Pre-ribosomal RNA synthesis in isolated rat-liver nucleoli.

Abstract

Isolated rat liver nucleoli represent a simple system to study transcription and processing of ribosomal RNA in vitro. These organelles retain their ability to synthesize ribosomal precursor RNA in vitro as measured by the incorporation of nucleotides into RNA. The incorporation of labelled nucleotides is not affected by α‐amanitin or rifampicin AF‐013. Low concentrations of ammonium sulfate (60 mM) stimulate the nucleolar synthetic activity considerably. The (A + U)/(G + C) ratio of nucleolar RNA synthesized in vitro is 0.5–0.55 and resembles that of ribosomal precursor RNA. Discrete peaks of high‐molecular‐weight RNA, widely distributed as to size, appear in analytical sucrose gradients. Molecules with sedimentation coefficients up to about 80 S have been obtained a surprising result in view of the fact that, in general, only 45‐S precursors are found in vivo. The amount of rapidly sedimenting RNA increases with prolonged incubation. It is uncertain whether or not the presence of ribosomal RNA precursor molecules with sedimentation coefficients greater than 45‐S reflects a normal transcription mechanism in the living cell which remains undetected due to rapid processing. It is also possible that isolated nucleoli may have lost some specific factors which in vivo cause the RNA polymerase to terminate the RNA chains at the end of the 45‐S transcription unit.