Recurrent insertion of 5'-terminal nucleotides and loss of the branchpoint motif in lineages of group II introns inserted in mitochondrial preribosomal RNAs

Discovery of highly reactive self-splicing group II introns within the mitochondrial genomes of human pathogenic fungi

Fungal pathogens represent an expanding global health threat for which treatment options are limited. Self-splicing group II introns have emerged as promising drug targets, but their development has been limited by a lack of information on their distribution and architecture in pathogenic fungi. To meet this challenge, we developed a bioinformatic workflow for scanning sequence data to identify unique RNA structural signatures within group II introns. Using this approach, we discovered a set of ubiquitous introns within thermally dimorphic fungi (genera of Blastomyces, Coccidioides and Histoplasma). These introns are the most biochemically reactive group II introns ever reported, and they self-splice rapidly under near-physiological conditions without protein cofactors. Moreover, we demonstrated the small molecule targetability of these introns by showing that they can be inhibited by the FDA-approved drug mitoxantrone in vitro. Taken together, our results highlight the utility of structure-based informatic searches for identifying riboregulatory elements in pathogens, revealing a striking diversity of reactive self-splicing introns with great promise as antifungal drug targets.

The fungal mitochondrial Nad5 pan-genic intron landscape

An intron landscape was prepared for the fungal mitochondrial nad5 gene. A hundred and eighty-eight fungal species were examined and a total of 265 introns were noted to be located in 29 intron insertion sites within the examined nad5 genes. Two hundred and sixty-three introns could be classified as group I types and two group II introns were noted. One additional group II intron module was identified nested within a composite group I intron. Based on features related to RNA secondary structures, introns can be classified into different subtypes and it was observed that intron insertion-sites are biased towards phase 0 and they appear to be specific to an intron type. Intron landscapes could be used as a guide map to predict the location of fungal mtDNA mobile introns, which are composite elements that include a ribozyme component and in some instances open reading frames encoding homing endonucleases or reverse transcriptases and all of these have applications in biotechnology.

DNA cleavage and reverse splicing of ribonucleoprotein particles reconstituted in vitro with linear RmInt1 RNA

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.

Small molecules that target group II introns are potent antifungal agents

Specific RNA structures control numerous metabolic processes that impact human health, and yet efforts to target RNA structures de novo have been limited. In eukaryotes, the self-splicing group II intron is a mitochondrial RNA tertiary structure that is absent in vertebrates but essential for respiration in plants, fungi and yeast. Here we show that this RNA can be targeted through a process of high-throughput in vitro screening, SAR and lead optimization, resulting in high-affinity compounds that specifically inhibit group IIB intron splicing in vitro and in vivo and lack toxicity in human cells. The compounds are potent growth inhibitors of the pathogen Candida parapsilosis, displaying antifungal activity comparable to that of amphotericin B. These studies demonstrate that RNA tertiary structures can be successfully targeted de novo, resulting in pharmacologically valuable compounds.

Group II introns in wheat mitochondria have degenerate structural features and varied splicing pathways

Mitochondrial introns in flowering plant genes are virtually all classified as members of the group II ribozyme family although certain structural features have degenerated to varying degrees over evolutionary time. We are interested in the impact that unconventional intron architecture might have on splicing biochemistry in vivo and we have focused in particular on intronic domains V and VI, which for self-splicing introns provide a key component of the catalytic core and the bulged branchpoint adenosine, respectively. Notably, the two transesterification steps in classical group II splicing are the same as for nuclear spliceosomal introns and release the intron as a lariat. Using RT-PCR and circularized RT-PCR, we had previously demonstrated that several wheat mitochondrial introns which lack a branchpoint adenosine have atypical splicing pathways, and we have now extended this analysis to the full set of wheat introns, namely six trans-splicing and sixteen cis-splicing ones. A number of introns are excised using non-lariat pathways and interestingly, we find that several introns which do have a conventional domain VI also use pathways that appear to exploit other internal or external nucleophiles, with the lariat form being relatively minor. Somewhat surprisingly, several introns with weakly-structured domain V/VI helices still exhibit classical lariat splicing, suggesting that accessory factors aid in restoring a splicing-competent conformation. Our observations illustrate that the loss of conventional group II features during evolution is correlated with altered splicing biochemistry in an intron-distinctive manner.

Multiple splicing pathways of group II trans-splicing introns in wheat mitochondria

Trans-splicing of discontinuous introns in plant mitochondria requires the assembly of independently-transcribed precursor RNAs into splicing-competent structures, and they are expected to be excised as Y-branched molecules ("broken lariats") because these introns belong to the group II ribozyme family. We now demonstrate that this is just one of several trans-splicing pathways for wheat mitochondrial nad1 intron 4 and nad5 intron 2; they also use a hydrolytic pathway and the liberated 5'-half-intron linear molecules are unexpectedly abundant in the RNA population. We also observe a third productive splicing pathway for nad5 intron 2 that yields full-length excised introns in which the termini are joined in vivo and possess non-encoded nucleotides. In the case of trans-splicing nad1 intron 1, which has a weakly-structured and poorly-conserved core sequence, excision appears to be solely through a hydrolytic pathway. When wheat embryos are germinated in the cold rather than at room temperature, an increased complexity in trans-splicing products is seen for nad1 intron 4, suggesting that there can be environmental effects on the RNA folding of bipartite introns. Our observations provide insights into intron evolution and the complexity of RNA processing events in plant mitochondria.

Activating the branch-forming splicing pathway by reengineering the ribozyme component of a natural group II intron

When assayed in vitro, group IIC self-splicing introns, which target bacterial Rho-independent transcription terminators, generally fail to yield branched products during splicing despite their possessing a seemingly normal branchpoint. Starting with intron O.i.I1 from Oceanobacillus iheyensis, whose crystallographically determined structure lacks branchpoint-containing domain VI, we attempted to determine what makes this intron unfit for in vitro branch formation. A major factor was found to be the length of the helix at the base of domain VI: 4 base pairs (bp) are required for efficient branching, even though a majority of group IIC introns have a 3-bp helix. Equally important for lariat formation is the removal of interactions between ribozyme domains II and VI, which are specific to the second step of splicing. Conversely, mismatching of domain VI and its proposed first-step receptor in subdomain IC1 was found to be detrimental; these data suggest that the intron-encoded protein may promote branch formation partly by modulating the equilibrium between conformations specific to the first and second steps of splicing. As a practical application, we show that by making just two changes to the O.i.I1 ribozyme, it is possible to generate sufficient amounts of lariat intron for the latter to be purified and used in kinetic assays in which folding and reaction are uncoupled.

Group II introns: versatile ribozymes and retroelements

Group II introns are catalytic RNAs (ribozymes) and retroelements found in the genomes of bacteria, archaebacteria, and organelles of some eukaryotes. The prototypical retroelement form consists of a structurally conserved RNA and a multidomain reverse transcriptase protein, which interact with each other to mediate splicing and mobility reactions. A wealth of biochemical, cross-linking, and X-ray crystal structure studies have helped to reveal how the two components cooperate to carry out the splicing and mobility reactions. In addition to the standard retroelement form, group II introns have evolved into derivative forms by either losing specific splicing or mobility characteristics, or becoming functionally specialized. Of particular interest are the eukaryotic derivatives-the spliceosome, spliceosomal introns, and non-LTR retroelements-which together make up approximately half of the human genome. On a practical level, the properties of group II introns have been exploited to develop group II intron-based biotechnological tools. WIREs RNA 2016, 7:341-355. doi: 10.1002/wrna.1339 For further resources related to this article, please visit the WIREs website.

Evolution of group II introns

Present in the genomes of bacteria and eukaryotic organelles, group II introns are an ancient class of ribozymes and retroelements that are believed to have been the ancestors of nuclear pre-mRNA introns. Despite long-standing speculation, there is limited understanding about the actual pathway by which group II introns evolved into eukaryotic introns. In this review, we focus on the evolution of group II introns themselves. We describe the different forms of group II introns known to exist in nature and then address how these forms may have evolved to give rise to spliceosomal introns and other genetic elements. Finally, we summarize the structural and biochemical parallels between group II introns and the spliceosome, including recent data that strongly support their hypothesized evolutionary relationship.

Novel RNA structural features of an alternatively splicing group II intron from Clostridium tetani

Group II introns are ribozymes in bacterial and organellar genomes that function as self-splicing introns and as retroelements. Previously, we reported that the group II intron C.te.I1 of Clostridium tetani alternatively splices in vivo to produce five distinct coding mRNAs. Accurate fusion of upstream and downstream reading frames requires a shifted 5' splice site located 8 nt upstream of the usual 5' GUGYG motif. This site is specified by the ribozyme through an altered intron/exon-binding site 1 (IBS1-EBS1) pairing. Here we use mutagenesis and self-splicing assays to investigate in more detail the significance of the structural features of the C.te.I1 ribozyme. The shifted 5' splice site is shown to be affected by structures in addition to IBS1-EBS1, and unlike other group II introns, C.te.I1 appears to require a spacer between IBS1 and the GUGYG motif. In addition, the mechanism of 3' exon recognition is modified from the ancestral IIB mechanism to a IIA-like mechanism that appears to be longer than the typical single base-pair interaction and may extend up to 4 bp. The novel ribozyme properties that have evolved for C.te.I1 illustrate the plasticity of group II introns in adapting new structural and catalytic properties that can be utilized to affect gene expression.

I-OmiI and I-OmiII: two intron-encoded homing endonucleases within the Ophiostoma minus rns gene

The mitochondrial small subunit ribosomal RNA (rns) gene of the ascomycetous fungus Ophiostoma minus [strain WIN(M)371] was found to contain a group IC2 and a group IIB1 intron at positions mS569 and mS952 respectively. Both introns have open reading frames (ORFs) embedded that encode double motif LAGLIDADG homing endonucleases (I-OmiI and I-OmiII respectively). Codon-optimized versions of I-OmiI and I-OmiII were synthesized for overexpression in Escherichia coli. The in vitro characterization of I-OmiII showed that it is a functional homing endonuclease that cleaves the rns target site two nucleotides upstream (sense strand) of the intron insertion site generating 4 nucleotide 3' overhangs. The endonuclease activity of I-OmiII was tested using linear and circular substrates and cleavage activity was evaluated at various temperatures. The I-OmiI protein was expressed in E. coli, but purification was difficult, thus the endonuclease activity of this protein was tested via in vivo assays. Overall this study showed that there are many native forms of functional homing endonucleases yet to be discovered among fungal mtDNA genomes.

Alternative splicing of a group II intron in a surface layer protein gene in Clostridium tetani

Group II introns are ribozymes and retroelements found in bacteria, and are thought to have been the ancestors of nuclear pre-mRNA introns. Whereas nuclear introns undergo prolific alternative splicing in some species, group II introns are not known to carry out equivalent reactions. Here we report a group II intron in the human pathogen Clostridium tetani, which undergoes four alternative splicing reactions in vivo. Together with unspliced transcript, five mRNAs are produced, each encoding a distinct surface layer protein isoform. Correct fusion of exon reading frames requires a shifted 5′ splice site located 8 nt upstream of the canonical boundary motif. The shifted junction is accomplished by an altered IBS1-EBS1 pairing between the intron and 5′ exon. Growth of C. tetani under a variety of conditions did not result in large changes in alternative splicing levels, raising the possibility that alternative splicing is constitutive. This work demonstrates a novel type of gene organization and regulation in bacteria, and provides an additional parallel between group II and nuclear pre-mRNA introns.

Impact of low temperature on splicing of atypical group II introns in wheat mitochondria

To investigate the impact of cold on group II intron splicing, we compared the physical forms of excised mitochondrial introns from wheat embryos germinated at room temperature and 4°C. For introns which deviate from the conventional branchpoint structure, we observed predominantly heterogeneous circularized introns in the cold rather than linear polyadenylated forms arising from a hydrolytic pathway as seen at room temperature. In addition, intron-containing precursors are elevated relative to mature mRNAs upon cold treatment. Our findings indicate that low temperature growth not only reduces splicing efficiency, but also shifts the splicing biochemistry of atypical group II introns to novel, yet productive, pathways.

RNA editing in mitochondrial trans-introns is required for splicing

In plant mitochondria, gene expression of translatable mRNAs is a complex process with two critical steps, RNA editing and splicing. We studied the role of RNA editing on non-coding regions of the mat-r-nad1e-nad5c transcript from wheat mitochondria. This RNA contains two trans-introns, 3'-nad1-I4 and 3'-nad5-I2, involved in different trans-splicing events, ensuring the association of nad1d-nad1e and nad5b-nad5c exons from nad1 and nad5 mRNAs respectively. The C-to-U editing changes studied here affect homologous positions on 3'-nad1-I4 and 3'-nad5-I2. It is proposed that these base changes are necessary to place an Adenosine residue in a bulging conformation characteristic of domain VI (D6) from group II introns. In this work, we investigated the role of RNA editing events on 3'-nad1-I4 and 3'-nad5-I2 in the trans-splicing process using in vivo and in organello approaches. When the branched intermediates formed during the splicing process were analyzed, the C residues from D6 intron domains from 3'-nad1-I4 and 3'-nad5-I2 were found changed to U, suggesting that RNA editing of these residues could be mandatory for splicing. This assumption was tested by expressing recombinant mat-r-nad1e transgenes introduced into mitochondria by electroporation. Mutation of the editing target residue dramatically affected trans-splicing. Interestingly, the exon joining efficiency was not recovered by compensatory mutations, suggesting that the role of RNA editing is not confined to the restoration of the secondary structure of domain D6 of the intron. Our results strongly support the hypothesis that RNA editing in trans-introns precedes maturation, and is required for the splicing reaction. In addition, this is the first report using an in organello approach to study the trans-splicing process, opening the way to future studies of this peculiar mechanism.

Homing endonucleases: DNA scissors on a mission

Buried within the genomes of many microorganisms are genetic elements that encode rare-cutting homing endonucleases that assist in the mobility of the elements that encode them, such as the self-splicing group I and II introns and in some cases inteins. There are several different families of homing endonucleases and their ability to initiate and target specific sequences for lateral transfers makes them attractive reagents for gene targeting. Homing endonucleases have been applied in promoting DNA modification or genome editing such as gene repair or "gene knockouts". This review examines the categories of homing endonucleases that have been described so far and their possible applications to biotechnology. Strategies to engineer homing endonucleases to alter target site specificities will also be addressed. Alternatives to homing endonucleases such as zinc finger nucleases, transcription activator-like effector nucleases, triplex forming oligonucleotide nucleases, and targetrons are also briefly discussed.

Multiple self-splicing introns in the 16S rRNA genes of giant sulfur bacteria

The gene encoding the small subunit rRNA serves as a prominent tool for the phylogenetic analysis and classification of Bacteria and Archaea owing to its high degree of conservation and its fundamental function in living organisms. Here we show that the 16S rRNA genes of not-yet-cultivated large sulfur bacteria, among them the largest known bacterium Thiomargarita namibiensis, regularly contain numerous self-splicing introns of variable length. The 16S rRNA genes can thus be enlarged to up to 3.5 kb. Remarkably, introns have never been identified in bacterial 16S rRNA genes before, although they are the most frequently sequenced genes today. This may be caused in part by a bias during the PCR amplification step that discriminates against longer homologs, as we show experimentally. Such length heterogeneity of 16S rRNA genes has so far never been considered when constructing 16S rRNA-based clone libraries, even though an elongation of rRNA genes due to intervening sequences has been reported previously. The detection of elongated 16S rRNA genes has profound implications for common methods in molecular ecology and may cause systematic biases in several techniques. In this study, catalyzed reporter deposition-fluorescence in situ hybridization on both ribosomes and rRNA precursor molecules as well as in vitro splicing experiments were performed and confirmed self-splicing of the introns. Accordingly, the introns do not inhibit the formation of functional ribosomes.

The retrohoming of linear group II intron RNAs in Drosophila melanogaster occurs by both DNA ligase 4-dependent and -independent mechanisms

Mobile group II introns are bacterial retrotransposons that are thought to have invaded early eukaryotes and evolved into introns and retroelements in higher organisms. In bacteria, group II introns typically retrohome via full reverse splicing of an excised intron lariat RNA into a DNA site, where it is reverse transcribed by the intron-encoded protein. Recently, we showed that linear group II intron RNAs, which can result from hydrolytic splicing or debranching of lariat RNAs, can retrohome in eukaryotes by performing only the first step of reverse splicing, ligating their 3' end to the downstream DNA exon. Reverse transcription then yields an intron cDNA, whose free end is linked to the upstream DNA exon by an error-prone process that yields junctions similar to those formed by non-homologous end joining (NHEJ). Here, by using Drosophila melanogaster NHEJ mutants, we show that linear intron RNA retrohoming occurs by major Lig4-dependent and minor Lig4-independent mechanisms, which appear to be related to classical and alternate NHEJ, respectively. The DNA repair polymerase θ plays a crucial role in both pathways. Surprisingly, however, mutations in Ku70, which functions in capping chromosome ends during NHEJ, have only moderate, possibly indirect effects, suggesting that both Lig4 and the alternate end-joining ligase act in some retrohoming events independently of Ku. Another potential Lig4-independent mechanism, reverse transcriptase template switching from the intron RNA to the upstream exon DNA, occurs in vitro, but gives junctions differing from the majority in vivo. Our results show that group II introns can utilize cellular NHEJ enzymes for retromobility in higher organisms, possibly exploiting mechanisms that contribute to retrotransposition and mitigate DNA damage by resident retrotransposons. Additionally, our results reveal novel activities of group II intron reverse transcriptases, with implications for retrohoming mechanisms and potential biotechnological applications.

Linking the branchpoint helix to a newly found receptor allows lariat formation by a group II intron

Like spliceosomal introns, the ribozyme-containing group II introns are excised as branched, lariat structures: a 2'-5' bond is created between the first nucleotide of the intron and an adenosine in domain VI, a component which is missing from available crystal structures of the ribozyme. Comparative sequence analysis, modelling and nucleotide substitutions point to the existence, and probable location, of a specific RNA receptor for the section of domain VI that lies just distal to the branchpoint adenosine. By designing oligonucleotides that tether domain VI to this novel binding site, we have been able to specifically activate lariat formation in an engineered, defective group II ribozyme. The location of the newly identified receptor implies that prior to exon ligation, the distal part of domain VI undergoes a major translocation, which can now be brought under control by the system of anchoring oligonucleotides we have developed. Interestingly, these oligonucleotides, which link the branchpoint helix and the binding site for intron nucleotides 3-4, may be viewed as counterparts of U2-U6 helix III in the spliceosome.