Ribosome-deficient Mutants of Zea mays
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Ribosomal protein
Wild type
Ribosomal protein
Eukaryotic Ribosome
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Phytohaemagglutinin
Polysome
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Ribosomal protein
Eukaryotic Ribosome
Differential centrifugation
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The inhibition of cell division and the ultimate loss of viability after removal of streptomycin from growing cultures of streptomycin-dependent bacteria are not the result of “unbalanced growth” or of the breakdown of ribosomes. The streptomycin-dependent strain of Escherichia coli K-12 studied continued to synthesize ribonucleic acid (RNA) and protein during streptomycin starvation. There was no evidence of a gross imbalance in the ratio of RNA to protein synthesized or of selective degradation of either protein or RNA. Using the sedimentation of subunits in sucrose as the criterion, normal ribosomes were synthesized even after 18 h of streptomycin deprivation, although the rates of appearance of mature 30 S and 50 S subunits decreased with time of deprivation. Once formed, these ribosomes appeared stable, as did those synthesized before the onset of starvation. Ribosomes isolated from starved dependent cells were as “functional” as ribosomes from cells grown with streptomycin in their capacity to bind aminoacyl-transfer RNA in response to polyuridylic acid or natural messenger RNA to interconvert between active and inactive transfer RNA binding states, and to synthesize proteins in cell-free systems. The effects are consistent with an impaired rate of synthesis of ribosomal components or assembly of ribosomes resulting in a continually diminishing rate of protein synthesis. The effect on cell division may be the result of a decreased rate of protein synthesis in general and the requirement for a specific protein(s) in particular.
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The hot tritium bombardment technique [(1976) Dokl. Akad. Nauk SSSR 228, 1237-1238] was used for studying the surface localization of ribosomal proteins on Escherichia coli ribosomes. The degree of tritium labeling of proteins was considered as a measure of their exposure (surface localization). Proteins S1, S4, S7, S9 and/or S11, S12 and/or L20, S13, S18, S20, S21, L5, L6, L7/L12, L10, L11, L16, L17, L24, L26 and L27 were shown to be the most exposed on the ribosome surface. The sets of exposed ribosomal proteins on the surface of 70 S ribosomes, on the one hand, and the surfaces of 50 S and 30 S ribosomal subunits in the dissociated state, on the other, were compared. It was found that the dissociation of ribosomes into subunits did not result in exposure of additional ribosomal proteins. The conclusion was drawn that proteins are absent from the contacting surfaces of the ribosomal subunits.
Ribosomal protein
Eukaryotic Ribosome
30S
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50S
Ribosomal protein
Eukaryotic Ribosome
30S
5S ribosomal RNA
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Ribosomal protein
Eukaryotic Ribosome
5S ribosomal RNA
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This review deals with results of affinity labelling experiments on proteins of the peptidyl-transferase centre (PTC) of animal ribosomes. From the studies it is concluded that ribosomal proteins L 5, L 21/23, L 32/33, and L 36 are the main candidates for the ribosomal P site and proteins L 6, L 18a, and L 28/29 for the ribosomal A site. Proteins L 10 and L 13/15 seem to be located in a more central region of the PTC. These findings are correlated with results obtained so far by using other techniques. Finally, the data are discussed, and a tentative model on the relative spatial arrangement of certain ribosomal proteins at the PTC is proposed.
Ribosomal protein
Peptidyl transferase
50S
Eukaryotic Ribosome
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An unexpected degree of heterogeneity for the ribosomal proteins of E. coli was initially demonstrated by Waller (1964). Definitive verification of the multiplicity of ribosomal proteins has been obtained in several laboratories through the purification and characterization of the individual proteins (Kaltschmidt et al., 1967; Fogel and Sypherd, 1968; Moore et al., 1968; Hardy et al., 1969; Craven et al., 1969). There are approximately 50 different proteins in the ribosomes of E. coli. Although it is possible to demonstrate the functional contribution of individual ribosomal proteins (Traub et al., 1967), the presence of so many different proteins in the ribosome is quite inexplicable. This is a good index of how little is understood about the mechanism of protein synthesis.
Ribosomal protein
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Ribosomal protein
Eukaryotic Ribosome
5S ribosomal RNA
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