Compact oligomers and nucleosome phasing.

Micrococcal nuclease (EC 3.1.4.7) digestion of histone H1- and H5-depleted chicken erythrocyte chromatin yields, in addition to 140-base-pair (bp) core particles, a series of nucleosome oligomers containing about 260 bp (compact dimer), 380 bp (compact trimer), etc. of DNA. These are postulated to represent members of a class of oligomers in which the DNA is tightly wound on stacked protein cores. The physical properties (melting, circular dichroism) as well as DNase I (EC 3.1.4.5) digestion patterns support this view. DNase I digestion of tight oligomers in which the 5' ends of the DNA have been labeled yields results consistent with this model and inconsistent with some other possible models. Several classes of such particles are postulated to exist, differing in DNA length by 10-bp increments. This may be an explanation of the 10-bp nucleosome "phasing" that has been observed in some nuclei.