Effect of Thyrotropin on the Sensitivity of Thyroid Nuclear Deoxyribonucleic Acid to Digestion by Micrococcal Nuclease*

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Abstract
Little is known about the mechanisms of action of polypeptide hormones on chromatin structure and nuclear function. We have employed micrococcal nuclease to examine the effect of TSH on the accessibility of DNA in thyroid nuclei. Brief digestion of nuclear suspensions with 0.05-0.2 U/ml micrococcal nuclease at 26–-28 C decreased their opacity at 600 nm. The decrease in opacity was linear with increasing nuclear concentration up to 0.2 mg/ml DNA. This response to nuclease was enhanced in nuclear suspensions prepared from thyroid slices that had been incubated with TSH (50 mU/ml) for 5 h (P < 0.001). To determine whether TSH also increased the digestion of DNA, we measured the amount of DNA released into 1200 × g supernatants by nuclease treatment of nuclei prepared from control and TSH-treated slices. When TSH-treated nuclei (110 μg/100μl) were digested with 0.2 U micrococcal nuclease/ml at 37 C for 30 sec, a mean of 12.6 μg ± 3.6 (SD) DNA appeared in the upernatant, as compared to 8.4 /×g± 1.9 DNA from control nuclei (P < 0.05). This increase in the sensitivity of nuclear DNA to microcpccal nuclease may reflect some conformational change in chromatin in response to TSH. Since micrococcal nuclease sensitivity may reflect transcriptional competence of DNA, we speculate that polypeptide hormones may enhance the accessibility of DNA to RNA polymerase or to endogenous stimulators of transcription.