Analysis of the Population of Native 40‐S Ribosomal Subunits in Mouse Plasmacytoma Cells Grown in Suspension Culture

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Abstract
The native ribosomal subunits of the mouse plasmacytoma cell line MPC‐11 were fractionated by CsCl gradient centrifugation into seven classes of particles designated p1, p2, p3, p4, p5, p6 and p7, which possessed densities of approximately 1.38, 1.43, 1.46, 1.49, 1.50, 1.51 and 1.53 g/cm3 respectively. The RNA was analysed by oligo(dT)‐cellulose chromatography and gel electrophoresis.In both well‐fed and starved cells p3 was generally the dominating class of particles. During metabolic shift‐up conditions, however, there was a great increase of p6 particles. Transfer of material containing rRNA and mRNA into this region of the CsCl gradient indicates the accumulation of an initiation complex containing mRNA.During metabolic shift‐down conditions a decrease of p6 particles and an increase of p2 and p3 particles was observed. The p2 fraction was rich in mRNA and may contain considerable quantities of messenger ribonucleoprotein particles. A substantial fraction of newly synthesised mRNA‐containing material in starved cells banded in fact in the p2 region of the CsCl gradients. Circumstantial evidence indicates the existence of a transient 40‐S initiation complex which bands in the p2‐p3 region of the gradients.Double‐labelling experiments demonstrated that radioactivity in 40‐S subunits first appeared in precursor particles with a density of 1.49–1.50 g/cm3 and only later in p3 and p6 particles. p7 particles were detected in starved cells. They were slowly labelled and were presumably derived from old ribosomes following completion of the polysome cycle.