Multiple forms of DNA-dependent RNA polymerase I from immature chick liver. Selective effect of 17beta-estradiol.

1. We found that a single injection of 17beta-estradiol into immature chicks resulted within 24 h in a 2-fold increase of the transcripitional capacity of liver nuclei which involved both endogenous RNA polymerase I and II actvities. Similarly, RNA polymerase activity I of purified nucleoli was also doubled. 2. During purification of RNA polymerase I from controls and treated chicks the difference in activity was almost completely lost. 3. RNA polymerase I could be resolved into two forms, IA and IB, on CM-Spehadex. Form IA increased and form IB decreased after estrogen treatment while the sum of the two remained constant. Form IA sedimented more slowly than form IB in glycerol gradients and was predominant in purified nucleoli. 4. These observations suggest that there may be an equilibrium between the two forms of RNA polymerase I which may change in order to produce the translational shift-up which occurs after estrogen treatment.