Supranucleosomal organization of sea urchin sperm chromatin in regularly arranged 40 to 50 nm large granular subunits.

The organization of the chromatin of sperm nuclei of sea urchin (Parcentrotus lividus and Sphaerechinus granularis) was studied by electron microscopy using thin sections of fixed cells and spread preparations of chromatin obtained after various procedures involving incubation in low salt buffers. In fixed cells and in moderately dispersed preparations chromatin appeared in the form of fibrillar chains formed by closely apposed, distinctly sized granules that were larger than chromatin subunit structures so far described in other kinds of chromatin (mean diameters of particle units determined were 36 nm, in thin sections, and 47 nm, in spread preparations). Upon prolonged incubation in low salt buffer an increasing number of these large granular units was transformed into extended nucleosomal chains (nucleofilaments). Estimations indicated that the large granules contained 20 to 26 nucleosomes, i.e. 4.8 to 6.2 kb DNA, resulting in a linear DNA contraction ratio of 33 to 55, which is in a similar range as values reported for the supranucleosomal chromatin particle of the SV40 minichromosome but greater than contraction ratios of "superbead" structures. The unravelling of the large supranucleosomal granule into the extended nucleofilament chain in low salt buffers seemed to be a gradual process since intermediate stages of variable particles sizes and shapes were observed. This novel type of chromatin particle is considered to represent an example of a naturally occurring globular supranucleosomal packing unit of chromatin which is predominating, if not the only forms of supranucleosomal levels of chromatin organization and in relation to the special DNA content and histone composition of sea urchin sperm nucleosomes.