Effect of thyrotropin on 32P-labelled histones H1 and H3 in specific populations of nucleosomes in the thyroid.

Thyrotropin (TSH) increases the labeling of histones of H1 and H3 in thyroid slices incubated with 32Pi. We have prepared nuclei from control and TSH-treated thyroid slices, digested them with micrococcal nuclease, and extracted specific populations of nucleosomes by salt fractionation. Mononucleosomes, derived from the most nuclease-sensitive regions of chromatin, appeared to be selectively enriched in 32P-labeled H1 and H3. However, we were able to detect TSH enhancement of H1 and H3 labeling only in nucleosome multimers derived from less nuclease-sensitive chromatin. Recent studies have indicated that transcriptionally competent regions of chromatin may be more susceptible to micrococcal nuclease digestion than inactive regions. Our results therefore suggest that H1 and H3 may be actively phosphorylated in transcriptionally competent chromatin; however, they suggest either that hormone-dependent phosphorylation of H1 and/or H3 does not confer transcriptional competence, or that not all transcriptionally competent chromatin is nuclease sensitive.