Nucleosomal organization of macronuclear chromatin in oxytricha fallax

The spacing of nucleosomes in chromatin from the macronucleus of the protozoan, Oxytricha fallax, was examined by staphylococcal nuclease digestion and gel electrophoresis. Macronuclear chromatin is ofmparticular interest because macronuclear DNA is not organized into chromosomes but instead exists as small molecules a few thousand base pairs long. When macronuclear chromatin is partially digested with staphylococcal nuclease the resulting fragments form a series of sizes which are multiples of 198 base pairs. Further digestion produces a ‘core’ DNA fragment 139 base pairs long. When chromatin from metabolically inactive cysts was examined it also showed a nucleosomal organization, with spacing identical to that in active vegetative cell chromatin. The spacing of nucleosomes in chromatin containing rRNA genes was examined also. DNA from staphylococcal nuclease digested macronuclear chromatin was electrophoresed in agarose gels, transferred to nitrocellulose and hybridized to I¹²⁵ labeled rRNA.The hybridization showed a series of rDNA fragments whose sizes were identical to the sizes of total DNA fragments. In the macronucleus of O. fallax, differences in chromatin function or activity do not seem to be reflected in differences in nucleosome spacing.