Localization of Escherichia coli ribosomal proteins S4 and S14 by electron microscopy of antibody-labeled subunits

Binding sites for antibodies specific for proteins S4 and S14 of the small subunit of E. coli ribosomes have been mapped on the surface of the subunit by electron microscopy. Antibody binding to reconstituted subunits was shown to depend specifically on the presence of E. coli S4 and S14. Anti-S14 IgG was found to bind to a limited region of the ribosome surface. In contrast anti-S4 IgG was found to bind to three separated regions of the ribosome surface, suggesting S4 has an elongated conformation in situ.

Stringent control of ribosomal protein gene expression in Escherichia coli

The regulation of the expression of ribosomal protein genes was examined during partial inhibition of tRNA aminoacylation in isogenic rel(+) and rel(-) strains of E. coli B carrying a temperature-sensitive mutation in valyl-tRNA synthetase (EC 6.1.1.9) gene. Measurements of the differential synthesis rate of ribosomal protein, alpha(r) (ribosomal protein synthesis rate/total protein synthesis rate), indicate that ribosomal protein synthesis is regulated directly or indirectly by availability of charged tRNA, and that the synthesis of ribosomal protein, like the synthesis of rRNA, is subject to the influence of the rel gene control system.

In vitro synthesis of ribosomal proteins directed by Escherichia coli DNA

In vitro synthesis of a number of E. coli 30S ribosomal proteins has been demonstrated in a cell-free system consisting of ribosomes, initiation factors, RNA polymerase, a fraction containing soluble enzymes and factors, and E. coli DNA. DNA-dependent synthesis of the following 30S proteins has been demonstrated: S4, S5, S7, S8, S9, S10, S13, S14, S16, S19, and S20.

Assembly of bacterial ribosomes

I have not mentioned the remarkable progress made mainly by Fellner and his co-workers (86) in the elucidation of the primary structure of rRNA's and by Wittmann and his co-workers (87) in determining the structure of several ribosomal proteins. Such knowledge of primary structures is certainly the basis of complete understanding of the structure of the ribosome. With the current progress in technology, complete elucidation of the primary structure of all the ribosomal components is probably a matter of time. As indicated in this article, a rough approximation of the three-dimensional structure of ribosomes is likely to emerge soon. Although not mentioned in this article, studies of ribosomes from higher organisms are also progressing. We must, therefore, consider what further studies should be conducted and what kinds of questions we would like to solve. Some groups of investigators aim to elucidate the complete three-dimensional structure of ribosomes and to find out how these complex cell organelles function; they hope to determine the conformational changes of many of the component molecules within the ribosome structure in response to external macromolecules and cofactors engaged in protein synthesis. Such knowledge will also be important in enabling us to understand the regulation of translation of genetic messages. Other groups of investigators aim to elucidate the complex series of events which originate in the transcription of the more than 60 genes and culminate in the formation of the specific structure of the organelle. Complete reproduction in vitro of all the assembly events that occur in vivo should not be difficult to achieve in principle. It should then become possible to study in vitro any factor regulating the biogenesis of the organelle. Although we do not know whether such studies would reveal any new fundamental principle that governs the complex circuits of interconnected macromolecular interactions, the achievement of such a complete in vitro system would represent a necessary step in the comprehensive understanding of biogenesis of organelles, and eventually, of the more complex behavior and genesis of cells (89).

Functional correspondence between 30S ribosomal proteins of Escherichia coli and Bacillus stearothermophilus

30S ribosomal proteins from Bacillus stearothermophilus (B. proteins) have been fractionated and characterized with respect to their ability to replace various E. coli 30S proteins (E. proteins) in the E. coli 30S ribosome reconstitution system. The functional counterparts of all the E. proteins, except S1, S6, S9, and S13, have been tested. In all cases, B. proteins can substitute for E. proteins. Several purified B. proteins are chemically different from their functionally homologous E. proteins. Five B. proteins are immunochemically related to E. proteins; this set includes two proteins that could not be tested in the reconstitution system (S9 and S13). Thus most, if not all, of the E. proteins have functionally equivalent counterparts among B. proteins, even though properties of the two ribosomes are different in several respects. These results suggest that the fundamental structural organization of ribosomes may be the same throughout prokaryotic organisms.

Expression of ribosomal protein genes as analyzed by bacteriophage Mu-induced mutations

The organization of ribosomal protein genes and the gene (fus) for a protein chain elongation factor, EF G, in Escherichia coli were studied with a merodiploid strain that has an episome with genetic markers, ery(r), spc(r), str(r), and fus(r), and a chromosome with markers ery(s), spc(s), str(s), and fus(s). The ery locus determines a 50S ribosomal protein and the spc and str loci determine 30S ribosomal proteins. The phenotype of the diploid strain is sensitive to all of the four antibiotics, erythromycin (Ery), spectinomycin (Spc), streptomycin (Str), and fusidic acid (Fus). Analysis of antibiotic-resistant mutants induced by bacteriophage Mu in the diploid strain indicates that these four genes, and probably many other ribosomal protein genes linked to them, are transcribed as a single unit, and the direction of the transcription is in the order of ery, spc, str, and fus.

Activity of ribosomes containing 5S RNA with a chemically modified 3'-terminus

Ribosomes containing 5S RNA in which the 3'-terminus has been modified chemically, either by periodate oxidation or by conversion of the terminal ribose to a morpholine derivative, are fully as active in peptidebond formation and polypeptide synthesis as those ribosomes with unmodified 5S RNA. These results exclude models for protein synthesis in which peptidyl-5S RNA is formed as an intermediate.

Role of 5S RNA in the functions of 50S ribosomal subunits

50S ribosomal subunits from Bacillus stearothermophilus can be reconstituted from their dissociated components, namely a 5S RNA-free protein fraction, a 5S RNA-free 23S ribosomal RNA fraction, and purified 5S RNA. The biological activity of reconstituted particles in polypeptide synthesis is dependent on the presence of 5S RNA. In the absence of 5S RNA, particles are produced that have greatly reduced activity in (a) polypeptide synthesis directed by synthetic, as well as natural, messenger RNA, (b) peptidyl transferase assay, (c) [(3)H]UAA binding dependent on peptide chain termination factor R1, (d) G factor-dependent [(3)H]GTP binding, and (e) codon-directed tRNA binding assayed in the presence of 30S subunits. Thus, 5S RNA is an essential 50S ribosomal component.

Specific inactivation of 16S ribosomal RNA induced by colicin E3 in vivo

Treatment of sensitive Escherichia coli cells with colicin E3 leads to inactivation of 30S ribosomal subunits. In vitro reconstitution of 30S subunits indicates that the E3-induced defect lies solely in the 16S RNA. 16S RNA from E3-treated cells lacks several T(1) RNase oligonucleotides of normal 16S RNA, including the one from the 3'-end of the 16S RNA, as analyzed by the fingerprint technique of Sanger. An RNA fragment about 50 nucleotides long has been isolated from E3-treated cells. This RNA contains the original 3'-terminal oligonucleotide and other oligonucleotides missing in the E3-16S RNA. The results show that colicin E3 treatment causes the cleavage of 16S RNA at a specific position near the 3'-terminus.

Structure and function of bacterial ribosomes. XI. Dependence of 50S ribosomal assembly on simultaneous assembly of 30S subunits

Some spectinomycin-resistant mutants of Escherichia coli are cold-sensitive. They are unable to assemble both 30S and 50S ribosomal subunits at low temperatures. They accumulate two kinds of incomplete particles, related to 30S and 50S subunits respectively. A single mutation, causing an alteration in a 30S ribosomal component, is responsible for these phenotypes. These results show that assembly of 50S subunits in vivo is dependent on simultaneous assembly of 30S subunits. On the other hand, the assembly of 30S subunits appears to be independent of 50S assembly.