The stimulation of RNA synthesis by ribosomes in vitro

Ribosomal precursors and ribonucleic acid synthesis in Escherichia coli

Ribosomes and immature ribonucleoprotein particles were isolated from extracts of log-phase cells grown under various conditions. Quantitative measurements were made to determine the relative amounts of immature particles present in the extracts. The results indicate that the steady-state level of ribosomal precursors accounted for essentially a constant fraction of the total ribonucleic acid (RNA) of the cells. For cells with RNA-protein ratios between 0.43 and 0.65, about 1.6% of the total RNA occurred as immature ribonucleoprotein particles. Further, increased levels of immature particles were shown to be correlated with a reduced rate of RNA synthesis in cells recovering from chloramphenicol inhibition. The reduction was found to vary directly with the duration of pretreatment in chloramphenicol and, consequently, with the level of immature particles present in the cells.

Isolation of two distinct classes of polysomes from a nuclear fraction of rat liver

Isolated rat liver nuclei were washed with Triton-X-100 in the presence of liver cell sap. This treatment liberated a fraction of polysomes which were isolated by differential centrifugation and were designated "outer membrane polysomes." The outer membrane polysomes synthesized protein in vivo. Shortly after injection of orotic acid-(14)C, the RNA of outer membrane polysomes had a higher specific activity than that of cytoplasmic polysomes. It was postulated that outer membrane polysomes may be an intermediate in the transfer of newly synthesized RNA from the nucleus to the cytoplasm. In other experiments, Triton-washed rat liver nuclei were lysed in the presence of deoxycholate and deoxyribonuclease. A ribonucleoprotein fraction was isolated from the lysate by differential centrifugation. This fraction contained "intranuclear ribosomes," which sedimented like partially degraded polysomes in sucrose gradients. This degradation could be partially prevented if intranuclear ribosomes were purified by sedimentation through heavy sucrose. The resulting pellets were termed "intranuclear polysomes" because they contained some undergraded polysomes. Intranuclear polysomes were highly radioactive after a brief pulse with orotic acid-(14)C, but did not appear to synthesize protein rapidly in vivo. Intranuclear polysomes may represent the initial stage of assembly of polyribosomes in the nucleus.

Polyribosome Metabolism in Escherichia coli II. Formation and Lifetime of Messenger RNA Molecules, Ribosomal Subunit Couples and Polyribosomes

During a continuous label with [3H]uracil of fragile Escherichia coli cells in exponential growth, all or very nearly all the hybridizable mRNA of the cells can be observed in polyribosomes during zonal sedimentation in sucrose gradients. In contrast, all the newly formed rRNA appears to be free of polyribosomes until it is part of complete ribosomal subunits. The separation of the two major classes of newly formed RNA permits the direct observation of the growth with time of mRNA molecules, of ribosomal subunits and of polyribosomes. A complete chain of ribosomal RNA forms in one to two minutes; completion of either a 30 s or 50 s ribosome requires a minimum of about five minutes more. The specific activities of free ribosomal subunits and those that are present in polyribosomes increase at identical rates, consistent with very rapid exchange of the two classes. Before any new subunits are finished, the 3H-labeled RNA in polyribosomes is the cellular mRNA, which can therefore be extracted and observed separated from ribosomal RNA. Assuming that their sedimentation rates in sucrose gradients are related to molecular weights as are those of ribosomal rRNA and that no degradation occurs dining extraction, most of the molecules of mRNA are large enough to code for only one or two protein chains of 30,000 molecular weight. The kinetics of entry of all the new mRNA into polyribosomes is in agreement with the view that a polyribosome forms as the corresponding messenger RNA is synthesized, over a period of at least a minute. 3% of the total cellular RNA in these cultures (doubling time 120 minutes) is mRNA, with an average chemical lifetime of 11 to 12·5 minutes.

Protein synthesis by long-lived messenger ribonucleic acid in bacteria

1. Experiments were performed to investigate two hypotheses about the function of long-lived messenger RNA in bacteria. After RNA synthesis had been stopped by the addition of actinomycin, continuing protein synthesis was used as a measure of persistent messenger RNA. 2. The hypothesis that messenger RNA responsible for the synthesis of membrane protein is exceptionally long-lived was tested in experiments with protoplasts of Bacillus megaterium. However, this messenger RNA proved to be of approximately average stability. 3. The hypothesis that long-lived messenger RNA is responsible for the synthesis of constitutive proteins was tested by comparing the synthesis of penicillinase in an inducible and a constitutive strain of Bacillus licheniformis. After the addition of actinomycin, penicillinase synthesis continued for far longer in the constitutive than in the inducible strain. This difference is attributed to a difference in stability of the penicillinase-messenger RNA in the two strains, which does not extend to all messenger RNA indiscriminately. 4. A model is tentatively proposed to account for the altered stability of messenger RNA in the constitutive mutant.