Function of aggregated reticulocyte ribosomes in protein synthesis

Abstract

The ribosome fraction of rabbit reticulocytes contains ribosomal units with a sedimentation coefficient of 80 s and some smaller components. In addition there is a spectrum of at least five more rapidly sedimenting fractions, comprising of the order of 30% or more of the material. The fast fractions are considered to be aggregates of ribosomes consisting of up to six ribosomal units. They disintegrate into the 80 s ribosomal units upon incubation with RNase. The fast fractions, separated from the slow fraction by sucrose gradient centrifugation, are shown to be more active both in whole cell and in cell-free incorporation of radioactive amino acids. Incorporation of [14C]phenylalanine is stimulated by the addition of polyuridylic acid as messenger RNA. The stimulation of activity is maximum for unaggregated ribosomes. Part of the active products of stimulation appear to be aggregates. The results indicate that many of the aggregated ribosomes contain messenger RNA, but are not receptive to added messenger RNA; whereas many of the unaggregated ribosomes do not contain messenger RNA, but are receptive to added messenger RNA molecules. The simplest interpretation is that the linkage of the ribosomes into aggregates is due to the messenger RNA itself. The findings support a theory of protein synthesis whereby several ribosomes can operate simultaneously on one molecule of messenger RNA.