Isolation and characterisation of a 167 bp core particle isolated from stripped chicken erythrocyte chromatin

We have digested chicken erythrocyte soluble chromatin, both unstripped and stripped of histones H1 and H5 with either 0.6 M NaCl or DNA-cellulose, with micrococcal nuclease (MNase). Digestion of unstripped chromatin to monomeric particles initially paused at 188 bp DNA; continued digestion resulted in another pause at 177 before the 167 bp chromatosome and 146 bp core particle were obtained. Digestion of stripped chromatin to monomeric particles paused transiently at 177 bp; continued digestion resulted in marked pauses at 167 and 156 before the 146 bp core particle was obtained. These results suggested that 167 bp DNA representing two complete turns are bound to the histone octamer. Histone H1/H5 binds an additional two helical turns of DNA, thereby protecting up to 188 bp DNA against nuclease digestion. Monomeric particles containing 167 bp DNA were isolated from stripped chromatin and found by DNase I digestion to be a homogeneous population with a 10 bp DNA extension to either end relative to the 146 bp core particle. Thermal denaturation and circular dichroism spectroscopy showed stronger histone-DNA interactions and increased DNA winding as the length of DNA attached to the core histone octamer was decreased. Thermal denaturation also showed three classes of histone-DNA interaction: the core particle containing 167 bp DNA had tight binding of ten helical turns of DNA, intermediate binding of two helical turns and looser binding of four helical turns.