The basis of chromatin fiber assembly within chromosomes studied by histone-DNA crosslinking followed by trypsin digestion.

To determine the structural basis of chromatin assembly that leads to chromosome formation in mitosis, crosslinks were introduced by formaldehyde between contiguous components within chromosomes. Crosslinked stable products were then observed by electronmicroscopy after non-crosslinked portions were briefly digested by trypsin to unfold chromosomes.--When the DNA-histone crosslink was the primary product, trypsin readily unfolded the whole chromosome structure while preserving the 250 A unit chromatin fiber intact; only a single unit fiber was tracked within the centromere region connecting the arms of each chromatid. When a histone polymer was formed by a prolonged formaldehyde crosslinking, trypsin digestion gave rise to chromatin fibers interacting with others at certain distances, and the typical chromosome structure remained unchanged. Regardless of the degree of crosslinking, there were neither thick supercoiled unit fibers nor proteinaceous cores.--These results suggest that the fiber connection may represent, to some extent, the interacting sites of folded chromatin fibers in situ within chromosomes, and also that the 250 A unit fibers are the sole, highest structural basis in chromosomes. Since virtually no appreciable histone digestion took place in the crosslinked chromosomes, the observation that even after DNA-histone crosslinking the fiber interacting sites were accessible to trypsin preferentially over other portions, may be consistent with our recent results that the exposed, lysine-rich tails of histones represent such interacting sites.