Ribosome biosynthesis in cultured lymphocytes: II. The role of ribosomal RNA production in the initiation and maintenance of lymphocyte growth.

T HE CYTOPLASM OF RESTING LYMPHOCYTES contains relatively few ribosomes when compared to the dense clusters of these organelles in immunoblasts.1 A previous publication2 has suggested that ribosome assembly is restricted in resting lymphocytes and that the augmented ribosome synthesis which accompanies phytohemagglutinin ( PHA)-induced transformation of resting lymphocytes into proliferating immunoblasts depends, at least in part, upon an increased efficiency of ribosome assembly. The present study will attempt to explore the association between ribosome assembly and the regulation of lymphocyte growth. In mammalian cells, cytoplasmic ribosomes may be dissociated into two subunits of unequal size.35 The RNA moieties contained within these subunits are distinctive and have been characterized by their sedimentation rates of 28S and 18S, respectively. Cytoplasmic ribosomal RNA ( rRNA) moieties ( 285 and 185 ) originate in the nucleolus where they are transcribed as a single 455 rRNA precursor molecule which becomes immediately methylated and then is cleaved nonconservatively into a 325 and an 185 moiety. The 18S RNA migrates into the cytoplasm. While still in the nucleolus, the 325 moiety undergoes further cleavage into a 28S moiety. Packaged into separate ribonucleoprotein particles ( RNP’s ), the 28S and the 18S RNA’s combine in the cytoplasm for ultimate ribosome assembly.35 A kinetic analysis of rRNA precursor transcription and of ultimate processing to mature 28S and 18S