Nonprotamine nucleoprotein ultrastructures in mature ram sperm nuclei.

Nuclei from mature ram spermatozoa were treated with a solution of 0.25 M 2-mercaptoethanol, 2 M NaCl, 4 to 8 M urea to dissociate protamines and some other proteins from DNA. The material remaining after such a treatment was spread for electron microscopy in the microcentrifugation chamber. Two types of structures were observed determined by proteins of nonprotamine nature. The first type was represented by protein bodies of an irregular ringlike shape to which DNA fibers were anchored to form a network. This structure determines the shape of the sperm head and may correspond to the nuclear skeleton described in somatic cells. The second type of structures were chromatin fibers containing beads of approximately nucleosomal size. These rough fibers were unevenly distributed in the nucleus and were much less frequent than the smooth fibers usually observed. Both types of structures were determined by unusually firmly bound proteins. They were resistant not only to the reduction of disulfide bonds and to high salt and urea concentrations but also to 2% sodium dodecysulfate and to 5 M guanidine chloride. These results show that apart from packing of DNA in a nucleoprotamine complex, two levels of DNA organization can be observed in the ram sperm nucleus: the first level consists of two kinds of DNA fibers (smooth and rough); in the second level of DNA is organized in domains fixed by a proteinous nuclear skeleton.