RNA Structure: Reading the Ribosome

RNA FRABASE version 1.0: an engine with a database to search for the three-dimensional fragments within RNA structures

The RNA FRABASE is a web-accessible engine with a relational database, which allows for the automatic search of user-defined, 3D RNA fragments within a set of RNA structures. This is a new tool to search and analyse RNA structures, directed at the 3D structure modelling. The user needs to input either RNA sequence(s) and/or secondary structure(s) given in a ‘dot-bracket’ notation. The algorithm searching for the requested 3D RNA fragments is very efficient. As of August 2007, the database contains: (i) RNA sequences and secondary structures, in the ‘dot-bracket’ notation, derived from 1065 protein data bank (PDB)-deposited RNA structures and their complexes, (ii) a collection of atom coordinates of unmodified and modified nucleotide residues occurring in RNA structures, (iii) calculated RNA torsion angles and sugar pucker parameters and (iv) information about base pairs. Advanced query involves filters sensitive to: modified residue contents, experimental method used and limits of conformational parameters. The output list of query-matching RNA fragments gives access to their coordinates in the PDB-format files, ready for direct download and visualization, conformational parameters and information about base pairs. The RNA FRABASE is automatically, monthly updated and is freely accessible at http://rnafrabase.ibch.poznan.pl (mirror at http://cerber.cs.put.poznan.pl/rnadb).

Molecular dynamics study of intrinsic stability in six RNA terminal loop motifs

Single stranded RNA molecules can assume a wide range of tertiary structures beyond the canonical A-form double helix. Certain sequences, termed motifs, are more common than a random distribution would suggest. The existence of such motifs can be rationalized in structural terms. In this study, we have investigated the intrinsic structural stability of RNA terminal loop motifs using multiple MD simulations in explicit water. Representative loops were chosen from the major tetraloop motifs, including also the U-turn motif. Not all loops retain their folded starting structure, but lowering the temperature to 277 K, or adding adjacent base pairs from the stem to which the motif is attached, helps stabilizing the folded loop structure.

Kinetics of RNA refolding in dynamic equilibrium by 1H-detected 15N exchange NMR spectroscopy

By implementing new NMR methods that were designed to map very slow exchange processes we have investigated and characterized the refolding kinetics of a thermodynamically stable 34mer RNA sequence in dynamic equilibrium. The RNA sequence was designed to undergo a topologically favored conformational exchange between different hairpin folds, serving as a model to estimate the minimal time required for more complex RNA folding processes. Chemically prepared RNA sequences with sequence-selective (15)N labels provided the required signal separation and allowed a straightforward signal assignment of the imino protons by HNN correlation experiments. The 2D version of the new (1)H-detected (15)N exchange spectroscopy (EXSY) pulse sequence provided cross-peaks for resonances belonging to different folds that interchange on the time scale of longitudinal relaxation of (15)N nuclei bound to imino protons. The 34mer RNA sequence exhibits two folds which exchange on the observable time scale (tau(obs) approximately T(1){(15)Nu} < 5 s) and a third fold which is static on this time scale. A 1D version of the (15)N exchange experiment allowed the measurement of the exchange rates between the two exchanging folds as a function of temperature and the determination of the corresponding activation energies E(a) and frequency factors A. We found that the refolding rates are strongly affected by an entropically favorable preorientation of the replacing strand. The activation energies are comparable to values obtained for the slow refolding of RNA sequences of similar thermodynamic stability but less favorable topology.

Viral RNA pseudoknots: versatile motifs in gene expression and replication

RNA pseudoknots are structural motifs in RNA that are increasingly recognized in viral and cellular RNAs. They have been shown to have a various roles in virus and cellular gene expression.

Pseudoknots are formed upon base pairing of a single-stranded region of RNA in the loop of a hairpin to a stretch of complementary nucleotides elsewhere in the RNA chain. This simple folding strategy can generate a large number of stable three-dimensional folds, which display a diverse range of highly specific functions.

Pseudoknot function is frequently associated with interactions with ribosomes. The inclusion of pseudoknots in an mRNA can thus confer unusual translational properties.

Many RNA viruses use pseudoknots in the control of viral RNA translation, replication and the switch between the two processes. Some satellite viruses encode ribozymes with active sites that are folded by a pseudoknot.

In cellular RNAs, pseudoknots are associated with all aspects of mRNA function and also ribosome function, as ribosomal RNAs contain numerous pseudoknots. Other essential cellular pseudoknots have been described in telomerase RNA and transfer messenger RNA.

Future research into pseudoknots will focus on structure–function relationships and bioinformatics identification of pseudoknots in genomes. The use of pseudoknots in antiviral applications could also become more widespread.

RNA pseudoknots have been identified in many different viral and cellular RNAs and are known to have various roles in virus and cellular gene expression. Here, Ian Brierley and colleagues review viral pseudoknots and the role of these structural motifs in virus gene expression and genome replication.

RNA pseudoknots are structural elements found in almost all classes of RNA. First recognized in the genomes of plant viruses, they are now established as a widespread motif with diverse functions in various biological processes. This Review focuses on viral pseudoknots and their role in virus gene expression and genome replication. Although emphasis is placed on those well defined pseudoknots that are involved in unusual mechanisms of viral translational initiation and elongation, the broader roles of pseudoknots are also discussed, including comparisons with relevant cellular counterparts. The relationship between RNA pseudoknot structure and function is also addressed.

Nucleoprotein assemblies at the nanoscale: medical implications

Bionanotechnology is exploiting the rich structural knowledge now available on DNA and DNA-protein interactions to construct nucleoprotein-based devices that have the potential not only to contribute to our understanding of the structure and function of the proteins and nucleic acids involved but also to new approaches to problems in medicine. Assemblies under development currently are poised to contribute to diagnosis and therapy. Here, I discuss recent work in this emerging field.

Theoretical study of the scalar coupling constants across the noncovalent contacts in RNA base pairs: the cis- and trans-watson-crick/sugar edge base pair family

The structure and function of RNA molecules are substantially affected by non-Watson-Crick base pairs actively utilizing the 2'-hydroxyl group of ribose. Here we correlate scalar coupling constants across the noncovalent contacts calculated for the cis- and trans-WC/SE (Watson-Crick/sugar edge) RNA base pairs with the geometry of base to base and sugar to base hydrogen bond(s). 23 RNA base pairs from the 32 investigated were found in RNA crystal structures, and the calculated scalar couplings are therefore experimentally relevant with regard to the binding patterns occurring in this class of RNA base pairs. The intermolecular scalar couplings 1hJ(N,H), 2hJ(N,N), 2hJ(C,H), and 3hJ(C,N) were calculated for the N-H...N and N-H...O=C base to base contacts and various noncovalent links between the sugar hydroxyl and RNA base. Also, the intramolecular 1J(N,H) and 2J(C,H) couplings were calculated for the amino or imino group of RNA base and the ribose 2'-hydroxyl group involved in the noncovalent interactions. The calculated scalar couplings have implications for validation of local geometry, show specificity for the amino and imino groups of RNA base involved in the linkage, and can be used for discrimination between the cis- and trans-WC/SE base pairs. The RNA base pairs within an isosteric subclass of the WC/SE binding patterns can be further sorted according to the scalar couplings calculated across different local noncovalent contacts. The effect of explicit water inserted in the RNA base pairs on the magnitude of the scalar couplings was calculated, and the data for discrimination between the water-inserted and direct RNA base pairs are presented. The calculated NMR data are significant for structural interpretation of the scalar couplings in the noncanonical RNA base pairs.

Insights from crystallographic studies into the structural and pairing properties of nucleic acid analogs and chemically modified DNA and RNA oligonucleotides

Chemically modified nucleic acids function as model systems for native DNA and RNA; as chemical probes in diagnostics or the analysis of protein-nucleic acid interactions and in high-throughput genomics and drug target validation; as potential antigene-, antisense-, or RNAi-based drugs; and as tools for structure determination (i.e., crystallographic phasing), just to name a few. Biophysical and structural investigations of chemically modified DNAs and RNAs, particularly of nucleic acid analogs with more significant alterations to the well-known base-sugar-phosphate framework (i.e., peptide or hexopyranose nucleic acids), can also provide insights into the properties of the natural nucleic acids that are beyond the reach of studies focusing on DNA or RNA alone. In this review we summarize results from crystallographic analyses of chemically modified DNAs and RNAs that are primarily of interest in the context of the discovery and development of oligonucleotide-based therapeutics. In addition, we re-examine recent structural data on nucleic acid analogs that are investigated as part of a systematic effort to rationalize nature's choice of pentose in the genetic system.

Detecting the coevolution of biosequences--an example of RNA interaction prediction

A probabilistic graphical model is proposed in order to detect the coevolution between different sites in biological sequences. The model extends the continuous-time Markov process of sequence substitution for single nucleic or amino acids and imposes general constraints regarding simultaneous changes on the substitution rate matrix. Given a multiple sequence alignment for each molecule of interest and a phylogenetic tree, the model can predict potential interactions within or between nucleic acids and proteins. Initial validation of the model is carried out using tRNA and 16S rRNA sequence data. The model accurately identifies the secondary interactions of tRNA as well as several known tertiary interactions. In addition, results on 16S rRNA data indicate this general and simple coevolutionary model outperforms several other parametric and nonparametric methods in predicting secondary interactions. Furthermore, the majority of the putative predictions exhibit either direct contact or proximity of the nucleotide pairs in the 3-dimensional structure of the Thermus thermophilus ribosomal small subunit. The results on RNA data suggest a general model of coevolution might be applied to other types of interactions between protein, DNA, and RNA molecules.

Simulations of RNA base pairs in a nanodroplet reveal solvation-dependent stability

We show that RNA base pairs have variable stability depending on their degree of solvation. This finding has far-reaching biological implications for nucleic acid structure in a partially solvated cellular environment such as inside RNA-protein complexes. Molecular dynamics simulations of partially solvated Watson-Crick RNA base pairs show that whereas water serves to destabilize a base pair by competing for and disrupting base-base hydrogen bonds, when sufficient water molecules are present, fewer hydrogen bonds are available to disrupt the base pairs and the destabilization effect is reduced. The result is that base pairs exist at a stability minimum when solvated in between 20 and 100 water molecules, the upper limit of which corresponds to the approximate number of water molecules contained in the first hydration shell.

Structural characterization of the ribosome maturation protein, RimM

The RimM protein has been implicated in the maturation of the 30S ribosomal subunit. It binds to ribosomal protein S19, located in the head domain of the 30S subunit. Multiple sequence alignments predicted that RimM possesses two domains in its N- and C-terminal regions. In the present study, we have produced Thermus thermophilus RimM in both the full-length form (162 residues) and its N-terminal fragment, spanning residues 1 to 85, as soluble proteins in Escherichia coli and have performed structural analyses by nuclear magnetic resonance spectroscopy. Residues 1 to 80 of the RimM protein fold into a single structural domain adopting a six-stranded beta-barrel fold. On the other hand, the C-terminal region of RimM (residues 81 to 162) is partly folded in solution. Analyses of 1H-15N heteronuclear single quantum correlation spectra revealed that a wide range of residues in the C-terminal region, as well as the residues in the vicinity of a hydrophobic patch in the N-terminal domain, were dramatically affected upon complex formation with ribosomal protein S19.

Informatics challenges in structured RNA

The world of regulatory RNAs is fast expanding into mainstream molecular biology as both a subject of intense mechanistic study and as a tool for functional characterization. The RNA world is one of complex structures that carry out catalysis, sense metabolites and synthesize proteins. The dynamic and structural nature of RNAs presents a whole new set of informatics challenges to the computational community. The ability to relate structure and dynamics to function will be key to understanding this complex world. I review several important classes of structured RNAs that present our community with a series of biologically novel informatics challenges. I also review available informatics tools that have been recently developed in the field.

Leading RNA tertiary interactions: structures, energies, and water insertion of A-minor and P-interactions. A quantum chemical view

Complex molecular shapes of ribosomal RNA molecules are stabilized by recurrent types of tertiary interactions involving highly specific and conserved non-Watson-Crick base pairs, triplets, and quartets. We analyzed the intrinsic structure and stability of the P-motif and the four basic types of A-minor interactions (types I, II, III, and 0), which represent the most prominent RNA tertiary interaction patterns refined in the course of evolution. In the studied interactions, the electron correlation component of the stabilization usually exceeds the Hartree-Fock (HF) term, leading to a strikingly different balance of forces as compared to standard base pairing stabilized primarily by the HF term. In other words, the A-minor and P-interactions are considerably more influenced by the dispersion energy as compared to canonical base pairs, which makes them particularly suitable to zip the folded RNA structures that are substantially hydrated even in their interior. Continuum solvent COSMO calculations confirm that the stability of the canonical GC base pair is affected (reduced) by the continuum solvent screening considerably more than the stability of the A-minor interaction. Among the studied systems, the strong A-minor II and weak A-minor III interactions require water molecules to stabilize the experimental geometry. Gas-phase optimization of the canonical A-minor II A/CG triplet without water results in a geometry that is clearly inconsistent with the RNA structure. The gas-phase structure of the P-interaction and the most stable A-minor I interaction nicely agrees with the geometries occurring in the ribosome. A-minor I can also adopt an alternative water-mediated substate rather often observed in X-ray and molecular dynamics studies. The A-minor I water bridge, however, does not appear to stabilize the tertiary contact, and its role is to provide structural flexibility to this binding pattern within the context of the RNA structure. Interestingly, the insertion of a polar water molecule in the A-minor I A/CG tertiary contact occurring in the A/C tertiary pair is stabilized primarily by the HF (electrostatic) interaction energy, while the dispersion-controlled A/G contact remains firmly bound. Thus, the intrinsic balance of forces as revealed by quantum mechanics (QM) calculations nicely correlates with many behavioral aspects of the studied interactions inside RNA. The comparison of interaction energies computed using quantum chemistry and an AMBER force field reveals that common molecular mechanics calculations perform rather well, except that the strength of the P-interaction is modestly overestimated. We also briefly discuss the non-negligible methodological differences when evaluating simple base-base nucleic acids base pairs and the complex RNA tertiary base pairing patterns using QM procedures.

Mutational analysis of the ribosome

The ribosome is responsible for protein synthesis, the translation of the genetic code, in all living organisms. Ribosomes are composed of RNA (ribosomal RNA) and protein (ribosomal protein). Soluble protein factors bind to the ribosome and facilitate different phases of translation. Genetic approaches have proved useful for the identification and characterization of the structural and functional roles of specific nucleotides in ribosomal RNA and of specific amino acids in ribosomal proteins and in ribosomal factors. This chapter summarizes examples of mutations identified in ribosomal RNA, ribosomal proteins, and ribosomal factors.

Complexes of tRNA and maturation enzymes: shaping up for translation

Several significant structures of transfer ribonucleic acid (tRNA) maturation enzymes complexed with precursor tRNA or fragments thereof have been published recently, providing detailed knowledge of enzyme-tRNA recognition and catalytic strategies. In addition to reinforcing the general principles of RNA-protein interaction, the new structures highlight both the features of composite RNA recognition by multiple enzyme subunits and the pronounced RNA structural flexibility in or near the active site in all cases. These structural principles provide plausible explanations for the exquisite specificity and catalytic power of these enzymes and, in the case of evolutionary adaptation, for the ability of some enzymes to develop novel specificities.

RNABindR: a server for analyzing and predicting RNA-binding sites in proteins

Understanding interactions between proteins and RNA is key to deciphering the mechanisms of many important biological processes. Here we describe RNABindR, a web-based server that identifies and displays RNA-binding residues in known protein–RNA complexes and predicts RNA-binding residues in proteins of unknown structure. RNABindR uses a distance cutoff to identify which amino acids contact RNA in solved complex structures (from the Protein Data Bank) and provides a labeled amino acid sequence and a Jmol graphical viewer in which RNA-binding residues are displayed in the context of the three-dimensional structure. Alternatively, RNABindR can use a Naive Bayes classifier trained on a non-redundant set of protein–RNA complexes from the PDB to predict which amino acids in a protein sequence of unknown structure are most likely to bind RNA. RNABindR automatically displays ‘high specificity’ and ‘high sensitivity’ predictions of RNA-binding residues. RNABindR is freely available at http://bindr.gdcb.iastate.edu/RNABindR.

Structure-based design, synthesis, and A-site rRNA cocrystal complexes of functionally novel aminoglycoside antibiotics: C2" ether analogues of paromomycin

A series of 2"-O-substituted ether analogues of paromomycin were prepared based on new site-selective functionalizations. X-ray cocrystal complexes of several such analogues revealed a new mode of binding in the A-site rRNA, whereby rings I and II adopted the familiar orientation and position previously observed with paromomycin, but rings III and IV were oriented differently. With few exceptions, all of the new analogues showed potent inhibitory activity equal or better than paromomycin against a sensitive strain of S. aureus. Single digit microM MIC values were obtained against E. coli, with some of the ether appendages containing polar or basic end groups. Two analogues showed excellent survival rate in a mouse septicemia protection assay. Preliminary histopathological analysis of the kidney showed no overt signs of toxicity, while controls with neomycin and kanamycin were toxic at lower doses.

Fabricating RNA microarrays with RNA-DNA surface ligation chemistry

A novel surface attachment strategy that utilizes RNA-DNA surface ligation chemistry to create renewable RNA microarrays from single-stranded DNA (ssDNA) microarrays on gold surfaces is demonstrated. The enzyme T4 DNA ligase was used to catalyze the formation of a phosphodiester bond between 5'-phosphate-modified ssDNA attached to the surface and the 3'-hydroxyl group of unlabeled RNA molecules from solution in the presence of a complementary template DNA strand. Surface plasmon resonance imaging (SPRI) measurements were performed to characterize the ligation process as well as to verify the bioactivity of the ssRNA microarray in terms of (i) the hybridization adsorption of complementary DNA onto the RNA array to form a surface RNA-DNA heteroduplex and (ii) the hydrolysis of the RNA microarrays with either ribonuclease S or ribonuclease H (RNase H). The hydrolysis of the surface-bound RNA with RNase H required the presence of a surface heteroduplex and, upon completion, regenerated the original 5'-phosphate-terminated ssDNA array elements. These ssDNA array elements could be ligated again to create a new RNA microarray. These RNA microarrays can be used in the study of RNA-protein/RNA/aptamer bioaffinity interactions and for the enzymatically amplified SPRI detection of DNA in the presence of RNase H.

G-ribo: a new structural motif in ribosomal RNA

Analysis of the available crystal structures of the ribosome and of its subunits has revealed a new RNA motif that we call G-ribo. The motif consists of two double helices positioned side-by-side and connected by an unpaired region. The juxtaposition of the two helices is kept by a complex system of tertiary interactions spread over several layers of stacked nucleotides. In the center of this arrangement, the ribose of a nucleotide from one helix is specifically packed with the ribose and the minor-groove edge of a guanosine from the other helix. In total, we found eight G-ribo motifs in both ribosomal subunits. The location of these motifs suggests that at least some of them play an important role in the formation of the ribosome structure and/or in its function.

6-hydroxy to 6'''-amino tethered ring-to-ring macrocyclic aminoglycosides as probes for APH(3')-IIIa kinase

Based on molecular modeling and available X-ray structure data on aminoglycosides complexed with a bacterial ribosomal surrogate or with a kinase, two analogues of paromomycin were prepared by tethering the 6-OH and the 6'''-NH(2) group with a five-carbon bridge. Only one of two possible hydroxyl groups was phosphorylated by the kinase. The application of ring closure metathesis is presented for the first time to construct bridged macrocyclic analogues in the aminoglycoside series.

Calculation of structural behavior of indirect NMR spin-spin couplings in the backbone of nucleic acids

Calculated indirect NMR spin-spin coupling constants (J-couplings) between (31)P, (13)C, and (1)H nuclei were related to the backbone torsion angles of nucleic acids (NAs), and it was shown that J-couplings can facilitate accurate and reliable structural interpretation of NMR measurements and help to discriminate between their distinct conformational classes. A proposed stepwise procedure suggests assignment of the J-couplings to torsion angles from the sugar part to the phosphodiester link. Some J-couplings show multidimensional dependence on torsion angles, the most prominent of which is the effect of the sugar pucker. J-couplings were calculated in 16 distinct nucleic acid conformations, two principal double-helical DNAs, B- and A-, the main RNA form, A-RNA, as well as in 13 other RNA conformations. High-level quantum mechanics calculations used a baseless dinucleoside phosphate as a molecular model, and the effect of solvent was included. The predicted J-couplings correlate reliably with available experimental data from the literature.

Size, shape, and flexibility of RNA structures

Determination of sizes and flexibilities of RNA molecules is important in understanding the nature of packing in folded structures and in elucidating interactions between RNA and DNA or proteins. Using the coordinates of the structures of RNA in the Protein Data Bank we find that the size of the folded RNA structures, measured using the radius of gyration R(G), follows the Flory scaling law, namely, R(G)=5.5N(1/3) A, where N is the number of nucleotides. The shape of RNA molecules is characterized by the asphericity Delta and the shape S parameters that are computed using the eigenvalues of the moment of inertia tensor. From the distribution of Delta, we find that a large fraction of folded RNA structures are aspherical and the distribution of S values shows that RNA molecules are prolate (S>0). The flexibility of folded structures is characterized by the persistence length l(p). By fitting the distance distribution function P(r), that is computed using the coordinates of the folded RNA, to the wormlike chain model we extracted the persistence length l(p). We find that l(p) approximately 1.5N(0.33) A which might reflect the large separation between the free energies that stabilize secondary and tertiary structures. The dependence of l(p) on N implies that the average length of helices should increase as the size of RNA grows. We also analyze packing in the structures of ribosomes (30S, 50S, and 70S) in terms of R(G), Delta, S, and l(p). The 70S and the 50S subunits are more spherical compared to most RNA molecules. The globularity in 50S is due to the presence of an unusually large number (compared to 30S subunit) of small helices that are stitched together by bulges and loops. Comparison of the shapes of the intact 70S ribosome and the constituent particles suggests that folding of the individual molecules might occur prior to assembly.

Evolution of RNA editing sites in the mitochondrial small subunit rRNA of the Myxomycota

Because of their unique and unprecedented character, it is often difficult to imagine how and why the different, diverse types of RNA editing have evolved. Information about the evolution of a particular RNA editing system can be obtained by comparing RNA editing characteristics in contemporary organisms whose phylogenetic relationships are known so that editing patterns in ancestral organisms can be inferred. This information can then be used to build models of the origins, constraints, variability, and mechanisms of RNA editing. As an example of the types of information that can be obtained from these analyses, we describe how we have used cDNA, covariation, and phylogenetic analyses to study the evolution of the variation in RNA editing site location in the core region of the small subunit rRNA gene in the mtDNA of seven myxomycetes, including Physarum polycephalum. We find that the unique type of insertional RNA editing present in mitochondria of P. polycephalum is also present in the mitochondrial small subunit (SSU) rRNA of the other six myxomycetes. As in Physarum, this editing predominantly consists of cytidine insertions, but also includes uridine insertions and certain dinucleotide insertions such that any of the four canonical ribonucleotides can be inserted. Although the characteristics of RNA editing in these organisms are the same as in Physarum, the location of the insertion sites varies among the seven organisms relative to the conserved primary sequence and secondary structure of the rRNA. Nucleotide insertions have been identified at 29 different sites within this core region of the rRNA, but no one organism has more than 10 of these insertion sites, suggesting that editing sites have been created and/or eliminated since the divergence of these organisms. To determine the order in which editing sites have been created or eliminated, the sequences of the mitochondrial SSU rRNA have been aligned and this alignment has been used to produce phylogenetic trees showing the sequence relationship of these organisms. These phylogenetic trees are congruent with phylogenetic trees predicted by alignment of nuclear rDNA sequences. These trees indicate that editing sites change rapidly relative to mtDNA sequence divergence and suggest that some editing sites have been created more than once during the evolution of the Myxomycota.

Modular evolution and increase of functional complexity in replicating RNA molecules

At early stages of biochemical evolution, the complexity of replicating molecules was limited by unavoidably high mutation rates. In an RNA world, prior to the appearance of cellular life, an increase in molecular length, and thus in functional complexity, could have been mediated by modular evolution. We describe here a scenario in which short, replicating RNA sequences are selected to perform a simple function. Molecular function is represented through the secondary structure corresponding to each sequence, and a given target secondary structure yields the optimal function in the environment where the population evolves. The combination of independently evolved populations may have facilitated the emergence of larger molecules able to perform more complex functions (including RNA replication) that could arise as a combination of simpler ones. We quantitatively show that modular evolution has relevant advantages with respect to the direct evolution of large functional molecules, among them the allowance of higher mutation rates, the shortening of evolutionary times, and the very possibility of finding complex structures that could not be otherwise directly selected.

Ribosomal protein gene knockdown causes developmental defects in zebrafish

The ribosomal proteins (RPs) form the majority of cellular proteins and are mandatory for cellular growth. RP genes have been linked, either directly or indirectly, to various diseases in humans. Mutations in RP genes are also associated with tissue-specific phenotypes, suggesting a possible role in organ development during early embryogenesis. However, it is not yet known how mutations in a particular RP gene result in specific cellular changes, or how RP genes might contribute to human diseases. The development of animal models with defects in RP genes will be essential for studying these questions. In this study, we knocked down 21 RP genes in zebrafish by using morpholino antisense oligos to inhibit their translation. Of these 21, knockdown of 19 RPs resulted in the development of morphants with obvious deformities. Although mutations in RP genes, like other housekeeping genes, would be expected to result in nonspecific developmental defects with widespread phenotypes, we found that knockdown of some RP genes resulted in phenotypes specific to each gene, with varying degrees of abnormality in the brain, body trunk, eyes, and ears at about 25 hours post fertilization. We focused further on the organogenesis of the brain. Each knocked-down gene that affected the morphogenesis of the brain produced a different pattern of abnormality. Among the 7 RP genes whose knockdown produced severe brain phenotypes, 3 human orthologs are located within chromosomal regions that have been linked to brain-associated diseases, suggesting a possible involvement of RP genes in brain or neurological diseases. The RP gene knockdown system developed in this study could be a powerful tool for studying the roles of ribosomes in human diseases.

Mutational analysis reveals two independent molecular requirements during transfer RNA selection on the ribosome

Accurate discrimination between cognate and near-cognate aminoacyl-tRNAs during translation relies on the specific acceleration of forward rate constants for cognate tRNAs. Such specific rate enhancement correlates with conformational changes in the tRNA and small ribosomal subunit that depend on an RNA-specific type of interaction, the A-minor motif, between universally conserved 16S ribosomal RNA nucleotides and the cognate codon-anticodon helix. We show that perturbations of these two components of the A-minor motif, the conserved rRNA bases and the codon-anticodon helix, result in distinct outcomes. Although both cause decreases in the rates of tRNA selection that are rescued by aminoglycoside antibiotics, only disruption of the codon-anticodon helix is overcome by a miscoding tRNA variant. On this basis, we propose that two independent molecular requirements must be met to allow tRNAs to proceed through the selection pathway, providing a mechanism for exquisite control of fidelity during this step in gene expression.

The interaction networks of structured RNAs

All pairwise interactions occurring between bases which could be detected in three-dimensional structures of crystallized RNA molecules are annotated on new planar diagrams. The diagrams attempt to map the underlying complex networks of base–base interactions and, especially, they aim at conveying key relationships between helical domains: co-axial stacking, bending and all Watson–Crick as well as non-Watson–Crick base pairs. Although such wiring diagrams cannot replace full stereographic images for correct spatial understanding and representation, they reveal structural similarities as well as the conserved patterns and distances between motifs which are present within the interaction networks of folded RNAs of similar or unrelated functions. Finally, the diagrams could help devising methods for meaningfully transforming RNA structures into graphs amenable to network analysis.

Amino acid residue doublet propensity in the protein-RNA interface and its application to RNA interface prediction

Protein-RNA interactions play essential roles in a number of regulatory mechanisms for gene expression such as RNA splicing, transport, translation and post-transcriptional control. As the number of available protein-RNA complex 3D structures has increased, it is now possible to statistically examine protein-RNA interactions based on 3D structures. We performed computational analyses of 86 representative protein-RNA complexes retrieved from the Protein Data Bank. Interface residue propensity, a measure of the relative importance of different amino acid residues in the RNA interface, was calculated for each amino acid residue type (residue singlet interface propensity). In addition to the residue singlet propensity, we introduce a new residue-based propensity, which gives a measure of residue pairing preferences in the RNA interface of a protein (residue doublet interface propensity). The residue doublet interface propensity contains much more information than the sum of two singlet propensities alone. The prediction of the RNA interface using the two types of propensities plus a position-specific multiple sequence profile can achieve a specificity of about 80%. The prediction method was then applied to the 3D structure of two mRNA export factors, TAP (Mex67) and UAP56 (Sub2). The prediction enables us to point out candidate RNA interfaces, part of which are consistent with previous experimental studies and may contribute to elucidation of atomic mechanisms of mRNA export.

Spatial codes in dendritic BC1 RNA

BC1 RNA is a dendritic untranslated RNA that has been implicated in local translational control mechanisms in neurons. Prerequisite for a functional role of the RNA in synaptodendritic domains is its targeted delivery along the dendritic extent. We report here that the targeting-competent 5' BC1 domain carries two dendritic targeting codes. One code, specifying somatic export, is located in the medial-basal region of the 5' BC1 stem-loop structure. It is defined by an export-determinant stem-bulge motif. The second code, specifying long-range dendritic delivery, is located in the apical part of the 5' stem-loop domain. This element features a GA kink-turn (KT) motif that is indispensable for distal targeting. It specifically interacts with heterogeneous nuclear ribonucleoprotein A2, a trans-acting targeting factor that has previously been implicated in the transport of MBP mRNA in oligodendrocytes and neurons. Our work suggests that a BC1 KT motif encodes distal targeting via the A2 pathway and that architectural RNA elements, such as KT motifs, may function as spatial codes in neural cells.

The world of subinhibitory antibiotic concentrations

Although antibiotics have long been known to have multiple effects on bacterial cells at low concentrations, it is only with the advent of genome transcription analyses that these activities have been studied in detail at the level of cell metabolism. It has been shown that all antibiotics, regardless of their receptors and mode of action, exhibit the phenomenon of hormesis and provoke considerable transcription activation at low concentrations. These analyses should be of value in providing information on antibiotic side-effects, in bioactive natural product discovery and antibiotic mode-of-action studies.

Characteristics of the nuclear (18S, 5.8S, 28S and 5S) and mitochondrial (12S and 16S) rRNA genes of Apis mellifera (Insecta: Hymenoptera): structure, organization, and retrotransposable elements

As an accompanying manuscript to the release of the honey bee genome, we report the entire sequence of the nuclear (18S, 5.8S, 28S and 5S) and mitochondrial (12S and 16S) ribosomal RNA (rRNA)-encoding gene sequences (rDNA) and related internally and externally transcribed spacer regions of Apis mellifera (Insecta: Hymenoptera: Apocrita). Additionally, we predict secondary structures for the mature rRNA molecules based on comparative sequence analyses with other arthropod taxa and reference to recently published crystal structures of the ribosome. In general, the structures of honey bee rRNAs are in agreement with previously predicted rRNA models from other arthropods in core regions of the rRNA, with little additional expansion in non-conserved regions. Our multiple sequence alignments are made available on several public databases and provide a preliminary establishment of a global structural model of all rRNAs from the insects. Additionally, we provide conserved stretches of sequences flanking the rDNA cistrons that comprise the externally transcribed spacer regions (ETS) and part of the intergenic spacer region (IGS), including several repetitive motifs. Finally, we report the occurrence of retrotransposition in the nuclear large subunit rDNA, as R2 elements are present in the usual insertion points found in other arthropods. Interestingly, functional R1 elements usually present in the genomes of insects were not detected in the honey bee rRNA genes. The reverse transcriptase products of the R2 elements are deduced from their putative open reading frames and structurally aligned with those from another hymenopteran insect, the jewel wasp Nasonia (Pteromalidae). Stretches of conserved amino acids shared between Apis and Nasonia are illustrated and serve as potential sites for primer design, as target amplicons within these R2 elements may serve as novel phylogenetic markers for Hymenoptera. Given the impending completion of the sequencing of the Nasonia genome, we expect our report eventually to shed light on the evolution of the hymenopteran genome within higher insects, particularly regarding the relative maintenance of conserved rDNA genes, related variable spacer regions and retrotransposable elements.

Displaying 3D data on RNA secondary structures: coloRNA

RNA performs a variety of diverse functions and therefore must adopt many different three-dimensional conformations. The number and complexity of RNA structures that are currently available are steadily increasing, necessitating the generation of versatile structure visualization tools. Here, we describe a new RNA secondary and tertiary structure visualization tool, the display program coloRNA. This program colors each nucleotide in a secondary structure schematic according to the value of an assigned property of the corresponding backbone phosphate group, such as the distance between corresponding residues in two atomic models of the same RNA molecule. To assist in analyzing tertiary structure, coloRNA also colors nucleotides based on the three-dimensional distances between a user-selected nucleotide and all others. Minimum and maximum thresholds can be used to focus in on, or eliminate, a particular value range. coloRNA can display a user-specified group of nucleotides by outlining the structure in an automatically assigned, but user-changeable color. As an example, we have used coloRNA to analyze a pair of recently published structures of the Escherichia coli 70S ribosome. When coloRNA is used to display the conformational difference between the two structures, the large movement of the small subunit head stands visually out from the background changes in the remaining domains of the small subunit.

BindN: a web-based tool for efficient prediction of DNA and RNA binding sites in amino acid sequences

BindN (http://bioinformatics.ksu.edu/bindn/) takes an amino acid sequence as input and predicts potential DNA or RNA-binding residues with support vector machines (SVMs). Protein datasets with known DNA or RNA-binding residues were selected from the Protein Data Bank (PDB), and SVM models were constructed using data instances encoded with three sequence features, including the side chain pK(a) value, hydrophobicity index and molecular mass of an amino acid. The results suggest that DNA-binding residues can be predicted at 69.40% sensitivity and 70.47% specificity, while prediction of RNA-binding residues achieves 66.28% sensitivity and 69.84% specificity. When compared with previous studies, the SVM models appear to be more accurate and more efficient for online predictions. BindN provides a useful tool for understanding the function of DNA and RNA-binding proteins based on primary sequence data.

The UAA/GAN internal loop motif: a new RNA structural element that forms a cross-strand AAA stack and long-range tertiary interactions

Analysis of aligned RNA sequences and high-resolution crystal structures has revealed a new RNA structural element, termed the UAA/GAN motif. Found in internal loops of the 23 S rRNA, as well as in RNase P RNA and group I and II introns, this six-nucleotide motif adopts a distinctive local structure that includes two base-pairs with non-canonical conformations and three conserved adenine bases, which form a cross-strand AAA stack in the minor groove. Most importantly, the motif invariably forms long-range tertiary contacts, as the AAA stack typically forms A-minor interactions and the flipped-out N nucleotide forms additional contacts that are specific to the structural context of each loop. The widespread presence of this motif and its propensity to form long-range contacts suggest that it plays a critical role in defining the architectures of structured RNAs.

YebU is a m5C Methyltransferase Specific for 16 S rRNA Nucleotide 1407

The rRNAs in Escherichia coli contain methylations at 24 nucleotides, which collectively are important for ribosome function. Three of these methylations are m5C modifications located at nucleotides C967 and C1407 in 16S rRNA and at nucleotide C1962 in 23S rRNA. Bacterial rRNA modifications generally require specific enzymes, and only one m5C rRNA methyltransferase, RsmB (formerly Fmu) that methylates nucleotide C967, has previously been identified. BLAST searches of the E.coli genome revealed a single gene, yebU, with sufficient similarity to rsmB to encode a putative m5C RNA methyltransferase. This suggested that the yebU gene product modifies C1407 and/or C1962. Here, we analysed the E.coli rRNAs by matrix assisted laser desorption/ionization mass spectrometry and show that inactivation of the yebU gene leads to loss of methylation at C1407 in 16 S rRNA, but does not interfere with methylation at C1962 in 23 S rRNA. Purified recombinant YebU protein retains its specificity for C1407 in vitro, and methylates 30 S subunits (but not naked 16 S rRNA or 70 S ribosomes) isolated from yebU knockout strains. Nucleotide C1407 is located at a functionally active region of the 30 S subunit interface close to the P site, and YebU-directed methylation of this nucleotide seems to be conserved in bacteria. The yebU knockout strains display slower growth and reduced fitness in competition with wild-type cells. We suggest that a more appropriate designation for yebU would be the rRNA small subunit methyltransferase gene rsmF, and that the nomenclature system be extended to include the rRNA methyltransferases that still await identification.

Following the dynamics of changes in solvent accessibility of 16 S and 23 S rRNA during ribosomal subunit association using synchrotron-generated hydroxyl radicals

We have probed the structure and dynamics of ribosomal RNA in the Escherichia coli ribosome using equilibrium and time-resolved hydroxyl radical (OH) RNA footprinting to explore changes in the solvent-accessible surface of the rRNA with single-nucleotide resolution. The goal of these studies is to better understand the structural transitions that accompany association of the 30 S and 50 S subunits and to build a foundation for the quantitative analysis of ribosome structural dynamics during translation. Clear portraits of the subunit interface surfaces for 16 S and 23 S rRNA were obtained by constructing difference maps between the OH protection maps of the free subunits and that of the associated ribosome. In addition to inter-subunit contacts consistent with the crystal structure, additional OH protections are evident in regions at or near the subunit interface that reflect association-induced conformational changes. Comparison of these data with the comparable difference maps of the solvent-accessible surface of the rRNA calculated for the Thermus thermophilus X-ray crystal structures shows extensive agreement but also distinct differences. As a prelude to time-resolved OH footprinting studies, the reactivity profiles obtained using Fe(II)EDTA and X-ray generated OH were comprehensively compared. The reactivity patterns are similar except for a small number of nucleotides that have decreased reactivity to OH generated from Fe(II)EDTA compared to X-rays. These nucleotides are generally close to ribosomal proteins, which can quench diffusing radicals by virtue of side-chain oxidation. Synchrotron X-ray OH footprinting was used to monitor the kinetics of association of the 30 S and 50 S subunits. The rates individually measured for the inter-subunit contacts are comparable within experimental error. The application of this approach to the study of ribosome dynamics during the translation cycle is discussed.

Non-coding RNA

The term non-coding RNA (ncRNA) is commonly employed for RNA that does not encode a protein, but this does not mean that such RNAs do not contain information nor have function. Although it has been generally assumed that most genetic information is transacted by proteins, recent evidence suggests that the majority of the genomes of mammals and other complex organisms is in fact transcribed into ncRNAs, many of which are alternatively spliced and/or processed into smaller products. These ncRNAs include microRNAs and snoRNAs (many if not most of which remain to be identified), as well as likely other classes of yet-to-be-discovered small regulatory RNAs, and tens of thousands of longer transcripts (including complex patterns of interlacing and overlapping sense and antisense transcripts), most of whose functions are unknown. These RNAs (including those derived from introns) appear to comprise a hidden layer of internal signals that control various levels of gene expression in physiology and development, including chromatin architecture/epigenetic memory, transcription, RNA splicing, editing, translation and turnover. RNA regulatory networks may determine most of our complex characteristics, play a significant role in disease and constitute an unexplored world of genetic variation both within and between species.

Mass fractal dimension of the ribosome and implication of its dynamic characteristics

Self-similar properties of the ribosome in terms of the mass fractal dimension are investigated. We find that both the 30S subunit and the 16S rRNA have fractal dimensions of 2.58 and 2.82, respectively; while the 50S subunit as well as the 23S rRNA has the mass fractal dimension close to 3, implying a compact three-dimensional macromolecule. This finding supports the dynamic and active role of the 30S subunit in the protein synthesis, in contrast to the pass role of the 50S subunit.

The RNA Ontology Consortium: an open invitation to the RNA community

The aim of the RNA Ontology Consortium (ROC) is to create an integrated conceptual framework-an RNA Ontology (RO)-with a common, dynamic, controlled, and structured vocabulary to describe and characterize RNA sequences, secondary structures, three-dimensional structures, and dynamics pertaining to RNA function. The RO should produce tools for clear communication about RNA structure and function for multiple uses, including the integration of RNA electronic resources into the Semantic Web. These tools should allow the accurate description in computer-interpretable form of the coupling between RNA architecture, function, and evolution. The purposes for creating the RO are, therefore, (1) to integrate sequence and structural databases; (2) to allow different computational tools to interoperate; (3) to create powerful software tools that bring advanced computational methods to the bench scientist; and (4) to facilitate precise searches for all relevant information pertaining to RNA. For example, one initial objective of the ROC is to define, identify, and classify RNA structural motifs described in the literature or appearing in databases and to agree on a computer-interpretable definition for each of these motifs. To achieve these aims, the ROC will foster communication and promote collaboration among RNA scientists by coordinating frequent face-to-face workshops to discuss, debate, and resolve difficult conceptual issues. These meeting opportunities will create new directions at various levels of RNA research. The ROC will work closely with the PDB/NDB structural databases and the Gene, Sequence, and Open Biomedical Ontology Consortia to integrate the RO with existing biological ontologies to extend existing content while maintaining interoperability.

Structural and evolutionary classification of G/U wobble basepairs in the ribosome

We present a comprehensive structural, evolutionary and molecular dynamics (MD) study of the G/U wobble basepairs in the ribosome based on high-resolution crystal structures, including the recent Escherichia coli structure. These basepairs are classified according to their tertiary interactions, and sequence conservation at their positions is determined. G/U basepairs participating in tertiary interactions are more conserved than those lacking any interactions. Specific interactions occurring in the G/U shallow groove pocket--like packing interactions (P-interactions) and some phosphate backbone interactions (phosphate-in-pocket interactions)--lead to higher G/U conservation than others. Two salient cases of unique phylogenetic compensation are discovered. First, a P-interaction is conserved through a series of compensatory mutations involving all four participating nucleotides to preserve or restore the G/U in the optimal orientation. Second, a G/U basepair forming a P-interaction and another one forming a phosphate-in-pocket interaction are replaced by GNRA loops that maintain similar tertiary contacts. MD simulations were carried out on eight P-interactions. The specific GU/CG signature of this interaction observed in structure and sequence analysis was rationalized, and can now be used for improving sequence alignments.

The expanding transcriptome: the genome as the 'Book of Sand'

The central dogma of molecular biology inspired by classical work in prokaryotic organisms accounts for only part of the genetic agenda of complex eukaryotes. First, post-transcriptional events lead to the generation of multiple mRNAs, proteins and functions from a single primary transcript, revealing regulatory networks distinct in mechanism and biological function from those controlling RNA transcription. Second, a variety of populous families of small RNAs (small nuclear RNAs, small nucleolar RNAs, microRNAs, siRNAs and shRNAs) assemble on ribonucleoprotein complexes and regulate virtually all aspects of the gene expression pathway, with profound biological consequences. Third, high-throughput methods of genomic analysis reveal that RNAs other than non-protein-coding RNAs (ncRNAs) represent a major component of the transcriptome that may perform novel functions in gene regulation and beyond. Post-transcriptional regulation, small RNAs and ncRNAs provide an expanding picture of the transcriptome that enriches our views of what genes are, how they operate, evolve and are regulated.

Mg2+-induced compaction of single RNA molecules monitored by tethered particle microscopy

We have applied tethered particle microscopy (TPM) as a single molecule analysis tool to studies of the conformational dynamics of poly-uridine(U) messenger (m)RNA and 16S ribosomal (r)RNA molecules. Using stroboscopic total internal reflection illumination and rigorous selection criteria to distinguish from nonspecific tethering, we have tracked the nanometer-scale Brownian motion of RNA-tethered fluorescent microspheres in all three dimensions at pH 7.5, 22 degrees C, in 10 mM or 100 mM NaCl in the absence or presence of 10 mM MgCl(2). The addition of Mg(2+) to low-ionic strength buffer results in significant compaction and stiffening of poly(U) mRNA, but not of 16S rRNA. Furthermore, the motion of poly(U)-tethered microspheres is more heterogeneous than that of 16S rRNA-tethered microspheres. Analysis of in-plane bead motion suggests that poly(U) RNA, but less so 16S rRNA, can be modeled both in the presence and absence of Mg(2+) by a statistical Gaussian polymer model. We attribute these differences to the Mg(2+)-induced compaction of the relatively weakly structured and structurally disperse poly(U) mRNA, in contrast to Mg(2+)-induced reinforcement of existing secondary and tertiary structure contacts in the highly structured 16S rRNA. Both effects are nonspecific, however, as they are dampened in the presence of higher concentrations of monovalent cations.

Restoration of native folding of single-stranded DNA sequences through reverse mutations: an indication of a new epigenetic mechanism

We used in vivo (biological), in silico (computational structure prediction), and in vitro (model sequence folding) analyses of single-stranded DNA sequences to show that nucleic acid folding conservation is the selective principle behind a high-frequency single-nucleotide reversion observed in a three-nucleotide mutated motif of the Maize streak virus replication associated protein (Rep) gene. In silico and in vitro studies showed that the three-nucleotide mutation adversely affected Rep nucleic acid folding, and that the single-nucleotide reversion [C(601)A] restored wild-type-like folding. In vivo support came from infecting maize with mutant viruses: those with Rep genes containing nucleotide changes predicted to restore a wild-type-like fold [A(601)/G(601)] preferentially accumulated over those predicted to fold differently [C(601)/T(601)], which frequently reverted to A(601) and displaced the original population. We propose that the selection of native nucleic acid folding is an epigenetic effect, which might have broad implications in the evolution of plants and their viruses.

Cooperative binding and self-assembling behavior of cationic low molecular-weight dendrons with RNA molecules

Tri(ethylene glycol) derived, low molecular-weight dendrons with various amine end groups were synthesized and characterized for their properties of binding and self-assembling with RNA using the Candida ribozyme as a model RNA molecule. These dendritic compounds form stable complexes and well-defined nanoscale particles with RNA molecules via electrostatic interactions and self-assembly process, while leaving the other terminal of the tri(ethylene glycol) chain accessible for targeting. This suggests that dendrimers of this type hold great promise for specific RNA targeting and RNA delivery.

Analysis of Conformational Changes in 16S rRNA During the Course of 30S Subunit Assembly

Ribosome biogenesis involves an integrated series of binding events coupled with conformational changes that ultimately result in the formation of a functional macromolecular complex. In vitro, Escherichia coli 30 S subunit assembly occurs in a cooperative manner with the ordered addition of 20 ribosomal proteins (r-proteins) with 16 S rRNA. The assembly pathway for 30 S subunits has been dissected in vitro into three steps, where specific r-proteins associate with 16 S rRNA early in 30 S subunit assembly, followed by a mid-assembly conformational rearrangement of the complex that then enables the remaining r-proteins to associate in the final step. Although the three steps of 30 S subunit assembly have been known for some time, few details have been elucidated about changes that occur as a result of these three specific stages. Here, we present a detailed analysis of the concerted early and late stages of small ribosomal subunit assembly. Conformational changes, roles for base-pairing and r-proteins at specific stages of assembly, and a polar nature to the assembly process have been revealed. This work has allowed a more comprehensive and global view of E.coli 30 S ribosomal subunit assembly to be obtained.