Transition of chromatin from the "10 nm" lower order structure, to the "30 nm" higher order structure as followed by small angle X-ray scattering.

Chromatin oligomers undergo a conformational change from a "10 nm" lower order structure at low concentration of salt to a "30 nm" higher order structure, with increasing NaCl or MgCl2 concentration. We have extended our previously reported hydrodynamic and light-scattering measurements of the folding of well-defined chicken erythrocyte chromatin fractions to include a study of the low angle X-ray scattering in solution. We show that it is feasible to identify the folding process with gradual compaction of a chain of freely joined filaments or a worm-like chain, within the limits of all the experimental data obtained. As the ionic strength is raised, the filament length of the oligomer, composed of Nz nucleosomes, decreases. At 75 mM-NaCl, the compacted model chains (Nz = 53) form structures that are, on average, cylindrically shaped with mean diameter 30 nm and length 104 nm. Helical symmetry need not be invoked in the modelling of the folding process and may, in particular, be difficult to establish in chicken erythrocyte chromatin, due to the non-uniform length of the DNA linker connecting the nucleosomes. Concerning the shape of the X-ray scattering profiles at various salt concentrations, it is possible in this way to rationalize two-slope cross-sectional plots, which have also been reported by other workers. Though this description represents a satisfactory conceptual presentation of a wealth of experimental data, it by no means represents a definitive solution to an exceedingly difficult problem.