Modification of rat liver RNA polymerase I after in vivo stimulation by hydrocortisone or methylisobutylxanthine.

Following in vivo administration of hydrocortisone or methylisobutylxanthine to rats, higher levels (1.5- to 2.3-fold) of RNA polymerase I activity are present in liver nuclei and nucleoli of the treated animals as compared to control animals. The elevated specific activity is retained after purification of the enzyme under conditions where the enzyme is dependent on exogenous template for activity. The elevated polymerase activity in nuclei, nucleoli, and soluble enzyme can be destroyed by mild trypsin treatment which results in a rapid decay of the specific activity to the control level. Under these conditions, the control polymerase I activity is stable. The results indicate that in vivo stimulation by hydrocortisone or methylisobutylxanthine results in a conversion of the enzyme to a form that is catalytically more active but has an increased sensitivity to proteolysis.