Triiodothyronine-Stimulated Formation of Poly(A)-Containing Nuclear RNA and mRNA in Rat Liver*

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Abstract
The formation of poly(A)-containing hn[heterogeneous nuclear]RNA and mRNA was studied by short-term orotic acid labeling in the livers of euthyroid-, hypothyroid- and triiodothyronine (T3)-injected hypothyroid animals. In order to calculate the rate of formation of these RNA species, the specific activity of the precursor UTP pool was measured. In euthyroid animals, the precursor specific activity was approximately twice that in hypothyroid animals. Since there was only a 17% reduction in the estimated precursor pool per gram in the liver of euthyroid animals, other factors must be responsible for the difference in specific activity, possibly an increased conversion of labeled orotic acid to UTP. Assuming identity in the base composition of various RNA species from hypothyroid and euthroid animals, from the counting rate of RNA species and the specific activity of the precursor pool it was calculated that the rate of formation of both the poly(A)-containing hnRNA and polysomal mRNA is approximately 60% greater in euthyroid than hypothyroid liver. Injection of T3 into hypothyroid animals resulted in an increased rate of formation of hnRNA and mRNA which was observed as early as 6 h after T3 injection. By 24 h the rate of formation of hnRNA and mRNA approximated that observed in euthyroid animals. In addition to the increased rate of mRNA formation a 32% increase in the content of mRNA was found in euthyroid rats compared with hypothyroid animals. Although the increase in thyroid hormone-induced poly(A)-containing hnRNA and mRNA most likely is due to an increase in the rate of gene transcription, the possibility that these changes are modulated by an alteration in the fractional RNA turnover has not been excluded.