Protein facilitation of group I intron splicing by assembly of the catalytic core and the 5' splice site domain

Expression of a reporter gene interrupted by the Candida albicans group I intron is inhibited by base analogs

We previously reported the identification of an intron (CaLSU) in the 25S ribosomal RNA of some Candida albicans yeast strains. CaLSU was shown to self-splice and has the potential to adopt a secondary structure typical of group I introns. The presence of CaLSU inC. albicans strains correlates with a high degree of susceptibility to base analog antifungal agents, 5-fluorocytosine (5-FC) or 5-fluorouracil (5-FU). Cell death, resulting from addition of base analogs to growing cultures, precluded demonstration of a causal relationship between CaLSU presence and susceptibility to base analogs. In the present study, CaLSU was inserted in a non-essential lacZ reporter gene and expression was examined in Saccharomyces cerevisiae. Different mutations affecting in vitro self-splicing also had similar effects on reporter gene expression in vivo. This indicates that in vivo removal of CaLSU from the reporter gene occurs through the typical self-splicing mechanism of group I introns. Base analogs inhibited expression of the reporter gene product in a concentration-dependent manner upon their addition to the cultures. This supports a model in which disruption of intron secondary structure, consecutive to the incorporation of nucleotide analogs, is a major factor determining the susceptibility of C.albicans cells to base analogs.

Hierarchy and dynamics of RNA folding

The evidence showing that the self-assembly of complex RNAs occurs in discrete transitions, each relating to the folding of sub-systems of increasing size and complexity starting from a state with most of the secondary structure, is reviewed. The reciprocal influence of the concentration of magnesium ions and nucleotide mutations on tertiary structure is analyzed. Several observations demonstrate that detrimental mutations can be rescued by high magnesium concentrations, while stabilizing mutations lead to a lesser dependence on magnesium ion concentration. Recent data point to the central controlling and monitoring roles of RNA-binding proteins that can bind to the different folding stages, either before full establishment of the secondary structure or at the molten globule state before the cooperative transition to the final three-dimensional structure.

New loop-loop tertiary interactions in self-splicing introns of subgroup IC and ID: a complete 3D model of the Tetrahymena thermophila ribozyme

BACKGROUND

Group I introns self-splice via two consecutive trans-esterification reactions in the presence of guanosine cofactor and magnesium ions. Comparative sequence analysis has established that a catalytic core of about 120 nucleotides is conserved in all known group I introns. This core is generally not sufficient for activity, however, and most self-splicing group I introns require non-conserved peripheral elements to stabilize the complete three-dimensional (3D) structure. The physico-chemical properties of group I introns make them excellent systems for unraveling the structural basis of the RNA-RNA interactions responsible for promoting the self-assembly of complex RNAs.

RESULTS

We present phylogenetic and experimental evidence for the existence of three additional tertiary base pairings between hairpin loops within peripheral components of subgroup IC1 and ID introns. Each of these new long range interactions, called P13, P14 and P16, involves a terminal loop located in domain 2. Although domains 2 of IC and ID introns share very strong sequence similarity, their terminal loops interact with domains 5 and 9 (subgroup IC1) and domain 6 (subgroup ID). Based on these tertiary contacts, comparative sequence analysis, and published experimental results such as Fe(II)-EDTA protection patterns, we propose 3D models for two entire group I introns, the subgroup IC1 intron in the large ribosomal precursor RNA of Tetrahymena thermophila and the SdCob.1 subgroup ID intron found in the cytochrome b gene of Saccharomyces douglasii.

CONCLUSIONS

Three-dimensional models of group I introns belonging to four different subgroups are now available. They all emphasize the modular and hierarchical organization of the architecture of group I introns and the widespread use of base-pairings between terminal hairpin loops for stabilizing the folded and active structures of large and complex RNA molecules.

In vitro mutagenesis of the mitochondrial leucyl tRNA synthetase of Saccharomyces cerevisiae shows that the suppressor activity of the mutant proteins is related to the splicing function of the wild-type protein

The NAM2 gene of Saccharomyces cerevisiae encodes the mitochondrial leucyl tRNA synthetase (mLRS), which is necessary for the excision of the fourth intron of the mitochondrial cytb gene (bI4) and the fourth intron of the mitochondrial coxI gene (aI4), as well as for mitochondrial protein synthesis. Some dominant mutant alleles of the gene are able to suppress mutations that inactivate the bI4 maturase, which is essential for the excision of the introns aI4 and bI4. Here we report mutagenesis studies which focus on the splicing and suppressor functions of the protein. Small deletions in the C-terminal region of the protein preferentially reduce the splicing, but not the synthetase activity; and all the C-terminal deletions tested abolish the suppressor activity. Mutations which increase the volume of the residue at position 240 in the wild-type mLRS without introducing a charge, lead to a suppressor activity. The mutant 238C, which is located in the suppressor region, has a reduced synthetase activity and no detectable splicing activity. These data show that the splicing and suppressor functions are linked and that the suppressor activity of the mutant alleles results from a modification of the wild-type splicing activity.

The Cbp2 protein suppresses splice site mutations in a group I intron

The Cbp2 protein facilitates the folding of a group I intron in the COB pre-mRNA of yeast mitochondria. Based on its ability to suppress mutations affecting the auto-catalytic reaction, the protein appears to play a role in the selection of splice sites. Adding Cbp2 did not overcome the effects of mutations in P1 whose primary effect was on the first step of splicing. In contrast, most mutations affecting the ligation of exons were suppressed in vitro by Cbp2. These included mutations in P1, P9.0 and P10. In fact, a mutant transcript lacking both P9.0 and P10 ligated efficiently in the presence of Cbp2. P9.0 and P10 mutations also reduced the rate of cleavage at the 5' splice junction, and this effect was only partially mitigated by adding Cbp2. A competitive secondary structure near the 3' splice junction blocked Cbp2-stimulated splicing, but this mutation could be suppressed by co-transcriptional splicing in the presence of Cbp2. Our data underscore the importance of the interaction between the 5' and 3' splice junctions in group I introns and suggest that nucleotide-nucleotide interactions that stabilize the structure of group I introns can be superceded by protein-RNA interactions.

Are engineered proteins getting competition from RNA?

Progress in several areas of research is pushing back the supposed limitations of nucleic acid structure and function. New ligand-binding and catalytic RNAs are being created at a rapid pace. Some engineered RNAs offer potential as therapeutic agents whereas others can be used as model systems to study the principles that direct structure formation, molecular recognition and catalytic function by nucleic acids.

The kinetic folding pathway of the Tetrahymena ribozyme reveals possible similarities between RNA and protein folding

We have probed the nature of the individual kinetic steps in the folding of the Tetrahymena ribozyme by studying the folding kinetics of mutant ribozymes. After rapid formation of the first structural subdomain, a slow step precedes stable formation of the second subdomain. The two central helices of the second subdomain form in an interdependent manner, and this structural subunit therefore also constitutes a kinetic folding unit. The slow folding step includes formation of tertiary interactions in a triple-helical scaffold that orients the two subdomains of the RNA. The rapid and early formation of short range secondary structure, the hierarchical formation of kinetic folding units corresponding to structural subdomains, and the formation of tertiary interactions between subdomains late during the folding process appear to be common features of the folding mechanism for both RNA and proteins.

Specificity of ribonucleoprotein interaction determined by RNA folding during complex formulation

Many proteins involved in pre-mRNA processing contain one or more copies of a 70-90-amino-acid alphabeta module called the ribonucleoprotein domain. RNA maturation depends on the specific recognition by ribonucleoproteins of RNA elements within pre-mRNAs and small nuclear RNAs. The human U1A protein binds an RNA hairpin during splicing, and regulates its own expression by binding an internal loop in the 3'-untranslated region of its pre-mRNA, preventing polyadenylation. Here we report the nuclear magnetic resonance structure of the complex between the regulatory element of the U1A 3'-untranslated region (UTR) and the U1A protein RNA-binding domain. Specific intermolecular recognition requires the interaction of the variable loops of the ribonucleoprotein domain with the well-structured helical regions of the RNA. Formation of the complex then orders the flexible RNA single-stranded loop against the protein beta-sheet surface, and reorganizes the carboxy-terminal region of the protein to maximize surface complementarity and functional group recognition.

The CBP2 gene from Saccharomyces douglasii is a functional homologue of the Saccharomyces cerevisiae gene and is essential for respiratory growth in the presence of a wild-type (intron-containing) mitochondrial genome

In Saccharomyces cerevisiae the only known role of the CBP2 gene is the excision of the fifth intron of the mitochondrial cyt b gene (bI5). We have cloned the CBP2 gene from Saccharomyces douglasii (a close relative of S. cerevisiae). A comparison of the S. douglasii and S. cerevisiae sequences shows that there are 14% nucleotide substitutions in the coding region, with transitions being three times more frequent than transversions. At the protein level sequence identity is 87%. We have demonstrated that the S. douglasii CBP2 gene is essential for respiratory growth in the presence of a wild-type S. douglasii mitochondrial genome, but not in the presence of an intronless S. cerevisiae mitochondrial genome. Also the S. douglasii and S. cerevisiae CBP2 genes are completely interchangeable, even though the intron bI5 is absent from the S. douglasii mitochondrial genome.

Assembly of a ribonucleoprotein catalyst by tertiary structure capture

CBP2 is an RNA tertiary structure binding protein required for efficient splicing of a yeast mitochondrial group I intron. CBP2 must wait for folding of the two RNA domains that make up the catalytic core before it can bind. In a subsequent step, association of the 5' domain of the RNA is stabilized by additional interactions with the protein. Thus, CBP2 functions primarily to capture otherwise transient RNA tertiary structures. This simple one-RNA, one-protein system has revealed how the kinetic pathway of RNA folding can direct the assembly of a specific ribonucleoprotein complex. There are parallels to steps in the formation of a much more complex ribonucleoprotein, the 30S ribosomal subunit.

Macromolecular complexes: how RNA and protein get together

Recent studies of protein-dependent RNA splicing have provided new insights into the way RNA-protein interactions drive the stepwise assembly of catalytic RNA structures.

Function of a pseudoknot in the suppression of an alternative splicing event in a group I intron

Like most mitochondrial group I introns with a free-standing open reading frame (ORF) located downstream of their catalytic core, the Sd.cob, 1 intron in the gene coding for the cytochrome b of Saccharomyces douglasii mitochondria possesses a putative proximal 3' splice site. However, incubation of Sd.cob, 1 preRNA transcripts under optimal in vitro splicing conditions essentially results in splicing at the authentic, distal 3' splice junction. The mechanism by which the proximal splicing event is suppressed in vitro involves formation of a tertiary interaction which is only found in the Sd.cob, 1 intron. Core nucleotides located in loop L5 block proximal splicing by forming Watson-Crick base pairs with the nucleotide sequence of the proximal 3' splice site. This tertiary base pairing, also important for the folding of the intron into an active conformation, may be regarded as equivalent to the L9/P5, GNRA-loop/helix interaction found in more than one-third of known group I introns.

Cotranscriptional splicing of a group I intron is facilitated by the Cbp2 protein

The nuclear CBP2 gene encodes a protein essential for the splicing of a mitochondrial group I intron in Saccharomyces cerevisiae. This intron (bI5) is spliced autocatalytically in the presence of high concentrations of magnesium and monovalent salt but requires the Cbp2 protein for splicing under physiological conditions. Addition of Cbp2 during RNA synthesis permitted cotranscriptional splicing. Splicing did not occur in the transcription buffer in the absence of synthesis. The Cbp2 protein appeared to modify the folding of the intron during RNA synthesis: pause sites for RNA polymerase were altered in the presence of the protein, and some mutant transcripts that did not splice after transcription did so during transcription in the presence of Cbp2. Cotranscriptional splicing also reduced hydrolysis at the 3' splice junction. These results suggest that Cbp2 modulates the sequential folding of the ribozyme during its synthesis. In addition, splicing during transcription led to an increase in RNA synthesis with both T7 RNA polymerase and mitochondrial RNA polymerase, implying a functional coupling between transcription and splicing.