THE CELLULAR SITES OF SYNTHESIS OF RIBOSOMAL AND 4S RNA

Abstract

Parallel auto-radiographic and sedimentation experiments were performed on tissue cultures of L strain fibroblasts. A comparison of data from normal and actinomycin-inhibited systems indicates the following The rapidly labeled nucleolar RNA (ribonucleic acid) consists of fast-sedimenting, heterogeneous components, some of which sediment appreciably faster than the heaviest stable ribosomal component (i.e.,> 32S); this heterogeneous, nucleolar RNA is an obligatory precursor of the ribosomal components of cytoplasmic RNA; a heterogeneous, labile component is also synthesized in the extranucleolar (chromatin) part of the nucleus; 4S RNA is first synthesized in the chromatin region of the nucleus and later appears in the cytoplasm.