Chromatin-bound and free RNA polymerase A activities in rat thymus cells following glucocorticoid treatment.

Treatment of rat thymus cell suspensions with dexamethasone resulted in inhibition of engaged RNA polymerase A, without significant change in free pool activity. Studies with the re-initiation inhibitor, rifamycin AF/0-13, and measurements of numbers of RNA polymerase A molecules and of elongation rates showed that the inhibition of pre-rRNA synthesis resulted from a decrease in elongation rate. This effect was selectively abolished by mild proteolysis of nuclei. It is concluded that glucocorticoid treatment of rat thymus cells suppresses 45 S rRNA synthesis primarily by decreasing the polyribonucleotide elongation rate, rather than by effecting a change in enzyme redistribution or concentration.