Abstract
Both nuclear RNA newly synthesized in vivo and RNA synthesized by isolated rat liver nuclei in vitro are rapidly degraded in an RNA synthesizing media of low ionic strength. The presence of either a crude preparation of rat liver cytoplasmic ribonuclease inhibitor or 0.64 M (NH 4) 2SO 4 in the incubation media markedly retards this degradation. The RNA synthesized by isolated nuclei in media of low ionic strength in the presence of ribonuclease inhibitor is largely of high molecular weight with sedimentation values of greater than 18 S; whereas the RNA synthesized in media of high ionic strength is of a wider range of molecular weights with s values between 8 and 45 S. This difference may be due to the existence of two nuclear RNA polymerase activities that differ in high and low ionic strength: one polymerase may synthesize ribosomal precursor RNA and show activity at low (and possibly also high) ionic strength, and a second polymerase may transcribe nonribosomal portions of the genome and be active only at high ionic strength.
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