Enhancement by thyroid hormone of free and bound RNA polymerase IB activities in rat liver nucleoli.

After having defined reliable assay conditions for free and template-bound RNA polymerase I activities, we examined in detail the effect of thyroid hormone in vivo on nucleolar RNA polymerase I in rat liver. The enzyme activity decreased markedly in the hypothyroid state produced by either thyroidectomy, hypophysectomy or by administration of propylthiouracil (PTU). One large dose (25 microgram/100 g body weight) of T3 given intraperitoneally caused a rapid recovery of the enzyme activity within 24 h either in the presence or absence of the pituitary. The enzyme activity of both free and chromatin-bound enzyme changed in parallel throughout. A similar recovery was noted after daily subcutaneous injections of a physiological dose (1 microgram/100 g body weight) of T3 for about 3 weeks. Analyses of subfractions of the enzyme indicated that only IB-type enzyme (the enzyme fraction containing the whole set of subunits) changed in response to thyroid hormone action, also irrespective of the presence or absence of the pituitary. The possibility that thyroid hormone is a major physiological regulator of the synthesis of RNA polymerase I in rat liver is discussed.