Stability of single-nucleotide bulge loops embedded in a GAAA RNA hairpin stem

Synthesis and properties of the anticodon stem-loop of human mitochondrial tRNAMet containing the disease-related G or m1G nucleosides at position 37

A single point mutation (A4435G) in the human mitochondrial tRNA^Met (hmt-tRNA^Met) gene causes severe mitochondrial disorders associated with hypertension, type 2 diabetes and LHON. This mutation leads to the exchange of A₃₇ in the anticodon loop of hmt-tRNA^Met for G₃₇ and 1-methylguanosine (m¹G₃₇). Here we present the first synthesis and structural/biophysical studies of the anticodon stem and loop of pathogenic hmt-tRNAs^Met.

How to benchmark RNA secondary structure prediction accuracy

RNA secondary structure prediction is widely used. As new methods are developed, these are often benchmarked for accuracy against existing methods. This review discusses good practices for performing these benchmarks, including the choice of benchmarking structures, metrics to quantify accuracy, the importance of allowing flexibility for pairs in the accepted structure, and the importance of statistical testing for significance.

Analysis of RNA nearest neighbor parameters reveals interdependencies and quantifies the uncertainty in RNA secondary structure prediction

RNA secondary structure prediction is often used to develop hypotheses about structure-function relationships for newly discovered RNA sequences, to identify unknown functional RNAs, and to design sequences. Secondary structure prediction methods typically use a thermodynamic model that estimates the free energy change of possible structures based on a set of nearest neighbor parameters. These parameters were derived from optical melting experiments of small model oligonucleotides. This work aims to better understand the precision of structure prediction. Here, the experimental errors in optical melting experiments were propagated to errors in the derived nearest neighbor parameter values and then to errors in RNA secondary structure prediction. To perform this analysis, the optical melting experimental values were systematically perturbed within the estimates of experimental error and alternative sets of nearest neighbor parameters were then derived from these error-bounded values. Secondary structure predictions using either the perturbed or reference parameter sets were then compared. This work demonstrated that the precision of RNA secondary structure prediction is more robust than suggested by previous work based on perturbation of the nearest neighbor parameters. This robustness is due to correlations between parameters. Additionally, this work identified weaknesses in the parameter derivation that makes accurate assessment of parameter uncertainty difficult. Considerations for experimental design are provided to mitigate these weaknesses are provided.

An asymmetric mesoscopic model for single bulges in RNA

Simple one-dimensional DNA or RNA mesoscopic models are of interest for their computational efficiency while retaining the key elements of the molecular interactions. However, they only deal with perfectly formed DNA or RNA double helices and consider the intra-strand interactions to be the same on both strands. This makes it difficult to describe highly asymmetric structures such as bulges and loops and, for instance, prevents the application of mesoscopic models to determine RNA secondary structures. Here we derived the conditions for the Peyrard-Bishop mesoscopic model to overcome these limitations and applied it to the calculation of single bulges, the smallest and simplest of these asymmetric structures. We found that these theoretical conditions can indeed be applied to any situation where stacking asymmetry needs to be considered. The full set of parameters for group I RNA bulges was determined from experimental melting temperatures using an optimization procedure, and we also calculated average opening profiles for several RNA sequences. We found that guanosine bulges show the strongest perturbation on their neighboring base pairs, considerably reducing the on-site interactions of their neighboring base pairs.

Influence of two bulge loops on the stability of RNA duplexes

Fifty-three RNA duplexes containing two single nucleotide bulge loops were optically melted in 1 M NaCl in order to determine the thermodynamic parameters ΔH°, ΔS°, ΔG°₃₇, and T_M for each duplex. Because of the large number of possible combinations and lack of sequence effects observed previously, we limited our initial investigation to adenosine bulges, the most common naturally occurring bulge. For example, the following duplexes were investigated: 5'GGCAXYAGGC/3'CCG YX CCG, 5'GGCAXY GCC/3'CCG YXACGG, and 5'GGC XYAGCC/3'CCGAYX CGG. The identity of XY (where XY are Watson-Crick base pairs) and the total number of base pairs in the terminal and central stems were varied. As observed for duplexes with a single bulge loop, the effect of the two bulge loops on duplex stability is primarily influenced by non-nearest neighbor interactions. In particular, the stability of the stems influences the destabilization of the duplex by the inserted bulge loops. The model proposed to predict the influence of multiple bulge loops on duplex stability suggests that the destabilization of each bulge is related to the stability of the adjacent stems. A database of RNA secondary structures was examined to determine the naturally occurring abundance of duplexes containing multiple bulge loops. Of the 2000 examples found in the database, over 65% of the two bulge loops occur within 3 base pairs of each other. A database of RNA three-dimensional structures was examined to determine the structure of duplexes containing two single nucleotide bulge loops. The structures of the bulge loops are described.

Determination of the secondary structure of group II bulge loops using the fluorescent probe 2-aminopurine

Eleven RNA hairpins containing 2-aminopurine (2-AP) in either base-paired or single nucleotide bulge loop positions were optically melted in 1 M NaCl; and, the thermodynamic parameters ΔH°, ΔS°, ΔG°37, and TM for each hairpin were determined. Substitution of 2-AP for an A (adenosine) at a bulge position (where either the 2-AP or A is the bulge) in the stem of a hairpin, does not affect the stability of the hairpin. For group II bulge loops such as AA/U, where there is ambiguity as to which of the A residues is paired with the U, hairpins with 2-AP substituted for either the 5' or 3' position in the hairpin stem have similar stability. Fluorescent melts were performed to monitor the environment of the 2-AP. When the 2-AP was located distal to the hairpin loop on either the 5' or 3' side of the hairpin stem, the change in fluorescent intensity upon heating was indicative of an unpaired nucleotide. A database of phylogenetically determined RNA secondary structures was examined to explore the presence of naturally occurring bulge loops embedded within a hairpin stem. The distribution of bulge loops is discussed and related to the stability of hairpin structures.

Non-nearest-neighbor dependence of stability for group III RNA single nucleotide bulge loops

Thirty-five RNA duplexes containing single nucleotide bulge loops were optically melted and the thermodynamic parameters for each duplex determined. The bulge loops were of the group III variety, where the bulged nucleotide is either a AG/U or CU/G, leading to ambiguity to the exact position and identity of the bulge. All possible group III bulge loops with Watson-Crick nearest-neighbors were examined. The data were used to develop a model to predict the free energy of an RNA duplex containing a group III single nucleotide bulge loop. The destabilization of the duplex by the group III bulge could be modeled so that the bulge nucleotide leads to the formation of the Watson-Crick base pair rather than the wobble base pair. The destabilization of an RNA duplex caused by the insertion of a group III bulge is primarily dependent upon non-nearest-neighbor interactions and was shown to be dependent upon the stability of second least stable stem of the duplex. In-line structure probing of group III bulge loops embedded in a hairpin indicated that the bulged nucleotide is the one positioned further from the hairpin loop irrespective of whether the resulting stem formed a Watson-Crick or wobble base pair. Fourteen RNA hairpins containing group III bulge loops, either 3' or 5' of the hairpin loop, were optically melted and the thermodynamic parameters determined. The model developed to predict the influence of group III bulge loops on the stability of duplex formation was extended to predict the influence of bulge loops on hairpin stability.