We have used salt extractions of nuclei and long agarose gels to dissect the chromatin fine structure of the histone gene repeat of Drosophila melanogaster. Extraction of nuclei with 0.35 M KCl removes many non-histone chromosomal proteins but does not significantly disturb the overall nucleosome arrangement of the repeat unit. After extraction of nuclei with 0.55 M KCl, which also removes histone HI, the basic arrangement of nucleosome core particles in the repeat unit is not greatly disturbed and the exposed DNA segments near the 5′ends of the histone genes are also retained. Extraction of nuclei with 0.75 M or higher KC1 concentrations causes extensive nucleosome sliding and rear rangement with accompanying changes in the nucleoprotein organization of the histone gene complex and loss of the 5′ hypersensitive sites. Our results indicate that the histone gene repeat displays a highly organized chromatin structure in vivo.
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