Visualization of nucleosomal substructure in native chromatin by atomic force microscopy.

Intact rDNA minichromosomes from Tetrahymena thermophila were isolated as native chromatin and imaged by atomic force microscopy (AFM). AFM measurements of condensed rDNA chromatin were consistent with a 30 nm fiber that frequently (87% of molecules observed) contained stretches of nucleosome cores arranged in a zig-zag conformation. Examination of rDNA chromatin in a dispersed conformation by tapping mode AFM in low humidity resulted in high resolution images of partially dissociated nucleosome cores and associated linker DNA. A majority of these nucleosome cores contained six to eight smaller particles with dimensions consistent with those of individual histones. Many of the nucleosome cores showed a striking resemblance to the wedge (35%), axial (15%), and front (6%) views of the nucleosome histone octamer modeled by Arents et al. [Arents, G., Burlingame, R. W., Wang, B.-C., Love, W.E., & Moudrianakis, E. N. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 10148-10152]. This direct visualization of histone subunits and nucleosomal substructure in native chromatin illustrates the potential use of AFM to localize individual proteins in condensed cellular chromatin.