Increased RNA polymerase activity in isolated liver nucleoli from thyroidectomized rats treated with triiodothyronine

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.