The role of Mg(II) in DNA cleavage site recognition in group II intron ribozymes: solution structure and metal ion binding sites of the RNA-DNA complex

Protein-free catalysis of DNA hydrolysis and self-integration by a ribozyme

Group II introns are ancient self-splicing ribozymes and retrotransposons. Though long speculated to have originated before translation, their dependence on intron-encoded proteins for splicing and mobility has cast doubt on this hypothesis. While some group II introns are known to retain part of their catalytic repertoire in the absence of protein cofactors, protein-free complete reverse splicing of a group II intron into a DNA target has never been demonstrated. Here, we demonstrate the complete independence of a group II intron from protein cofactors in all intron-catalyzed reactions. The ribozyme is capable of fully reverse splicing into single-stranded DNA targets in vitro, readily hydrolyzes DNA substrates and is even able to unwind and react with stably duplexed DNA. Our findings make a protein-free origin for group II introns plausible by expanding their known catalytic capabilities beyond what would be needed to survive the transition from RNA to DNA genomes. Furthermore, the intron's capacity to react with both single and double-stranded DNA in conjunction with its expanded sequence recognition may represent a promising starting point for the development of protein-free genomic editing tools.

Hydrolytic endonucleolytic ribozyme (HYER) is programmable for sequence-specific DNA cleavage

Ribozymes are catalytic RNAs with diverse functions including self-splicing and polymerization. This work aims to discover natural ribozymes that behave as hydrolytic and sequence-specific DNA endonucleases, which could be repurposed as DNA manipulation tools. Focused on bacterial group II-C introns, we found that many systems without intron-encoded protein propagate multiple copies in their resident genomes. These introns, named HYdrolytic Endonucleolytic Ribozymes (HYERs), cleaved RNA, single-stranded DNA, bubbled double-stranded DNA (dsDNA), and plasmids in vitro. HYER1 generated dsDNA breaks in the mammalian genome. Cryo-electron microscopy analysis revealed a homodimer structure for HYER1, where each monomer contains a Mg2+-dependent hydrolysis pocket and captures DNA complementary to the target recognition site (TRS). Rational designs including TRS extension, recruiting sequence insertion, and heterodimerization yielded engineered HYERs showing improved specificity and flexibility for DNA manipulation.

Gas-phase hydration of Mg2+ complexes with deprotonated uracil, thymine, uridine, and thymidine

The gas-phase hydration of Mg²⁺ complexes with deprotonated uracil (U), thymine (T), uridine (U_r , uracil riboside), and thymidine (T_dr , thymine deoxyriboside) was studied. The aim of the work was to analyze the hydration of product ions (eg, [2U-H+Mg]⁺ ) formed as a result of the collision induced dissociation of the respective parent ion (eg, [3U_r -H+Mg]⁺ ). The efficiency of gas-phase hydration of the ions [2U-H+Mg]⁺ and [2T-H+Mg]⁺ was similar. However, the efficiency of gas-phase hydration of the ion [U+U_r -H+Mg]⁺ was much higher than that of gas-phase hydration of the ion [T+T_dr -H+Mg]⁺ . On the basis of the mass spectra obtained and the performed molecular modelling, it was concluded that in the ion [T+T_dr -H+Mg]⁺ , we deal with a steric hindrance due to the presence of a sugar moiety, which affects water attachment. In the ion [U+U_r -H+Mg]⁺ , the position of the sugar moiety does not affect water attachment.

Metal ions and sugar puckering balance single-molecule kinetic heterogeneity in RNA and DNA tertiary contacts

The fidelity of group II intron self-splicing and retrohoming relies on long-range tertiary interactions between the intron and its flanking exons. By single-molecule FRET, we explore the binding kinetics of the most important, structurally conserved contact, the exon and intron binding site 1 (EBS1/IBS1). A comparison of RNA-RNA and RNA-DNA hybrid contacts identifies transient metal ion binding as a major source of kinetic heterogeneity which typically appears in the form of degenerate FRET states. Molecular dynamics simulations suggest a structural link between heterogeneity and the sugar conformation at the exon-intron binding interface. While Mg2+ ions lock the exon in place and give rise to long dwell times in the exon bound FRET state, sugar puckering alleviates this structural rigidity and likely promotes exon release. The interplay of sugar puckering and metal ion coordination may be an important mechanism to balance binding affinities of RNA and DNA interactions in general.

An atomistic view on carbocyanine photophysics in the realm of RNA

Carbocyanine dyes have a long-standing tradition in fluorescence imaging and spectroscopy, due to their photostability and large spectral separation between individual dye species. Herein, we explore the versatility of cyanine dyes to probe the dynamics of nucleic acids and we report on the interrelation of fluorophores, RNA, and metal ions, namely K⁺ and Mg²⁺. Photophysical parameters including the fluorescence lifetime, quantum yield and dynamic anisotropy are monitored as a function of the nucleic acid composition, conformation, and metal ion abundance. Occasional excursions to a non-fluorescent cis-state hint at the remarkable sensitivity of carbocyanines to their local environment. Comparison of time-correlated single photon experiments with all-atom molecular dynamics simulations demonstrate that the propensity of photoisomerization is dictated by sterical constraints imposed on the fluorophore. Structural features in the vicinity of the dye play a crucial role in RNA recognition and have far-reaching implications on the mobility of the fluorescent probe. An atomic level description of the mutual interactions will ultimately benefit the quantitative interpretation of single-molecule FRET measurements on large RNA systems.

Properties and reactivity of nucleic acids relevant to epigenomics, transcriptomics, and therapeutics

Developments in epigenomics, toxicology, and therapeutic nucleic acids all rely on a precise understanding of nucleic acid properties and chemical reactivity. In this review we discuss the properties and chemical reactivity of each nucleobase and attempt to provide some general principles for nucleic acid targeting or engineering. For adenine-thymine and guanine-cytosine base pairs, we review recent quantum chemical estimates of their Watson-Crick interaction energy, π-π stacking energies, as well as the nuclear quantum effects on tautomerism. Reactions that target nucleobases have been crucial in the development of new sequencing technologies and we believe further developments in nucleic acid chemistry will be required to deconstruct the enormously complex transcriptome.

Cation-induced kinetic heterogeneity of the intron-exon recognition in single group II introns

RNA is commonly believed to undergo a number of sequential folding steps before reaching its functional fold, i.e., the global minimum in the free energy landscape. However, there is accumulating evidence that several functional conformations are often in coexistence, corresponding to multiple (local) minima in the folding landscape. Here we use the 5'-exon-intron recognition duplex of a self-splicing ribozyme as a model system to study the influence of Mg(2+) and Ca(2+) on RNA tertiary structure formation. Bulk and single-molecule spectroscopy reveal that near-physiological M(2+) concentrations strongly promote interstrand association. Moreover, the presence of M(2+) leads to pronounced kinetic heterogeneity, suggesting the coexistence of multiple docked and undocked RNA conformations. Heterogeneity is found to decrease at saturating M(2+) concentrations. Using NMR, we locate specific Mg(2+) binding pockets and quantify their affinity toward Mg(2+). Mg(2+) pulse experiments show that M(2+) exchange occurs on the timescale of seconds. This unprecedented combination of NMR and single-molecule Förster resonance energy transfer demonstrates for the first time to our knowledge that a rugged free energy landscape coincides with incomplete occupation of specific M(2+) binding sites at near-physiological M(2+) concentrations. Unconventional kinetics in nucleic acid folding frequently encountered in single-molecule experiments are therefore likely to originate from a spectrum of conformations that differ in the occupation of M(2+) binding sites.