Distribution of thyroid hormone receptors, glucocorticoid receptors, and growth hormone gene sequences in chromatin from cultured rat pituitary cells.

Chromatin from rat pituitary tumor (GH3) cells was fractionated into transcriptionally active and inactive domains. When the various chromatin fractions were assayed for their content of growth hormone gene sequences, it was found that these sequences were highly enriched in those chromatin fractions most sensitive to micrococcal nuclease. Fraction S1 showed the highest enrichment in growth hormone genes and was impoverished in histones H3 and H4. The distribution of specifically bound thyroid and glucocorticoid receptors in chromatin fractions enriched and depleted in growth hormone gene sequences was also examined. Both thyroid and glucocorticoid receptors are enriched to different extents in transcriptionally active chromatin. Within active chromatin, both types of receptors exist in more than one molecular form.