Abstract
Purified F(2)a(1) histone molecules assemble into organized structures observable by electron microscopy. The basic structure observed at pH 8 and ionic strength of 0.15 has the shape of a bent rod with an average width of 22 A. The average circumferential length of the rod is 220 A and the average distance between the tips of the rod is 150 A. When the ionic strength is increased the rods align lengthwise into intertwined fiber-like structures. In some cases bent rods assemble "face-to-face" to give circular structures. At high protein concentrations the long fibers form paracrystalline arrays. Examination of these arrays by optical diffraction yielded a meridional reflection with a spacing of 150 A. The main additional reflections are compatible with a structure having a repeat unit of 300 A. We suggest that in chromatin the DNA is packed around organized periodic histone structures and that the periodicity of the histone structure may dictate the periodicity of the repeating unit in chromatin.