The Journal of Physical Chemistry B

Conformational dynamics plays crucial roles in RNA functions about sensing and responding to environmental signals. The liquid-liquid phase separation of RNAs and the formation of stress granules partly relies on RNAs conformational plasticity and its ability to engage in multivalent interactions. Recent experiments with homopolymeric and low-complexity RNAs have revealed significant differences in phase separations due to differences in base chemistry of RNA units. We hypothesize that differences in RNA phase-transition dynamics can be traced back to the differences in conformational dynamics of single RNA chains. In the present contribution, we utilize atomistic simulations with numerous unsupervised learning to map temperature dependence conformational free energy landscapes for homopolymeric RNA chains. These landscapes reveal a variety of metastable excited states influenced by the nature of base chemistry. We shed light on the distinct contributions of the polyphosphate backbone versus base chemistry in shaping conformational ensembles of different RNAs. We demonstrate that the experimentally observed temperature-driven shifts in metastable state populations align with experimental phase diagrams for homopolymeric RNAs. The work establishes a microscopic framework to reason about base-specific RNA propensity for phase separation. We believe our work will be valuable for designing novel RNA sensors for biological and synthetic applications.

Phase separation plays an important role in the formation of membraneless compartments within the cell, and intrinsically disordered proteins with low-complexity sequences can drive this compartmentalisation. Various intermolecular forces, such as aromatic-aromatic and cation-aromatic interactions, promote phase separation. However, little is known about how the ability of proteins to phase separate under physiological conditions is encoded in their energy landscapes, and this is the focus of the present investigation. Our results provide a first glimpse into how the energy landscapes of minimal peptides that contain{pi} -{pi} and cation-{pi} interactions differ from the peptides that lack amino acids with such interactions. The peaks in the heat capacity (CV) as a function of temperature report on alternative low-lying conformations that differ significantly in terms of their enthalpic and entropic contributions. The CV analysis and subsequent quantification of frustration of the energy landscape suggest that the interactions that promote phase separation leads to features (peaks or inflection points) at low temperatures in CV, more features may occur for peptides containing residues with better phase separation propensity and the energy landscape is more frustrated for such peptides. Overall, this work links the features in the underlying single-molecule potential energy landscapes to their collective phase separation behaviour, and identifies quantities (CV and frustration metric) that can be utilised in soft material design.

RNA folding and functioning require the binding of metal ions in specific cavities of the folded structure. This property is critical to the functioning of riboswitches that especially regulate the metal ions concentration in bacteria. However, the fundamental principles governing the specific binding of metal ions in RNA are unclear. We probed the condensation mechanism of biologically relevant alkali (Na+ and K+), alkaline earth (Mg2+ and Ca2+), and transition metals (Mn2+, Co2+, Ni2+ and Zn2+) on a part of the Ni2+ and Co2+ (NiCo) sensing riboswitch aptamer domain using computer simulations. The selected structure has multiple secondary structural elements and a single site for the specific binding of a metal ion. We show that three factors primarily determine the binding of a metal ion to an RNA site - (1) The varying structural constraints from different RNA secondary structural elements strongly influence the metal ion binding. The mode of ion binding depends on the local structure around the RNAs ion-binding pocket. (2) The arrangement of water molecules in the ion hydration shell, and (3) the energy barrier for the ion to lose a water molecule from its hydration shell and transition from an outer to an inner shell interaction, which is primarily influenced by the metal ion charge density. These results have implications for designing biocompatible sensors using riboswitches to probe the concentration of intracellular metal ions.

{beta}-sheets in proteins are formed by extended polypeptide chains, called {beta}-strands. While there is a general consensus on two types of {beta}-strands, viz. edge strands (or edges) and inner strands (or central strands), the possibility of distinguishing between different regions of inner strands remains less explored. In this paper, we address the portions of inner strands of {beta}-sheets that stick out on either or both sides. We call these portions the indent strands or indents because they give the typical indented appearance to {beta}-sheets. Similar to the edge strands, the indent strands also have {beta}-bridge partner residues on one side while the other side is still open for backbone hydrogen bonds. Despite this similarity, the indent strands differ from the edge strands in terms of various properties such as {beta}-bulges and amino acid composition due to their localization within {beta}-sheets and therefore within folded proteins to certain extent. The localization of indents and edges within folded proteins seems to govern the strategies deployed to deter unhindered {beta}-sheet propagation through {beta}-strand stacking interactions. Our findings suggest that, edges and indents differ in their strategies to avoid further {beta}-strand stacking. Short length itself is a good strategy to avoid stacking and a majority of indents are two residue or shorter in length. Edge strands on the other hand are overall longer. While long edges are known to use various negative design strategies like {beta}-bulges, prolines, strategically placed charges, inward-pointing charged side chains and loop coverage to avoid further {beta}-strand stacking, long indents seem to favor mechanisms such as enrichment in flexible residues with high solvation potential and depletion in hydrophobic residues in response to their less solvent exposed nature. Such subtle differences between indents and edges could be leveraged for designing novel {beta}-sheet architectures.

Riboswitches are non-coding RNA that regulate gene expression by folding into specific three-dimensional structures (holo-form) upon binding by their cognate ligand in the presence of Mg2+. Riboswitch functioning is also hypothesized to be under kinetic control requiring large cognate ligand concentrations. We ask the question under thermodynamic conditions, can the riboswitches populate holo-form like structures in the absence of their cognate ligands only in the presence of Mg2+. We addressed this question using thiamine pyrophosphate (TPP) riboswitch as a model system and computer simulations using a coarse-grained model for RNA. The folding free energy surface (FES) shows that with the initial increase in Mg2+ concentration ([Mg2+]), TPP AD undergoes a barrierless collapse in its dimensions. On further increase in [Mg2+], intermediates separated by barriers appear on the FES, and one of the intermediates has a TPP ligand-binding competent structure. We show that site-specific binding of the Mg2+ aids in the formation of tertiary contacts. For [Mg2+] greater than physiological concentration, AD folds into its holo-form like structure even in the absence of the TPP ligand. The folding kinetics shows that it populates an intermediate due to the misalignment of the two arms in the TPP AD, which acts as a kinetic trap leading to larger folding timescales. The predictions of the intermediate structures from the simulations are amenable for experimental verification.

Understanding the emergence and role of lipid packing defects in the detection and subsequent partitioning of antimicrobial agents into bacterial membranes is essential for gaining insights into general antimicrobial mechanisms. Herein, using methacrylate polymers as a model platform, we investigate the effects of inclusion of various functional groups in the biomimetic antimicrobial polymer design on the aspects of lipid packing defects in model bacterial membranes. Two antimicrobial polymers are considered: ternary polymers composed of cationic, hydrophobic and polar moieties and binary polymers with only cationic and hydrophobic moieties. We find that differing modes of insertion of these two polymers lead to different packing defects in the bacterial membrane. While insertion of both binary and ternary polymers leads to an enhanced number of deep defects in the upper leaflet, shallow defects are moderately enhanced upon interaction with ternary polymers only. We provide conclusive evidence that insertion of antimicrobial polymers in bacterial membrane is preceded by sensing of interfacial lipid packing defects. Our simulation results show that the hydrophobic groups are inserted at a single co-localized deep defect site for both binary and ternary polymers. However, the presence of polar groups in the ternary polymers use the shallow defects close to the lipid-water interface, in addition, to insert into the membrane, which leads to a more folded conformation of the ternary polymer in the membrane environment, and hence a different membrane partitioning mechanism compared to the binary polymer, which acquires an amphiphilic conformation.

Riboswitches are regulatory elements present in bacterial messenger RNA acting as sensors of small molecules and consequently playing a vital role in bacterial gene regulation. The SAM-II riboswitch is a class of riboswitches, that recognizes S-adenosyl methionine. It has been previously illustrated that the presence of Mg2+ ions stabilizes the pre-existing minor state of the riboswitch, which is structurally characterised by having a nucleated pseudoknot, leading to the increase of its probability. In this study, an analytical equilibrium model is developed to describe the impact of Mg2+ ions concentration on the folding of the SAM-II riboswitch, linking RNA folding and tertiary interactions energetics to ligand binding, and, hence enabling quantitative predictions. The method was used to study the role of the P1 helix sequence in determining the fraction of binding competent conformers of the SAM-II riboswitch, by simulating the Mg2+ titration curves of various mutants. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=127 SRC="FIGDIR/small/439486v2_ufig1.gif" ALT="Figure 1"> View larger version (21K): org.highwire.dtl.DTLVardef@19367ecorg.highwire.dtl.DTLVardef@8be5ecorg.highwire.dtl.DTLVardef@a3fec6org.highwire.dtl.DTLVardef@ab9d65_HPS_FORMAT_FIGEXP M_FIG C_FIG

Post-transcriptional modifications in RNA can significantly impact their structure and function. In particular, transfer RNAs (tRNAs) are heavily modified, with around 100 different naturally occurring nucleotide modifications contributing to codon bias and decoding efficiency. Here, we describe our efforts to investigate the impact of RNA modifications on the structure and stability of tRNA Phenylalanine (tRNAPhe) from S. cerevisiae using molecular dynamics (MD) simulations. Through temperature replica exchange MD (T-REMD) studies, we explored the unfolding pathway to understand how RNA modifications influence the conformational dynamics of tRNAPhe, both in the presence and absence of magnesium ions (Mg2+). We observe that modified nucleotides in key regions of the tRNA establish a complex network of hydrogen bonds and stacking interactions which is essential for tertiary structure stability of the tRNA. Furthermore, our simulations show that modifications facilitate the formation of ion binding sites on the tRNA. However, high concentrations of Mg2+ ions can stabilize the tRNA tertiary structure in the absence of modifications. Our findings illuminate the intricate interactions between modifications, magnesium ions, and RNA structural stability. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=168 SRC="FIGDIR/small/584441v1_ufig1.gif" ALT="Figure 1"> View larger version (35K): org.highwire.dtl.DTLVardef@a8d8e1org.highwire.dtl.DTLVardef@136a038org.highwire.dtl.DTLVardef@150bf34org.highwire.dtl.DTLVardef@391279_HPS_FORMAT_FIGEXP M_FIG C_FIG

Metabotropic glutamate receptor 2 (mGluR2), a subclass C member of the G protein-coupled receptor (GPCR) superfamily, is essential for regulating neurotransmitter signaling and facilitating synaptic adaptability in the central nervous system. This receptor, like other GPCRs, is highly sensitive to its surrounding lipid environment, where specific lipid compositions can influence its stability, conformational dynamics, and function. In particular, cholesteryl hemisuccinate (CHS) plays a critical role in stabilizing mGluR2 and modulating its structural states within cellular membranes and micellar environments. However, the molecular basis for this lipid-mediated modulation remains largely unexplored. To investigate the effects of CHS and lipid composition on mGluR2, we employed all-atom molecular dynamics simulations of mGluR2 embedded in both detergent micelles (BLMNG and CHS) and a POPC lipid bilayer containing 0%, 10%, and 25% CHS. These simulations were conducted for both active and inactive states of the receptor. Our findings reveal that CHS concentration modulates mGluR2s structural stability and conformational behavior, with a marked impact observed within transmembrane helices TM1, TM2, and TM3, which constitute the core of the receptors transmembrane domain. In micelle environments, mGluR2 displayed unique conformational dynamics influenced by CHS, underscoring the receptors sensitivity to its lipid surroundings. Notably, a CHS concentration of 10% elicited more pronounced conformational changes than either cholesterol-depleted (0%) or cholesterol-enriched (25%) systems, indicating an optimal CHS range for maintaining structural stability. Our study provides atomistic insights into how lipid composition and CHS concentration impact mGluR2s conformational landscape in distinct micelle and bilayer environments. These findings advance our understanding of lipid-mediated modulation in GPCR function, highlighting potential avenues for receptor-targeted drug design, particularly in cases where lipid interactions play a significant role in therapeutic efficacy.

Vasoactive intestinal polypeptide receptor (VIP1R) is a class B G-protein coupled receptor (GPCR) that is widely distributed throughout the central nervous system, T-lymphocytes, and peripheral tissues of organs like lungs and liver. Critical functions of these receptors render them potential pharmacological targets for the treatment of a broad spectrum of inflammatory and neurodegenerative diseases. Here we use atomistic studies to show that phospholipids can act as potent regulators of peptide binding on to the receptor. We simulated the binding of neuropeptide pituitary adenylate cyclase-activating peptide (PACAP27) into the transmembrane bundle of the receptor. The simulations reveal two lipid binding sites on the peptidic ligand for the negatively charged phosphodiester of phospholipids in the extracellular leaflet which lower the peptide-receptor binding free energy by ~8kBT. We further simulated the effect of anionic lipids phosphatidylinositol-4,5-bisphosphate (PIP2). These lipids show much stronger interaction, lowering the peptide-receptor binding energy by an additional ~7kBT compared to POPC lipids. These findings suggest that lipids can play an active role in catalyzing peptide-receptor binding and activating vasoactive intestinal polypeptide receptors.

Magnesium ions (Mg{superscript 2}) play a critical role in RNA structure stabilization by forming various coordinated complexes, preferentially interacting with the backbone phosphate groups. Using extensive atomistic and free energy simulations across simple models and RNA structures of varying complexity, we characterized critical components of the RNA-ion-atmosphere. Radial distribution function analysis reveals distinct peak positions for direct (inner) and solvent-separated (outer-sphere) Mg2+-phosphate coordination layers, aligning with solution X-ray diffraction data. Addressing forcefield limitations, the free energy calculations quantify the kinetic barriers for Mg{superscript 2}-phosphate binding, benchmarking parameters against {superscript 2}Mg NMR measurement. Free energy calculations further explore Mg{superscript 2} chelation with bi-phosphate coordinated Mg2+ systems, identifying a dynamic ensemble of pre-chelate complexes, in addition to a chelated and outer-sphere hexa-hydrated state of Mg2+. In the pre-chelated states, Mg{superscript 2} maintains one inner-sphere interaction while simultaneously coordinating with multiple other phosphates in a solvent-separated manner, referred to as meta-sphere coordination. The pre-chelated complexes from different solvents-separated layers undergo a frequent transition and mediate a unique oxygen exchange mechanism between phosphate and water ligands. Insights into the free energy landscape of SAM-I RNA aptamer further emphasize the significance of pre-chelate complexes for complex RNA structure stabilization, where a number of such solvent-separated dynamic phosphate groups are found to influence Mg2+-RNA coordination. The comprehensive thermodynamic analysis of Mg{superscript 2} chelation and quantitative characterizations of various RNA-ion coordination modes, including this new meta-sphere coordination, provides vital insights for advancing RNA modelling and experimental exploration of complex phosphate networks in the RNA structures.

Antimicrobial peptides (AMPs) initiate killing of bacteria by binding to and destabilizing their membranes. The multiple peptide resistance factor (MprF) provides a defence mechanism for bacteria against a broad range of AMPs. MprF reduces the negative charge of both Gram-positive and Gram--negative bacterial membranes through enzymatic conversion of the anionic lipid phosphatidyl glycerol (PG) to either zwitterionic alanyl-phosphatidyl glycerol (Ala-PG) or cationic lysylphosphatidyl glycerol (Lys-PG). The resulting change in membrane charge is suggested to reduce AMP-membrane binding and hinder downstream AMP activity. Using molecular dynamics to investigate the effects of these modified lipids on AMP-binding to model membranes, we show that AMPs have substantially reduced affinity for model membranes containing Ala-PG or Lys-PG. A total of ~7000 simulations are used to define the relationship between bilayer composition and binding for 5 different membrane active peptides. The reduction of degree of interaction of a peptide with the membrane is shown to correlate with the change in membrane surface charge density. Free energy profile (potential of mean force) calculations reveal that these lipid modifications alter the energy barrier to peptide helix penetration of the bilayer. These results will enable us to guide design of novel peptides which address the issue of resistance via MprF-mediated membrane modification.

With rising bacterial resistance, antimicrobial peptides (AMPs) have been widely investigated as potential antibacterial molecules to replace conventional antibiotics. Our understanding of the molecular mechanism for membrane disruption are largely based on AMP interactions with the inner phospholipid bilayers of both Gram-negative and Grampositive bacteria. Mechanisms for AMP translocation across the outer membrane of Gram-negative bacteria composed of lipopolysaccharides and the asymmetric lipid bilayer are incompletely understood. In the current study, we have employed atomistic molecular dynamics and umbrella sampling simulations with an aggregate duration of ~ 8 microseconds to understand the free energy landscape of CM15 peptide within the OM of Gram-negative bacteria, E. coli. The peptide has a favourable binding free energy (-130 kJ mol-1) in the O-antigen region with a large barrier (150 kJ mol-1) at the interface between the anionic coresaccharides and upper bilayer leaflet made up of lipid A molecules. We have analyzed the peptide and membrane properties at each of the 100 ns duration umbrella sampling windows to study variations in the membrane and the peptide structure during the translocation through the OM. Interestingly the peptide is seen to elongate, adopting a membrane perpendicular orientation in the phospholipid region resulting in the formation of a transient water channel during its translocation through the bilayer. The presence of the peptide at the lipid A and core-saccharide interface results in a 11% increase in the membrane area with the peptide adopting a predominantly membrane parallel orientation in this cation rich region. Additionally, the lateral displacement of the peptide is significantly reduced in this region, and increases toward the inner phospholipid leaflet and the outer O-antigen regions of the membrane. The peptide is found to be sufficiently hydrated across both the hydrophilic as well as hydrophobic regions of the membrane and remains unstructured without any gain in helical content. Our study unravels the complex free energy landscape for the translocation of the AMP CM15 across the outer membrane of Gram-negative bacteria and we discuss the implications of our findings with the broader question of how AMPs overcome this barrier during antimicrobial activity.

Thermodynamics and structural transitions on protein surfaces remain relatively understudied and poorly understood. Wrapping of DNA on proteins provides a paradigm for studying protein surfaces. We used magnetic tweezers to investigate a prototypical DNA-interacting protein, i.e., the single-stranded DNA binding protein (SSB). SSB binds DNA with distinct binding modes the mechanism of which is still elusive. The measured thermodynamic parameters relevant to the SSB-DNA complex are salt-dependent and discontinuous at the bind-mode transitions. Our data indicate that free SSB undergoes salt-induced first-order structural transitions. The conclusion was supported by the infrared spectroscopy of SSB in salt solutions. Ultrafast infrared spectroscopy further suggests that the transitions are correlated with surface salt bridges. Our work not only unravels a long-standing mystery of the different binding site sizes of SSB, but also would inspire interests in thermodynamics of protein surfaces.

Understanding the mechanism of ligands binding to their protein targets and the influence of various factors governing the binding thermodynamics is essential for rational drug design. The solution pH is one of the critical factors that can influence ligand binding to a protein cavity, especially in enzymes whose function is sensitive to the pH. Using computer simulations, we studied the pH effect on the binding of a guanidinium ion (Gdm+) to the active site of hen-egg white lysozyme (HEWL). HEWL serves as a model system for enzymes with two acidic residues in the active site and ligands with Gdm+ moieties, which can bind to the active sites of such enzymes and are present in several approved drugs treating various disorders. The computed free energy surface (FES) shows that Gdm+ binds to the HEWL active site using two dominant binding pathways populating multiple intermediates. We show that the residues close to the active site that can anchor the ligand could play a critical role in ligand binding. Using a Markov state model, we quantified the lifetimes and kinetic pathways connecting the different states in the FES. The protonation and deprotonation of the acidic residues in the active site in response to the pH change strongly influence the Gdm+ binding. There is a sharp jump in the ligand-binding rate constant when the pH approaches the largest pKa of the acidic residue present in the active site. The simulations reveal that, at most, three Gdm+ can bind at the active site, with the Gdm+ bound in the cavity of the active site acting as a scaffold for the other two Gdm+ ions binding. This result implies the possibility of designing single large molecules containing multiple Gdm+ moieties that can have high binding affinities to inhibit the function of enzymes with two acidic residues in their active site.

Understanding the thermodynamics that drives liquid-liquid phase separation (LLPS) is quite important given the many numbers of diverse biomolecular systems undergoing this phenomenon. Regardless of the diversity, the processes underlying the formation of condensates exhibit physical similarities. Many studies have focused on condensates of long polymers, but very few systems of short polymer condensates have been observed and yet studied. Here we study a short polymer system of various lengths of poly-Adenine RNA and peptide formed by the RGRGG sequence repeats to understand the underlying thermodynamics of LLPS. We carried out MD simulations using the recently developed COCOMO coarse-grained (CG) model which revealed the possibility of condensates for lengths as short as 5-10 residues, which was then confirmed by experiment, making this one of the smallest LLPS systems yet observed. Condensation depends on polymer length and concentration, and phase boundaries were identified. A free energy model was also developed. Results show that the length dependent condensation is driven solely by entropy of confinement and identifies a negative free energy (-{Delta}G) of phase separation, indicating the stability of the condensates. The simplicity of this system will provide the basis for understanding more biologically realistic systems.

Intrinsically disordered proteins (IDPs) and proteins with intrinsically disordered regions (IDRs) can modulate cellular responses to environmental conditions by undergoing coil-to-globule transitions and phase separation. However, the molecular mechanisms of these phenomena still need to be fully understood. Here, we use Monte Carlo calculations of a model incorporating waters effects on the systems free energy to investigate how an IDP responds to a hydrophobic surface under different conditions. We show that a slit pore confinement without top-down symmetry enhances the unfolding and adsorption of the IDP in both random coil and globular states. Moreover, we demonstrate that the hydration water modulates this behavior depending on the thermodynamic parameters. Our findings provide insights into how IDPs and IDRs can sense and adjust to external stimuli such as nanointerfaces or stresses.

TAR DNA binding protein 43 (TDP-43) is involved in key processes in RNA metabolism such as splicing, stability and transcription. TDP-43 dysfunction is frequently implicated in many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and fronto-temporal dementia (FTD). The prion-like, disordered C-terminal domain (CTD) of TDP-43 is aggregation-prone and harbors the majority (~90%) of all ALS-related mutations. Recent studies have established that TDP-43 CTD can undergo liquid-liquid phase separation (LLPS) in isolation and is important for phase separation (PS) of the full-length protein under physiological conditions. While a short conserved helical region (CR, spanning residues 319-341) promotes oligomerization and is essential for LLPS, aromatic residues in the flanking disordered regions (IDR1/2) have also been found to play a critical role in PS and aggregation. However, TDP-43 CTD has a distinct sequence composition compared with other phase separating proteins, including many aliphatic residues. These residues have been suggested to modulate the apparent viscosity of the resulting phases, but their direct contribution to phase separation has been relatively ignored. Here, we utilized a multiscale simulation and experimental approach to assess the residue-level determinants of TDP-43 CTD phase separation. Single chain and condensed phase simulations performed at the atomistic and coarse-grained level respectively, identified the importance of aromatic residues (previously established) while also suggesting an essential role for aliphatic methionine residues in LLPS. In vitro experiments confirmed the role of phenylalanine, methionine, and leucine (but not alanine) residues in driving the phase separation of CTD, which have not been previously considered essential for describing the molecular grammar of PS. Finally, NMR experiments also showed that phenylalanine residues in the disordered flanking regions and methionine residues both within and outside the CR contribute important contacts to CTD interactions. Broadly, our work highlights the importance of non-alanine aliphatic residues such as methionine and leucine, and potentially valine and isoleucine, in determining the LLPS propensity, expanding the molecular grammar of protein phase separation to include critical contributions from aliphatic residues.

Group I Introns are non-coding regions of pre-mRNA that catalyze their splicing from the RNA sequence by folding to a specific structure. We used computer simulations to study the folding mechanism of the P4-P6 domain in the Tetrahymena thermophila group I intron, focusing on the GAAA tetraloop-receptor (TL-R) interaction, which is a ubiquitous tertiary interaction in RNA structures. We show that the intron folds via a multistep pathway, populating seven states with distinct tertiary contacts. Under physiological Mg2+ concentrations ([Mg2+]), the loop-bulge-P4 tertiary interaction is essential to stabilize the docked TL-R complex, whereas in high [Mg2+], the TL-R complex is stable by itself. The solvated Mg2+ ions modulate the TL-R docking-undocking dynamics and stabilize non-native intermediate states. The condensation of Mg2+ in the major grooves of the TL and R helices is critical for them to attain specific stiffness essential for their facile docking. The results highlight the critical role of Mg2+ ions in facilitating TL-R interaction formation, which stabilizes long-range tertiary contacts in RNA structures. For Table of Contents Use Only O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=111 SRC="FIGDIR/small/700762v1_ufig1.gif" ALT="Figure 1"> View larger version (26K): org.highwire.dtl.DTLVardef@142852org.highwire.dtl.DTLVardef@1632ad1org.highwire.dtl.DTLVardef@190021aorg.highwire.dtl.DTLVardef@17a1261_HPS_FORMAT_FIGEXP M_FIG C_FIG

Riboswitches in bacteria regulate gene expression and are targets for antibiotic development. The fluoride riboswitch is essential for bacterias survival as it is critical to maintaining the F- ion concentration below the toxic level. The anionic cognate ligand, F- ion, is encapsulated by three Mg2+ ions in a trigonal pyramidal arrangement bound to the ligand-binding domain (LBD) of the riboswitch. The assembly mechanism of this intriguing LBD structure and its role in transcription initiation are not clear. Computer simulations using both coarse-grained and all-atom RNA models show that F- and Mg2+ binding to the LBD are essential to stabilize the LBD structure and tertiary stacking interactions. We propose that the first two Mg2+ ions sequentially bind to the LBD through water-mediated outer-shell coordination. The first bound Mg2+ should undergo a transition to a direct inner shell interaction through dehydration to strengthen its interaction with LBD before the binding of the second Mg2+ ion. The binding of the third Mg2+ and F- to the LBD occurs in two modes. In the first mode, the third Mg2+ binds first to the LBD, followed by F- binding. In the second mode, Mg2+ and F- form a water-mediated ion pair and bind to the LBD simultaneously, which we propose to be the efficient binding mode. We show that the linchpin hydrogen bonds involved in the antiterminator helix formation and transcription initiation are stable only after F- binding. The intermediates populated during riboswitch folding and cognate-ligand binding are potential targets for discovering new antibiotics.