A peripheral element assembles the compact core structure essential for group I intron self-splicing

A maturase that specifically stabilizes and activates its cognate group I intron at high temperatures

Folding of large structured RNAs into their functional tertiary structures at high temperatures is challenging. Here we show that I-TnaI protein, a small LAGLIDADG homing endonuclease encoded by a group I intron from a hyperthermophilic bacterium, acts as a maturase that is essential for the catalytic activity of this intron at high temperatures and physiological cationic conditions. I-TnaI specifically binds to and induces tertiary packing of the P4-P6 domain of the intron; this RNA-protein complex might serve as a thermostable platform for active folding of the entire intron. Interestingly, the binding affinity of I-TnaI to its cognate intron RNA largely increases with temperature; over 30-fold stronger binding at higher temperatures relative to 37 °C correlates with a switch from an entropy-driven (37 °C) to an enthalpy-driven (55-60 °C) interaction mode. This binding mode may represent a novel strategy how an RNA binding protein can promote the function of its target RNA specifically at high temperatures.

Catalytic activity as a probe of native RNA folding

As RNAs fold to functional structures, they traverse complex energy landscapes that include many partially folded and misfolded intermediates. For structured RNAs that possess catalytic activity, this activity can provide a powerful means of monitoring folding that is complementary to biophysical approaches. RNA catalysis can be used to track accumulation of the native RNA specifically and quantitatively, readily distinguishing the native structure from intermediates that resemble it and may not be differentiated by other approaches. Here, we outline how to design and interpret experiments using catalytic activity to monitor RNA folding, and we summarize adaptations of the method that have been used to probe aspects of folding well beyond determination of the folding rates.

A phage RNA-binding protein binds to a non-cognate structured RNA and stabilizes its core structure

Recent studies suggest that some RNA-binding proteins facilitate the folding of non-cognate RNAs. Here, we report that bacteriophage MS2 coat protein (MS2 CP) bound and promoted the catalytic activity of Candida group I ribozyme. Cloning of the MS2-bound RNA segments showed that this protein primarily interacts with the P5ab-P5 structure. Ultraviolet cross-linking and the T1 footprinting assay further showed that MS2 binding stabilized tertiary interactions, including the conserved L9-P5 interaction, and led to a more compact core structure. This mechanism is similar to that of the yeast mitochondrial tyrosyl-tRNA synthetase on other group I introns, suggesting that different RNA-binding proteins may use common mechanisms to support RNA structures.

Coordination of two sequential ester-transfer reactions: exogenous guanosine binding promotes the subsequent omegaG binding to a group I intron

Self-splicing of group I introns is accomplished by two sequential ester-transfer reactions mediated by sequential binding of two different guanosine ligands, but it is yet unclear how the binding is coordinated at a single G-binding site. Using a three-piece trans-splicing system derived from the Candida intron, we studied the effect of the prior GTP binding on the later ωG binding by assaying the ribozyme activity in the second reaction. We showed that adding GTP simultaneously with and prior to the esterified ωG in a substrate strongly accelerated the second reaction, suggesting that the early binding of GTP facilitates the subsequent binding of ωG. GTP-mediated facilitation requires C2 amino and C6 carbonyl groups on the Watson–Crick edge of the base but not the phosphate or sugar groups, suggesting that the base triple interactions between GTP and the binding site are important for the subsequent ωG binding. Strikingly, GTP binding loosens a few local structures of the ribozyme including that adjacent to the base triple, providing structural basis for a rapid exchange of ωG for bound GTP.

Splicing and evolution of an unusually small group I intron

Introns are common in the rRNA gene loci of fungal genomes, but biochemical studies to investigate splicing are rare. Here, self-splicing of a very small (67 nucleotide) group I intron is demonstrated. The PaSSU intron (located within the rRNA small subunit gene of Phialophora americana) splices in vitro under group I intron conditions. Most group I ribozymes contain pairing regions P1-P10, with a conserved G.U pair at the 5' splice site, and a G at the 3' intron border. The PaSSU intron contains only P1, P7, and P10. While it contains the G.U pair at the 5' splice, a U is found at the 3' end of the intron instead of a G. Phylogenetic analysis places it within subgroup IC1, whose members are found in the nuclear rRNA genes of fungi. The structural elements are similar to those in the centermost regions of other group I introns. Its size can be explained by a single large deletion that removed P2 through much of P9. Part of the original P9 region has assumed the function of P7. Its small size and genealogy makes it an excellent model to study RNA catalysis and evolution.

Pentamidine binds to tRNA through non-specific hydrophobic interactions and inhibits aminoacylation and translation

The selective and potent inhibition of mitochondrial translation in Saccharomyces cerevisiae by pentamidine suggests a novel antimicrobial action for this drug. Electrophoresis mobility shift assay, T1 ribonuclease footprinting, hydroxyl radical footprinting and isothermal titration calorimetry collectively demonstrated that pentamidine non-specifically binds to two distinct classes of sites on tRNA. The binding was driven by favorable entropy changes indicative of a large hydrophobic interaction, suggesting that the aromatic rings of pentamidine are inserted into the stacked base pairs of tRNA helices. Pentamidine binding disrupts the tRNA secondary structure and masks the anticodon loop in the tertiary structure. Consistently, we showed that pentamidine specifically inhibits tRNA aminoacylation but not the cognate amino acid adenylation. Pentamidine inhibited protein translation in vitro with an EC(50) equivalent to that binds to tRNA and inhibits tRNA aminoacylation in vitro, but drastically higher than that inhibits translation in vivo, supporting the established notion that the antimicrobial activity of pentamidine is largely due to its selective accumulation by the pathogen rather than by the host cell. Therefore, interrupting tRNA aminoacylation by the entropy-driven non-specific binding is an important mechanism of pentamidine in inhibiting protein translation, providing new insights into the development of antimicrobial drugs.

Monovalent cations use multiple mechanisms to resolve ribozyme misfolding

Recent efforts have been made to unravel the independent roles of monovalent cations in RNA folding, primarily using the Tetrahymena ribozyme as a model. Here we report how monovalent cations impact the folding of the Candida ribozyme. Interestingly, this ribozyme requires an order of magnitude less monovalent cations (Na+ and Tris+) to commit to a new folding starting state in which the J3/4:P6 base triple is partially formed and mispairing in the L2.1 and L6 terminal loops is resolved. When Mg2+-induced ribozyme folding proceeded on the same energy landscape, the altered starting state led to a rapid assembly of the correct ribozyme core and a fivefold to 10-fold increase in the ribozyme activity. Moreover, when the ribozyme folding was started from a misfolding-prone state, high millimolar concentrations of monovalent cations moderately elevated the ribozyme activity by efficiently resolving the misfolding of a peripheral element, P5abc.

Atomic level architecture of group I introns revealed

Twenty-two years after their discovery as ribozymes, the self-splicing group I introns are finally disclosing their architecture at the atomic level. The crystal structures of three group I introns solved at moderately high resolution (3.1-3.8A) reveal a remarkably conserved catalytic core bound to the metal ions required for activity. The structure of the core is stabilized by an intron-specific set of long-range interactions that involves peripheral elements. Group I intron structures thus provide much awaited and extremely valuable snapshots of how these ribozymes coordinate substrate binding and catalysis.