Structural transitions of chromatin at low salt concentrations: a flow linear dichroism study

We have studied the structure of nuclease-solubilized chromatin from Ehrlich ascites cells by flow linear dichroism (LD) using the anisotropic absorption of the DNA bases and of two intercalated dyes, ethidium bromide and methylene blue. It is confirmed that intercalation occurs preferentially in the linker part of the chromatin fiber, at binding ratios (dye/base) below 0.020. Using this information, we determined the orientation of the linker in relation to the average DNA organization in chromatin. The LD measurements indicate that the conformation of chromatin is considerably changed in the ionic strength interval 0.1-10 mM NaCl: with increasing salt concentration, the LD of the intrinsic DNA base absorption changes signs, from negative to positive, at approximately 2.5 mM NaCl. The LD of the intercalated dyes also changes signs, however, at a somewhat higher salt concentration. The results are analyzed in terms of possible allowed combinations of tilt angles of nucleosomes and pitch or tilt angles of linker DNA sections relative to the fiber axis, at different salt concentrations in the interval 0.1-10 mM NaCl. Two models for the salt-induced structural change of chromatin are discussed.