Abstract
Hen oviduct chromatin was digested with DNase II and separated into two fractions. The MgCl 2 - insoluble chromatin fraction (43% of the total DNA) was enriched in nucleosome-like particles, which sedimented at 11 S and contained 185 base pairs of DNA. The MgCl 2 soluble chromatin fraction (5 % of the total DNA) was characterized by 5 S and 14 S peaks in sucrose gradients. Estrogen receptors in the chromatin fractions were labelled with { 3 H} estradiol using the steroid exchange assay. The concentration of receptors in the MgCl 2 soluble chromatin was 4.5 times higher than that in the MgCl 2 insoluble chromatin. In sucrose gradient analysis the 11 S particles displayed a negligible specific radioactivity suggesting that estrogen receptors mainly bind to extranucleosomal chromatin.