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Molina-Sánchez MD, García-Rodríguez FM, Andrés-León E, Toro N. Identification of Group II Intron RmInt1 Binding Sites in a Bacterial Genome. Front Mol Biosci 2022; 9:834020. [PMID: 35281263 PMCID: PMC8914252 DOI: 10.3389/fmolb.2022.834020] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2021] [Accepted: 02/07/2022] [Indexed: 11/13/2022] Open
Abstract
RmInt1 is a group II intron encoding a reverse transcriptase protein (IEP) lacking the C-terminal endonuclease domain. RmInt1 is an efficient mobile retroelement that predominantly reverse splices into the transient single-stranded DNA at the template for lagging strand DNA synthesis during host replication, a process facilitated by the interaction of the RmInt1 IEP with DnaN at the replication fork. It has been suggested that group II intron ribonucleoprotein particles bind DNA nonspecifically, and then scan for their correct target site. In this study, we investigated RmInt1 binding sites throughout the Sinorhizobium meliloti genome, by chromatin-immunoprecipitation coupled with next-generation sequencing. We found that RmInt1 binding sites cluster around the bidirectional replication origin of each of the three replicons comprising the S. meliloti genome. Our results provide new evidence linking group II intron mobility to host DNA replication.
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Affiliation(s)
- María Dolores Molina-Sánchez
- Structure, Dynamics and Function of Rhizobacterial Genomes, Estación Experimental del Zaidín, Department of Soil Microbiology and Symbiotic Systems, Spanish National Research Council (CSIC), Granada, Spain
| | - Fernando Manuel García-Rodríguez
- Structure, Dynamics and Function of Rhizobacterial Genomes, Estación Experimental del Zaidín, Department of Soil Microbiology and Symbiotic Systems, Spanish National Research Council (CSIC), Granada, Spain
| | - Eduardo Andrés-León
- Bioinformatics Unit, Institute of Parasitology and Biomedicine “López-Neyra” (IPBLN), Spanish National Research Council (CSIC), Granada, Spain
| | - Nicolás Toro
- Structure, Dynamics and Function of Rhizobacterial Genomes, Estación Experimental del Zaidín, Department of Soil Microbiology and Symbiotic Systems, Spanish National Research Council (CSIC), Granada, Spain
- *Correspondence: Nicolás Toro,
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2
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Liu G, Jiang H, Sun W, Zhang J, Chen D, Murchie AIH. The function of twister ribozyme variants in non-LTR retrotransposition in Schistosoma mansoni. Nucleic Acids Res 2021; 49:10573-10588. [PMID: 34551436 PMCID: PMC8501958 DOI: 10.1093/nar/gkab818] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2021] [Revised: 08/23/2021] [Accepted: 09/08/2021] [Indexed: 12/13/2022] Open
Abstract
The twister ribozyme is widely distributed over numerous organisms and is especially abundant in Schistosoma mansoni, but has no confirmed biological function. Of the 17 non-LTR retrotransposons known in S. mansoni, none have thus far been associated with ribozymes. Here we report the identification of novel twister variant (T-variant) ribozymes and their function in S. mansoni non-LTR retrotransposition. We show that T-variant ribozymes are located at the 5′ end of Perere-3 non-LTR retrotransposons in the S. mansoni genome. T-variant ribozymes were demonstrated to be catalytically active in vitro. In reporter constructs, T-variants were shown to cleave in vivo, and cleavage of T-variants was sufficient for the translation of downstream reporter genes. Our analysis shows that the T-variants and Perere-3 are transcribed together. Target site duplications (TSDs); markers of target-primed reverse transcription (TPRT) and footmarks of retrotransposition, are located adjacent to the T-variant cleavage site and suggest that T-variant cleavage has taken place inS. mansoni. Sequence heterogeneity in the TSDs indicates that Perere-3 retrotransposition is not site-specific. The TSD sequences contribute to the 5′ end of the terminal ribozyme helix (P1 stem). Based on these results we conclude that T-variants have a functional role in Perere-3 retrotransposition.
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Affiliation(s)
- Getong Liu
- Fudan University Pudong Medical Center, and Institutes of Biomedical Sciences, Shanghai Medical College, Key Laboratory of Medical Epigenetics and Metabolism, Fudan University, Shanghai 200032, China.,Key Laboratory of Metabolism and Molecular Medicine, Ministry of Education, School of Basic Medical Sciences, Fudan University, Shanghai 200032, China
| | - Hengyi Jiang
- Fudan University Pudong Medical Center, and Institutes of Biomedical Sciences, Shanghai Medical College, Key Laboratory of Medical Epigenetics and Metabolism, Fudan University, Shanghai 200032, China.,Key Laboratory of Metabolism and Molecular Medicine, Ministry of Education, School of Basic Medical Sciences, Fudan University, Shanghai 200032, China
| | - Wenxia Sun
- Fudan University Pudong Medical Center, and Institutes of Biomedical Sciences, Shanghai Medical College, Key Laboratory of Medical Epigenetics and Metabolism, Fudan University, Shanghai 200032, China.,Key Laboratory of Metabolism and Molecular Medicine, Ministry of Education, School of Basic Medical Sciences, Fudan University, Shanghai 200032, China
| | - Jun Zhang
- Fudan University Pudong Medical Center, and Institutes of Biomedical Sciences, Shanghai Medical College, Key Laboratory of Medical Epigenetics and Metabolism, Fudan University, Shanghai 200032, China.,Key Laboratory of Metabolism and Molecular Medicine, Ministry of Education, School of Basic Medical Sciences, Fudan University, Shanghai 200032, China
| | - Dongrong Chen
- Fudan University Pudong Medical Center, and Institutes of Biomedical Sciences, Shanghai Medical College, Key Laboratory of Medical Epigenetics and Metabolism, Fudan University, Shanghai 200032, China.,Key Laboratory of Metabolism and Molecular Medicine, Ministry of Education, School of Basic Medical Sciences, Fudan University, Shanghai 200032, China
| | - Alastair I H Murchie
- Fudan University Pudong Medical Center, and Institutes of Biomedical Sciences, Shanghai Medical College, Key Laboratory of Medical Epigenetics and Metabolism, Fudan University, Shanghai 200032, China.,Key Laboratory of Metabolism and Molecular Medicine, Ministry of Education, School of Basic Medical Sciences, Fudan University, Shanghai 200032, China
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García-Rodríguez FM, Neira JL, Marcia M, Molina-Sánchez MD, Toro N. A group II intron-encoded protein interacts with the cellular replicative machinery through the β-sliding clamp. Nucleic Acids Res 2019; 47:7605-7617. [PMID: 31127285 PMCID: PMC6698660 DOI: 10.1093/nar/gkz468] [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: 06/29/2018] [Revised: 05/14/2019] [Accepted: 05/16/2019] [Indexed: 12/15/2022] Open
Abstract
Group II introns are self-splicing mobile genetic retroelements. The spliced intron RNA and the intron-encoded protein (IEP) form ribonucleoprotein particles (RNPs) that recognize and invade specific DNA target sites. The IEP is a reverse transcriptase/maturase that may bear a C-terminal endonuclease domain enabling the RNP to cleave the target DNA strand to prime reverse transcription. However, some mobile introns, such as RmInt1, lack the En domain but nevertheless retrohome efficiently to transient single-stranded DNA target sites at a DNA replication fork. Their mobility is associated with host DNA replication, and they use the nascent lagging strand as a primer for reverse transcription. We searched for proteins that interact with RmInt1 RNPs and direct these RNPs to the DNA replication fork. Co-immunoprecipitation assays suggested that DnaN (the β-sliding clamp), a component of DNA polymerase III, interacts with the protein component of the RmInt1 RNP. Pulldown assays, far-western blots and biolayer interferometry supported this interaction. Peptide binding assays also identified a putative DnaN-interacting motif in the RmInt1 IEP structurally conserved in group II intron IEPs. Our results suggest that intron RNP interacts with the β-sliding clamp of the DNA replication machinery, favouring reverse splicing into the transient ssDNA at DNA replication forks.
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Affiliation(s)
- Fernando M García-Rodríguez
- Structure, Dynamics and Function of Rhizobacterial Genomes (Grupo de Ecología Genética de la Rizosfera), Department of Soil Microbiology and Symbiotic Systems, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, C/Profesor Albareda 1, 18008 Granada, Spain
| | - José L Neira
- Instituto de Biología Molecular y Celular, Universidad Miguel Hernández, 03202 Elche (Alicante), Spain.,Instituto de Biocomputación y Física de Sistemas Complejos, Joint Units IQFR-CSIC-BIFI, and GBsC-CSIC-BIFI, Universidad de Zaragoza, 50009 Zaragoza
| | - Marco Marcia
- European Molecular Biology Laboratory (EMBL), Grenoble Outstation, 71 Avenue des Martyrs, Grenoble 38042, France
| | - María D Molina-Sánchez
- Structure, Dynamics and Function of Rhizobacterial Genomes (Grupo de Ecología Genética de la Rizosfera), Department of Soil Microbiology and Symbiotic Systems, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, C/Profesor Albareda 1, 18008 Granada, Spain
| | - Nicolás Toro
- Structure, Dynamics and Function of Rhizobacterial Genomes (Grupo de Ecología Genética de la Rizosfera), Department of Soil Microbiology and Symbiotic Systems, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, C/Profesor Albareda 1, 18008 Granada, Spain
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4
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Molina-Sánchez MD, Toro N. DNA cleavage and reverse splicing of ribonucleoprotein particles reconstituted in vitro with linear RmInt1 RNA. RNA Biol 2019; 16:930-939. [PMID: 30943851 DOI: 10.1080/15476286.2019.1601379] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022] Open
Abstract
The RmInt1 group II intron is an efficient self-splicing mobile retroelement that catalyzes its own excision as lariat, linear and circular molecules. In vivo, the RmInt1 lariat and the reverse transcriptase (IEP) it encodes form a ribonucleoprotein particle (RNP) that recognizes the DNA target for site-specific full intron insertion via a two-step reverse splicing reaction. RNPs containing linear group II intron RNA are generally thought to be unable to complete the reverse splicing reaction. Here, we show that reconstituted in vitro RNPs containing linear RmInt1 ΔORF RNA can mediate the cleavage of single-stranded DNA substrates in a very precise manner with the attachment of the intron RNA to the 3´exon as the first step of a reverse splicing reaction. Notably, we also observe molecules in which the 5´exon is linked to the RmInt1 RNA, suggesting the completion of the reverse splicing reaction, albeit rather low and inefficiently. That process depends on DNA target recognition and can be successful completed by RmInt1 RNPs with linear RNA displaying 5´ modifications.
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Affiliation(s)
- María Dolores Molina-Sánchez
- a Structure, Dynamics and Function of Rhizobacterial Genomes, Grupo de Ecología Genética de la Rizosfera, Department of Soil Microbiology and Symbiotic Systems , Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas , Granada , Spain
| | - Nicolás Toro
- a Structure, Dynamics and Function of Rhizobacterial Genomes, Grupo de Ecología Genética de la Rizosfera, Department of Soil Microbiology and Symbiotic Systems , Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas , Granada , Spain
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5
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Mohr G, Kang SYS, Park SK, Qin Y, Grohman J, Yao J, Stamos JL, Lambowitz AM. A Highly Proliferative Group IIC Intron from Geobacillus stearothermophilus Reveals New Features of Group II Intron Mobility and Splicing. J Mol Biol 2018; 430:2760-2783. [PMID: 29913158 DOI: 10.1016/j.jmb.2018.06.019] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2018] [Revised: 05/01/2018] [Accepted: 06/05/2018] [Indexed: 12/13/2022]
Abstract
The thermostable Geobacillus stearothermophilus GsI-IIC intron is among the few bacterial group II introns found to proliferate to high copy number in its host genome. Here, we developed a bacterial genetic assay for retrohoming and biochemical assays for protein-dependent and self-splicing of GsI-IIC. We found that GsI-IIC, like other group IIC introns, retrohomes into sites having a 5'-exon DNA hairpin, typically from a bacterial transcription terminator, followed by short intron-binding sequences (IBSs) recognized by base pairing of exon-binding sequences (EBSs) in the intron RNA. Intron RNA insertion occurs preferentially but not exclusively into the parental lagging strand at DNA replication forks, using a nascent lagging strand DNA as a primer for reverse transcription. In vivo mobility assays, selections, and mutagenesis indicated that a variety of GC-rich DNA hairpins of 7-19 bp with continuous base pairs or internal elbow regions support efficient intron mobility and identified a critically recognized nucleotide (T-5) between the hairpin and IBS1, a feature not reported previously for group IIC introns. Neither the hairpin nor T-5 is required for intron excision or lariat formation during RNA splicing, but the 5'-exon sequence can affect the efficiency of exon ligation. Structural modeling suggests that the 5'-exon DNA hairpin and T-5 bind to the thumb and DNA-binding domains of GsI-IIC reverse transcriptase. This mode of DNA target site recognition enables the intron to proliferate to high copy number by recognizing numerous transcription terminators and then finding the best match for the EBS/IBS interactions within a short distance downstream.
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Affiliation(s)
- Georg Mohr
- Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712, USA; Department of Molecular Biosciences, University of Texas at Austin, Austin, TX 78712, USA
| | - Sean Yoon-Seo Kang
- Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712, USA; Department of Molecular Biosciences, University of Texas at Austin, Austin, TX 78712, USA
| | - Seung Kuk Park
- Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712, USA; Department of Molecular Biosciences, University of Texas at Austin, Austin, TX 78712, USA
| | - Yidan Qin
- Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712, USA; Department of Molecular Biosciences, University of Texas at Austin, Austin, TX 78712, USA
| | - Jacob Grohman
- Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712, USA; Department of Molecular Biosciences, University of Texas at Austin, Austin, TX 78712, USA
| | - Jun Yao
- Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712, USA; Department of Molecular Biosciences, University of Texas at Austin, Austin, TX 78712, USA
| | - Jennifer L Stamos
- Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712, USA; Department of Molecular Biosciences, University of Texas at Austin, Austin, TX 78712, USA
| | - Alan M Lambowitz
- Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712, USA; Department of Molecular Biosciences, University of Texas at Austin, Austin, TX 78712, USA.
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Wu DC, Lambowitz AM. Facile single-stranded DNA sequencing of human plasma DNA via thermostable group II intron reverse transcriptase template switching. Sci Rep 2017; 7:8421. [PMID: 28827600 PMCID: PMC5566474 DOI: 10.1038/s41598-017-09064-w] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2017] [Accepted: 07/20/2017] [Indexed: 01/30/2023] Open
Abstract
High-throughput single-stranded DNA sequencing (ssDNA-seq) of cell-free DNA from plasma and other bodily fluids is a powerful method for non-invasive prenatal testing, and diagnosis of cancers and other diseases. Here, we developed a facile ssDNA-seq method, which exploits a novel template-switching activity of thermostable group II intron reverse transcriptases (TGIRTs) for DNA-seq library construction. This activity enables TGIRT enzymes to initiate DNA synthesis directly at the 3′ end of a DNA strand while simultaneously attaching a DNA-seq adapter without end repair, tailing, or ligation. Initial experiments using this method to sequence E. coli genomic DNA showed that the TGIRT enzyme has surprisingly robust DNA polymerase activity. Further experiments showed that TGIRT-seq of plasma DNA from a healthy individual enables analysis of nucleosome positioning, transcription factor-binding sites, DNA methylation sites, and tissues-of-origin comparably to established methods, but with a simpler workflow that captures precise DNA ends.
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Affiliation(s)
- Douglas C Wu
- Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas, 78712, USA.,Department of Molecular Biosciences, University of Texas at Austin, Austin, Texas, 78712, USA
| | - Alan M Lambowitz
- Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas, 78712, USA. .,Department of Molecular Biosciences, University of Texas at Austin, Austin, Texas, 78712, USA.
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Molina-Sánchez MD, García-Rodríguez FM, Toro N. Functionality of In vitro Reconstituted Group II Intron RmInt1-Derived Ribonucleoprotein Particles. Front Mol Biosci 2016; 3:58. [PMID: 27730127 PMCID: PMC5037169 DOI: 10.3389/fmolb.2016.00058] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2016] [Accepted: 09/12/2016] [Indexed: 01/22/2023] Open
Abstract
The functional unit of mobile group II introns is a ribonucleoprotein particle (RNP) consisting of the intron-encoded protein (IEP) and the excised intron RNA. The IEP has reverse transcriptase activity but also promotes RNA splicing, and the RNA-protein complex triggers site-specific DNA insertion by reverse splicing, in a process called retrohoming. In vitro reconstituted ribonucleoprotein complexes from the Lactococcus lactis group II intron Ll.LtrB, which produce a double strand break, have recently been studied as a means of developing group II intron-based gene targeting methods for higher organisms. The Sinorhizobium meliloti group II intron RmInt1 is an efficient mobile retroelement, the dispersal of which appears to be linked to transient single-stranded DNA during replication. The RmInt1IEP lacks the endonuclease domain (En) and cannot cut the bottom strand to generate the 3' end to initiate reverse transcription. We used an Escherichia coli expression system to produce soluble and active RmInt1 IEP and reconstituted RNPs with purified components in vitro. The RNPs generated were functional and reverse-spliced into a single-stranded DNA target. This work constitutes the starting point for the use of group II introns lacking DNA endonuclease domain-derived RNPs for highly specific gene targeting methods.
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Affiliation(s)
- Maria D Molina-Sánchez
- Structure, Dynamics and Function of Rhizobacterial Genomes, Department of Soil Microbiology and Symbiotic Systems, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas Granada, Spain
| | - Fernando M García-Rodríguez
- Structure, Dynamics and Function of Rhizobacterial Genomes, Department of Soil Microbiology and Symbiotic Systems, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas Granada, Spain
| | - Nicolás Toro
- Structure, Dynamics and Function of Rhizobacterial Genomes, Department of Soil Microbiology and Symbiotic Systems, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas Granada, Spain
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8
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Nisa-Martínez R, Molina-Sánchez MD, Toro N. Host Factors Influencing the Retrohoming Pathway of Group II Intron RmInt1, Which Has an Intron-Encoded Protein Naturally Devoid of Endonuclease Activity. PLoS One 2016; 11:e0162275. [PMID: 27588750 PMCID: PMC5010178 DOI: 10.1371/journal.pone.0162275] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2016] [Accepted: 08/21/2016] [Indexed: 11/21/2022] Open
Abstract
Bacterial group II introns are self-splicing catalytic RNAs and mobile retroelements that have an open reading frame encoding an intron-encoded protein (IEP) with reverse transcriptase (RT) and RNA splicing or maturase activity. Some IEPs carry a DNA endonuclease (En) domain, which is required to cleave the bottom strand downstream from the intron-insertion site for target DNA-primed reverse transcription (TPRT) of the inserted intron RNA. Host factors complete the insertion of the intron. By contrast, the major retrohoming pathway of introns with IEPs naturally lacking endonuclease activity, like the Sinorhizobium meliloti intron RmInt1, is thought to involve insertion of the intron RNA into the template for lagging strand DNA synthesis ahead of the replication fork, with possible use of the nascent strand to prime reverse transcription of the intron RNA. The host factors influencing the retrohoming pathway of such introns have not yet been described. Here, we identify key candidates likely to be involved in early and late steps of RmInt1 retrohoming. Some of these host factors are common to En+ group II intron retrohoming, but some have different functions. Our results also suggest that the retrohoming process of RmInt1 may be less dependent on the intracellular free Mg2+ concentration than those of other group II introns.
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Affiliation(s)
- Rafael Nisa-Martínez
- Structure, Dynamics and Function of Rhizobacterial Genomes, Department of Soil Microbiology and Symbiotic Systems, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, Calle Profesor Albareda 1, 18008, Granada, Spain
| | - María Dolores Molina-Sánchez
- Structure, Dynamics and Function of Rhizobacterial Genomes, Department of Soil Microbiology and Symbiotic Systems, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, Calle Profesor Albareda 1, 18008, Granada, Spain
| | - Nicolás Toro
- Structure, Dynamics and Function of Rhizobacterial Genomes, Department of Soil Microbiology and Symbiotic Systems, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, Calle Profesor Albareda 1, 18008, Granada, Spain
- * E-mail:
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Abstract
R2 elements are sequence specific non-LTR retrotransposons that exclusively insert in the 28S rRNA genes of animals. R2s encode an endonuclease that cleaves the insertion site and a reverse transcriptase that uses the cleaved DNA to prime reverse transcription of the R2 transcript, a process termed target primed reverse transcription. Additional unusual properties of the reverse transcriptase as well as DNA and RNA binding domains of the R2 encoded protein have been characterized. R2 expression is through co-transcription with the 28S gene and self-cleavage by a ribozyme encoded at the R2 5' end. Studies in laboratory stocks and natural populations of Drosophila suggest that R2 expression is tied to the distribution of R2-inserted units within the rDNA locus. Most individuals have no R2 expression because only a small fraction of their rRNA genes need to be active, and a contiguous region of the locus free of R2 insertions can be selected for activation. However, if the R2-free region is not large enough to produce sufficient rRNA, flanking units - including those inserted with R2 - must be activated. Finally, R2 copies rapidly turnover within the rDNA locus, yet R2 has been vertically maintained in animal lineages for hundreds of millions of years. The key to this stability is R2's ability to remain dormant in rDNA units outside the transcribed regions for generations until the stochastic nature of the crossovers that drive the concerted evolution of the rDNA locus inevitably reshuffle the inserted and uninserted units, resulting in transcription of the R2-inserted units.
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Abstract
Reverse transcriptases (RTs) are usually thought of as eukaryotic enzymes, but they are also present in bacteria and likely originated in bacteria and migrated to eukaryotes. Only three types of bacterial retroelements have been substantially characterized: group II introns, diversity-generating retroelements, and retrons. Recent work, however, has identified a myriad of uncharacterized RTs and RT-related sequences in bacterial genomes, which exhibit great sequence diversity and a range of domain structures. Apart from group II introns, none of these putative RTs show evidence of active retromobility. Instead, available information suggests that they are involved in useful processes, sometimes related to phages or phage resistance. This article reviews our knowledge of both characterized and uncharacterized RTs in bacteria. The range of their sequences and genomic contexts promises the discovery of new biochemical reactions and biological phenomena.
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Abstract
This review focuses on recent developments in our understanding of group II intron function, the relationships of these introns to retrotransposons and spliceosomes, and how their common features have informed thinking about bacterial group II introns as key elements in eukaryotic evolution. Reverse transcriptase-mediated and host factor-aided intron retrohoming pathways are considered along with retrotransposition mechanisms to novel sites in bacteria, where group II introns are thought to have originated. DNA target recognition and movement by target-primed reverse transcription infer an evolutionary relationship among group II introns, non-LTR retrotransposons, such as LINE elements, and telomerase. Additionally, group II introns are almost certainly the progenitors of spliceosomal introns. Their profound similarities include splicing chemistry extending to RNA catalysis, reaction stereochemistry, and the position of two divalent metals that perform catalysis at the RNA active site. There are also sequence and structural similarities between group II introns and the spliceosome's small nuclear RNAs (snRNAs) and between a highly conserved core spliceosomal protein Prp8 and a group II intron-like reverse transcriptase. It has been proposed that group II introns entered eukaryotes during bacterial endosymbiosis or bacterial-archaeal fusion, proliferated within the nuclear genome, necessitating evolution of the nuclear envelope, and fragmented giving rise to spliceosomal introns. Thus, these bacterial self-splicing mobile elements have fundamentally impacted the composition of extant eukaryotic genomes, including the human genome, most of which is derived from close relatives of mobile group II introns.
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Truong DM, Hewitt FC, Hanson JH, Cui X, Lambowitz AM. Retrohoming of a Mobile Group II Intron in Human Cells Suggests How Eukaryotes Limit Group II Intron Proliferation. PLoS Genet 2015; 11:e1005422. [PMID: 26241656 PMCID: PMC4524724 DOI: 10.1371/journal.pgen.1005422] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2015] [Accepted: 07/05/2015] [Indexed: 12/22/2022] Open
Abstract
Mobile bacterial group II introns are evolutionary ancestors of spliceosomal introns and retroelements in eukaryotes. They consist of an autocatalytic intron RNA (a “ribozyme”) and an intron-encoded reverse transcriptase, which function together to promote intron integration into new DNA sites by a mechanism termed “retrohoming”. Although mobile group II introns splice and retrohome efficiently in bacteria, all examined thus far function inefficiently in eukaryotes, where their ribozyme activity is limited by low Mg2+ concentrations, and intron-containing transcripts are subject to nonsense-mediated decay (NMD) and translational repression. Here, by using RNA polymerase II to express a humanized group II intron reverse transcriptase and T7 RNA polymerase to express intron transcripts resistant to NMD, we find that simply supplementing culture medium with Mg2+ induces the Lactococcus lactis Ll.LtrB intron to retrohome into plasmid and chromosomal sites, the latter at frequencies up to ~0.1%, in viable HEK-293 cells. Surprisingly, under these conditions, the Ll.LtrB intron reverse transcriptase is required for retrohoming but not for RNA splicing as in bacteria. By using a genetic assay for in vivo selections combined with deep sequencing, we identified intron RNA mutations that enhance retrohoming in human cells, but <4-fold and not without added Mg2+. Further, the selected mutations lie outside the ribozyme catalytic core, which appears not readily modified to function efficiently at low Mg2+ concentrations. Our results reveal differences between group II intron retrohoming in human cells and bacteria and suggest constraints on critical nucleotide residues of the ribozyme core that limit how much group II intron retrohoming in eukaryotes can be enhanced. These findings have implications for group II intron use for gene targeting in eukaryotes and suggest how differences in intracellular Mg2+ concentrations between bacteria and eukarya may have impacted the evolution of introns and gene expression mechanisms. Mobile group II introns are bacterial retrotransposons that are evolutionary ancestors of spliceosomal introns and retroelements in eukaryotes. They consist of an autocatalytic intron RNA (a ribozyme) and an intron-encoded reverse transcriptase, which together promote intron mobility to new DNA sites by a mechanism called retrohoming. Although found in bacteria, archaea and eukaryotic organelles, group II introns are absent from eukaryotic nuclear genomes, where host defenses impede their expression and lower intracellular Mg2+ concentrations limit their ribozyme activity. Here, we developed a mobile group II intron expression system that bypasses expression barriers and show that simply adding Mg2+ to culture medium enables group II intron retrohoming into plasmid and chromosomal target sites in human cells at appreciable frequencies. Genetic selections and deep sequencing identified intron RNA mutations that moderately enhance retrohoming in human cells, but not without added Mg2+. Thus, low Mg2+ concentrations in human cells are a natural barrier to efficient retrohoming that is not readily overcome by mutational variation and selection. Our results have implications for group II intron use for gene targeting in higher organisms and highlight the impact of different intracellular environments on intron evolution and gene expression mechanisms in bacteria and eukarya.
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Affiliation(s)
- David M. Truong
- Institute for Cellular and Molecular Biology, Department of Molecular Biosciences, The University of Texas at Austin, Austin, Texas, United States of America
| | - F. Curtis Hewitt
- Institute for Cellular and Molecular Biology, Department of Molecular Biosciences, The University of Texas at Austin, Austin, Texas, United States of America
| | - Joseph H. Hanson
- Institute for Cellular and Molecular Biology, Department of Molecular Biosciences, The University of Texas at Austin, Austin, Texas, United States of America
| | - Xiaoxia Cui
- Institute for Cellular and Molecular Biology, Department of Molecular Biosciences, The University of Texas at Austin, Austin, Texas, United States of America
| | - Alan M. Lambowitz
- Institute for Cellular and Molecular Biology, Department of Molecular Biosciences, The University of Texas at Austin, Austin, Texas, United States of America
- * E-mail:
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Guha TK, Hausner G. A homing endonuclease with a switch: Characterization of a twintron encoded homing endonuclease. Fungal Genet Biol 2014; 65:57-68. [DOI: 10.1016/j.fgb.2014.01.004] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2013] [Revised: 01/22/2014] [Accepted: 01/23/2014] [Indexed: 10/25/2022]
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Enyeart PJ, Mohr G, Ellington AD, Lambowitz AM. Biotechnological applications of mobile group II introns and their reverse transcriptases: gene targeting, RNA-seq, and non-coding RNA analysis. Mob DNA 2014; 5:2. [PMID: 24410776 PMCID: PMC3898094 DOI: 10.1186/1759-8753-5-2] [Citation(s) in RCA: 57] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2013] [Accepted: 11/19/2013] [Indexed: 12/21/2022] Open
Abstract
Mobile group II introns are bacterial retrotransposons that combine the activities of an autocatalytic intron RNA (a ribozyme) and an intron-encoded reverse transcriptase to insert site-specifically into DNA. They recognize DNA target sites largely by base pairing of sequences within the intron RNA and achieve high DNA target specificity by using the ribozyme active site to couple correct base pairing to RNA-catalyzed intron integration. Algorithms have been developed to program the DNA target site specificity of several mobile group II introns, allowing them to be made into ‘targetrons.’ Targetrons function for gene targeting in a wide variety of bacteria and typically integrate at efficiencies high enough to be screened easily by colony PCR, without the need for selectable markers. Targetrons have found wide application in microbiological research, enabling gene targeting and genetic engineering of bacteria that had been intractable to other methods. Recently, a thermostable targetron has been developed for use in bacterial thermophiles, and new methods have been developed for using targetrons to position recombinase recognition sites, enabling large-scale genome-editing operations, such as deletions, inversions, insertions, and ‘cut-and-pastes’ (that is, translocation of large DNA segments), in a wide range of bacteria at high efficiency. Using targetrons in eukaryotes presents challenges due to the difficulties of nuclear localization and sub-optimal magnesium concentrations, although supplementation with magnesium can increase integration efficiency, and directed evolution is being employed to overcome these barriers. Finally, spurred by new methods for expressing group II intron reverse transcriptases that yield large amounts of highly active protein, thermostable group II intron reverse transcriptases from bacterial thermophiles are being used as research tools for a variety of applications, including qRT-PCR and next-generation RNA sequencing (RNA-seq). The high processivity and fidelity of group II intron reverse transcriptases along with their novel template-switching activity, which can directly link RNA-seq adaptor sequences to cDNAs during reverse transcription, open new approaches for RNA-seq and the identification and profiling of non-coding RNAs, with potentially wide applications in research and biotechnology.
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Affiliation(s)
| | | | | | - Alan M Lambowitz
- Departments of Molecular Biosciences and Chemistry, Institute for Cellular and Molecular Biology, The University of Texas at Austin, Austin, TX 78712, USA.
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Contreras LM, Huang T, Piazza CL, Smith D, Qu G, Gelderman G, Potratz JP, Russell R, Belfort M. Group II intron-ribosome association protects intron RNA from degradation. RNA (NEW YORK, N.Y.) 2013; 19:1497-1509. [PMID: 24046482 PMCID: PMC3851717 DOI: 10.1261/rna.039073.113] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 03/11/2013] [Accepted: 07/26/2013] [Indexed: 06/02/2023]
Abstract
The influence of the cellular environment on the structures and properties of catalytic RNAs is not well understood, despite great interest in ribozyme function. Here we report on ribosome association of group II introns, which are ribozymes that are important because of their putative ancestry to spliceosomal introns and retrotransposons, their retromobility via an RNA intermediate, and their application as gene delivery agents. We show that group II intron RNA, in complex with the intron-encoded protein from the native Lactoccocus lactis host, associates strongly with ribosomes in vivo. Ribosomes have little effect on intron ribozyme activities; rather, the association with host ribosomes protects the intron RNA against degradation by RNase E, an enzyme previously shown to be a silencer of retromobility in Escherichia coli. The ribosome interacts strongly with the intron, exerting protective effects in vivo and in vitro, as demonstrated by genetic and biochemical experiments. These results are consistent with the ribosome influencing the integrity of catalytic RNAs in bacteria in the face of degradative nucleases that regulate intron mobility.
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Affiliation(s)
- Lydia M. Contreras
- Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712-2100, USA
- Wadsworth Center, New York State Department of Health, Albany, New York 12201-2002, USA
- Department of Chemistry and Biochemistry, Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas 78712-2100, USA
| | - Tao Huang
- Wadsworth Center, New York State Department of Health, Albany, New York 12201-2002, USA
- SUNY Downstate Medical Center, University Hospital, Brooklyn, New York 11203, USA
| | - Carol Lyn Piazza
- Wadsworth Center, New York State Department of Health, Albany, New York 12201-2002, USA
- Department of Biological Sciences, RNA Institute, University at Albany, SUNY, Albany, New York 12222, USA
| | - Dorie Smith
- Wadsworth Center, New York State Department of Health, Albany, New York 12201-2002, USA
- Department of Biological Sciences, RNA Institute, University at Albany, SUNY, Albany, New York 12222, USA
| | - Guosheng Qu
- Department of Biological Sciences, RNA Institute, University at Albany, SUNY, Albany, New York 12222, USA
| | - Grant Gelderman
- Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712-2100, USA
| | - Jeffrey P. Potratz
- Department of Chemistry and Biochemistry, Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas 78712-2100, USA
| | - Rick Russell
- Department of Chemistry and Biochemistry, Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas 78712-2100, USA
| | - Marlene Belfort
- Wadsworth Center, New York State Department of Health, Albany, New York 12201-2002, USA
- Department of Biological Sciences, RNA Institute, University at Albany, SUNY, Albany, New York 12222, USA
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16
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Enhanced group II intron retrohoming in magnesium-deficient Escherichia coli via selection of mutations in the ribozyme core. Proc Natl Acad Sci U S A 2013; 110:E3800-9. [PMID: 24043808 DOI: 10.1073/pnas.1315742110] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
Mobile group II introns are bacterial retrotransposons thought to be evolutionary ancestors of spliceosomal introns and retroelements in eukaryotes. They consist of a catalytically active intron RNA ("ribozyme") and an intron-encoded reverse transcriptase, which function together to promote RNA splicing and intron mobility via reverse splicing of the intron RNA into new DNA sites ("retrohoming"). Although group II introns are active in bacteria, their natural hosts, they function inefficiently in eukaryotes, where lower free Mg(2+) concentrations decrease their ribozyme activity and constitute a natural barrier to group II intron proliferation within nuclear genomes. Here, we show that retrohoming of the Ll.LtrB group II intron is strongly inhibited in an Escherichia coli mutant lacking the Mg(2+) transporter MgtA, and we use this system to select mutations in catalytic core domain V (DV) that partially rescue retrohoming at low Mg(2+) concentrations. We thus identified mutations in the distal stem of DV that increase retrohoming efficiency in the MgtA mutant up to 22-fold. Biochemical assays of splicing and reverse splicing indicate that the mutations increase the fraction of intron RNA that folds into an active conformation at low Mg(2+) concentrations, and terbium-cleavage assays suggest that this increase is due to enhanced Mg(2+) binding to the distal stem of DV. Our findings indicate that DV is involved in a critical Mg(2+)-dependent RNA folding step in group II introns and demonstrate the feasibility of selecting intron variants that function more efficiently at low Mg(2+) concentrations, with implications for evolution and potential applications in gene targeting.
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Mohr G, Hong W, Zhang J, Cui GZ, Yang Y, Cui Q, Liu YJ, Lambowitz AM. A targetron system for gene targeting in thermophiles and its application in Clostridium thermocellum. PLoS One 2013; 8:e69032. [PMID: 23874856 PMCID: PMC3706431 DOI: 10.1371/journal.pone.0069032] [Citation(s) in RCA: 51] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/15/2013] [Accepted: 06/03/2013] [Indexed: 01/04/2023] Open
Abstract
Background Targetrons are gene targeting vectors derived from mobile group II introns. They consist of an autocatalytic intron RNA (a “ribozyme”) and an intron-encoded reverse transcriptase, which use their combined activities to achieve highly efficient site-specific DNA integration with readily programmable DNA target specificity. Methodology/Principal Findings Here, we used a mobile group II intron from the thermophilic cyanobacterium Thermosynechococcus elongatus to construct a thermotargetron for gene targeting in thermophiles. After determining its DNA targeting rules by intron mobility assays in Escherichia coli at elevated temperatures, we used this thermotargetron in Clostridium thermocellum, a thermophile employed in biofuels production, to disrupt six different chromosomal genes (cipA, hfat, hyd, ldh, pta, and pyrF). High integration efficiencies (67–100% without selection) were achieved, enabling detection of disruptants by colony PCR screening of a small number of transformants. Because the thermotargetron functions at high temperatures that promote DNA melting, it can recognize DNA target sequences almost entirely by base pairing of the intron RNA with less contribution from the intron-encoded protein than for mesophilic targetrons. This feature increases the number of potential targetron-insertion sites, while only moderately decreasing DNA target specificity. Phenotypic analysis showed that thermotargetron disruption of the genes encoding lactate dehydrogenase (ldh; Clo1313_1160) and phosphotransacetylase (pta; Clo1313_1185) increased ethanol production in C. thermocellum by decreasing carbon flux toward lactate and acetate. Conclusions/Significance Thermotargetron provides a new, rapid method for gene targeting and genetic engineering of C. thermocellum, an industrially important microbe, and should be readily adaptable for gene targeting in other thermophiles.
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Affiliation(s)
- Georg Mohr
- Section of Molecular Genetics and Microbiology, Department of Chemistry and Biochemistry, Institute for Cellular and Molecular Biology, School of Biological Sciences, University of Texas at Austin, Austin, Texas, United States of America
| | - Wei Hong
- Shandong Provincial Key Laboratory of Energy Genetics, and Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, People’s Republic of China
- University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing, People’s Republic of China
| | - Jie Zhang
- Shandong Provincial Key Laboratory of Energy Genetics, and Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, People’s Republic of China
- University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing, People’s Republic of China
| | - Gu-zhen Cui
- Shandong Provincial Key Laboratory of Energy Genetics, and Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, People’s Republic of China
| | - Yunfeng Yang
- State Key Joint Laboratory of Environment, Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, People’s Republic of China
| | - Qiu Cui
- Shandong Provincial Key Laboratory of Energy Genetics, and Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, People’s Republic of China
| | - Ya-jun Liu
- Shandong Provincial Key Laboratory of Energy Genetics, and Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, People’s Republic of China
- * E-mail: (AL); (YL)
| | - Alan M. Lambowitz
- Section of Molecular Genetics and Microbiology, Department of Chemistry and Biochemistry, Institute for Cellular and Molecular Biology, School of Biological Sciences, University of Texas at Austin, Austin, Texas, United States of America
- * E-mail: (AL); (YL)
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