On the site of cortisol inhibition of thymus ribonucleic acid synthesis

Cortisol, in doses shown to inhibit RNA synthesis by thymus aggregate enzyme, did not alter maximum template activity of chromatin isolated from thymus glands of steroid-treated rats when assayed in the presence of E. coli RNA polymerase. Cortisol was shown, however, to decrease the amount of endogenous thymic RNA polymerase activity extractable from purified nuclear preparations. The magnitude of this effect was similar to the decrease in [ 3H]-UMP incorporation seen with intact nuclei or aggregate enzyme preparations when assayed without added bacterial RNA polymerase. When aggregate enzyme was obtained from cortisol-treated rats and assayed at low ionic strength (no NH 4Cl added to assay system) in the presence of added E. coli RNA polymerase, the usual cortisol-induced inhibition of RNA synthesis disappeared; when this same preparation was assayed in the presence of 0.3 M NH 4Cl, however, the inhibitory effect of cortisol was paradoxically converted to a stimulatory effect. Removal (by extraction) of endogenous RNA polymerase from control aggregate enzyme-like preparations was also shown to cause a stimulation in [ 3H]-UMP incorporation when the resulting preparation was assayed in the presence of 0.3 M NH 4C1 and added E. coli RNA polymerase. The paradoxical stimulation in [ 3H]-UMP incorporation seen after cortisol treatment was therefore explained on the basis that the presence of less endogenous RNA polymerase might allow greater access of the more efficient E. coli RNA polymerase to the template. These results are compatible with the suggestion that cortisol may inhibit thymic RNA synthesis by decreasing the amount, availability or activity of thymic RNA polymerase rather than by an effect on template activity.