1
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Pujari N, Saundh SL, Acquah FA, Mooers BHM, Ferré-D’Amaré AR, Leung AKW. Engineering Crystal Packing in RNA Structures I: Past and Future Strategies for Engineering RNA Packing in Crystals. CRYSTALS 2021; 11:952. [PMID: 34745656 PMCID: PMC8570644 DOI: 10.3390/cryst11080952] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
X-ray crystallography remains a powerful method to gain atomistic insights into the catalytic and regulatory functions of RNA molecules. However, the technique requires the preparation of diffraction-quality crystals. This is often a resource- and time-consuming venture because RNA crystallization is hindered by the conformational heterogeneity of RNA, as well as the limited opportunities for stereospecific intermolecular interactions between RNA molecules. The limited success at crystallization explains in part the smaller number of RNA-only structures in the Protein Data Bank. Several approaches have been developed to aid the formation of well-ordered RNA crystals. The majority of these are construct-engineering techniques that aim to introduce crystal contacts to favor the formation of well-diffracting crystals. A typical example is the insertion of tetraloop-tetraloop receptor pairs into non-essential RNA segments to promote intermolecular association. Other methods of promoting crystallization involve chaperones and crystallization-friendly molecules that increase RNA stability and improve crystal packing. In this review, we discuss the various techniques that have been successfully used to facilitate crystal packing of RNA molecules, recent advances in construct engineering, and directions for future research in this vital aspect of RNA crystallography.
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Affiliation(s)
- Narsimha Pujari
- Department of Veterinary Biomedical Sciences, University of Saskatchewan, Saskatoon, SK S7N 5B4, Canada
| | - Stephanie L. Saundh
- Department of Veterinary Biomedical Sciences, University of Saskatchewan, Saskatoon, SK S7N 5B4, Canada
| | - Francis A. Acquah
- Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
| | - Blaine H. M. Mooers
- Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
- Stephenson Cancer Center, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
| | - Adrian R. Ferré-D’Amaré
- Biochemistry and Biophysics Center, National Heart, Lung and Blood Institute, Bethesda, MD 20892, USA
| | - Adelaine Kwun-Wai Leung
- Department of Veterinary Biomedical Sciences, University of Saskatchewan, Saskatoon, SK S7N 5B4, Canada
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2
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Richardson KE, Adams MS, Kirkpatrick CC, Gohara DW, Znosko BM. Identification and Characterization of New RNA Tetraloop Sequence Families. Biochemistry 2019; 58:4809-4820. [PMID: 31714066 DOI: 10.1021/acs.biochem.9b00535] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
There is an abundance of RNA sequence information available due to the efforts of sequencing projects. However, current techniques implemented to solve the tertiary structures of RNA, such as NMR and X-ray crystallography, are difficult and time-consuming. Therefore, biophysical techniques are not able to keep pace with the abundance of sequence information available. Because of this, there is a need to develop quick and efficient ways to predict RNA tertiary structure from sequence. One promising approach is to identify structural patterns within previously solved 3D structures and apply these patterns to new sequences. RNA tetraloops are one of the most common naturally occurring secondary structure motifs. Here, we use RNA Characterization of Secondary Structure Motifs (CoSSMos), Dissecting the Spatial Structure of RNA (DSSR), and a bioinformatic approach to search for and characterize tertiary structure patterns among tetraloops. Not surprising, we identified the well-known GNRA and UNCG tetraloops, as well as the previously identified RNYA tetraloop. However, some previously identified characteristics of these families were not observed in this data set, and some new characteristics were identified. In addition, we also identified and characterized three new tetraloop sequence families: YGAR, UGGU, and RMSA. This new structural information sheds light on the tertiary structure of tetraloops and contributes to the efforts of RNA tertiary structure prediction from sequence.
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Affiliation(s)
- Katherine E Richardson
- Department of Chemistry , Saint Louis University , Saint Louis , Missouri 63103 , United States
| | - Miranda S Adams
- Department of Chemistry , Saint Louis University , Saint Louis , Missouri 63103 , United States
| | - Charles C Kirkpatrick
- Department of Chemistry , Saint Louis University , Saint Louis , Missouri 63103 , United States
| | - David W Gohara
- Department of Biochemistry and Molecular Biology , Saint Louis University , Saint Louis , Missouri 63103 , United States
| | - Brent M Znosko
- Department of Chemistry , Saint Louis University , Saint Louis , Missouri 63103 , United States
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3
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D'Ascenzo L, Vicens Q, Auffinger P. Identification of receptors for UNCG and GNRA Z-turns and their occurrence in rRNA. Nucleic Acids Res 2019; 46:7989-7997. [PMID: 29986118 PMCID: PMC6125677 DOI: 10.1093/nar/gky578] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2018] [Accepted: 07/01/2018] [Indexed: 12/17/2022] Open
Abstract
In contrast to GNRA tetraloop receptors that are common in RNA, receptors for the more thermostable UNCG loops have remained elusive for almost three decades. An analysis of all RNA structures with resolution ≤3.0 Å from the PDB allowed us to identify three previously unnoticed receptors for UNCG and GNRA tetraloops that adopt a common UNCG fold, named ‘Z-turn’ in agreement with our previously published nomenclature. These receptors recognize the solvent accessible second Z-turn nucleotide in different but specific ways. Two receptors participating in a complex network of tertiary interactions are associated with the rRNA UUCG and GAAA Z-turns capping helices H62 and H35a in rRNA large subunits. Structural comparison of fully assembled ribosomes and comparative sequence analysis of >6500 rRNA sequences helped us recognize that these motifs are almost universally conserved in rRNA, where they may contribute to organize the large subunit around the subdomain-IV four-way junction. The third UCCG receptor was identified in a rRNA/protein construct crystallized at acidic pH. These three non-redundant Z-turn receptors are relevant for our understanding of the assembly of rRNA and other long-non-coding RNAs, as well as for the design of novel folding motifs for synthetic biology.
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Affiliation(s)
- Luigi D'Ascenzo
- Architecture et Réactivité de l'ARN, Université de Strasbourg, Institut de Biologie Moléculaire et Cellulaire du CNRS, 67084 Strasbourg, France.,Department of Integrative Structural and Computational Biology, The Scripps Research Institute, La Jolla, CA 92037, USA
| | - Quentin Vicens
- Architecture et Réactivité de l'ARN, Université de Strasbourg, Institut de Biologie Moléculaire et Cellulaire du CNRS, 67084 Strasbourg, France.,Department of Biochemistry and Molecular Genetics, RNA Bioscience Initiative, University of Colorado, Denver School of Medicine, Aurora, CO 80045, USA
| | - Pascal Auffinger
- Architecture et Réactivité de l'ARN, Université de Strasbourg, Institut de Biologie Moléculaire et Cellulaire du CNRS, 67084 Strasbourg, France
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4
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Šponer J, Bussi G, Krepl M, Banáš P, Bottaro S, Cunha RA, Gil-Ley A, Pinamonti G, Poblete S, Jurečka P, Walter NG, Otyepka M. RNA Structural Dynamics As Captured by Molecular Simulations: A Comprehensive Overview. Chem Rev 2018; 118:4177-4338. [PMID: 29297679 PMCID: PMC5920944 DOI: 10.1021/acs.chemrev.7b00427] [Citation(s) in RCA: 336] [Impact Index Per Article: 56.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2017] [Indexed: 12/14/2022]
Abstract
With both catalytic and genetic functions, ribonucleic acid (RNA) is perhaps the most pluripotent chemical species in molecular biology, and its functions are intimately linked to its structure and dynamics. Computer simulations, and in particular atomistic molecular dynamics (MD), allow structural dynamics of biomolecular systems to be investigated with unprecedented temporal and spatial resolution. We here provide a comprehensive overview of the fast-developing field of MD simulations of RNA molecules. We begin with an in-depth, evaluatory coverage of the most fundamental methodological challenges that set the basis for the future development of the field, in particular, the current developments and inherent physical limitations of the atomistic force fields and the recent advances in a broad spectrum of enhanced sampling methods. We also survey the closely related field of coarse-grained modeling of RNA systems. After dealing with the methodological aspects, we provide an exhaustive overview of the available RNA simulation literature, ranging from studies of the smallest RNA oligonucleotides to investigations of the entire ribosome. Our review encompasses tetranucleotides, tetraloops, a number of small RNA motifs, A-helix RNA, kissing-loop complexes, the TAR RNA element, the decoding center and other important regions of the ribosome, as well as assorted others systems. Extended sections are devoted to RNA-ion interactions, ribozymes, riboswitches, and protein/RNA complexes. Our overview is written for as broad of an audience as possible, aiming to provide a much-needed interdisciplinary bridge between computation and experiment, together with a perspective on the future of the field.
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Affiliation(s)
- Jiří Šponer
- Institute of Biophysics of the Czech Academy of Sciences , Kralovopolska 135 , Brno 612 65 , Czech Republic
| | - Giovanni Bussi
- Scuola Internazionale Superiore di Studi Avanzati , Via Bonomea 265 , Trieste 34136 , Italy
| | - Miroslav Krepl
- Institute of Biophysics of the Czech Academy of Sciences , Kralovopolska 135 , Brno 612 65 , Czech Republic
- Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science , Palacky University Olomouc , 17. listopadu 12 , Olomouc 771 46 , Czech Republic
| | - Pavel Banáš
- Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science , Palacky University Olomouc , 17. listopadu 12 , Olomouc 771 46 , Czech Republic
| | - Sandro Bottaro
- Structural Biology and NMR Laboratory, Department of Biology , University of Copenhagen , Copenhagen 2200 , Denmark
| | - Richard A Cunha
- Scuola Internazionale Superiore di Studi Avanzati , Via Bonomea 265 , Trieste 34136 , Italy
| | - Alejandro Gil-Ley
- Scuola Internazionale Superiore di Studi Avanzati , Via Bonomea 265 , Trieste 34136 , Italy
| | - Giovanni Pinamonti
- Scuola Internazionale Superiore di Studi Avanzati , Via Bonomea 265 , Trieste 34136 , Italy
| | - Simón Poblete
- Scuola Internazionale Superiore di Studi Avanzati , Via Bonomea 265 , Trieste 34136 , Italy
| | - Petr Jurečka
- Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science , Palacky University Olomouc , 17. listopadu 12 , Olomouc 771 46 , Czech Republic
| | - Nils G Walter
- Single Molecule Analysis Group and Center for RNA Biomedicine, Department of Chemistry , University of Michigan , Ann Arbor , Michigan 48109 , United States
| | - Michal Otyepka
- Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science , Palacky University Olomouc , 17. listopadu 12 , Olomouc 771 46 , Czech Republic
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5
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Mapping the Universe of RNA Tetraloop Folds. Biophys J 2017; 113:257-267. [PMID: 28673616 DOI: 10.1016/j.bpj.2017.06.011] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/06/2017] [Revised: 06/08/2017] [Accepted: 06/08/2017] [Indexed: 11/22/2022] Open
Abstract
We report a map of RNA tetraloop conformations constructed by calculating pairwise distances among all experimentally determined four-nucleotide hairpin loops. Tetraloops with similar structures are clustered together and, as expected, the two largest clusters are the canonical GNRA and UNCG folds. We identify clusters corresponding to known tetraloop folds such as GGUG, RNYA, AGNN, and CUUG. These clusters are represented in a simple two-dimensional projection that recapitulates the relationship among the different folds. The cluster analysis also identifies 20 novel tetraloop folds that are peculiar to specific positions in ribosomal RNAs and that are stabilized by tertiary interactions. In our RNA tetraloop database we find a significant number of non-GNRA and non-UNCG sequences adopting the canonical GNRA and UNCG folds. Conversely, we find a significant number of GNRA and UNCG sequences adopting non-GNRA and non-UNCG folds. Our analysis demonstrates that there is not a simple one-to-one, but rather a many-to-many mapping between tetraloop sequence and tetraloop fold.
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6
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D'Ascenzo L, Leonarski F, Vicens Q, Auffinger P. Revisiting GNRA and UNCG folds: U-turns versus Z-turns in RNA hairpin loops. RNA (NEW YORK, N.Y.) 2017; 23:259-269. [PMID: 27999116 PMCID: PMC5311481 DOI: 10.1261/rna.059097.116] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/08/2016] [Accepted: 11/29/2016] [Indexed: 06/06/2023]
Abstract
When thinking about RNA three-dimensional structures, coming across GNRA and UNCG tetraloops is perceived as a boon since their folds have been extensively described. Nevertheless, analyzing loop conformations within RNA and RNP structures led us to uncover several instances of GNRA and UNCG loops that do not fold as expected. We noticed that when a GNRA does not assume its "natural" fold, it adopts the one we typically associate with a UNCG sequence. The same folding interconversion may occur for loops with UNCG sequences, for instance within tRNA anticodon loops. Hence, we show that some structured tetranucleotide sequences starting with G or U can adopt either of these folds. The underlying structural basis that defines these two fold types is the mutually exclusive stacking of a backbone oxygen on either the first (in GNRA) or the last nucleobase (in UNCG), generating an oxygen-π contact. We thereby propose to refrain from using sequences to distinguish between loop conformations. Instead, we suggest using descriptors such as U-turn (for "GNRA-type" folds) and a newly described Z-turn (for "UNCG-type" folds). Because tetraloops adopt for the largest part only two (inter)convertible turns, we are better able to interpret from a structural perspective loop interchangeability occurring in ribosomes and viral RNA. In this respect, we propose a general view on the inclination for a given sequence to adopt (or not) a specific fold. We also suggest how long-noncoding RNAs may adopt discrete but transient structures, which are therefore hard to predict.
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Affiliation(s)
- Luigi D'Ascenzo
- Université de Strasbourg, CNRS, Architecture et Réactivité de l'ARN, UPR 9002, F-67000 Strasbourg, France
| | - Filip Leonarski
- Université de Strasbourg, CNRS, Architecture et Réactivité de l'ARN, UPR 9002, F-67000 Strasbourg, France
- Faculty of Chemistry, University of Warsaw, 02-093 Warsaw, Poland
| | - Quentin Vicens
- Université de Strasbourg, CNRS, Architecture et Réactivité de l'ARN, UPR 9002, F-67000 Strasbourg, France
| | - Pascal Auffinger
- Université de Strasbourg, CNRS, Architecture et Réactivité de l'ARN, UPR 9002, F-67000 Strasbourg, France
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7
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Chan RT, Keating KS, Go MC, Toor N. Identification of a GUAAY Pentaloop Sequence Involved in a Novel RNA Loop-Helix Interaction. J Mol Biol 2016; 428:4882-4889. [PMID: 27771480 PMCID: PMC5138090 DOI: 10.1016/j.jmb.2016.10.015] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2016] [Revised: 10/11/2016] [Accepted: 10/11/2016] [Indexed: 11/25/2022]
Abstract
Large RNAs often utilize GNRA tetraloops as structural elements to stabilize the overall tertiary fold. These tetraloop-receptor (TR) interactions have a conserved geometry in which the tetraloop docks into the receptor at an angle of ~15° from the helix containing the receptor. Here, we show that the conserved GUAAY pentaloop found in domain III of group IIB1 introns participates in a novel class of RNA tertiary interaction with a geometry and mode of binding that are significantly different from that found in GNRA TR interactions. This pentaloop is highly conserved within the IIB1 class and interacts with the minor groove of the catalytic domain V. The base planes of the loop and receptor nucleotides are not coplanar and greatly deviate from standard A-minor motifs. The helical axis of the GUAAY stem loop diverges ~70° from the angle of insertion found in a typical GNRA TR interaction. Therefore, the loop architecture and insertion orientation are distinctive, with in vitro splicing data indicating that a GNRA tetraloop is incompatible at this position. The GUAAY pentaloop-receptor motif is also found in the structure of the eukaryotic thiamine pyrophosphate riboswitch in the context of a hexanucleotide loop sequence. We therefore propose, based on phylogenetic, structural, and biochemical data, that the GUAAY pentaloop-receptor interaction represents a novel structural motif that is present in multiple structured RNAs.
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Affiliation(s)
- Russell T Chan
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, 92093, USA
| | | | - Michaela C Go
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, 92093, USA
| | - Navtej Toor
- Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, 92093, USA.
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8
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Analysis of stacking overlap in nucleic acid structures: algorithm and application. J Comput Aided Mol Des 2014; 28:851-67. [DOI: 10.1007/s10822-014-9767-6] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2014] [Accepted: 06/23/2014] [Indexed: 10/25/2022]
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9
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Somarowthu S, Legiewicz M, Keating KS, Pyle AM. Visualizing the ai5γ group IIB intron. Nucleic Acids Res 2013; 42:1947-58. [PMID: 24203709 PMCID: PMC3919574 DOI: 10.1093/nar/gkt1051] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
It has become apparent that much of cellular metabolism is controlled by large well-folded noncoding RNA molecules. In addition to crystallographic approaches, computational methods are needed for visualizing the 3D structure of large RNAs. Here, we modeled the molecular structure of the ai5γ group IIB intron from yeast using the crystal structure of a bacterial group IIC homolog. This was accomplished by adapting strategies for homology and de novo modeling, and creating a new computational tool for RNA refinement. The resulting model was validated experimentally using a combination of structure-guided mutagenesis and RNA structure probing. The model provides major insights into the mechanism and regulation of splicing, such as the position of the branch-site before and after the second step of splicing, and the location of subdomains that control target specificity, underscoring the feasibility of modeling large functional RNA molecules.
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Affiliation(s)
- Srinivas Somarowthu
- Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, CT 06511, USA, Department of Chemistry, Yale University, New Haven, CT 06511, USA and Howard Hughes Medical Institute, Chevy Chase, MD 20815, USA
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10
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Thapar R, Denmon AP, Nikonowicz EP. Recognition modes of RNA tetraloops and tetraloop-like motifs by RNA-binding proteins. WILEY INTERDISCIPLINARY REVIEWS-RNA 2013; 5:49-67. [PMID: 24124096 DOI: 10.1002/wrna.1196] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Subscribe] [Scholar Register] [Received: 02/05/2013] [Revised: 08/13/2013] [Accepted: 08/18/2013] [Indexed: 12/19/2022]
Abstract
RNA hairpins are the most commonly occurring secondary structural elements in RNAs and serve as nucleation sites for RNA folding, RNA-RNA, and RNA-protein interactions. RNA hairpins are frequently capped by tetraloops, and based on sequence similarity, three broad classes of RNA tetraloops have been defined: GNRA, UNCG, and CUYG. Other classes such as the UYUN tetraloop in histone mRNAs, the UGAA in 16S rRNA, the AUUA tetraloop from the MS2 bacteriophage, and the AGNN tetraloop that binds RNase III have also been characterized. The tetraloop structure is compact and is usually characterized by a paired interaction between the first and fourth nucleotides. The two unpaired nucleotides in the loop are usually involved in base-stacking or base-phosphate hydrogen bonding interactions. Several structures of RNA tetraloops, free and complexed to other RNAs or proteins, are now available and these studies have increased our understanding of the diverse mechanisms by which this motif is recognized. RNA tetraloops can mediate RNA-RNA contacts via the tetraloop-receptor motif, kissing hairpin loops, A-minor interactions, and pseudoknots. While these RNA-RNA interactions are fairly well understood, how RNA-binding proteins recognize RNA tetraloops and tetraloop-like motifs remains unclear. In this review, we summarize the structures of RNA tetraloop-protein complexes and the general themes that have emerged on sequence- and structure-specific recognition of RNA tetraloops. We highlight how proteins achieve molecular recognition of this nucleic acid motif, the structural adaptations observed in the tetraloop to accommodate the protein-binding partner, and the role of dynamics in recognition.
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Affiliation(s)
- Roopa Thapar
- Department of Structural Biology, Hauptman-Woodward Medical Research Institute, Buffalo, NY, USA; Department of Structural Biology, SUNY at Buffalo, Buffalo, NY, USA; Department of Biochemistry and Cell Biology, Rice University, Houston, TX, USA
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11
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Appasamy SD, Ramlan EI, Firdaus-Raih M. Comparative sequence and structure analysis reveals the conservation and diversity of nucleotide positions and their associated tertiary interactions in the riboswitches. PLoS One 2013; 8:e73984. [PMID: 24040136 PMCID: PMC3764141 DOI: 10.1371/journal.pone.0073984] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2013] [Accepted: 07/25/2013] [Indexed: 12/17/2022] Open
Abstract
The tertiary motifs in complex RNA molecules play vital roles to either stabilize the formation of RNA 3D structure or to provide important biological functionality to the molecule. In order to better understand the roles of these tertiary motifs in riboswitches, we examined 11 representative riboswitch PDB structures for potential agreement of both motif occurrences and conservations. A total of 61 unique tertiary interactions were found in the reference structures. In addition to the expected common A-minor motifs and base-triples mainly involved in linking distant regions the riboswitch structures three highly conserved variants of A-minor interactions called G-minors were found in the SAM-I and FMN riboswitches where they appear to be involved in the recognition of the respective ligand’s functional groups. From our structural survey as well as corresponding structure and sequence alignments, the agreement between motif occurrences and conservations are very prominent across the representative riboswitches. Our analysis provide evidence that some of these tertiary interactions are essential components to form the structure where their sequence positions are conserved despite a high degree of diversity in other parts of the respective riboswitches sequences. This is indicative of a vital role for these tertiary interactions in determining the specific biological function of riboswitch.
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Affiliation(s)
- Sri D Appasamy
- School of Biosciences and Biotechnology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, UKM Bangi, Selangor, Malaysia
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12
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Abstract
Nearly two decades after Westhof and Michel first proposed that RNA tetraloops may interact with distal helices, tetraloop–receptor interactions have been recognized as ubiquitous elements of RNA tertiary structure. The unique architecture of GNRA tetraloops (N=any nucleotide, R=purine) enables interaction with a variety of receptors, e.g., helical minor grooves and asymmetric internal loops. The most common example of the latter is the GAAA tetraloop–11 nt tetraloop receptor motif. Biophysical characterization of this motif provided evidence for the modularity of RNA structure, with applications spanning improved crystallization methods to RNA tectonics. In this review, we identify and compare types of GNRA tetraloop–receptor interactions. Then we explore the abundance of structural, kinetic, and thermodynamic information on the frequently occurring and most widely studied GAAA tetraloop–11 nt receptor motif. Studies of this interaction have revealed powerful paradigms for structural assembly of RNA, as well as providing new insights into the roles of cations, transition states and protein chaperones in RNA folding pathways. However, further research will clearly be necessary to characterize other tetraloop–receptor and long-range tertiary binding interactions in detail – an important milestone in the quantitative prediction of free energy landscapes for RNA folding.
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13
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Ishikawa J, Furuta H, Ikawa Y. An in vitro-selected RNA receptor for the GAAC loop: modular receptor for non-GNRA-type tetraloop. Nucleic Acids Res 2013; 41:3748-59. [PMID: 23382175 PMCID: PMC3616724 DOI: 10.1093/nar/gkt040] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
Although artificial RNA motifs that can functionally replace the GNRA/receptor interaction, a class of RNA-RNA interacting motifs, were isolated from RNA libraries and used to generate designer RNA structures, receptors for non-GNRA tetraloops have not been found in nature or selected from RNA libraries. In this study, we report successful isolation of a receptor motif interacting with GAAC, a non-GNRA tetraloop, from randomized sequences embedded in a catalytic RNA. Biochemical characterization of the GAAC/receptor interacting motif within three structural contexts showed its binding affinity, selectivity and structural autonomy. The motif has binding affinity comparable with that of a GNRA/receptor, selectivity orthogonal to GNRA/receptors and structural autonomy even in a large RNA context. These features would be advantageous for usage of the motif as a building block for designer RNAs. The isolated motif can also be used as a query sequence to search for unidentified naturally occurring GANC receptor motifs.
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Affiliation(s)
- Junya Ishikawa
- Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan
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14
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Wu L, Chai D, Fraser ME, Zimmerly S. Structural variation and uniformity among tetraloop-receptor interactions and other loop-helix interactions in RNA crystal structures. PLoS One 2012; 7:e49225. [PMID: 23152878 PMCID: PMC3494683 DOI: 10.1371/journal.pone.0049225] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2012] [Accepted: 10/08/2012] [Indexed: 11/19/2022] Open
Abstract
Tetraloop-receptor interactions are prevalent structural units in RNAs, and include the GAAA/11-nt and GNRA-minor groove interactions. In this study, we have compiled a set of 78 nonredundant loop-helix interactions from X-ray crystal structures, and examined them for the extent of their sequence and structural variation. Of the 78 interactions in the set, only four were classical GAAA/11-nt motifs, while over half (48) were GNRA-minor groove interactions. The GNRA-minor groove interactions were not a homogeneous set, but were divided into five subclasses. The most predominant subclass is characterized by two triple base pair interactions in the minor groove, flanked by two ribose zipper contacts. This geometry may be considered the "standard" GNRA-minor groove interaction, while the other four subclasses are alternative ways to form interfaces between a minor groove and tetraloop. The remaining 26 structures in the set of 78 have loops interacting with mostly idiosyncratic receptors. Among the entire set, a number of sequence-structure correlations can be identified, which may be used as initial hypotheses in predicting three-dimensional structures from primary sequences. Conversely, other sequence patterns are not predictive; for example, GAAA loop sequences and GG/CC receptors bind to each other with three distinct geometries. Finally, we observe an example of structural evolution in group II introns, in which loop-receptor motifs are substituted for each other while maintaining the larger three-dimensional geometry. Overall, the study gives a more complete view of RNA loop-helix interactions that exist in nature.
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Affiliation(s)
- Li Wu
- Department of Biological Sciences, University of Calgary, Calgary, Canada
| | - Dinggeng Chai
- Department of Biological Sciences, University of Calgary, Calgary, Canada
| | - Marie E. Fraser
- Department of Biological Sciences, University of Calgary, Calgary, Canada
| | - Steven Zimmerly
- Department of Biological Sciences, University of Calgary, Calgary, Canada
- * E-mail:
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15
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Sahu B, Khade PK, Joseph S. Functional replacement of two highly conserved tetraloops in the bacterial ribosome. Biochemistry 2012; 51:7618-26. [PMID: 22938718 DOI: 10.1021/bi300930r] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Ribosomes are RNA-protein complexes responsible for protein synthesis. A dominant structural motif in the rRNAs is an RNA helix capped with a four-nucleotide loop, called a tetraloop. The sequence of the tetraloop is invariant at some positions in the rRNAs but is highly variable at other positions. The biological reason for the conservation of the tetraloop sequence at specific positions in the rRNAs is not clear. In the 16S rRNA, the GAAA tetraloop in helix 8 and the UACG tetraloop in helix 14 are highly conserved and located near the binding site for EF-Tu and EF-G. To investigate whether the structural stability of the tetraloop or the precise sequence of the tetraloop is important for function, we separately changed the GAAA tetraloop in helix 8 to a UACG tetraloop and the UACG tetraloop in helix 14 to a GAAA tetraloop. The effects of the tetraloop replacements on protein synthesis were analyzed in vivo and in vitro. Replacement of the tetraloops in helices 8 and 14 did not significantly affect the growth rate of the Escherichia coli (Δ7rrn) strain. However, the mutant ribosomes showed a slightly reduced rate of protein synthesis in vitro. In addition, we observed a 2-fold increase in the error rate of translation with the mutant ribosomes, which is consistent with an earlier report. Our results suggest that the tetraloops in helices 8 and 14 are highly conserved mainly for their structural stability and the precise sequences of these tetraloops are not critical for protein synthesis.
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Affiliation(s)
- Bhubanananda Sahu
- Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0314, USA
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16
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Keating KS, Pyle AM. RCrane: semi-automated RNA model building. ACTA CRYSTALLOGRAPHICA SECTION D: BIOLOGICAL CRYSTALLOGRAPHY 2012; 68:985-95. [PMID: 22868764 PMCID: PMC3413212 DOI: 10.1107/s0907444912018549] [Citation(s) in RCA: 75] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/20/2011] [Accepted: 04/25/2012] [Indexed: 11/22/2022]
Abstract
RCrane is a new tool for the partially automated building of RNA crystallographic models into electron-density maps of low or intermediate resolution. This tool helps crystallographers to place phosphates and bases into electron density and then automatically predicts and builds the detailed all-atom structure of the traced nucleotides. RNA crystals typically diffract to much lower resolutions than protein crystals. This low-resolution diffraction results in unclear density maps, which cause considerable difficulties during the model-building process. These difficulties are exacerbated by the lack of computational tools for RNA modeling. Here, RCrane, a tool for the partially automated building of RNA into electron-density maps of low or intermediate resolution, is presented. This tool works within Coot, a common program for macromolecular model building. RCrane helps crystallographers to place phosphates and bases into electron density and then automatically predicts and builds the detailed all-atom structure of the traced nucleotides. RCrane then allows the crystallographer to review the newly built structure and select alternative backbone conformations where desired. This tool can also be used to automatically correct the backbone structure of previously built nucleotides. These automated corrections can fix incorrect sugar puckers, steric clashes and other structural problems.
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Affiliation(s)
- Kevin S Keating
- Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, CT 06511, USA
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17
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Butcher SE, Pyle AM. The molecular interactions that stabilize RNA tertiary structure: RNA motifs, patterns, and networks. Acc Chem Res 2011; 44:1302-11. [PMID: 21899297 DOI: 10.1021/ar200098t] [Citation(s) in RCA: 239] [Impact Index Per Article: 18.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
RNA molecules adopt specific three-dimensional structures critical to their function. Many essential metabolic processes, including protein synthesis and RNA splicing, are carried out by RNA molecules with elaborate tertiary structures (e.g. 3QIQ, right). Indeed, the ribosome and self-splicing introns are complex RNA machines. But even the coding regions in messenger RNAs and viral RNAs are flanked by highly structured untranslated regions, which provide regulatory information necessary for gene expression. RNA tertiary structure is defined as the three-dimensional arrangement of RNA building blocks, which include helical duplexes, triple-stranded structures, and other components that are held together through connections collectively termed RNA tertiary interactions. The structural diversity of these interactions is now a subject of intense investigation, involving the techniques of NMR, X-ray crystallography, chemical genetics, and phylogenetic analysis. At the same time, many investigators are using biophysical techniques to elucidate the driving forces for tertiary structure formation and the mechanisms for its stabilization. RNA tertiary folding is promoted by maximization of base stacking, much like the hydrophobic effect that drives protein folding. RNA folding also requires electrostatic stabilization, both through charge screening and site binding of metals, and it is enhanced by desolvation of the phosphate backbone. In this Account, we provide an overview of the features that specify and stabilize RNA tertiary structure. A major determinant for overall tertiary RNA architecture is local conformation in secondary-structure junctions, which are regions from which two or more duplexes project. At junctions and other structures, such as pseudoknots and kissing loops, adjacent helices stack on one another, and these coaxial stacks play a major role in dictating the overall architectural form of an RNA molecule. In addition to RNA junction topology, a second determinant for RNA tertiary structure is the formation of sequence-specific interactions. Networks of triple helices, tetraloop-receptor interactions, and other sequence-specific contacts establish the framework for the overall tertiary fold. The third determinant of tertiary structure is the formation of stabilizing stacking and backbone interactions, and many are not sequence specific. For example, ribose zippers allow 2'-hydroxyl groups on different RNA strands to form networks of interdigitated hydrogen bonds, serving to seal strands together and thereby stabilize adjacent substructures. These motifs often require monovalent and divalent cations, which can interact diffusely or through chelation to specific RNA functional groups. As we learn more about the components of RNA tertiary structure, we will be able to predict the structures of RNA molecules from their sequences, thereby obtaining key information about biological function. Understanding and predicting RNA structure is particularly important given the recent discovery that although most of our genome is transcribed into RNA molecules, few of them have a known function. The prevalence of RNA viruses and pathogens with RNA genomes makes RNA drug discovery an active area of research. Finally, knowledge of RNA structure will facilitate the engineering of supramolecular RNA structures, which can be used as nanomechanical components for new materials. But all of this promise depends on a better understanding of the RNA parts list, and how the pieces fit together.
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Affiliation(s)
- Samuel E. Butcher
- Department of Biochemistry, University of Wisconsin—Madison, 433 Babcock
Drive, Madison, Wisconsin 53706-1544, United States
| | - Anna Marie Pyle
- Department of Molecular, Cellular
and Developmental Biology and Department of Chemistry, Yale University, New Haven, Connecticut, United States
- Howard Hughes Medical Institute
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18
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Abstract
Unlike proteins, the RNA backbone has numerous degrees of freedom (eight, if one counts the sugar pucker), making RNA modeling, structure building and prediction a multidimensional problem of exceptionally high complexity. And yet RNA tertiary structures are not infinite in their structural morphology; rather, they are built from a limited set of discrete units. In order to reduce the dimensionality of the RNA backbone in a physically reasonable way, a shorthand notation was created that reduced the RNA backbone torsion angles to two (η and θ, analogous to φ and ψ in proteins). When these torsion angles are calculated for nucleotides in a crystallographic database and plotted against one another, one obtains a plot analogous to a Ramachandran plot (the η/θ plot), with highly populated and unpopulated regions. Nucleotides that occupy proximal positions on the plot have identical structures and are found in the same units of tertiary structure. In this review, we describe the statistical validation of the η/θ formalism and the exploration of features within the η/θ plot. We also describe the application of the η/θ formalism in RNA motif discovery, structural comparison, RNA structure building and tertiary structure prediction. More than a tool, however, the η/θ formalism has provided new insights into RNA structure itself, revealing its fundamental components and the factors underlying RNA architectural form.
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19
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Pyle AM. The tertiary structure of group II introns: implications for biological function and evolution. Crit Rev Biochem Mol Biol 2010; 45:215-32. [PMID: 20446804 DOI: 10.3109/10409231003796523] [Citation(s) in RCA: 91] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
Group II introns are some of the largest ribozymes in nature, and they are a major source of information about RNA assembly and tertiary structural organization. These introns are of biological significance because they are self-splicing mobile elements that have migrated into diverse genomes and played a major role in the genomic organization and metabolism of most life forms. The tertiary structure of group II introns has been the subject of many phylogenetic, genetic, biochemical and biophysical investigations, all of which are consistent with the recent crystal structure of an intact group IIC intron from the alkaliphilic eubacterium Oceanobacillus iheyensis. The crystal structure reveals that catalytic intron domain V is enfolded within the other intronic domains through an elaborate network of diverse tertiary interactions. Within the folded core, DV adopts an activated conformation that readily binds catalytic metal ions and positions them in a manner appropriate for reaction with nucleic acid targets. The tertiary structure of the group II intron reveals new information on motifs for RNA architectural organization, mechanisms of group II intron catalysis, and the evolutionary relationships among RNA processing systems. Guided by the structure and the wealth of previous genetic and biochemical work, it is now possible to deduce the probable location of DVI and the site of additional domains that contribute to the function of the highly derived group IIB and IIA introns.
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Affiliation(s)
- Anna Marie Pyle
- Department of Molecular Biophysics and Biochemistry, Howard Hughes Medical Institute and Yale University, New Haven, CT, USA.
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20
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Mohan S, Hsiao C, Bowman JC, Wartell R, Williams LD. RNA Tetraloop Folding Reveals Tension between Backbone Restraints and Molecular Interactions. J Am Chem Soc 2010; 132:12679-89. [DOI: 10.1021/ja104387k] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Srividya Mohan
- School of Chemistry and Biochemistry, School of Biology, and Parker H. Petit Institute of Bioengineering and Biosciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
| | - Chiaolong Hsiao
- School of Chemistry and Biochemistry, School of Biology, and Parker H. Petit Institute of Bioengineering and Biosciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
| | - Jessica C. Bowman
- School of Chemistry and Biochemistry, School of Biology, and Parker H. Petit Institute of Bioengineering and Biosciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
| | - Roger Wartell
- School of Chemistry and Biochemistry, School of Biology, and Parker H. Petit Institute of Bioengineering and Biosciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
| | - Loren Dean Williams
- School of Chemistry and Biochemistry, School of Biology, and Parker H. Petit Institute of Bioengineering and Biosciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
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21
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Masquida B, Beckert B, Jossinet F. Exploring RNA structure by integrative molecular modelling. N Biotechnol 2010; 27:170-83. [PMID: 20206310 DOI: 10.1016/j.nbt.2010.02.022] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
RNA molecular modelling is adequate to rapidly tackle the structure of RNA molecules. With new structured RNAs constituting a central class of cellular regulators discovered every year, the need for swift and reliable modelling methods is more crucial than ever. The pragmatic method based on interactive all-atom molecular modelling relies on the observation that specific structural motifs are recurrently found in RNA sequences. Once identified by a combination of comparative sequence analysis and biochemical data, the motifs composing the secondary structure of a given RNA can be extruded in three dimensions (3D) and used as building blocks assembled manually during a bioinformatic interactive process. Comparing the models to the corresponding crystal structures has validated the method as being powerful to predict the RNA topology and architecture while being less accurate regarding the prediction of base-base interactions. These aspects as well as the necessary steps towards automation will be discussed.
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Affiliation(s)
- Benoît Masquida
- Architecture et Réactivité de l'ARN, Université de Strasbourg, IBMC, CNRS, 15 rue René Descartes, Strasbourg, France.
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22
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Keating KS, Toor N, Perlman PS, Pyle AM. A structural analysis of the group II intron active site and implications for the spliceosome. RNA (NEW YORK, N.Y.) 2010; 16:1-9. [PMID: 19948765 PMCID: PMC2802019 DOI: 10.1261/rna.1791310] [Citation(s) in RCA: 97] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/23/2009] [Accepted: 08/12/2009] [Indexed: 05/20/2023]
Abstract
Group II introns are self-splicing, mobile genetic elements that have fundamentally influenced the organization of terrestrial genomes. These large ribozymes remain important for gene expression in almost all forms of bacteria and eukaryotes and they are believed to share a common ancestry with the eukaryotic spliceosome that is required for processing all nuclear pre-mRNAs. The three-dimensional structure of a group IIC intron was recently determined by X-ray crystallography, making it possible to visualize the active site and the elaborate network of tertiary interactions that stabilize the molecule. Here we describe the molecular features of the active site in detail and evaluate their correspondence with prior biochemical, genetic, and phylogenetic analyses on group II introns. In addition, we evaluate the structural significance of RNA motifs within the intron core, such as the major-groove triple helix and the domain 5 bulge. Having combined what is known about the group II intron core, we then compare it with known structural features of U6 snRNA in the eukaryotic spliceosome. This analysis leads to a set of predictions for the molecular structure of the spliceosomal active site.
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Affiliation(s)
- Kevin S Keating
- Program in Computational Biology and Bioinformatics, Yale University, New Haven, Connecticut 06520, USA
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23
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Toor N, Keating KS, Fedorova O, Rajashankar K, Wang J, Pyle AM. Tertiary architecture of the Oceanobacillus iheyensis group II intron. RNA (NEW YORK, N.Y.) 2010; 16:57-69. [PMID: 19952115 PMCID: PMC2802037 DOI: 10.1261/rna.1844010] [Citation(s) in RCA: 56] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/11/2023]
Abstract
Group II introns are large ribozymes that act as self-splicing and retrotransposable RNA molecules. They are of great interest because of their potential evolutionary relationship to the eukaryotic spliceosome, their continued influence on the organization of many genomes in bacteria and eukaryotes, and their potential utility as tools for gene therapy and biotechnology. One of the most interesting features of group II introns is their relative lack of nucleobase conservation and covariation, which has long suggested that group II intron structures are stabilized by numerous unusual tertiary interactions and backbone-mediated contacts. Here, we provide a detailed description of the tertiary interaction networks within the Oceanobacillus iheyensis group IIC intron, for which a crystal structure was recently solved to 3.1 A resolution. The structure can be described as a set of several intricately constructed tertiary interaction nodes, each of which contains a core of extended stacking networks and elaborate motifs. Many of these nodes are surrounded by a web of ribose zippers, which appear to further stabilize local structure. As predicted from biochemical and genetic studies, the group II intron provides a wealth of new information on strategies for RNA folding and tertiary structural organization.
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Affiliation(s)
- Navtej Toor
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06511, USA
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24
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Vorechovsky I. Transposable elements in disease-associated cryptic exons. Hum Genet 2009; 127:135-54. [PMID: 19823873 DOI: 10.1007/s00439-009-0752-4] [Citation(s) in RCA: 82] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2009] [Accepted: 09/27/2009] [Indexed: 11/28/2022]
Abstract
Transposable elements (TEs) make up a half of the human genome, but the extent of their contribution to cryptic exon activation that results in genetic disease is unknown. Here, a comprehensive survey of 78 mutation-induced cryptic exons previously identified in 51 disease genes revealed the presence of TEs in 40 cases (51%). Most TE-containing exons were derived from short interspersed nuclear elements (SINEs), with Alus and mammalian interspersed repeats (MIRs) covering >18 and >16% of the exonized sequences, respectively. The majority of SINE-derived cryptic exons had splice sites at the same positions of the Alu/MIR consensus as existing SINE exons and their inclusion in the mRNA was facilitated by phylogenetically conserved changes that improved both traditional and auxiliary splicing signals, thus marking intronic TEs amenable for pathogenic exonization. The overrepresentation of MIRs among TE exons is likely to result from their high average exon inclusion levels, which reflect their strong splice sites, a lack of splicing silencers and a high density of enhancers, particularly (G)AA(G) motifs. These elements were markedly depleted in antisense Alu exons, had the most prominent position on the exon-intron gradient scale and are proposed to promote exon definition through enhanced tertiary RNA interactions involving unpaired (di)adenosines. The identification of common mechanisms by which the most dynamic parts of the genome contribute both to new exon creation and genetic disease will facilitate detection of intronic mutations and the development of computational tools that predict TE hot-spots of cryptic exon activation.
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Affiliation(s)
- Igor Vorechovsky
- Division of Human Genetics, University of Southampton School of Medicine, MP808, Tremona Road, Southampton SO16 6YD, UK.
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25
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Toor N, Keating KS, Pyle AM. Structural insights into RNA splicing. Curr Opin Struct Biol 2009; 19:260-6. [PMID: 19443210 DOI: 10.1016/j.sbi.2009.04.002] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2009] [Accepted: 04/07/2009] [Indexed: 11/16/2022]
Abstract
Intron splicing is a fundamental biological process whereby noncoding sequences are removed from precursor RNAs. Recent work has provided new insights into the structural features and reaction mechanisms of two introns that catalyze their own splicing from precursor RNA: the group I and II introns. In addition, there is an increasing amount of structural information on the spliceosome, which is a ribonucleoprotein machine that catalyzes nuclear pre-mRNA splicing in eukaryotes. Here, we compare structures and catalytic mechanisms of self-splicing RNAs and we discuss the possible implications for spliceosomal reaction mechanisms.
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Affiliation(s)
- Navtej Toor
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06511, USA
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26
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The ribozyme core of group II introns: a structure in want of partners. Trends Biochem Sci 2009; 34:189-99. [DOI: 10.1016/j.tibs.2008.12.007] [Citation(s) in RCA: 70] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2008] [Revised: 12/10/2008] [Accepted: 12/12/2008] [Indexed: 11/23/2022]
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