The linear form of a group II intron catalyzes efficient autocatalytic reverse splicing, establishing a potential for mobility

Unraveling the role of the enigmatic MatK maturase in chloroplast group IIA intron excision

Maturases are prokaryotic enzymes that aid self-excision of introns in precursor RNAs and have evolutionary ties to the nuclear spliceosome. Both the mitochondria and chloroplast, due to their prokaryotic origin, encode a single intron maturase, MatR for the mitochondria and MatK for the chloroplast. MatK is proposed to aid excision of seven different chloroplast group IIA introns that reside within precursor RNAs for essential elements of chloroplast function. We have developed an in vitro activity assay to test chloroplast group IIA intron excision. Using this assay, we demonstrate self-excision of the group IIA intron of the second intron of rps12 and the group IIA intron of rpl2. We further show that the addition of heterologously expressed MatK protein increases efficiency of group IIA intron self-splicing for the second intron of rps12 but not the group IIA intron of rpl2. These data, to our knowledge, provide the first direct evidence of MatK's maturase activity.

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.

Structural Insights into the Mechanism of Group II Intron Splicing

While the major architectural features and active-site components of group II introns have been known for almost a decade, information on the individual stages of splicing has been lacking. Recent advances in crystallography and cryo-electron microscopy (cryo-EM) have provided major new insights into the structure of intact lariat introns. Conformational changes that mediate the steps of splicing and retrotransposition are being elucidated, revealing the dynamic, highly coordinated motions that are required for group II intron activity. Finally, these ribozymes can now be viewed in their larger, more natural context as components of holoenzymes that include encoded maturase proteins. These studies expand our understanding of group II intron structural diversity and evolution, while setting the stage for rigorous mechanistic analysis of RNA splicing machines.

Crystal structure of a eukaryotic group II intron lariat

The formation of branched lariat RNA is an evolutionarily conserved feature of splicing reactions for both group II and spliceosomal introns. The lariat is important for the fidelity of 5' splice-site selection and consists of a 2'-5' phosphodiester bond between a bulged adenosine and the 5' end of the intron. To gain insight into this ubiquitous intramolecular linkage, we determined the crystal structure of a eukaryotic group IIB intron in the lariat form at 3.7 Å. This revealed that two tandem tetraloop-receptor interactions, η-η' and π-π', place domain VI in the core to position the lariat bond in the post-catalytic state. On the basis of structural and biochemical data, we propose that π-π' is a dynamic interaction that mediates the transition between the two steps of splicing, with η-η' serving an ancillary role. The structure also reveals a four-magnesium-ion cluster involved in both catalysis and positioning of the 5' end. Given the evolutionary relationship between group II and nuclear introns, it is likely that this active site configuration exists in the spliceosome as well.

Vulnerabilities on the lagging-strand template: opportunities for mobile elements

Mobile genetic elements have the ability to move between positions in a genome. Some of these elements are capable of targeting one of the template strands during DNA replication. Examples found in bacteria include (a) Red recombination mediated by bacteriophage λ, (b) integration of group II mobile introns that reverse splice and reverse transcribe into DNA, (c) HUH endonuclease elements that move as single-stranded DNA, and (d) Tn7, a DNA cut-and-paste transposon that uses a target-site-selecting protein to target transposition into certain forms of DNA replication. In all of these examples, the lagging-strand template appears to be targeted using a variety of features specific to this strand. These features appear especially available in certain situations, such as when replication forks stall or collapse. In this review, we address the idea that features specific to the lagging-strand template represent vulnerabilities that are capitalized on by mobile genetic elements.

Selective forces for the origin of spliceosomes

It has been proposed that eukaryotic spliceosomes evolved from bacterial group II introns via constructive neutral changes. However, a more likely interpretation is that spliceosomes and group II introns share a common undefined RNA ancestor--a proto-spliceosome. Although, the constructive neutral evolution may have probably played some roles in the development of complexity including the evolution of modern spliceosomes, in fact, the origin, losses and the retention of spliceosomes can be explained straight-forwardly mainly by positive and negative selection: (1) proto-spliceosomes evolved in the RNA world as a mechanism to excise functional RNAs from an RNA genome and to join non-coding information (ancestral to exons) possibly designed to be degraded. (2) The complexity of proto-spliceosomes increased with the invention of protein synthesis in the RNP world and they were adopted for (a) the addition of translation signal to RNAs via trans-splicing, and for (b) the exon-shuffling such as to join together exons coding separate protein domains, to translate them as a single unit and thus to facilitate the molecular interaction of protein domains needed to be assembled to functional catalytic complexes. (3) Finally, the spliceosomes were adopted for cis-splicing of (mainly) non-coding information (contemporary introns) to yield translatable mRNAs. (4) Spliceosome-negative organisms (i.e., prokaryotes) have been selected in the DNA-protein world to save a lot of energy. (5) Spliceosome-positive organisms (i.e., eukaryotes) have been selected, because they have been completely spliceosome-dependent.

Group II introns: mobile ribozymes that invade DNA

Group II introns are mobile ribozymes that self-splice from precursor RNAs to yield excised intron lariat RNAs, which then invade new genomic DNA sites by reverse splicing. The introns encode a reverse transcriptase that stabilizes the catalytically active RNA structure for forward and reverse splicing, and afterwards converts the integrated intron RNA back into DNA. The characteristics of group II introns suggest that they or their close relatives were evolutionary ancestors of spliceosomal introns, the spliceosome, and retrotransposons in eukaryotes. Further, their ribozyme-based DNA integration mechanism enabled the development of group II introns into gene targeting vectors ("targetrons"), which have the unique feature of readily programmable DNA target specificity.

Relevance of the branch point adenosine, coordination loop, and 3' exon binding site for in vivo excision of the Sinorhizobium meliloti group II intron RmInt1

Excision of the bacterial group II intron RmInt1 has been demonstrated in vivo, resulting in the formation of both intron lariat and putative intron RNA circles. We show here that the bulged adenosine in domain VI of RmInt1 is required for splicing via the branching pathway, but branch site mutants produce small numbers of RNA molecules in which the first G residue of the intron is linked to the last C residue. Mutations in the coordination loop in domain I reduced splicing efficiency, but branched templates clearly predominated among splicing products. We also found that a single substitution at the EBS3 position (G329C), preventing EBS3-IBS3 pairing, resulted in the production of 50 to 100 times more RNA molecules in which the 5' and 3' extremities were joined. We provide evidence that these intron molecules may correspond to both, intron circles linked by a 2'-5' phosphodiester bond, and tandem, head-to-tail intron copies.

DEAD-box protein facilitated RNA folding in vivo

In yeast mitochondria the DEAD-box helicase Mss116p is essential for respiratory growth by acting as group I and group II intron splicing factor. Here we provide the first structure-based insights into how Mss116p assists RNA folding in vivo. Employing an in vivo chemical probing technique, we mapped the structure of the ai5γ group II intron in different genetic backgrounds to characterize its intracellular fold. While the intron adopts the native conformation in the wt yeast strain, we found that the intron is able to form most of its secondary structure, but lacks its tertiary fold in the absence of Mss116p. This suggests that ai5γ is largely unfolded in the mss116-knockout strain and requires the protein at an early step of folding. Notably, in this unfolded state misfolded substructures have not been observed. As most of the protein-induced conformational changes are located within domain D1, Mss116p appears to facilitate the formation of this largest domain, which is the scaffold for docking of other intron domains. These findings suggest that Mss116p assists the ordered assembly of the ai5γ intron in vivo.

The tertiary structure of group II introns: implications for biological function and evolution

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.

The 2'-OH group at the group II intron terminus acts as a proton shuttle

Group II introns are self-splicing ribozymes that excise themselves from precursor RNAs and catalyze the joining of flanking exons. Excised introns can behave as parasitic RNA molecules, catalyzing their own insertion into DNA and RNA via a reverse-splicing reaction. Previous studies have identified mechanistic roles for various functional groups located in the catalytic core of the intron and within target molecules. Here we introduce a new method for synthesizing long RNA molecules with a modified nucleotide at the 3′-terminus. This modification allows us to examine the mechanistic role of functional groups adjacent to the reaction nucleophile. During reverse-splicing, the 3′-OH group of the intron terminus attacks the phosphodiester linkage of spliced exon sequences. Here we show that the adjacent 2′-OH group on the intron terminus plays an essential role in activating the nucleophile by stripping away a proton from the 3′-OH and then shuttling it from the active-site.

Linear group II intron RNAs can retrohome in eukaryotes and may use nonhomologous end-joining for cDNA ligation

Mobile group II introns retrohome by an RNP-based mechanism in which the excised intron lariat RNA fully reverse splices into a DNA site via 2 sequential transesterification reactions and is reverse transcribed by the associated intron-encoded protein. However, linear group II intron RNAs, which can arise by either hydrolytic splicing or debranching of lariat RNA, cannot carry out both reverse-splicing steps and were thus expected to be immobile. Here, we used facile microinjection assays in 2 eukaryotic systems, Xenopus laevis oocyte nuclei and Drosophila melanogaster embryos, to show that group II intron RNPs containing linear intron RNA can retrohome by carrying out the first step of reverse splicing into a DNA site, thereby ligating the 3' end of the intron RNA to the 5' end of the downstream exon DNA. The attached linear intron RNA is then reverse transcribed, yielding an intron cDNA whose free end is linked to the upstream exon DNA. Some of these retrohoming events result in the precise insertion of full-length intron. Most, however, yield aberrant 5' junctions with 5' exon resections, 5' intron truncations, and/or extra nucleotide residues, hallmarks of nonhomologous end-joining. Our findings reveal a mobility mechanism for linear group II intron RNAs, show how group II introns can co-opt different DNA repair pathways for retrohoming, and suggest that linear group II intron RNAs might be used for site-specific DNA integration in gene targeting.