INVITRO CORE PARTICLE AND NUCLEOSOME ASSEMBLY AT PHYSIOLOGICAL IONIC-STRENGTH

Nucleosome core particles have been efficiently assembled in vitro by direct interaction of histones and DNA at physiological ionic strength, as assayed by digestion with DNAses, supercoiling of relaxed circular DNA, and electron microscopy. Reconstitution was achieved either by the simultaneous addition of all core histones, or by the sequential binding of H3-H4 tetramer and H2A-H2B dimer to DNA. Micrococcal nuclease digestion and electron microscopy studies indicated that there is heterogeneity in the spacings at which core partices are assembled on the DNA. Length measurements of oligomeric DNA produced during the course of the digestion suggest that the core histone octamer can organize 167 (±4) rather than 145 base pairs of DNA, the extra 20 base pairs being quickly digested. Binding of histone H1 to core particles resulted in the protection of about 165 base pairs of DNA from nuclease attack. Because the core histone octamer is fully dissociated into H3-H4 tetramer and H2A-H2B dimer at physiological ionic strength, our results would suggest that in vivo core particle assembly may also occur by interaction of these two complexes on the nascent DNA.