CHROMATIN CONFORMATION IN LIVING CELLS - SUPPORT FOR A ZIGZAG MODEL OF THE 30 NM CHROMATIN FIBER

A new method was used to probe the conformation of chromatin in living mammalian cells. The method employs ionizing radiation and is based o n the concept that such radiation induces correlated breaks in DNA str ands that are in spatial proximity. Human dermal fibroblasts in G(0) p hase of the cell cycle and Chinese hamster ovary cells in mitosis were irradiated by X-rays or accelerated ions. Following lysis of the cell s, DNA fragments induced by correlated breaks were end-labeled and sep arated according to size on denaturing polyacrylamide gels. A characte ristic peak was obtained for a fragment size of 78 bases, which is the size that corresponds to one turn of DNA around the nucleosome. Addit ional peaks between 175 and 450 bases reflect the relative position of nearest-neighbor nucleosomes. Theoretical calculations that simulate the indirect and direct effect of radiation on DNA demonstrate that th e fragment size distributions are closely related to the chromatin str ucture model used. Comparison of the experimental data with theoretica l results support a zig-zag model of the chromatin fiber rather than a simple helical model. Thus, radiation-induced damage analysis can pro vide information on chromatin structure in the living cell. (C) 1998 A cademic Press.