On the location of linker DNA in the chromatin fiber. Studies with immobilized and soluble micrococcal nuclease.

The structure of chicken erythrocyte chromatin fibers has been probed using micrococcal nuclease, both membrane-immobilized and free in solution. Under the extremely mild digestion conditions used, the linker DNA is almost completely protected against digestion with either immobilized or free enzyme in the 30 nm fibers, whereas it is readily accessible in the more extended structures. Control experiments with glutaraldehyde-fixed chromatin fibers gave essentially the same results. Experiments with fibers of intermediate degree of condensation revealed a direct relationship between the degree of compaction and the resistance of linker DNA to digestion. Our results favor models in which access to the linkers is limited by local steric hindrance due to the high compaction, rather than by internalization in the center of the fibers.