title: Pathways of assembly of nucleohistone complexes formed in vitro under physiological conditions. Implications for the structure of the nucleosome.
abstract: The composition and structure of three discrete nucleohistone particles produced by micrococcal nuclease digestion of the complex formed when the four core histones are mixed directly with DNA under conditions that are close to physiological have been reported in accompanying articles (Ellison, M. J., and Pulleyblank, D. E. (1983) J. Biol. Chem. 258, 13307-13313; 13314-13320). These include a peak containing a compact hexameric complex composed of one pair of (H3,H4) and two pairs of (H2A,H2B) (P2) and a peak containing complexes with the composition and sedimentation properties of nucleosome cores (P3). We have examined the pathways of assembly of P2 and P3 by determining the effect of order of histone addition on the yields of these species. The assembly of P2 is initiated by the binding of an (H2A,H2B) pair to the DNA which then directs the placement of an (H3,H4) pair. The incorporation of the final (H2A,H2B) pair to complete the particle is probably cooperative. The assembly of P3 is less dependent than the assembly of P2 upon the order in which the histone pairs are added to the DNA. More than one pathway has been implicated in the formation of these particles. P3 contains a component which has many but not all of the properties of nucleosome cores assembled at elevated ionic strength. Alternative pathways of assembly of histone pairs onto DNA are discussed which could lead to the formation of the discrete histone DNA structures that have been isolated here. A model is proposed in which the nucleosome unfolds and refolds into an alternative configuration.
mesh_terms: D000818:Animals; D002467:Cell Nucleus / Q000378:metabolism; D002645:Chickens; D004247:DNA / Q000378:metabolism*; D004912:Erythrocytes; D006657:Histones / Q000378:metabolism*; D007700:Kinetics; D046911:Macromolecular Substances; D008297:Male; D008836:Micrococcal Nuclease; D008961:Models, Structural; D009707:Nucleosomes / Q000648:ultrastructure*; D012474:Salmon; D013094:Spermatozoa
chemical_list: D006657:Histones; D046911:Macromolecular Substances; D009707:Nucleosomes; D004247:DNA; D008836:Micrococcal Nuclease