Glycols modulate terminator stem stability and ligand-dependency of a glycine riboswitch

Development of Allosteric Ribozymes for ATP and l-Histidine Based on the R3C Ligase Ribozyme

During the evolution of the RNA, short RNAs are thought to have joined together to form long RNAs, enhancing their function as ribozymes. Previously, the artificial R3C ligase ribozyme (73 nucleotides) was successfully reduced to 46 nucleotides; however, its activity decreased significantly. Therefore, we aimed to develop allosteric ribozymes, whose activities could be regulated by effector compounds, based on the reduced R3C ligase ribozyme (R3C-A). Among the variants prepared by fusing an ATP-binding aptamer RNA with R3C-A, one mutant showed increased ligation activity in an ATP-dependent manner. Melting temperature measurements of the two RNA mutants suggested that the region around the aptamer site was stabilized by the addition of ATP. This resulted in a suitable conformation for the reaction at the ligation site. Another ribozyme was prepared by fusing R3C-A with a l-histidine-binding aptamer RNA, and the ligase activity increased with increasing l-histidine concentrations. Both ATP and l-histidine play prominent roles in current molecular biology and the interaction of RNAs and these molecules could be a key step in the evolution of the world of RNAs. Our results suggest promise in the development of general allosteric ribozymes that are independent of the type of effector molecule and provide important clues to the evolution of the RNA world.

Elucidation of productive alanine recognition mechanism by Escherichia coli alanyl-tRNA synthetase

Alanyl-tRNA synthetase (AlaRS) incorrectly recognizes both a slightly smaller glycine and a slightly larger serine in addition to alanine, and the probability of incorrect identification is extremely low at 1/300 and 1/170, respectively. Alanine is the second smallest amino acid after glycine; however, the mechanism by which AlaRS specifically identifies small differences in side chains with high accuracy remains unknown. In this study, using a malachite green assay, we aimed to elucidate the alanine recognition mechanism of a fragment (AlaRS368N) containing only the amino acid activation domain of Escherichia coli AlaRS. This method quantifies monophosphate by decomposing pyrophosphate generated during aminoacyl-AMP production. AlaRS368N produced far more pyrophosphate when glycine or serine was used as a substrate than when alanine was used. Among several mutants tested, an AlaRS mutant in which the widely conserved aspartic acid at the 235th position (D235) near the active center was replaced with glutamic acid (D235E) increased pyrophosphate release for the alanine substrate, compared to that from glycine and serine. These results suggested that D235 is optimal for AlaRS to specifically recognize alanine. Alanylation activities of an RNA minihelix by the mutants of valine at the 214th position (V214) of another fragment (AlaRS442N), which is the smallest AlaRS with alanine charging activity, suggest the existence of the van der Waals-like interaction between the side chain of V214 and the methyl group of the alanine substrate.

Molecular Anatomy of the Class I Ligase Ribozyme for Elucidation of the Activity-Generating Unit

The class I ligase ribozyme consists of 121 nucleotides and shows a high catalytic rate comparable to that found in natural proteinaceous polymerases. In this study, we aimed to identify the smaller active unit of the class I ligase ribozyme comprising ~50 nucleotides, comparable to the estimated length of prebiotically synthesized RNA. Based on the three-dimensional structure of the class I ligase ribozyme, mutants were prepared and their ligation activities were analyzed. Sufficient ligation activity was maintained even when shortening to 94 nucleotides. However, because it would be difficult to approach the target of ~50 nucleotides by removing only the partial structure, the class I ligase ribozyme was then split into two molecules. The ligation activity was maintained even when splitting into two molecules of 55 and 39 nucleotides. Using a system with similar split ribozymes, we analyzed the ligation activity of mutants C30, C47, and A71, which have been previously identified as the positions that contribute to catalytic activity, and discussed the structural basis of the activity of these bases. Our findings suggest the rationale for the class I ligase ribozyme's assembling from multiple fragments that would be achievable with prebiotic synthesis.

Acquisition of Dual Ribozyme-Functions in Nonfunctional Short Hairpin RNAs through Kissing-Loop Interactions

The acquisition of functions via the elongation of nucleotides is an important factor in the development of the RNA world. In our previous study, we found that the introduction of complementary seven-membered kissing loops into inactive R3C ligase ribozymes revived their ligation activity. In this study, we applied the kissing complex formation-induced rearrangement of RNAs to two nonfunctional RNAs by introducing complementary seven-membered loops into each of them. By combining these two forms of RNAs, the ligase activity (derived from the R3C ligase ribozyme) as well as cleavage activity (derived from the hammerhead ribozyme) was obtained. Thus, effective RNA evolution toward the formation of a life system may require the achievement of “multiple” functions via kissing-loop interactions, as indicated in this study. Our results point toward the versatility of kissing-loop interactions in the evolution of RNA, i.e., two small nonfunctional RNAs can gain dual functions via a kissing-loop interaction.

Crystal structure of Nanoarchaeum equitans tyrosyl-tRNA synthetase and its aminoacylation activity toward tRNATyr with an extra guanosine residue at the 5'-terminus

tRNA^Tyr of Nanoarchaeum equitans has a remarkable feature with an extra guanosine residue at the 5'-terminus. However, the N. equitans tRNA^Tyr mutant without extra guanosine at the 5'-end was tyrosylated by tyrosyl-tRNA synthase (TyrRS). We solved the crystal structure of N. equitans TyrRS at 2.80 Å resolution. By comparing the present solved structure with the complex structures TyrRS with tRNA^Tyr of Thermus thermophilus and Methanocaldococcus jannaschii, an arginine substitution mutant of N. equitans TyrRS at Ile200 (I200R), which is the putative closest candidate to the 5'-phosphate of C1 of N. equitans tRNA^Tyr, was prepared. The I200R mutant tyrosylated not only wild-type tRNA^Tyr but also the tRNA without the G-1 residue. Further tyrosylation analysis revealed that the second base of the anticodon (U35), discriminator base (A73), and C1:G72 base pair are strong recognition sites.

G:U-Independent RNA Minihelix Aminoacylation by Nanoarchaeum equitans Alanyl-tRNA Synthetase: An Insight into the Evolution of Aminoacyl-tRNA Synthetases

Nanoarchaeum equitans is a species of hyperthermophilic archaea with the smallest genome size. Its alanyl-tRNA synthetase genes are split into AlaRS-α and AlaRS-β, encoding the respective subunits. In the current report, we surveyed N. equitans AlaRS-dependent alanylation of RNA minihelices, composed only of the acceptor stem and the T-arm of tRNAAla. Combination of AlaRS-α and AlaRS-β showed a strong alanylation activity specific to a single G3:U70 base pair, known to mark a specific tRNA for charging with alanine. However, AlaRS-α alone had a weak but appreciable alanylation activity that was independent of the G3:U70 base pair. The shorter 16-mer RNA tetraloop substrate mimicking only the first four base pairs of the acceptor stem of tRNAAla, but with C3:G70 base pair, was also successfully aminoacylated by AlaRS-α. The end of the acceptor stem, including CCA-3' terminus and the discriminator A73, was able to function as a minimal structure for the recognition by the enzyme. Our findings imply that aminoacylation by N. equitans AlaRS-α may represent a vestige of a primitive aminoacylation system, before the appearance of the G3:U70 pair as an identity element for alanine.

Aminoacylation of short hairpin RNAs through kissing-loop interactions indicates evolutionary trend of RNA molecules

The unique G3:U70 base pair in the acceptor stem of tRNA^Ala has been shown to be a critical recognition site by alanyl-tRNA synthetase (AlaRS). The base pair resides on one of the arms of the L-shaped structure of tRNA (minihelix) and the genetic code has likely evolved from a primordial tRNA-aaRS (aminoacyl-tRNA synthetase) system. In terms of the evolution of tRNA, incorporation of a G:U base pair in the structure would be important. Here, we found that two independent short hairpin RNAs change their conformation through kissing-loop interactions, finally forming a minihelix-like structure, in which the G3:U70 base pair is incorporated. The RNA system can be properly aminoacylated by the minimal Escherichia coli AlaRS variant with alanylation activity (AlaRS442N). Thus, characteristic structural features produced via kissing-loop interactions may provide important clues into the evolution of RNA.

RNA Aptamers for a tRNA-Binding Protein from Aeropyrum pernix with Homologous Counterparts Distributed Throughout Evolution

In the present in vitro selection study, we isolated and characterized RNA aptamers for a tRNA-binding protein (Trbp) from an extremophile archaeon Aeropyrum pernix. Trbp-like structures are frequently found not only in aminoacyl-tRNA synthetases but also in diverse types of proteins from different organisms. They likely arose early in evolution and have played important roles in evolution through interactions with key RNA structures. RNA aptamers specific for A. pernix Trbp were successfully selected from a pool of RNAs composed of 60 nucleotides, including a random 30-nucleotide region. From the secondary structures, we obtained a shortened sequence composed of 21 nucleotides, of which the 3′-terminal single stranded CA nucleotides are essential for binding. This may be related to the initial evolutionary role of the universal CCA-3′ terminus of tRNA in the interaction with Trbp-like structures.

Glycyl-tRNA synthetase from Nanoarchaeum equitans: The first crystal structure of archaeal GlyRS and analysis of its tRNA glycylation

This study reports the X-ray crystallographic structure of the glycyl-tRNA synthetase (GlyRS) of Nanoarchaeum equitans - a hyperthermophilic archaeal species. This is the first archaeal GlyRS crystal structure elucidated. The GlyRS comprises an N-terminal catalytic domain and a C-terminal anticodon-binding domain with a long β-sheet inserted between these domains. An unmodified transcript of the wild-type N. equitans tRNA^Gly was successfully glycylated using GlyRS. Substitution of the discriminator base A73 of tRNA^Gly with any other nucleotide caused a significant decrease in glycylation activity. Mutational analysis of the second base-pair C2G71 of the acceptor stem of tRNA^Gly elucidated the importance of the base-pair, especially G71, as an identity element for recognition by GlyRS. Glycylation assays using tRNA^Gly G71 substitution mutants and a GlyRS mutant where Arg223 is mutated to alanine strengthen the possibility that the carbonyl oxygen at position 6 of G71 would hydrogen-bond with the guanidine nitrogen of Arg223 in N. equitans GlyRS.

Effects of complementary loop composition in truncated R3C ligase ribozymes on kiss switch activation

The formation of a kissing-loop through the introduction of complementary 7-membered loops is known to dramatically increase the activity of truncated R3C ligase ribozymes that otherwise display reduced activity. Restoration of activity is thought to result from kissing complex formation-induced rearrangement of two molecules with complementary loops. By combining two types of R3C ligase ribozyme mutants, <A> and <hairpin-ΔU>, the influence of loop composition on ligation activity was investigated. Substrate ligation occurred in <hairpin-ΔU>, but not in <A>, despite the absence of a substrate-binding site in <hairpin-ΔU>. Loop-loop interactions of <A>- and <hairpin-ΔU>-variants with complementary 6-membered loops also resulted in proper kissing-complex formation-induced substrate ligation. However, heterogeneous combinations of 7- and 6-membered loops, and/or of 6- and 5-membered loops had distinct results that depended upon the sequence and bulged nucleotides of the loop regions. These differences suggest that both thermodynamic and kinetic controls act upon the kissing-loop interaction-mediated rearrangement of the shortened trans-R3C ribozymes.

The Kiss Switch Brings Inactive R3C Ligase Ribozyme Back to Life

R3C ligase ribozyme catalyzes the nucleophilic attack by a 3'-hydroxyl on a 5'-α-phosphorus of triphosphates to form a 3'-5'-phosphodiester bond. In the present study, although the truncation of R3C ribozyme was accompanied by a large reduction in ligation activity (decrease by two orders of magnitude compared to that of the ligated product of full-length R3C ribozyme after 18.5 h at 23 °C), the introduction of complementary seven-membered kissing-loops served as a "switch" to reactivate the truncated R3C ribozyme with approximately one-fifth of the activity of the full-length R3C ribozyme. This reactivation occurred in a trans-manner, and the grip region and substrate-binding site of the truncated R3C ribozyme were necessary to locate the substrate in the proper position for ligation with the other molecule. Reactivation resulted from complex tertiary interactions between two ribozymes, including kissing-loop interaction-induced annealing and the formation of a stable duplex. The drastic increase of the activity of poorly active ribozymes through the kissing-loop interaction may provide an important clue into the acquisition of substantial activity during the evolution of the RNA world.

Binding Properties of Split tRNA to the C-terminal Domain of Methionyl-tRNA Synthetase of Nanoarchaeum equitans

The C-terminal domain of methionyl-tRNA synthetase (MetRS-C) from Nanoarchaeum equitans is homologous to a tRNA-binding protein consisting of 111 amino acids (Trbp111) from Aquifex aeolicus. The crystal structure of MetRS-C showed that it existed as a homodimer, and that each monomer possessed an oligonucleotide/oligosaccharide-binding fold (OB-fold). Analysis using a quartz crystal microbalance indicated that MetRS-C freshly isolated from N. equitans was bound to tRNA. However, binding of the split 3'-half tRNA species was stronger than that of the 5'-half species. The T-loop and the 3'-end regions of the split 3'-half tRNA were found to be responsible for the binding. The minimum structure for binding to MetRS-C might be a minihelix-like stem-loop with single-stranded 3'-terminus. After successive duplications of such a small hairpin structure with the assistance of a Trbp-like structure, the interaction of the T-loop region of the 3'-half with a Trbp-like structure could have been evolutionarily replaced by RNA-RNA interactions, along with many combinational tertiary interactions, to form the modern tRNA structure.

Development of a Functionally Minimized Mutant of the R3C Ligase Ribozyme Offers Insight into the Plausibility of the RNA World Hypothesis

The R3C ligase ribozyme is an artificial ligase ribozyme produced by modification of the ribozyme that lacks cytidine. Here, we attempted to modify the original R3C ribozyme (73 nucleotides) by reducing the number of nucleotides while maintaining the maximum possible catalytic efficiency. By partially deleting both the "grip" (P4 + P5) and "hammer" (P3) stem-loops, we found the critical border to retain activity comparable to that of full-length R3C. The three-way junction structure was necessary to maintain enzymatic function and the stability of the "grip" (P4 + P5) stem had a large influence on the catalytic activity of R3C. The final minimized ribozyme we obtained comprised ~50 nucleotides, comparable to the estimated length of prebiotically synthesized RNA. Our findings suggest that the autocatalytic function in ribozymes is indeed possible to obtain using sequence lengths achievable with prebiotic synthesis.