Enrichment of estradiol-receptor complexes in a transcriptionally active fraction of chromatin from MCF-7 cells

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Abstract
The interaction of the estradiol [E2] receptor molecule with chromatin in MCF-7 cells, a human breast tumor cell line responsive to E2, was examined. Receptor was associated with the various nucleosomal products produced by digestion with micrococcal nuclease. To determine whether these receptor binding sites were distributed in a random or nonrandom manner within the chromatin, MCF-7 cell chromatin was fractionated into transcriptionally active and inactive fractions by limited micrococcal nuclease digestion followed by Mg2+ precipitation. A comparison of the Mg2+-soluble and insoluble chromatin fractions showed that the Mg2+-soluble fraction: was composed predominantly of mononucleosomes; was enriched in nonhistone proteins; apparently lacked histone H1; was enriched approximately 5-fold in transcribed sequences as measured by a c[complementary]DNA probe to cytoplasmic poly(A)-RNA sequences; was depleted at least 5-fold of globin sequences, which is presumably a nontranscribed gene in these cells. When these cells were stimulated with .beta.-[3H]E2, the Mg2+-soluble fraction showed a significant enrichment in chromatin-bound E2 receptor: the Mg2+-soluble mononucleosomes showed a 3- to 4-fold enrichment and the di- and trinucleosomes, a 7- to 19-fold enrichment, when compared to the corresponding subunits in the Mg2+-insoluble chromatin fraction. This cofractionation of chromatin enriched in transcribed sequences and bound E2 receptor indicated that receptor binding to MCF-7 cell chromatin was not random but, rather, occurred preferentially in specific regions of the chromatin.