Nuclear thyroid hormone receptor binding to chromatin subunits: Implications from digestion studies with micrococcal nuclease

Abstract

Nuclear thyroid hormone receptors are putative regulatory proteins which copurify with chromattn components and remain associated with chromatin following hydrodynamic shear. Rat liver chromatin containing bound receptors labeled in vivo with 125I-labeled L-triiodothyronine (T 3) is cleaved by micrococcal nuclease at 2°C into a series of nucleosomes or chromatin subunits. After velocity sedimentation of these subunits in glycerol gradients, receptors remain associated with oligomeric subunits but not with monomers. In studies with isolated nuclei, thyroid hormone receptors are concentrated in a particulate ehromatin fraction prepared by low temperature digestion of rat liver nuclei with micrococcal nuclease and isolation of the unsheared ‘core’ chromatin. On brief incubation at 37°C, core chromatin is digested by residual nuclease, yielding monomer subunits. Thyroid hormone receptors dissociate from nucleoprotein binding sites during this incubation. These observations with two different chromatin preparations suggest that receptor binding sites are preserved in nuclease-generated chromatin oligomers but not in monomer subunits. It is proposed that receptors bind to DNA segments linking monomer subunits, rather than to the subunits.