Evidence for a subunit structure of chromatin in mouse myeloma cells

If micrococcal nuclease is allowed to digest chromatin as it exists inside intact nuclei isolated from mouse myeloma tissue culture cells, more than 60% of the DNA can be isolated as a homogeneous fragment on a sucrose gradient. Analytical ultracentrifugation indicates that the protected DNA is native, unnicked, and about 140 +/- 10 base pairs long. After less extensive nuclease digestion, the protected DNA migrates in gels in lengths which are integral multiples of this 140 base pair "monomer" band. A submonomer band, 105 "/- 10 base pairs long, can also be detected. Similar digestion patterns were obtained by two different nuclear isolation procedures and even when intact cells were gently lysed directly in the digestion medium. These results confirm and extend the chromatin digestion studies of previous investigators and provide support for a subunit model for eukaryotic chromatin. The single strand specific S1 nuclease did not digest intranuclear chromatin under the conditions used.