Abstract

The nuclear RNA polymerase system ("aggregate enzyme" of Weiss) isolated from thymus glands of cortisol-treated rats was shown to incorporate ³H-UTP into RNA to a lesser extent than the same system isolated from control animals. The degree of inhibition varied with time after injection and with cortisol dosage and was specific for steroids which cause thymic involution. No difference in nucleotide triphosphatase activity was observed when thymus aggregate enzyme preparations isolated from cortisol-treated rats were compared with identical preparations isolated from control animals. The rate of ³H-UTP incorporation into RNA by thymus aggregate enzyme was found to be dependent on ionic strength. Maximum rates of incorporation and maximum steroid effects were found when NH₄Cl was present in the incubation mixture at a final concentration of 0.75 M. Increasing NH₄Cl concentration in the incubation mixture was found to stimulate control thymus aggregate enzyme to a greater extent than thymus aggregate enzyme isolated from cortisol-treated rats. Cortisol was without effect when added directly to the aggregate enzyme incubation mixture at a final concentration of 10^-5 M.