CONTRIBUTIONS OF LINKER HISTONES AND HISTONE H3 TO CHROMATIN STRUCTURE - SCANNING FORCE MICROSCOPY STUDIES ON TRYPSINIZED FIBERS

Little is known about the mechanisms that organize linear arrays of nu cleosomes into the three-dimensional structures of extended and conden sed chromatin fibers. We have earlier defined, from scanning force mic roscopy (SFM) and mathematical modeling, a set of simple structural de terminants of extended fiber morphology, the critical parameters being the entry-exit angle between consecutive linkers and linker length. H ere we study the contributions of the structural domains of the linker histones (LHs) and of the N-terminus of histone H3 to extended fiber morphology by SFM imaging of progressively trypsinized chromatin fiber s. We find that cleavage of LH tails is associated with a lengthening of the internucleosomal center-to-center distance, and that the somewh at later cleavage of the N-terminus of histone H3 is associated with a flattening of the fiber. The persistence of the ''zigzag'' fiber morp hology, even at the latest stages of trypsin digestion, can be attribu ted to the retention of the globular domain of LH in the fiber.