Native ribosomal subunits labeled in their ribosomal RNA have been obtained from HeLa cells. These ribosomal subunits enter into polyribosomes and are converted to ribosomes when added to a rabbit reticulocyte cell-free system active in protein synthesis. Incubation of the ribosomal subunits in the cell-free system from which polyribosomes have been removed fails to promote conversion to ribosomes. Control experiments have shown that this conversion is dependent on protein synthesis. The rate of entry into polyribosomes of two types of small ribosomal subunits and of the large ribosomal subunits has been estimated. This rate is slower than the rate expected if all initiations are carried out by free ribosomal subunits. This observation has been explained by postulating that ribosomal subunits are continually produced by dissociation of ribosomes involved in translation, upon termination of the synthesis of polypeptide chains, and that these ribosomal subunits are preferentially utilized for initiation. Experiments in which the rate of entry of ribosomal subunits into polyribosomes has been measured at different dilutions suggest that topographical factors may be responsible for this preferential utilization.
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