. The closed rings formed by a nucleosomal chain have a close relevance to the two marked characteristics of the mild method develo ped in this work for preparing H1-depleted chromatin. The described method may provide a new reliable way to study the behaviour of the extended chrom atin fibre at low ionic strength. These observations may provide a useful c lue to elucidate how the structure of the extended chromatin fibre affects DNA transcription, replication and repair. Copyright (C) 2001 John Wiley & Sons, Ltd.