Free DNA stretches in histone H1-depleted chromatin and their possible relation to chromomere structure

The many length scales of DNA packaging

This collection of reviews focuses on the most exciting areas of DNA packaging at the current time. Many of the new discoveries are driven by the development of molecular or imaging techniques, and these are providing insights into the complex world of chromatin. As these new techniques continue to improve, we will be able to answer many of the questions we have now, while likely raising many new ones.

Changes in chromatin properties after partial extraction of non-histone proteins

By treatment with tRNA in the presence of 1 mM MgCl2, a chromatin preparation was obtained containing all five major histone fractions but lacking a considerable portion of non-histone proteins. This chromatin preparation as well as chromatin extracted with 0.6 M NaCl (depleted of H1 histone and some non-histone proteins) were characterized in respect of solubility and chromatin DNA accessibility. Both samples possessed practically the same solubility in the presence of 0.15 M NaCl and 1 mM MgCl2. The solubility of tRNA-treated chromatin in 5 and 10 mM MgCl2 was higher than that of salt-extracted chromation. The accessibility of the DNA of these chromatin preparations was tested with DNA-dependent RNA polymerase of Escherichia coli as a probe, using procedure that permits measurement of binding site frequency. Both tRNA-treated and salt-extracted chromatin contained as many as 33% and untreated chromatin as few as 4% of the number of binding sites found on protein-free DNA. These results demonstrate that at least in part the non-histone proteins are responsible for salt-induced insolubility and low DNA accessibility of chromatin, thus revealing the importance of non-histone proteins in the maintenance of an overall chromatin structure.