In isolated liver nucleoli from thyroidectomized rats the activity of the two RNA polymerase I populations, one of which is active and the other inactive towards the endogenous chromatin template, is greatly enhanced 10 and 24h after a single ip injection triiodothyronine (T3). When the nucleolar enzyme is solubilized and assayed with exogenous DNA as template, it retains, after T3 treatment, the same increase in activity as observed in intact nucleoli. On the contrary, the template availability, as judged by the binding capacity of isolated nucleoli for [3H]actinomycin D, does not appear to be modified by the hormone. These observations support the conclusion that the enhanced nucleolar RNA synthesis following T3 administration is due to an increased activity of the RNA polymerase I itself rather than to a greater availability of ribosomal RNA cistrons. The hormonal stimulation of both nucleolar RNA polymerase activities depends on continuous protein synthesis since it is almost completely abolished by the administration of cycloheximide.

A single ip injection of triiodothyronine (T3; 30 mug/100 g BW) to thyroidectomized rats markedly stimulates RNA synthesis in isolated liver nuclei. The increased level of RNA synthesized in vitro by isolated nuclei does not depend on a reduced degradation of the nascent RNA molecules, since ribonuclease activities are not affected by the administration of T3. In addition, our results have confirmed previous findings of Tata et al. that the increase in nucleolar alpha-amanitin-resistant RNA polymerase I activity at low ionic strength always preceded the rise of the nucleoplasmic alpha-amanitin-sensitive RNA polymerase II activity at high ionic strength. Moreover, it has been found that a significant increase in an alpha-amanitin-resistant activity at high ionic strength occurs as early as 10 h after hormone injection. This enzyme, which forms RNA with a U to G ratio significantly higher than that of RNA synthesized by the nucleolar alpha-amanitin-resistant enzyme, is probably nucleoplasmic RNA polymerase III which is though to synthesize 5S and transfer RNAs. The possible role and the mechanism(s) of the early and concomitant increase in nucleolar and nucleoplasmic alpha-aminitin-resistant activities, and of the subsequent rise of RNA polymerase II activity following T3 administration are discussed.

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