Major role of the histones H3-H4 in the folding of the chromatin fiber.

We have characterized the hydrodynamic behavior of nucleosome arrays in which the N- and C-terminal "tails" of the histone H2A-H2B and H3-H4 domains have been selectively removed by digestion with immobilized trypsin. The sedimentation coefficient of the polynucleosome fibers lacking the histone H2A-H2B tails exhibited a salt dependence close to that of the non-trypsinized nucleosome arrays. In contrast, the salt-dependent behavior of the H3-H4-trypsinized polynucleosome fibers was found to be closer to that observed for the nucleosome arrays on which all the histones were trypsinized. This indicates that the N- and C-terminal domains of histones H3-H4 play a major role in the folding of the chromatin fiber. Magnesium titration of the polynucleosome fibers consisting of these trypsinized histone octamer hybrids at low ionic strength indicates that the histone H3-H4 tails also play an important role in the association of the polynucleosome fibers. These findings suggest that, after linker histones (histones of the H1 family), the tails of the histone H3-H4 domains are the major players in the processes that lead to the intra-association (folding) and inter-association of the chromatin fiber.