Salt-induced folding of sea urchin sperm chromatin.

We have studied the folding in solution of chromatin from sea urchin sperm, which has the longest reported nucleosome repeat length (approximately equal to 240 base pairs) and an H2B with a basic N-terminal extension. The same two size- and ionic-strength-dependent transitions in sedimentation behaviour are observed as for other chromatins (rat liver, chicken erythrocyte) with shorter DNA repeat lengths. The results support the notion of an essentially similar folding of chromatins with different linker lengths of 0-80 base pairs. We interpret the hydrodynamic data in terms of a solenoid with about six nucleosomes per turn, which has greater stability in sea urchin sperm than in the other cases. The higher values of the sedimentation coefficients for sea urchin sperm oligomers indicate lower frictional coefficients, which probably result from accommodating the additional linker DNA and the extra mass of larger H1 and H2B in the central hole and/or the surface interstices of the solenoid.