Monkey fibroblasts maintained in culture regulate their levels of intracellular protein throughout the growth cycle by means of variations in the rate of protein biosynthesis. Cytoplasmic mRNA in stationary phase cells was compared to that in exponential phase cells. In stationary phase cells 56% of the cytoplasmic polyadenylated RNA was found in the 40--90S postpolysomal region of sucrose sedimentation gradients, while only 23% was found in this region in exponential phase cells. Analysis of electron micrographs of sectioned exponential and stationary phase cells revealed that this shift in polyadenylated RNA location is accompanied by a loss of polysome-like aggregates of ribosomes. Most if not all of this species of postpolysomal polyadenylated RNA is not being translated by single ribosomes since no detectable amounts of nascent peptide were present in this region. This nonpolysomal polyadenylated RNA is comparable in size to polysomal polyadenylated RNA. The length of the 3'-poly(A) tract was also comparable for these two species. The extent of capping of poly(A)-containing molecules was also comparable for these two species. The template activity of nonpolysomal RNA in a wheat germ extract was comparable to that of polysomal RNA. The peptides produced by these two preparations were of a similar large size. Furthermore, most of the nonpolysomal polyadenylated RNA of stationary phase cells was driven into polysomes in the presence of a low dose of cycloheximide. Therefore, we conclude that the untranslated mRNA that accumulates in stationary phase cells is structurally intact, is fully capable of being translated, and is not being translated due to the operation of a translational initiation block.

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