Rat liver HMG1: a physiological nucleosome assembly factor.

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Abstract
Incubation of rat liver single‐stranded DNA‐binding protein HMG1 with the four core histones at 0.15 M NaCl favors histone association primarily into tetramers and, to a lesser extent, into octamers. The assembly of pre‐formed histone‐HMG1 complexes with DNA yields nucleosome‐like subunits which satisfy most of the criteria defining native core particles: (i) the circular DNA extracted from the complexes is supercoiled indicating that the initially relaxed DNA acquired superhelical turns during complex formation in the presence of topoisomerase I; (ii) the digestion of the complexes with micrococcal nuclease yields a DNA fragment of approximately 140 bp in length; (iii) electron microscopy of the reconstituted complexes shows a beaded structure with the DNA wrapped around the histone cores, leading to a reduction in the contour length of the genome compared with free DNA. Moreover, in the presence of HMG1, nucleosome assembly occurs rapidly at 0.15 M NaCl. Therefore, in addition to its DNA‐binding properties, HMG1 mediates the assembly of nucleosomes in vitro under conditions of physiological ionic strength. The possible involvement of these properties in the DNA replication process is discussed.