Location of the ethidium binding sites of high affinity in chromatin.

The influence of H1 and H5 histones proteins upon the accessibility of ethidium bromide into chromatin is studied by steady-state fluorescence anisotropy in the range of r-values ([Dye]/[Phosphate]) smaller than 0.01. This corresponds to the very strong binding process. When H1 and H5 are present, the DNA segment which contains the binding sites is 25-30 base pairs long, even if H1 and H5 are digested by trypsin or by natural proteolysis, but presumably still interacting with the DNA chromatin. On the contrary, when H1 or H5 are separated from chromatin by an increase of the ionic strength, ethidium binds to a segment of DNA about 55-60 base pairs long. We may explain the results by assuming that the ethidium sites are located on a continuous segment constituting about one half of the linker, the other half interacting with H1 and H5. When chromatin is depleted from these proteins, the high affinity sites are distributed all along the linker.