Glucocorticoid regulation of rat thymus RNA polymerase activity: the role of RNA and protein synthesis.

Treatment of rat thymus cells with the glucocorticoids cortisol and dexamethasone resulted in the stimulation of RNA polymerase B activity within 10 min of steroid addition. This early effect was followed by the inhibition of both RNA polymerase A and B activities. These effects were glucocorticoid-specific and were inhibited by the antiglucocorticoid cortexolone. The inhibitory effect of dexamethasone on RNA polymerase A activity was abolished by prior treatment of the cells with alpha-amanitin, cordycepin or cycloheximide, but cycloheximide was only capable of inhibiting the steroid effect measured at 3 h if added within 10--20 min after steroid addition. Cycloheximide had no effect on the steroid-mediated inhibition of RNA polymerase B activity. Control RNA polymerase A activities were unaffected by the presence of inhibitors of RNA and protein synthesis. It is concluded that the inhibition of ribosomal RNA synthesis by glucocorticoids is dependent on protein synthesis, but that basal RNA polymerase A activity in rat thymus cells is not stringently coupled to protein synthesis.