Stepwise unfolding of chromatin by urea. A flow linear dichroism and photoaffinity labeling study.

The unfolding of chromatin by urea (0-7 M) was studied by means of flow linear dichroism, photoaffinity labeling and nuclease digestion. The linear dichroism results indicate that the unfolding of the DNA is accomplished through two distinct transitions at 1-2 M urea and 6-8 M urea, respectively. The photoaffinity labeling studies indicate that an opening of the nucleosome histone core occurs above 2 M urea, accompanied by general loosening of the structure. Based on the results a model for the unfolding of chromatin fibers by urea is proposed, which includes a stretching of the linker DNA (0-2 M urea) followed by a "loosening" of the nucleosome core, possibly to a one-loop DNA conformation (2-6 M urea), and finally resulting in an almost total stretching of the DNA (greater than 6 M urea).