NUCLEAR‐CYTOSOL INTERACTIONS THAT FACILITATE RELEASE OF RNA FROM MOUSE BRAIN NUCLEI

— Mouse brain nuclei were incubated in vitro under conditions that primarily lead to the synthesis of radioactive polydisperse and messengerlike nuclear RNA. After incubation the effects of Mg 2 concentrations, nucleoside triphosphate levels and brain cytosol were examined with regard to their ability to influence the release of RNA from brain nuclei. The presence of 8 mM ‐MgCl 2 and a total of 0.3 mM‐nuclcoside triphosphates during the labelling procedure allowed only a minimal amount of RNA to be released. However, when the MgCl 2 was decreased to 2 mM and the nucleoside triphosphates were increased to 1 mM, a stimulation of RNA release was observed. The addition of unfractionated brain cytosol under these conditions resulted in an inhibition of RNA release. G‐100 Sephadex filtration removed detectable RNase activity from the cytosol preparations and allowed the identification of fractions that were able to facilitate nuclear RNA release by 3‐fold. The fractions that stimulated release did not have detectable levels of RNase, protease or DNA‐dependenl RNA polymerase. Under conditions that provided maximum nuclear RNA release by both labelled mouse brain and neuroblastoma nuclei, no release of DNA could be measured. The cytosol fractions that facilitated RNA release did not have a high affinity for nuclear RNA or an ability to stimulate nuclear RNA synthesis. However, other components in the cytosol were shown to stimulate RNA metabolism in isolated mouse brain nuclei and to have a relatively high binding affinity to nuclear RNA. Further purification of the RNA release components in the brain cytosol by DEAF. Sephadex chromatography allowed an increase in specific activity of at least 40‐fold. The thermal lability, effective filtration size, and solubility in phenol suggested that the cytosol factors that facilitiated nuclear RNA release were associated with cellular proteins.