Model for the three-dimensional folding of 16 S ribosomal RNA

Endogenous rRNA Sequence Variation Can Regulate Stress Response Gene Expression and Phenotype

Prevailing dogma holds that ribosomes are uniform in composition and function. Here, we show that nutrient limitation-induced stress in E. coli changes the relative expression of rDNA operons to alter the rRNA composition within the actively translating ribosome pool. The most upregulated operon encodes the unique 16S rRNA, rrsH, distinguished by conserved sequence variation within the small ribosomal subunit. rrsH-bearing ribosomes affect the expression of functionally coherent gene sets and alter the levels of the RpoS sigma factor, the master regulator of the general stress response. These impacts are associated with phenotypic changes in antibiotic sensitivity, biofilm formation, and cell motility and are regulated by stress response proteins, RelA and RelE, as well as the metabolic enzyme and virulence-associated protein, AdhE. These findings establish that endogenously encoded, naturally occurring rRNA sequence variation can modulate ribosome function, central aspects of gene expression regulation, and cellular physiology.

Most organisms encode multiple, distinct copies of rRNA genes, rendering the composition of the ribosome pool intrinsically heterogeneous. Here, Kurylo et al. show that nutrient limitation in E. coli upregulates the expression of ribosomes bearing conserved sequence variation in 16S rRNA that can regulate gene expression and phenotype.

A metastable rRNA junction essential for bacterial 30S biogenesis

Tertiary sequence motifs encode interactions between RNA helices that create the three-dimensional structures of ribosomal subunits. A Right Angle motif at the junction between 16S helices 5 and 6 (J5/6) is universally conserved amongst small subunit rRNAs and forms a stable right angle in minimal RNAs. J5/6 does not form a right angle in the mature ribosome, suggesting that this motif encodes a metastable structure needed for ribosome biogenesis. In this study, J5/6 mutations block 30S ribosome assembly and 16S maturation in Escherichia coli. Folding assays and in-cell X-ray footprinting showed that J5/6 mutations favor an assembly intermediate of the 16S 5' domain and prevent formation of the central pseudoknot. Quantitative mass spectrometry revealed that mutant pre-30S ribosomes lack protein uS12 and are depleted in proteins uS5 and uS2. Together, these results show that impaired folding of the J5/6 right angle prevents the establishment of inter-domain interactions, resulting in global collapse of the 30S structure observed in electron micrographs of mutant pre-30S ribosomes. We propose that the J5/6 motif is part of a spine of RNA helices that switch conformation at distinct stages of assembly, linking peripheral domains with the 30S active site to ensure the integrity of 30S biogenesis.

Design, synthesis and characterization of novel chromone based-copper(ii) antitumor agents with N,N-donor ligands: comparative DNA/RNA binding profile and cytotoxicity

A series of new chromone based-Cu(ii) complexes 1–3 derived from bioactive pharmacophore, 3-formylchromone and N,N-donor ligands viz., 1,10-phenanthroline, 2,2′-bipyridine and 1R,2R-DACH were synthesized as potential antitumor agents and thoroughly characterized by UV-vis, FT-IR, EPR, ESI-MS and elemental analysis. Single X-crystal diffraction studies of complex 2 revealed triclinic P1̄ space group with square pyramidal geometry around the Cu(ii) center. Comparative in vitro binding studies with ct-DNA and tRNA were carried out using absorption and emission titration experiments which revealed intercalative mode of binding with higher binding propensity of complexes 1–3 towards tRNA as compared to ct-DNA. Additionally, complex 1 exhibited high binding affinity among all the three complexes due to the involvement of phen co-ligands via π-stacking interactions in between nucleic acid base pairs. Furthermore, Hirshfeld surface analysis was carried out for complex 2 to investigate various intra and intermolecular non-covalent interactions (H-bonding, C–H⋯π etc.) accountable for stabilization of crystal lattice. The cleavage activity of complex 1 was performed by gel electrophoretic assay with pBR322 DNA and tRNA which revealed efficient DNA/tRNA cleaving ability of complex, suggesting tRNA cleavage both concentration and time dependent. Furthermore, in vitro cytotoxic activity of complexes 1–3 on a selected panel of human cancer cell lines was performed which revealed that all three complexes exhibited remarkably good cytotoxic activity with GI₅₀ value < 10 μg mL⁻¹ (<20 μM).

New chromone-based Cu(ii) tRNA targeted complexes 1–3 as potential anticancer agents have been synthesized and thoroughly characterized.

Evolution of protein-coupled RNA dynamics during hierarchical assembly of ribosomal complexes

Assembly of 30S ribosomes involves the hierarchical addition of ribosomal proteins that progressively stabilize the folded 16S rRNA. Here, we use three-color single molecule FRET to show how combinations of ribosomal proteins uS4, uS17 and bS20 in the 16S 5′ domain enable the recruitment of protein bS16, the next protein to join the complex. Analysis of real-time bS16 binding events shows that bS16 binds both native and non-native forms of the rRNA. The native rRNA conformation is increasingly favored after bS16 binds, explaining how bS16 drives later steps of 30S assembly. Chemical footprinting and molecular dynamics simulations show that each ribosomal protein switches the 16S conformation and dampens fluctuations at the interface between rRNA subdomains where bS16 binds. The results suggest that specific protein-induced changes in the rRNA dynamics underlie the hierarchy of 30S assembly and simplify the search for the native ribosome structure.

Ribosomes assemble through the hierarchical addition of proteins to a ribosomal RNA scaffold. Here the authors use three-color single-molecule FRET to show how the dynamics of the rRNA dictate the order in which multiple proteins assemble on the 5′ domain of the E. coli 16S rRNA.

A single mutation in the 15S rRNA gene confers non sense suppressor activity and interacts with mRF1 the release factor in yeast mitochondria

We have determined the nucleotide sequence of the mim3-1 mitochondrial ribosomal suppressor, acting on ochre mitochondrial mutations and one frameshift mutation in Saccharomyces cerevisiae. The 15s rRNA suppressor gene contains a G633 to C transversion. Yeast mitochondrial G633 corresponds to G517 of the E.coli 15S rRNA, which is occupied by an invariant G in all known small rRNA sequences. Interestingly, this mutation has occurred at the same position as the known MSU1 mitochondrial suppressor which changes G633 to A. The suppressor mutation lies in a highly conserved region of the rRNA, known in E.coli as the 530-loop, interacting with the S4, S5 and S12 ribosomal proteins. We also show an interesting interaction between the mitochondrial mim3-1 and the nuclear nam3-1 suppressors, both of which have the same action spectrum on mitochondrial mutations: nam3-1 abolishes the suppressor effect when present with mim3-1 in the same haploid cell. We discuss these results in the light of the nature of Nam3, identified by 1 as the yeast mitochondrial translation release factor. A hypothetical mechanism of suppression by "ribosome shifting" is also discussed in view of the nature of mutations suppressed and not suppressed.

30S Subunit-dependent activation of the Sorangium cellulosum So ce56 aminoglycoside resistance-conferring 16S rRNA methyltransferase Kmr

Methylation of bacterial 16S rRNA within the ribosomal decoding center confers exceptionally high resistance to aminoglycoside antibiotics. This resistance mechanism is exploited by aminoglycoside producers for self-protection while functionally equivalent methyltransferases have been acquired by human and animal pathogenic bacteria. Here, we report structural and functional analyses of the Sorangium cellulosum So ce56 aminoglycoside resistance-conferring methyltransferase Kmr. Our results demonstrate that Kmr is a 16S rRNA methyltransferase acting at residue A1408 to confer a canonical aminoglycoside resistance spectrum in Escherichia coli. Kmr possesses a class I methyltransferase core fold but with dramatic differences in the regions which augment this structure to confer substrate specificity in functionally related enzymes. Most strikingly, the region linking core β-strands 6 and 7, which forms part of the S-adenosyl-l-methionine (SAM) binding pocket and contributes to base flipping by the m(1)A1408 methyltransferase NpmA, is disordered in Kmr, correlating with an exceptionally weak affinity for SAM. Kmr is unexpectedly insensitive to substitutions of residues critical for activity of other 16S rRNA (A1408) methyltransferases and also to the effects of by-product inhibition by S-adenosylhomocysteine (SAH). Collectively, our results indicate that adoption of a catalytically competent Kmr conformation and binding of the obligatory cosubstrate SAM must be induced by interaction with the 30S subunit substrate.

Uptake and localisation of small-molecule fluorescent probes in living cells: a critical appraisal of QSAR models and a case study concerning probes for DNA and RNA

Small-molecule fluorochromes are used in biology and medicine to generate informative microscopic and macroscopic images, permitting identification of cell structures, measurement of physiological/physicochemical properties, assessment of biological functions and assay of chemical components. Modes of uptake and precise intracellular localisation of a probe are typically significant factors in its successful application. These processes and localisations can be predicted using quantitative structure activity relations (QSAR) models, which correlate aspects of the physicochemical properties of the probes (expressed numerically) with the uptake/localisation. Pay-offs of such modelling include better understanding and trouble-shooting of current and novel probes, and easier design of future probes ("guided synthesis"). Uptake models discussed consider adsorptive (to lipid or protein domains), phagocytic and pinocytotic endocytosis, as well as passive diffusion. Localisation models discussed include those for cytosol, endoplasmic reticulum, Golgi apparatus, lipid droplets, lysosomes, mitochondria, nucleus and plasma membrane. A case example illustrates how such QSAR modelling of probe interactions can clarify localisation and mode of binding of probes to intracellular nucleic acids of living cells, including not only eukaryotic chromatin DNA and ribosomal RNA, but also prokaryote chromosomes.

Differential assembly of 16S rRNA domains during 30S subunit formation

Rapid and accurate assembly of the ribosomal subunits, which are responsible for protein synthesis, is required to sustain cell growth. Our best understanding of the interaction of 30S ribosomal subunit components (16S ribosomal RNA [rRNA] and 20 ribosomal proteins [r-proteins]) comes from in vitro work using Escherichia coli ribosomal components. However, detailed information regarding the essential elements involved in the assembly of 30S subunits still remains elusive. Here, we defined a set of rRNA nucleotides that are critical for the assembly of the small ribosomal subunit in E. coli. Using an RNA modification interference approach, we identified 54 nucleotides in 16S rRNA whose modification prevents the formation of a functional small ribosomal subunit. The majority of these nucleotides are located in the head and interdomain junction of the 30S subunit, suggesting that these regions are critical for small subunit assembly. In vivo analysis of specific identified sites, using engineered mutations in 16S rRNA, revealed defective protein synthesis capability, aberrant polysome profiles, and abnormal 16S rRNA processing, indicating the importance of these residues in vivo. These studies reveal that specific segments of 16S rRNA are more critical for small subunit assembly than others, and suggest a hierarchy of importance.

The ribosome at atomic resolution

The 2009 Nobel Prize in Chemistry has been awarded to Venki Ramakrishnan, Tom Steitz, and Ada Yonath for crystallographic studies of the ribosome. The atomic resolution structures of the ribosomal subunits provide an extraordinary context for understanding one of the most fundamental aspects of cellular function: protein synthesis.

The genesis of ribosome structure: how a protein generates RNA structure in real time

Ribosomal subunit assembly is initiated by the binding of several primary binding proteins. Results from chemical modification studies show that 16S ribosomal RNA undergoes striking structural rearrangements when protein S17 is bound. For the first time, we are able to distinguish and order these structural rearrangements by using time-dependent chemical probing. Initially, protein S17 binds to a portion of helix 11, inducing a kink-turn in that helix that bends helix 7 toward the S17-helix 11 complex in a hairpin-like manner, allowing helix 7 to bind to protein S17. This structural change is rapidly stabilized by interactions at the distal and proximal ends of both RNA helices. Identifying the dynamic nature of interactions between RNA and proteins is not only essential in unraveling ribosome assembly, but also has more general application to all protein-RNA interactions.

Thermodynamic characterization of naturally occurring RNA single mismatches with G-U nearest neighbors

Due to their prevalence and roles in biological systems, single mismatches adjacent to G-U pairs are important RNA structural elements. Since there are only limited experimental values for the stability of single mismatches adjacent to G-U pairs, current algorithms using free energy minimization to predict RNA secondary structure from sequence assign predicted thermodynamic values to these types of single mismatches. Here, thermodynamic data are reported for frequently occurring single mismatches adjacent to at least one G-U pair. This experimental data can be used in place of predicted thermodynamic values in algorithms that predict secondary structure from sequence using free energy minimization. When predicting the thermodynamic contributions of previously unmeasured single mismatches, most algorithms apply the same thermodynamic penalty for an A-U pair adjacent to a single mismatch and a G-U pair adjacent to a single mismatch. A recent study, however, suggests that the penalty for a G-U pair adjacent to a tandem mismatch should be 1.2 +/- 0.1 kcal/mol, and the penalty for an A-U pair adjacent to a tandem mismatch should be 0.5 +/- 0.2 kcal/mol [Christiansen, M. E. and Znosko, B. M. (2008) Biochemistry 47, 4329-4336]. Therefore, the data reported here are combined with the existing thermodynamic dataset of single mismatches, and nearest neighbor parameters are derived for an A-U pair adjacent to a single mismatch (1.1 +/- 0.1 kcal/mol) and a G-U pair adjacent to a single mismatch (1.4 +/- 0.1 kcal/mol).

The Contribution of the Zinc-Finger Motif to the Function of Thermus thermophilus Ribosomal Protein S14

In the crystal structure of the 30S ribosomal subunit from Thermus thermophilus, cysteine 24 of ribosomal protein S14 (TthS14) occupies the first position in a CXXC-X12-CXXC motif that coordinates a zinc ion. The structural and functional importance of cysteine 24, which is widely conserved from bacteria to humans, was studied by its replacement with serine and by incorporating the resulting mutant into Escherichia coli ribosomes. The capability of such modified ribosomes in binding tRNA at the P and A-sites was equal to that obtained with ribosomes incorporating wild-type TthS14. In fact, both chimeric ribosomal species exhibited 20% lower tRNA affinity compared with native E. coli ribosomes. In addition, replacement of the native E. coli S14 by wild-type, and particularly by mutant TthS14, resulted in reduced capability of the 30S subunit for association with 50S subunits. Nevertheless, ribosomes from transformed cells sedimented normally and had a full complement of proteins. Unexpectedly, the peptidyl transferase activity in the chimeric ribosomes bearing mutant TthS14 was much lower than that measured in ribosomes incorporating wild-type TthS14. The catalytic center of the ribosome is located within the 50S subunit and, therefore, it is unlikely to be directly affected by changes in the structure of S14. More probably, the perturbing effects of S14 mutation on the catalytic center seem to be propagated by adjacent intersubunit bridges or the P-site tRNA molecule, resulting in weak donor-substrate reactivity. This hypothesis was verified by molecular dynamics simulation analysis.

A novel single amino acid change in small subunit ribosomal protein S5 has profound effects on translational fidelity

S5 is a small subunit ribosomal protein (r-protein) linked to the functional center of the 30S ribosomal subunit. In this study we have identified a unique amino acid mutation in Escherichia coli S5 that produces spectinomycin-resistance and cold sensitivity. This mutation significantly alters cell growth, folding of 16S ribosomal RNA, and translational fidelity. While translation initiation is not affected, both +1 and -1 frameshifting and nonsense suppression are greatly enhanced in the mutant strain. Interestingly, this S5 ribosome ambiguity-like mutation is spatially remote from previously identified S5 ribosome ambiguity (ram) mutations. This suggests that the mechanism responsible for ram phenotypes in the novel mutant strain is possibly distinct from those proposed for other known S5 (and S4) ram mutants. This study highlights the importance of S5 in ribosome function and cell physiology, and suggests that translational fidelity can be regulated in multiple ways.

Isolation and biodiversity of hitherto undescribed soil bacteria related to Bacillus niacini

The hitherto largely not described phylogenetic neighborhood of Bacillus niacini has been explored by a comprehensive cultivation experiment and genomic variety studies. Previous culture-independent studies demonstrated that approximately 15% of all Bacillus 16S rDNA directly extracted from soils worldwide was affiliated to B. niacini. Seven different media were inoculated with soil suspensions in serial dilutions and incubated at different temperatures. Then, bacterial colonies were picked and analyzed by sequencing. A mineral medium with acetate as carbon source yielded a B. niacini rate of >3% of all picked colonies. Other media were less efficient but also successful. Applying this culturing approach, we succeeded in obtaining 64 isolates from different Dutch soils. The isolates turned out to be diverse, although closely related to B. niacini as revealed by 16S rDNA sequencing. Close matches with environmental clones were also found, thus demonstrating much more diversity beyond previously known 16S rDNA sequences. The rep-PCR fingerprinting method revealed a high genomic variety, redundancy could not be observed among our isolates. Hence, the hitherto neglected B. niacini lineage, apparently among the most abundant soil Bacillus, was accessible to our cultivation approach.

A structural model for the assembly of the 30S subunit of the ribosome

The order in which proteins bind to 16S rRNA, the assembly map, was determined by Nomura and co-workers in the early 1970s. The assembly map shows the dependencies of binding of successive proteins but fails to address the relationship of these dependencies to the three-dimensional folding of the ribosome. Here, using molecular mechanics techniques, we rationalize the order of protein binding in terms of ribosomal folding. We determined the specific contacts between the ribosomal proteins and 16S rRNA from a crystal structure of the 30S subunit (1FJG). We then used these contacts as restraints in a rigid body Monte-Carlo simulation with reduced-representation models of the RNA and proteins. Proteins were added sequentially to the RNA in the order that they appear in the assembly map. Our results show that proteins nucleate the folding of the head, platform, and body domains, but they do not strongly restrict the orientations of the domains relative to one another. We also examined the contributions of individual proteins to the formation of binding sites for sequential proteins in the assembly process. Binding sites for the primary binding proteins are generally more ordered in the naked RNA than those for other proteins. Furthermore, we examined one pathway in the assembly map and found that the addition of early binding proteins helps to organize the RNA around the binding sites of proteins that bind later. It appears that the order of assembly depends on the degree of pre-organization of each protein's binding site at a given stage of assembly, and the impact that the binding of each protein has on the organization of the remaining unoccupied binding sites.

Assembly of the 30S ribosomal subunit

Ribosomes are large macromolecular complexes responsible for cellular protein synthesis. The smallest known cytoplasmic ribosome is found in prokaryotic cells; these ribosomes are about 2.5 MDa and contain more than 4000 nucleotides of RNA and greater than 50 proteins. These components are distributed into two asymmetric subunits. Recent advances in structural studies of ribosomes and ribosomal subunits have revealed intimate details of the interactions within fully assembled particles. In contrast, many details of how these massive ribonucleoprotein complexes assemble remain elusive. The goal of this review is to discuss some crucial aspects of 30S ribosomal subunit assembly.

Evolution of a protein-rich mitochondrial ribosome: implications for human genetic disease

Mitochondrial ribosomes comprise the most diverse group of ribosomes known. The mammalian mitochondrial ribosomes (55S) differ unexpectedly from bacterial (70S) and cytoplasmic ribosomes (80S), as well as other kinds of mitochondrial ribosomes. The bovine mitochondrial ribosome has been developed as a model system for the study of human mitochondrial ribosomes to address several questions related to the structure, function, biosynthesis and evolution of these interesting ribosomes. Bovine mitochondrial ribosomal proteins (MRPs) from each subunit have been identified and characterized with respect to individuality and electrophoretic properties, amino acid sequence, topographic disposition, RNA binding properties, evolutionary relationships and reaction with affinity probes of ribosomal functional domains. Several distinctive properties of these ribosomes are being elucidated, including their antibiotic susceptibility and composition. Mammalian mitochondrial ribosomes lack several of the major RNA stem structures of bacterial ribosomes but they contain a correspondingly higher protein content (as many as 80 proteins), suggesting a model where proteins have replaced RNA structural elements during the evolution of these ribosomes. Despite their lower RNA content they are physically larger than bacterial ribosomes, because of the 'extra' proteins they contain. The extra proteins in mitochondrial ribosomes are 'new' in the sense that they are not homologous to proteins in bacterial or cytoplasmic ribosomes. Some of the new proteins appear to be bifunctional. All of the mammalian MRPs are encoded in nuclear genes (a separate set from those encoding cytoplasmic ribosomal proteins) which are evolving more rapidly than those encoding cytoplasmic ribosomal proteins. The MRPs are imported into mitochondria where they assemble coordinately with mitochondrially transcribed rRNAs into ribosomes that are responsible for translating the 13 mRNAs for essential proteins of the oxidative phosphorylation system. Interest is growing in the structure, organization, chromosomal location and expression of genes for human MRPs. Proteins which are essential for mitoribosome function are candidates for involvement in human genetic disease.

Construction and analysis of base-paired regions of the 16S rRNA in the 30S ribosomal subunit determined by constraint satisfaction molecular modelling

Structure models for each of the secondary structure regions from the Escherichia coli 16S rRNA (58 separate elements) were constructed using a constraint satisfaction modelling program to determine which helices deviated from classic A-form geometry. Constraints for each rRNA element included the comparative secondary structure, H-bonding conformations predicted from patterns of base-pair covariation, tertiary interactions predicted from covariation analysis, chemical probing data, rRNA-rRNA crosslinking information, and coordinates from solved structures. Models for each element were built using the MC-SYM modelling algorithm and subsequently were subjected to energy minimization to correct unfavorable geometry. Approximately two-thirds of the structures that result from the input data are very similar to A-form geometry. In the remaining instances, the presence of internal loops and bulges, some sequences (and sequence covariants) and accessory information require deviation from A-form geometry. The structures of regions containing more complex base-pairing arrangements including the central pseudoknot, the 530 region, and the pseudoknot involving base-pairing between G570-U571/A865-C866 and G861-C862/G867-C868 were predicted by this approach. These molecular models provide insight into the connection between patterns of H-bonding, the presence of unpaired nucleotides, and the overall geometry of each element.

Crystal structure of the 30 S ribosomal subunit from Thermus thermophilus: structure of the proteins and their interactions with 16 S RNA

We present a detailed analysis of the protein structures in the 30 S ribosomal subunit from Thermus thermophilus, and their interactions with 16 S RNA based on a crystal structure at 3.05 A resolution. With 20 different polypeptide chains, the 30 S subunit adds significantly to our data base of RNA structure and protein-RNA interactions. In addition to globular domains, many of the proteins have long, extended regions, either in the termini or in internal loops, which make extensive contact to the RNA component and are involved in stabilizing RNA tertiary structure. Many ribosomal proteins share similar alpha+beta sandwich folds, but we show that the topology of this domain varies considerably, as do the ways in which the proteins interact with RNA. Analysis of the protein-RNA interactions in the context of ribosomal assembly shows that the primary binders are globular proteins that bind at RNA multihelix junctions, whereas proteins with long extensions assemble later. We attempt to correlate the structure with a large body of biochemical and genetic data on the 30 S subunit.

Mapping proteins of the 50S subunit from Escherichia coli ribosomes

Mapping of protein positions in the ribosomal subunits was first achieved for the 30S subunit by means of neutron scattering about 15 years ago. Since the 50S subunit is almost twice as large as the 30S subunit and consists of more proteins, it was difficult to apply classical contrast variation techniques for the localisation of the proteins. Polarisation dependent neutron scattering (spin-contrast variation) helped to overcome this restriction. Here a map of 14 proteins within the 50S subunit from Escherichia coli ribosomes is presented including the proteins L17 and L20 that are not present in archeal ribosomes. The results are compared with the recent crystallographic map of the 50S subunit from the archea Haloarcula marismortui.

Structure and function of the conserved 690 hairpin in Escherichia coli 16 S ribosomal RNA. II. NMR solution structure

The solution structure of the conserved 690 hairpin from Escherichia coli 16 S rRNA was determined by NMR spectroscopy. The 690 loop is located at the surface of the 30 S subunit in the platform region and has been implicated in interactions with P-site bound tRNA, E-site mRNA, S11 binding, IF3 binding, and in RNA-RNA interactions with the 790 loop of 16 S rRNA and domain IV of 23 S rRNA. The structure reveals a novel sheared type G690.U697 base-pair with a single hydrogen bond from the G690 amino to U697-04. G691 and A696 also form a sheared pair and U692 forms a U-turn with an H-bond to the A695 non-bridging phosphate oxygen. The sheared pairs and U-turn result in the continuous single-stranded stacking of five residues from 6693 to U697 with their Watson-Crick functional groups exposed in the minor groove. The overall fold of the 690 hairpin is similar to the anticodon loop of tRNA. The structure provides an explanation for chemical protection patterns in the loop upon interaction with tRNA, the 50 S subunit, and S11. In vivo genetic studies demonstrate the functional importance of the motifs observed in the solution structure of the 690 hairpin.

An adenosyl-cobalamin (coenzyme-B12)-repressed translational enhancer in the cob mRNA of Salmonella typhimurium

Expression of the cobalamin (Cbl) biosynthetic cob operon in Salmonella typhimurium is repressed by the end-product. This regulation is conferred mainly at the translational level and involves a cobalamin-induced folding of an RNA hairpin that sequesters the ribosomal binding site (RBS) of the cob mRNA and prevents translation initiation. A combined structural and mutational analysis shows that a cis-acting translational enhancer (TE) element, located 83 nucleotides upstream of the Shine-Dalgarno sequence in the 5'-untranslated region (5'-UTR) of the cob mRNA, is required to unfold the inhibitory RBS hairpin in the absence of cobalamin. The TE element, which consists of 5 nucleotides, is proposed to confer its enhancer function in the absence of cobalamin by interacting with nucleotides in the stem of the RBS hairpin. This interaction destabilizes the RNA hairpin and allows ribosome binding. In the presence of cobalamin, the enhancer function is inhibited. As a result, the RBS hairpin forms and prevents translation initiation. Several additional RNA hairpins in the 5'-UTR were also identified and are suggested to be important for repression. The above data suggest that normal cobalamin repression of the cob operon requires that the 5'-UTR has a defined secondary and tertiary structure.

The bacterial ribosome at atomic resolution

X-ray crystallographic structures have just been published for the 30S ribosomal subunit of Thermus thermophilus at 3.4 A resolution and for the 50S subunit of Haloarcula marismortui at 2.4 A. These eagerly awaited structures will provide an enormous boost to research into the mechanisms involved in protein biosynthesis.

Cirripede phylogeny using a novel approach: molecular morphometrics

We present a new method using nucleic acid secondary structure to assess phylogenetic relationships among species. In this method, which we term "molecular morphometrics," the measurable structural parameters of the molecules (geometrical features, bond energies, base composition, etc.) are used as specific characters to construct a phylogenetic tree. This method relies both on traditional morphological comparison and on molecular sequence comparison. Applied to the phylogenetic analysis of Cirripedia, molecular morphometrics supports the most recent morphological analyses arguing for the monophyly of Cirripedia sensu stricto (Thoracica + Rhizocephala + Acrothoracica). As a proof, a classical multiple alignment was also performed, either using or not using the structural information to realign the sequence segments considered in the molecular morphometrics analysis. These methods yielded the same tree topology as the direct use of structural characters as a phylogenetic signal. By taking into account the secondary structure of nucleic acids, the new method allows investigators to use the regions in which multiple alignments are barely reliable because of a large number of insertions and deletions. It thus appears to be complementary to classical primary sequence analysis in phylogenetic studies.

Structure of the 30S ribosomal subunit

Genetic information encoded in messenger RNA is translated into protein by the ribosome, which is a large nucleoprotein complex comprising two subunits, denoted 30S and 50S in bacteria. Here we report the crystal structure of the 30S subunit from Thermus thermophilus, refined to 3 A resolution. The final atomic model rationalizes over four decades of biochemical data on the ribosome, and provides a wealth of information about RNA and protein structure, protein-RNA interactions and ribosome assembly. It is also a structural basis for analysis of the functions of the 30S subunit, such as decoding, and for understanding the action of antibiotics. The structure will facilitate the interpretation in molecular terms of lower resolution structural data on several functional states of the ribosome from electron microscopy and crystallography.

Initiation factor 3-induced structural changes in the 30 S ribosomal subunit and in complexes containing tRNA(f)(Met) and mRNA

Initiation factor 3 (IF3) acts to switch the decoding preference of the small ribosomal subunit from elongator to initiator tRNA. The effects of IF3 on the 30 S ribosomal subunit and on the 30 S.mRNA. tRNA(f)(Met) complex were determined by UV-induced RNA crosslinking. Three intramolecular crosslinks in the 16 S rRNA (of the 14 that were monitored by gel electrophoresis) are affected by IF3. These are the crosslinks between C1402 and C1501 within the decoding region, between C967xC1400 joining the end loop of a helix of 16 S rRNA domain III and the decoding region, and between U793 and G1517 joining the 790 end loop of 16 S rRNA domain II and the end loop of the terminal helix. These changes occur even in the 30 S.IF3 complex, indicating they are not mediated through tRNA(f)(Met) or mRNA. UV-induced crosslinks occur between 16 S rRNA position C1400 and tRNA(f)(Met) position U34, in tRNA(f)(Met) the nucleotide adjacent to the 5' anticodon nucleotide, and between 16 S rRNA position C1397 and the mRNA at positions +9 and +10 (where A of the initiator AUG codon is +1). The presence of IF3 reduces both of these crosslinks by twofold and fourfold, respectively. The binding site for IF3 involves the 790 region, some other parts of the 16 S rRNA domain II and the terminal stem/loop region. These are located in the front bottom part of the platform structure in the 30 S subunit, a short distance from the decoding region. The changes that occur in the decoding region, even in the absence of mRNA and tRNA, may be induced by IF3 from a short distance or could be caused by the second IF3 structural domain.

A map of protein-rRNA distribution in the 70 S Escherichia coli ribosome

Neutron scattering exploits the enormous scattering difference between protons and deuterons. A set of 42 x-ray and neutron solution scattering curves from hybrid Escherichia coli ribosomes was obtained, where the proteins and rRNA moieties in the subunits were either protonated or deuterated in all possible combinations. This extensive data set is analyzed using a novel method. The volume defined by the cryoelectron microscopic model of Frank and co-workers (Frank, J., Zhu, J., Penczek, P., Li, Y. H., Srivastava, S., Verschoor, A., Radermacher, M., Grassucci, R., Lata, R. K., and Agrawal, R. K. (1995) Nature 376, 441-444) is divided into 7890 densely packed spheres of radius 0.5 nm. Simulated annealing is employed to assign each sphere to solvent, protein, or rRNA moieties to simultaneously fit all scattering curves. Twelve independent reconstructions starting from random approximations yielded reproducible results. The resulting model at a resolution of 3 nm represents the volumes occupied by rRNA and protein moieties at 95% probability threshold and displays 15 and 20 protein subvolumes in the 30 S and 50 S, respectively, connected by rRNA. 17 proteins with known atomic structure can be tentatively positioned into the protein subvolumes within the ribosome in agreement with the results from other methods. The protein-rRNA map enlarges the basis for the models of the rRNA folding and can further help to localize proteins in high-resolution crystallographic density maps.

The 3D arrangement of the 23 S and 5 S rRNA in the Escherichia coli 50 S ribosomal subunit based on a cryo-electron microscopic reconstruction at 7.5 A resolution

The Escherichia coli 23 S and 5 S rRNA molecules have been fitted helix by helix to a cryo-electron microscopic (EM) reconstruction of the 50 S ribosomal subunit, using an unfiltered version of the recently published 50 S reconstruction at 7.5 A resolution. At this resolution, the EM density shows a well-defined network of fine structural elements, in which the major and minor grooves of the rRNA helices can be discerned at many locations. The 3D folding of the rRNA molecules within this EM density is constrained by their well-established secondary structures, and further constraints are provided by intra and inter-rRNA crosslinking data, as well as by tertiary interactions and pseudoknots. RNA-protein cross-link and foot-print sites on the 23 S and 5 S rRNA were used to position the rRNA elements concerned in relation to the known arrangement of the ribosomal proteins as determined by immuno-electron microscopy. The published X-ray or NMR structures of seven 50 S ribosomal proteins or RNA-protein complexes were incorporated into the EM density. The 3D locations of cross-link and foot-print sites to the 23 S rRNA from tRNA bound to the ribosomal A, P or E sites were correlated with the positions of the tRNA molecules directly observed in earlier reconstructions of the 70 S ribosome at 13 A or 20 A. Similarly, the positions of cross-link sites within the peptidyl transferase ring of the 23 S rRNA from the aminoacyl residue of tRNA were correlated with the locations of the CCA ends of the A and P site tRNA. Sites on the 23 S rRNA that are cross-linked to the N termini of peptides of different lengths were all found to lie within or close to the internal tunnel connecting the peptidyl transferase region with the presumed peptide exit site on the solvent side of the 50 S subunit. The post-transcriptionally modified bases in the 23 S rRNA form a cluster close to the peptidyl transferase area. The minimum conserved core elements of the secondary structure of the 23 S rRNA form a compact block within the 3D structure and, conversely, the points corresponding to the locations of expansion segments in 28 S rRNA all lie on the outside of the structure.

Independent in vitro assembly of all three major morphological parts of the 30S ribosomal subunit of Thermus thermophilus

Fragments of the 16S rRNA of Thermus thermophilus representing the 3' domain (nucleotides 890-1515) and the 5' domain (nucleotides 1-539) have been prepared by transcription in vitro. Incubation of these fragments with total 30S ribosomal proteins of T. thermophilus resulted in formation of specific RNPs. The particle assembled on the 3' RNA domain contained seven proteins corresponding to Escherichia coli ribosomal proteins S3, S7, S9, S10, S13, S14, and S19. All of them have previously been shown to interact with the 3' domain of the 16S RNA and to be localized in the head of the 30S ribosomal subunit. The particle formed on the 5' RNA domain contained five ribosomal proteins corresponding to E. coli proteins S4, S12, S17, S16, and S20. These proteins are known to be localized in the main part of the body of the 30S subunit. Both types of particle were compact and had sedimentation coefficients of 15.5 S and 13 S, respectively. Together with our recent demonstration of the reconstitution of the RNA particle representing the platform of the T. thermophilus 30S ribosomal subunit [Agalarov, S.C., Zheleznyakova, E.N., Selivanova, O.M., Zheleznaya, L.A., Matvienko, N.I., Vasiliev, V.D. & Spirin, A.S. (1998) Proc. Natl Acad. Sci. USA 95, 999-1003], these experiments establish that all three main structural lobes of the small ribosomal subunit can be reconstituted independently of each other and prepared in the individual state.

Characterization of transient RNA-RNA interactions important for the facilitated structure formation of bacterial ribosomal 16S RNA

The co-transcribed leader sequences of bacterial rRNA are known to affect the structure and function of the small ribosomal subunits. Base changes in the leader nut -like sequence elements have been shown to cause misfolded but correctly processed 16S rRNA structures at low growth temperature. Transient interactions of leader sequences with the nascent 16S rRNA are considered to guide rRNA folding and to facilitate correct structure formation. In order to understand this chaperone-like activity of the leader RNA we have analyzed the thermodynamic stabilities of wild-type and mutant leader transcripts. We show here that base changes cause subtle differences in the melting profiles of the corresponding leader transcripts. Furthermore, we show that direct interaction between leader transcripts and the 16S rRNA is limited to the 5'-domain of the 16S rRNA for both wild-type and mutant leaders. Binding studies of mutant and wild-type leader transcripts to 16S rRNA revealed small changes in the affinities and the thermal stabilities as a consequence of the base changes. Different complex stabilities as a function of the Mg(2+) ion concentration indicated that mutant and wild-type leader transcripts interact differently with the 16S rRNA, consistent with a less stable and tightly folded structure of the mutant leader. Employing time-resolved oligonucleotide hybridization assays we could show different folding kinetics for 16S rRNA molecules when linked to wild-type leader, mutant leader or in the absence of leader RNA. The studies help to understand how bacterial rRNA leader transcripts may affect the folding of the small subunit rRNA.

Structure of a beheaded 30 S ribosomal subunit from Thermus thermophilus

The 22 S ribonucleoproten particles containing the 5' (body) and the central (platform) domains of the Thermus thermophilus 30 S subunit has been studied by sedimentation, neutron scattering and electron microscopy. The RNP particles have been obtained by oligonucleotide-directed cleavage of 16 S RNA with ribonulease H in the region of the 900th nucleotide of the protein-deficient derivatives of the 30 S subunits. It is shown that these RNP particles are very compact, though their form and dimensions differ slightly from those expected from the electron microscopy model of the 30 S subunit beheaded by computer simulation. The particles are subdivided into two structural domains whose mutual arrangement differs from that of the corresponding morphological parts of the native 30 S subunit. Electron microscopy demonstrates that the mutual arrangement of domains in the RNP particles is not strictly fixed suggesting that interaction with the third domain of the 30 S subunit is a requisite for their correct fitting.

Location of translational initiation factor IF3 on the small ribosomal subunit

The location of translational initiation factor IF3 bound to the 30S subunit of the Thermus thermophilus ribosome has been determined by cryoelectron microscopy. Both the 30S.IF3 complex and control 30S subunit structures were determined to 27-A resolution. The difference map calculated from the two reconstructions reveals three prominent lobes of positive density. The previously solved crystal structure of IF3 fits very well into two of these lobes, whereas the third lobe probably arises from conformational changes induced in the 30S subunit as a result of IF3 binding. Our placement of IF3 on the 30S subunit allows an understanding in structural terms of the biochemical functions of this initiation factor, namely its ability to dissociate 70S ribosomes into 30S and 50S subunits and the preferential selection of initiator tRNA by IF3 during initiation.

Structure and structural variations of the Escherichia coli 30 S ribosomal subunit as revealed by three-dimensional cryo-electron microscopy

A three-dimensional reconstruction of the 30 S subunit of the Escherichia coli ribosome was obtained at 23 A resolution. Because of the improved resolution, many more structural details are seen as compared to those obtained in earlier studies. Thus, the new structure is more suitable for comparison with the 30 S subunit part of the 70 S ribosome, whose structure is already known at a better resolution. In addition, we observe relative and, to some extent, independent movements of three main structural domains of the 30 S subunit, namely head, platform and the main body, which lead to partial blurring of the reconstructed volume. An attempt to subdivide the data set into conformationally defined subsets reveals the existence of conformers in which these domains have different orientations with respect to one another. This result suggests the existence of dynamic properties of the 30 S subunit that might be required for facilitating its interactions with mRNA, tRNA and other ligands during protein biosynthesis.

Purification, cloning, and characterization of the 16S RNA m5C967 methyltransferase from Escherichia coli

The methyltransferase that forms m5C967 in Escherichia coli small subunit ribosomal RNA has been purified, cloned, and characterized. The gene was identified from the N-terminal sequence of the purified enzyme. The gene is a fusion of two open reading frames, fmu and fmv, previously believed to be distinct due to a DNA sequencing error. The gene, here named rsmB, encodes a 429-amino acid protein that has a number of homologues in prokaryotes, Archaea, and eukaryotes. C-Terminal sequencing of the overexpressed and affinity-purified protein by mass spectrometry methods verified the sequence expected for the gene product. The recombinant protein exhibited the same specificity as the previously described native enzyme; that is, it formed only m5C and only at position 967. C1407, which is also m5C in natural 16S RNA, was not methylated. In vitro, the enzyme only recognized free 16S RNA. 30S ribosomal subunits were not a substrate. There was no requirement for added magnesium, suggesting that extensive secondary or tertiary structure in the RNA substrate may not be a requirement for recognition.

Directed hydroxyl radical probing of 16S ribosomal RNA in 70S ribosomes from internal positions of the RNA

Directed hydroxyl radical probing of 16S ribosomal RNA from Fe(II) tethered to specific sites within the RNA was used to determine RNA-RNA proximities in 70S ribosomes. We have transcribed 16S ribosomal RNA in vitro as two separate fragments, covalently attached an Fe(II) probe to a 5'-guanosine-alpha-phosphorothioate at the junction between the two fragments, and reconstituted 30S subunits with the two separate pieces of RNA and the small subunit proteins. Reconstituted 30S subunits capable of association with 50S subunits were selected by isolation of 70S ribosomes. Hydroxyl radicals, generated in situ from the tethered Fe(II), cleaved sites in the 16S rRNA backbone that were close in three-dimensional space to the Fe(II), and a primer extension was used to identify these sites of cleavage. Two sets of 16S ribosomal RNA fragments, 1-360/361-1542 and 1-448/449-1542, were reconstituted into active 30S subunits. Fe(II) tethered to position 361 results in cleavage of 16S rRNA around nucleotides 34, 160, 497, 512, 520, 537, 552, and 615, as well as around positions 1410, 1422, 1480, and 1490. Fe(II) tethered to position 449 induces cleavage around nucleotide 488 and around positions 42 and 617. Fe(II) tethered to the 5' end of 16S rRNA induces cleavage of the rRNA around nucleotides 5, 601, 615, and 642. These results provide constraints for the positioning of these regions of 16S rRNA, for which there has previously been only limited structural information, within the 30S subunit.

Site-directed hydroxyl radical probing of 30S ribosomal subunits by using Fe(II) tethered to an interruption in the 16S rRNA chain

Two in vitro transcripts, one corresponding to the 5' and central domains (residues 1-920) of 16S rRNA and the other corresponding to its 3' domain (residues 922-1542), assemble efficiently in trans with 30S ribosomal proteins to form a compact ribonucleoprotein particle that cosediments with natural 30S subunits. Isolated particles are similar in appearance to natural 30S subunits with electron microscopy and contain a full complement of the small subunit ribosomal proteins. The particles have a reduced ability to bind tRNA (attributable to the location of the discontinuity in a conserved region of the rRNA) near features that have been implicated in tRNA binding. Association of these two halves of 16S rRNA in trans must be stabilized by either previously unidentified RNA-RNA contacts or interactions mediated by ribosomal proteins because there are no known direct interactions between them. The trans construct was used to probe the three-dimensional RNA neighborhood around position 922 of 16S rRNA by generating hydroxyl radicals from Fe(II) tethered to the 5' end of the 3' transcript. Hydroxyl radical-induced cuts in the 16S rRNA chain were localized by primer extension to nucleotides 923-929 and 1192-1198, providing evidence for the mutual proximity of the 920 and 1192 regions.

Conformational variability in Escherichia coli 70S ribosome as revealed by 3D cryo-electron microscopy

During protein biosynthesis, ribosomes are believed to go through a cycle of conformational transitions. We have identified some of the most variable regions of the E. coli 70S ribosome and its subunits, by means of cryo-electron microscopy and three-dimensional (3D) reconstruction. Conformational changes in the smaller 30S subunit are mainly associated with the functionally important domains of the subunit, such as the neck and the platform, as seen by comparison of heat-activated, non-activated and 50S-bound states. In the larger 50S subunit the most variable regions are the L7/L12 stalk, central protuberance and the L1-protein, as observed in various tRNA-70S ribosome complexes. Difference maps calculated between 3D maps of ribosomes help pinpoint the location of ribosomal regions that are most strongly affected by conformational transitions. These results throw direct light on the dynamic behavior of the ribosome and help in understanding the role of these flexible domains in the translation process.

Ribosomal protein L15 as a probe of 50 S ribosomal subunit structure

L15, a 15 kDa protein of the large ribosomal subunit, interacts with over ten other proteins during 50 S assembly in vitro. We have probed the interaction L15 with 23 S rRNA in 50 S ribosomal subunits by chemical footprinting, and have used localized hydroxyl radical probing, generated from Fe(II) tethered to unique sites of L15, to characterize the three-dimensional 23 S rRNA environment of L15. Footprinting of L15 was done by reconstituting purified, recombinant L15 with core particles derived from Escherichia coli 50 S subunits by treatment with 2 M LiCl. The cores migrate as compact 50 S-like particles in sucrose gradients, contain 23 S and 5 S rRNA, and lack a subset of the 50 S proteins, including L15. Using both Fe(II).EDTA and dimethyl sulfate, we have identified a strong footprint for L15 in the region spanning nucleotides 572-654 in domain II of 23 S rRNA. This footprint cannot be detected when L15 is incubated with "naked" 23 S rRNA, indicating that formation of the L15 binding site requires a partially assembled particle.Protein-tethered hydroxyl radical probing was done using mutants of L15 containing single cysteine residues at amino acid positions 68, 71 and 115. The mutant proteins were derivatized with 1-[p-(bromo-acetamido)benzyl]-EDTA. Fe(II), bound to core particles, and hydroxyl radical cleavage was initiated. Distinct but overlapping sets of cleavages were obtained in the footprinted region of domain II, and in specific regions of domains I, IV and V of 23 S rRNA. These data locate L15 in proximity to several 23 S rRNA elements that are dispersed in the secondary structure, consistent with its central role in the latter stages of 50 S subunit assembly. Furthermore, these results indicate the proximity of these rRNA regions to one another, providing constraints on the tertiary folding of 23 S rRNA.

To knot or not to knot? Examination of 16S ribosomal RNA models

The presence of topological knots in large RNA structures is highly unlikely given that 1) no RNA structures determined thus far contain topological knots, 2) secondary structure maps for most RNA molecules are knot free, 3) there are no known RNA topoisomerases, and 4) it is difficult to imagine how knots could be formed specifically and uniquely during transcription. Since native RNA structures probably lack topological knots, models of these RNA molecules should be free of knots as well. Therefore, we have examined four existing models for the 30S ribosomal subunit to determine if any of the three domains of the 16S rRNA molecule is knotted. We found that all but one model had at least one knotted domain. We conclude that models of large RNA molecules should be examined for knotting before publication.

A common motif organizes the structure of multi-helix loops in 16 S and 23 S ribosomal RNAs

Phylogenetic and chemical probing data indicate that a modular RNA motif, common to loop E of eucaryotic 5 S ribosomal RNA (rRNA) and the alpha-sarcin/ricin loop of 23 S rRNA, organizes the structure of multi-helix loops in 16 S and 23 S ribosomal RNAs. The motif occurs in the 3' domain of 16 S rRNA at positions 1345-1350/1372-1376 (Escherichia coli numbering), within the three-way junction loop, which binds ribosomal protein S7, and which contains nucleotides that help to form the binding site for P-site tRNA in the ribosome. The motif also helps to structure a three-way junction within domain I of 23 S, which contains many universally conserved bases and which lies close in the primary and secondary structure to the binding site of r-protein L24. Several other highly conserved hairpin, internal, and multi-helix loops in 16 S and 23 S rRNA contain the motif, including the core junction loop of 23 S and helix 27 in the core of 16 S rRNA. Sequence conservation and range of variation in bacteria, archaea, and eucaryotes as well as chemical probing and cross-linking data, provide support for the recurrent and autonomous existence of the motif in ribosomal RNAs. Besides its presence in the hairpin ribozyme, the loop E motif is also apparent in helix P10 of bacterial RNase P, in domain P7 of one sub-group of group I introns, and in domain 3 of one subgroup of group II introns.

Small angle scattering in ribosomal structure research: localization of the messenger RNA within ribosomal elongation states

Besides EM and biochemical studies small angle scattering (SAS) examinations have contributed significantly to our current knowledge about the ribosomal structure. SAS does not only allow the validation of competing models but permits independent model building. However, the major contribution of SAS to ribosomal structure research derived from its ability to reveal the spatial distribution of the individual ribosomal components (57 in the E. coli ribosome) within the ribosomal structure. More recently, an improved scattering method (proton-spin contrast variation) made it possible also to address the question of mapping functional ligands in defined ribosomal elongation states. Here, we review the contributions of SAS to the current understanding of the ribosome. Furthermore we present the direct localization of a small mRNA fragment within 70S elongation complexes and describe its movement upon the translocation reaction. The successful mapping of this fragment comprising only about 0.6% of the total mass of the complex proves that proton-spin contrast-variation is a powerful tool in modern ribosome research.

Ribosomal proteins S5 and L6: high-resolution crystal structures and roles in protein synthesis and antibiotic resistance

Antibiotic resistance is rapidly becoming a major medical problem. Many antibiotics are directed against bacterial ribosomes, and mutations within both the RNA and protein components can render them ineffective. It is well known that the majority of these antibiotics act by binding to the ribosomal RNA, and it is of interest to understand how mutations in the ribosomal proteins can produce resistance. Translational accuracy is one important target of antibiotics, and a number of ribosomal protein mutations in Escherichia coli are known to modulate the proofreading mechanism of the ribosome. Here we describe the high-resolution structures of two such ribosomal proteins and characterize these mutations. The S5 protein, from the small ribosomal unit, is associated with two types of mutations: those that reduce translational fidelity and others that produce resistance to the antibiotic spectinomycin. The L6 protein, from the large subunit, has mutations that cause resistance to several aminoglycoside antibiotics, notably gentamicin. In both proteins, the mutations occur within their putative RNA-binding sites. The L6 mutations are particularly drastic because they result in large deletions of an RNA-binding region. These results support the hypothesis that the mutations create local distortions of the catalytic RNA component.When combined with a variety of structural and biochemical data, these mutations also become important probes of the architecture and function of the translational machinery. We propose that the C-terminal half of S5, which contains the accuracy mutations, organizes RNA structures associated with the decoding region, and the N-terminal half, which contains the spectinomycin-resistance mutations, directly interacts with an RNA helix that binds this antibiotic. As regards L6, we suggest that the mutations indirectly affect proofreading by locally distorting the EF-Tu.GTP.aminoacyl tRNA binding site on the large subunit.

Ribosomal tRNA binding sites: three-site models of translation

The first models of translation described protein synthesis in terms of two operationally defined tRNA binding sites, the P-site for the donor substrate, the peptidyl-tRNA, and the A-site for the acceptor substrates, the aminoacyl-tRNAs. The discovery and analysis of the third tRNA binding site, the E-site specific for deacylated tRNAs, resulted in the allosteric three-site model, the two major features of which are (1) the reciprocal relationship of A-site and E-site occupation, and (2) simultaneous codon-anticodon interactions of both tRNAs present at the elongating ribosome. However, structural studies do not support the three operationally defined sites in a simple fashion as three topographically fixed entities, thus leading to new concepts of tRNA binding and movement: (1) the hybrid-site model describes the tRNAs' movement through the ribosome in terms of changing binding sites on the 30S and 50S subunits in an alternating fashion. The tRNAs thereby pass through hybrid binding states. (2) The alpha-epsilon model introduces the concept of a movable tRNA-binding domain comprising two binding sites, termed alpha and epsilon. The translocation movement is seen as a result of a conformational change of the ribosome rather than as a diffusion process between fixed binding sites. The alpha-epsilon model reconciles most of the experimental data currently available.

The importance of base pairing in the penultimate stem of Escherichia coli 16S rRNA for ribosomal subunit association

The influence of base pairing in the penultimate stem of Escherichia coli 16S rRNA (defined as nt 1409-1491) on ribosome function has been addressed by the construction of mutations in this region of rRNA. Two sets of mutations were made on either side of a structurally conserved region in the penultimate stem that disrupted base pairing, while a third set of mutations replaced the wild-type sequence with other base pair combinations. The effects of these mutations were analyzed in vivo and in vitro . The mutations that disrupted base pairing caused significant increases in cell doubling times as well as a severe subunit association defect and a modest increase in frame shifting and stop codon read-through. Restoration of base pairing restored wild-type growth rates, decoding and subunit association, indicating that base pairing in this region is essential for proper ribosome function.

Mutations in the leader region of ribosomal RNA operons cause structurally defective 30 S ribosomes as revealed by in vivo structural probing

The biogenesis of functional ribosomes is regulated in a very complex manner, involving different proteins and RNA molecules. RNAs are not only essential components of both ribosomal subunits but also transiently interacting factors during particle formation. In eukaryotes snoRNAs act as molecular chaperones to assist maturation, modification and assembly. In a very similar way highly conserved leader sequences of bacterial rRNA operons are involved in the correct formation of 30 S ribosomal subunits. Certain mutations in the rRNA leader region cause severe growth defects due to malfunction of ribosomes which are assembled from such transcription units. To understand how the leader sequences act to facilitate the formation of the correct 30 S subunits we performed in vivo chemical probing to assess structural differences between ribosomes assembled either from rRNA transcribed from wild-type operons or from operons which contain mutations in the rRNA leader region. Cells transformed with plasmids containing the respective rRNA operons were reacted with dimethylsulphate (DMS). Ribosomes were isolated by sucrose gradient centrifugation and modified nucleotides within the 16 S rRNA were identified by primer extension reaction. Structural differences between ribosomes from wild-type and mutant rRNA operons occur in several clusters within the 16 S rRNA secondary structure. The most prominent differences are located in the central domain including the universally conserved pseudoknot structure which connects the 5', the central and the 3' domain of 16 S rRNA. Two other clusters with structural differences fall in the 5' domain where the leader had been shown to interact with mature 16 S rRNA and within the ribosomal protein S4 binding site. The other differences in structure are located in sites which are also known as sites for the action of several antibiotics. The data explain the functional defects of ribosomes from rRNA operons with leader mutations and help to understand the altered biogenesis pathway from mutations in an rRNA leader region to the formation of functionally defective ribosomes.