Optical model studies of the salt-induced 10-30-nm fiber transition in chromatin

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Abstract
Fractionated chicken erythrocyte chromatin fibers consisting of 10-mer and 75-mer polynucleosomes were studied by flow birefringence and viscosity over a range of Na+ and Mg2+ concentrations sufficient to span the 10-30 nm fiber transition. Negative intrinsic flow bifringence was observed under all solvent conditions investigated. The intrinsic birefringence, obtained from the reduced birefringence to intrinsic viscosity ratio, was used to evaluate various optical models for the DNA conformation in the fiber. Results are consistent with an extended chromatosome-linker necklace model for the unfolded, low-salt fiber and with a solenoidal model of edge-stacked chromatosomes for the condensed fiber at high salts.

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