Phylogenetic footprinting of non-coding RNA: hammerhead ribozyme sequences in a satellite DNA family of Dolichopoda cave crickets (Orthoptera, Rhaphidophoridae)

Is radon emission in caves causing deletions in satellite DNA sequences of cave-dwelling crickets?

The most stable isotope of radon, ²²²Rn, represents the major source of natural radioactivity in confined environments such as mines, caves and houses. In this study, we explored the possible radon-related effects on the genome of Dolichopoda cave crickets (Orthoptera, Rhaphidophoridae) sampled in caves with different concentrations of radon. We analyzed specimens from ten populations belonging to two genetically closely related species, D. geniculata and D. laetitiae, and explored the possible association between the radioactivity dose and the level of genetic polymorphism in a specific family of satellite DNA (pDo500 satDNA). Radon concentration in the analyzed caves ranged from 221 to 26000 Bq/m³. Specimens coming from caves with the highest radon concentration showed also the highest variability estimates in both species, and the increased sequence heterogeneity at pDo500 satDNA level can be explained as an effect of the mutation pressure induced by radon in cave. We discovered a specific category of nuclear DNA, the highly repetitive satellite DNA, where the effects of the exposure at high levels of radon-related ionizing radiation are detectable, suggesting that the satDNA sequences might be a valuable tool to disclose harmful effects also in other organisms exposed to high levels of radon concentration.

Plus ça change - evolutionary sequence divergence predicts protein subcellular localization signals

BACKGROUND

Protein subcellular localization is a central problem in understanding cell biology and has been the focus of intense research. In order to predict localization from amino acid sequence a myriad of features have been tried: including amino acid composition, sequence similarity, the presence of certain motifs or domains, and many others. Surprisingly, sequence conservation of sorting motifs has not yet been employed, despite its extensive use for tasks such as the prediction of transcription factor binding sites.

RESULTS

Here, we flip the problem around, and present a proof of concept for the idea that the lack of sequence conservation can be a novel feature for localization prediction. We show that for yeast, mammal and plant datasets, evolutionary sequence divergence alone has significant power to identify sequences with N-terminal sorting sequences. Moreover sequence divergence is nearly as effective when computed on automatically defined ortholog sets as on hand curated ones. Unfortunately, sequence divergence did not necessarily increase classification performance when combined with some traditional sequence features such as amino acid composition. However a post-hoc analysis of the proteins in which sequence divergence changes the prediction yielded some proteins with atypical (i.e. not MPP-cleaved) matrix targeting signals as well as a few misannotations.

CONCLUSION

We report the results of the first quantitative study of the effectiveness of evolutionary sequence divergence as a feature for protein subcellular localization prediction. We show that divergence is indeed useful for prediction, but it is not trivial to improve overall accuracy simply by adding this feature to classical sequence features. Nevertheless we argue that sequence divergence is a promising feature and show anecdotal examples in which it succeeds where other features fail.

High-pressure analysis of a hammerhead ribozyme from Chrysanthemum chlorotic mottle viroid reveals two different populations of self-cleaving molecule

The activity of the full-length hammerhead ribozyme requires a tertiary interaction between its distal loops leading to the closure of the molecule and its stabilization in the active conformation. In this study, the conformational changes accompanying the cis-cleavage reaction of Chrysanthemum chlorotic mottle viroid hammerhead ribozyme were investigated by high-pressure experiments on the complete cleavage reaction. Two activation volumes (ΔV(≠)) were measured, pointing to the presence of two different populations of molecules corresponding to fast-cleaving and slow-cleaving ribozymes in the reaction mixture. The fast population, with a small ΔV(≠) of 2.6 mL·mol(-1), most likely represents molecules in the near-active conformation, whereas the slow population, with a larger ΔV(≠) of 11.6 mL·mol(-1 , represents molecules that need a larger conformational change to induce activity. In addition, pH-dependence experiments suggest that the group whose deprotonation is required for activity intervenes in the formation of the transition state or in the chemistry of the reaction, but not in the conformational change that precedes it.

Conformational heterogeneity and the determinants of tertiary stabilization in the hammerhead ribozyme from Dolichopoda cave crickets

Repetitive DNA elements in Dolichopoda cave cricket genomes contain extended hammerhead ribozymes that are functional in adult crickets, but that exhibit very low self-cleavage activity in vitro relative to other extended hammerhead ribozymes. We find that the parental ribozyme tends to misfold into alternate secondary structures in vitro, complicating analysis of contributions by specific nucleotides to activity under biologically relevant magnesium concentrations. However, minor sequence alterations that stabilize the active secondary structure, without altering candidate tertiary interacting nucleotides, boosted observed rates more than 50-fold (4.4 ± 1.7 min(-1)) and doubled the cleavage extent (>60%) in submillimolar magnesium. Productive alterations included flipping two base pairs in stem I, lengthening stem I and opening stem III to generate a trans-cleaving ribozyme. Specific peripheral nucleotides involved in tertiary stabilization were then identified through kinetic analysis for a series of sequence variants and by correlating plateau cleavage values with band intensity in native gel electrophoresis. These results demonstrate that conformational heterogeneity governs self-cleavage by the wild-type Dolichopoda hammerhead ribozyme in vitro, and they suggest a strategy for improving activity and enhancing the suitability of HHRz for intracellular and biotechnology applications.

Identification of hammerhead ribozymes in all domains of life reveals novel structural variations

Hammerhead ribozymes are small self-cleaving RNAs that promote strand scission by internal phosphoester transfer. Comparative sequence analysis was used to identify numerous additional representatives of this ribozyme class than were previously known, including the first representatives in fungi and archaea. Moreover, we have uncovered the first natural examples of "type II" hammerheads, and our findings reveal that this permuted form occurs in bacteria as frequently as type I and III architectures. We also identified a commonly occurring pseudoknot that forms a tertiary interaction critical for high-speed ribozyme activity. Genomic contexts of many hammerhead ribozymes indicate that they perform biological functions different from their known role in generating unit-length RNA transcripts of multimeric viroid and satellite virus genomes. In rare instances, nucleotide variation occurs at positions within the catalytic core that are otherwise strictly conserved, suggesting that core mutations are occasionally tolerated or preferred.