A thermodynamic framework and cooperativity in the tertiary folding of a Mg2+-dependent ribozyme

The folding thermodynamics of the catalytic domain from the Bacillus subtilis RNase P RNA is analyzed using circular dichroism and fluorescence spectroscopies, hydroxyl radical protection, and catalytic activity. Folding of this 255-nucleotide ribozyme can be described with three populated species: unfolded (U), intermediate (I), and native (N) states. The U-to-I transition primarily involves secondary structure formation, whereas the I-to-N transition is dominated by tertiary structure formation. The I-to-N transition is highly cooperative as indicated by the coincidence of the four probes applied here. Two isothermal methods are used to determine the stability of the N state relative to the I state at 10 and 37 degrees C. The first method measures the extent of Mg(2+)-induced folding without urea or at constant urea concentrations. The second method measures the extent of urea-induced unfolding at constant Mg(2+) concentrations. Via application of a cooperative binding analysis, the Mg(2+) transition midpoint (K(Mg)), the Hill constant (n), and the urea-dependent surface burial parameter (m value) determined by both methods are identical, indicating that they report the same, reversible folding event. Three conclusions can be drawn from these results. (i) The folding free energy of a Mg(2+)-dependent tertiary RNA structure can be described by the K(Mg) and n parameters according to a cooperative Mg(2+) binding model. (ii) The Hill constant for this tertiary RNA structure probably represents the differential number of Mg(2+) ions bound in the I-to-N transition. (iii) Under physiological conditions, the stability of this large ribozyme is similar to that of small globular proteins.     

The tertiary structure of the hairpin ribozyme is formed through a slow conformational search

The hairpin ribozyme is a small catalytic RNA comprised of two internal loops carried on two adjacent arms of a four-way helical junction (4WJ). To achieve catalytic activity, the ribozyme folds into a compact conformation that facilitates the formation of tertiary interactions between the two loops. We have investigated the folding kinetics of the natural 4WJ form of the hairpin ribozyme, as well as a minimal construct consisting of just the two loop-containing duplexes, by means of stopped-flow fluorescence resonance energy transfer between donor and acceptor probes attached to the ends of the loop-bearing arms. Folding was initiated by the addition of Mg(2+) ions or a pseudosubstrate strand to the ribozyme, and the ensuing changes in the emission of both donor and acceptor were monitored over time. Both ribozyme constructs exhibited slow, biphasic kinetic behavior, attributed to two parallel folding pathways leading to compact, docked structures. Two distinct folding rates were observed across a range of Mg(2+) concentrations, and increasing amounts of Mg(2+) accelerated both rates. Notably, both rates were essentially independent of temperature, indicating that the corresponding activation enthalpies were negligible, in contrast to the large activation enthalpies generally observed for RNA folding processes. Instead, the slow folding was due to unfavorable entropy changes in reaching the transition state, indicating that the ribozyme tertiary structure forms through a slow conformational search. These features were observed in both forms of the ribozyme, indicating that the conformational search is confined to the two loop regions and is largely independent of the overall ribozyme architecture. Conformational search may be a general mechanism of tertiary structure formation in RNA.     

Folding mechanism of the Tetrahymena ribozyme P4-P6 domain

Synchrotron X-ray-dependent hydroxyl radical footprinting was used to probe the folding kinetics of the P4-P6 domain of the Tetrahymena group I ribozyme, which forms a stable, closely packed tertiary structure. The 160-nt domain folds independently at a similar rate (approximately 2 s(-1)) as it does in the ribozyme, when folding is measured in 10 mM sodium cacodylate and 10 mM MgCl(2). Surprisingly, tertiary interactions around a three-helix junction (P5abc) within the P4-P6 domain fold at least 25 times more rapidly (k >/= 50 s(-1)) in isolation, than when part of the wild-type P4-P6 RNA. This difference implies that long-range interactions in the P4-P6 domain can interfere with folding of P5abc. P4-P6 was observed to fold much faster at higher ionic strength than in 10 mM sodium cacodylate. Analytical centrifugation was used to measure the sedimentation and diffusion coefficients of the unfolded RNA. The hydrodynamic radius of the RNA decreased from 58 to 46 A over the range of 0-100 mM NaCl. We propose that at low ionic strength, the addition of Mg(2+) causes the domain to collapse to a compact intermediate where P5abc is trapped in a non-native structure. At high ionic strength, the RNA rapidly collapses to the native structure. Faster folding most likely results from a different average initial conformation of the RNA in higher salt conditions.     

Magnesium dependence of the amplified conformational switch in the trans-acting hepatitis delta virus ribozyme

The ability of divalent metal ions to participate in both structure formation and catalytic chemistry of RNA enzymes (ribozymes) has made it difficult to separate their cause and effect in ribozyme function. For example, the recently solved crystal structures of precursor and product forms of the cis-cleaving genomic hepatitis delta virus (HDV) ribozyme show a divalent metal ion bound in the active site that is released upon catalysis due to an RNA conformational change. This conformational switch is associated with a repositioning of the catalytically involved base C75 in the active-site cleft, thus controlling catalysis. These findings confirm previous data from fluorescence resonance energy transfer (FRET) on a trans-acting form of the HDV ribozyme that found a global conformational change to accompany catalysis. Here, we further test the conformational switch model by measuring the Mg(2+) dependence of the global conformational change of the trans-acting HDV ribozyme, using circular dichroism and time-resolved FRET as complementary probes of secondary and tertiary structure formation, respectively. We observe significant differences in both structure and Mg(2+) affinity of the precursor and product forms, in the presence and absence of 300 mM Na(+) background. The precursor shortens while the product extends with increasing Mg(2+) concentration, essentially amplifying the structural differences observed in the crystal structures. In addition, the precursor has an approximately 2-fold and approximately 13-fold lower Mg(2+) affinity than the product in secondary and tertiary structure formation, respectively. We also have compared the C75 wild-type with the catalytically inactive C75U mutant and find significant differences in global structure and Mg(2+) affinity for both their precursor and product forms. Significantly, the Mg(2+) affinity of the C75 wild-type is 1.7-2.1-fold lower than that of the C75U mutant, in accord with the notion that C75 is essential for a catalytic conformational change that leads to a decrease in the local divalent metal ion affinity and release of a catalytic metal. Thus, a consistent picture emerges in which divalent metal ions and RNA functional groups are intimately intertwined in affecting structural dynamics and catalysis in the HDV ribozyme.     

Concordant exploration of the kinetics of RNA folding from global and local perspectives

Time-resolved small-angle X-ray scattering (SAXS) with millisecond time-resolution reveals two discrete phases of global compaction upon Mg2+-mediated folding of the Tetrahymena thermophila ribozyme. Electrostatic relaxation of the RNA occurs rapidly and dominates the first phase of compaction during which the observed radius of gyration (R(g)) decreases from 75 angstroms to 55 angstroms. A further decrease in R(g) to 45 angstroms occurs in a well-defined second phase. An analysis of mutant ribozymes shows that the latter phase depends upon the formation of long-range tertiary contacts within the P4-P6 domain of the ribozyme; disruption of the three remaining long-range contacts linking the peripheral helices has no effect on the 55-45 angstroms compaction transition. A better understanding of the role of specific tertiary contacts in compaction was obtained by concordant time-resolved hydroxyl radical (OH) analyses that report local changes in the solvent accessibility of the RNA backbone. Comparison of the global and local measures of folding shows that formation of a subset of native tertiary contacts (i.e. those defining the ribozyme core) can occur within a highly compact ensemble whose R(g) is close to that of the fully folded ribozyme. Analyses of additional ribozyme mutants and reaction conditions establish the generality of the rapid formation of a partially collapsed state with little to no detectable tertiary structure. These studies directly link global RNA compaction with formation of tertiary structure as the molecule acquires its biologically active structure, and underscore the strong dependence on salt of both local and global measures of folding kinetics.     

Distinct reaction pathway promoted by non-divalent-metal cations in a tertiary stabilized hammerhead ribozyme

Divalent ion sensitivity of hammerhead ribozymes is significantly reduced when the RNA structure includes appropriate tertiary stabilization. Therefore, we investigated the activity of the tertiary stabilized "RzB" hammerhead ribozyme in several nondivalent ions. Ribozyme RzB is active in spermidine and Na(+) alone, although the cleavage rates are reduced by more than 1,000-fold relative to the rates observed in Mg(2+) and in transition metal ions. The trivalent cobalt hexammine (CoHex) ion is often used as an exchange-inert analog of hydrated magnesium ion. Trans-cleavage rates exceeded 8 min(-1) in 20 mM CoHex, which promoted cleavage through outersphere interactions. The stimulation of catalysis afforded by the tertiary structural interactions within RzB does not require Mg(2+), unlike other extended hammerhead ribozymes. Site-specific interaction with at least one Mg(2+) ion is suggested by CoHex competition experiments. In the presence of a constant, low concentration of Mg(2+), low concentrations of CoHex decreased the rate by two to three orders of magnitude relative to the rate in Mg(2+) alone. Cleavage rates increased as CoHex concentrations were raised further, but the final fraction cleaved was lower than what was observed in CoHex or Mg(2+) alone. These observations suggest that Mg(2+) and CoHex compete for binding and that they cause misfolded structures when they are together. The results of this study support the existence of an alternate catalytic mechanism used by nondivalent ions (especially CoHex) that is distinct from the one promoted by divalent metal ions, and they imply that divalent metals influence catalysis through a specific nonstructural role.     

Tm studies of a tertiary structure from the human hepatitis delta agent which functions in vitro as a ribozyme control element.

Viroids and other circular subviral RNA pathogens, such as the hepatitis delta agent, use a rolling circle replication cycle requiring an intact circular RNA. However, many infectious RNAs have the potential to form self-cleavage structures, whose formation must be controlled in order to preserve the circular replication template. The native structure of delta RNA contains a highly conserved element of local tertiary structure which is composed of sequences partially overlapping those needed to form the self-cleavage motif. A bimolecular complex containing the tertiary structure can be made. We show that when it is part of this bimolecular complex the potential cleavage site is protected and is not cleaved by the delta ribozyme, demonstrating that the element of local tertiary structure can function as a ribozyme control element in vitro. Physical studies of the complex containing this element were carried out. The complex binds magnesium ions and is not readily dissociated by EDTA under the conditions tested; > 50% of the complexes remain following incubation in 1 mM EDTA at 60 degrees C for 81 min. The thermal stability of the complex is reduced in the presence of sodium ions. A DNA complex and a perfect RNA duplex studied in parallel showed a similar effect, but of lesser magnitude. The RNA complex melts at temperatures approximately 10 degrees C lower in buffers containing 0.5 mM MgCl2 and 100 mM NaCl than in buffers containing 0.5 mM MgCl2 with no NaCl (78.1 compared with 87.7 degrees C). The element of local tertiary structure in delta genomic RNA appears to be a molecular clamp whose stability is highly sensitive to ion concentration in the physiological range.     

Principles of RNA compaction: insights from the equilibrium folding pathway of the P4-P6 RNA domain in monovalent cations

Counterions are required for RNA folding, and divalent metal ions such as Mg(2+) are often critical. To dissect the role of counterions, we have compared global and local folding of wild-type and mutant variants of P4-P6 RNA derived from the Tetrahymena group I ribozyme in monovalent and in divalent metal ions. A remarkably simple picture of the folding thermodynamics emerges. The equilibrium folding pathway in monovalent ions displays two phases. In the first phase, RNA molecules that are initially in an extended conformation enforced by charge-charge repulsion are relaxed by electrostatic screening to a state with increased flexibility but without formation of long-range tertiary contacts. At higher concentrations of monovalent ions, a state that is nearly identical to the native folded state in the presence of Mg(2+) is formed, with tertiary contacts that involve base and backbone interactions but without the subset of interactions that involve specific divalent metal ion-binding sites. The folding model derived from these and previous results provides a robust framework for understanding the equilibrium and kinetic folding of RNA.     

Long-range tertiary interactions in single hammerhead ribozymes bias motional sampling toward catalytically active conformations

Enzymes generally are thought to derive their functional activity from conformational motions. The limited chemical variation in RNA suggests that such structural dynamics may play a particularly important role in RNA function. Minimal hammerhead ribozymes are known to cleave efficiently only in ～ 10-fold higher than physiologic concentrations of Mg(2+) ions. Extended versions containing native loop-loop interactions, however, show greatly enhanced catalytic activity at physiologically relevant Mg(2+) concentrations, for reasons that are still ill-understood. Here, we use Mg(2+) titrations, activity assays, ensemble, and single molecule fluorescence resonance energy transfer (FRET) approaches, combined with molecular dynamics (MD) simulations, to ask what influence the spatially distant tertiary loop-loop interactions of an extended hammerhead ribozyme have on its structural dynamics. By comparing hammerhead variants with wild-type, partially disrupted, and fully disrupted loop-loop interaction sequences we find that the tertiary interactions lead to a dynamic motional sampling that increasingly populates catalytically active conformations. At the global level the wild-type tertiary interactions lead to more frequent, if transient, encounters of the loop-carrying stems, whereas at the local level they lead to an enrichment in favorable in-line attack angles at the cleavage site. These results invoke a linkage between RNA structural dynamics and function and suggest that loop-loop interactions in extended hammerhead ribozymes-and Mg(2+) ions that bind to minimal ribozymes-may generally allow more frequent access to a catalytically relevant conformation(s), rather than simply locking the ribozyme into a single active state.     

Altering the intermediate in the equilibrium folding of unmodified yeast tRNAPhe with monovalent and divalent cations

The isothermal equilibrium folding of the unmodified yeast tRNA(Phe) is studied as a function of Na(+), Mg(2+), and urea concentration with hydroxyl radical protection, circular dichroism, and diethyl pyrocarbonate (DEPC) modification. These assays indicate that this tRNA folds in Na(+) alone. Similar to folding in Mg(2+), folding in Na(+) can be described by two transitions, unfolded-to-intermediate-to-native. The I-to-N transition has a Na(+) midpoint of approximately 0.5 M and a Hill constant of approximately 4. Unexpectedly, the urea m-value, the dependence of free energy on urea concentration, for the I-to-N transition is significantly smaller in Na(+) than in Mg(2+), 0.4 versus 1.7 kcal mol(-1) M(-1), indicating that more structure is formed in the Mg(2+)-induced transition. DEPC modification indicates that the I state in Na(+)-induced folding contains all four helices of tRNA and the I-to-N transition primarily corresponds to the formation of the tertiary structure. In contrast, the intermediate in Mg(2+)-induced folding contains only three helices, and the I-to-N transition corresponds to the formation of the acceptor stem plus tertiary structure. The cation dependence of the intermediates arises from the differences in the stability of the acceptor stem and the tertiary structure. The acceptor stem is stable at a lower Na(+) concentration than required for the tertiary structure formation. The relative stability is reversed in Mg(2+) so that the acceptor stem and the tertiary structure form simultaneously in the I-to-N transition. These results demonstrate that formation of the RNA secondary structure can be independent or coupled to the formation of the tertiary structure depending on their relative stability in monovalent and divalent ions.     

EPR spectroscopic analysis of U7 hammerhead ribozyme dynamics during metal ion induced folding

Electron paramagnetic resonance (EPR) spectroscopy was used to examine changes in internal structure and dynamics of the hammerhead ribozyme upon metal ion induced folding, changes in pH, and the presence and absence of ribozyme inhibitors. A nitroxide spin-label was attached to nucleotide U7 of the HH16 catalytic core, and this modified ribozyme was observed to retain catalytic activity. U7 was shown by EPR spectroscopy to be more mobile in the ribozyme-product complex than in either the unfolded ribozyme or the ribozyme-substrate complex. A two-step divalent metal ion dependent folding pathway was observed for the ribozyme-substrate complex with a weak first transition observed at 0.25 mM Mg2+ and a strong second transition observed around 10 mM Mg2+, in agreement with studies using other biophysical and biochemical techniques. Previously, ribozyme activity was observed in the absence of divalent metal ions and the presence of high concentrations of monovalent metal ions, although the activity was less than that observed in the presence of divalent metal ions. Here, we observed similar dynamics for U7 in the presence of 4 M Na+ or Li+, which were distinctively different than that observed in the presence of 10 mM Mg2+, indicating that U7 of the catalytic core forms a different microenvironment under monovalent versus divalent metal ion conditions. Interestingly, the catalytically efficient microenvironment of U7 was similar to that observed in a solution containing 1 M Na+ upon addition of one divalent metal ion per ribozyme. In summary, these results demonstrate that changes in local dynamics, as detected by EPR spectroscopy, can be used to study conformational changes associated with RNA folding and function.     

Mg2+-linked oligomerization modulates the catalytic activity of the Lon (La) protease from Mycobacterium smegmatis

Lon (La) proteases are multimeric enzymes that are activated by ATP and Mg(2+) ions and stimulated by unfolded proteins such as alpha-casein. The peptidase activity of the Lon protease from Mycobacterium smegmatis (Ms-Lon) is dependent upon both its concentration and that of Mg(2+). Addition of alpha-casein partially substitutes for Mg(2+) in activating the enzyme. In chemical dissociation experiments, higher concentrations of urea were required to inhibit Ms-Lon's catalytic activities after an addition of alpha-casein. Analytical ultracentrifugation was used to directly probe the effect of activators of peptidase activity on Ms-Lon self-association. Sedimentation velocity experiments reveal that Ms-Lon monomers are in a reversible equilibrium with oligomeric forms of the protein and that the self-association reaction is facilitated by Mg(2+) ions but not by AMP-PNP or ATP gamma S. NaCl at 100 mM facilitates oligomerization and stimulates peptidase activity at suboptimal concentrations of MgCl(2). Sedimentation equilibrium analysis shows that Ms-Lon associates to a hexamer at 50 mM Tris and 10 mM MgCl(2), at pH 8.0 and 20 degrees C, and that the assembly reaction is Mg(2+) dependent; the mole fraction of hexamer decreases with decreasing MgCl(2) to undetectable levels in 10 mM EDTA. The analysis of experiments conducted at a series of initial protein and MgCl(2) concentrations yields two assembly models: dimer <--> tetramer <--> hexamer and timer <--> hexamer, equally consistent with the data. Limited trypsin digestion, CD, and tryptophan fluorescence suggest only minor changes in secondary and tertiary structure upon Mg(2+)-linked oligomerization. These results show that activation of Ms-Lon peptidase activity requires oligomerization and that Ms-Lon self-association reaction is facilitated by its activator, Mg(2+), and stimulator, unfolded protein.     

Roles of Mg2+ in the mechanism of formation and dissociation of open complexes between Escherichia coli RNA polymerase and the lambda PR promoter: kinetic evidence for a second open complex requiring Mg2+.

Comparative studies of the effects of Mg2+ vs Na+ and of acetate (OAc-) vs Cl- on the kinetics of formation and dissociation of E. coli RNA polymerase (E sigma 70)-lambda PR promoter open complexes have been used to probe the mechanism of this interaction. Composite second-order association rate constants ka and first-order dissociation rate constants kd, and their power dependences on salt concentration SKa (SKa identical to d log ka/d log [salt]) and Skd (Skd identical to d log kd/d log [salt]), were determined in MgCl2 and NaOAc to compare with the results of Roe and Record (1985) in NaCl. Replacement of NaCl by MgCl2 reduces the magnitude of Ska 2-fold (Ska = -11.9 +/- 1.1 in NaCl; Ska = -5.2 +/- 0.3 in MgCl2) and (by extrapolation) drastically reduces the magnitude of ka at any specified salt concentration (e.g., approximately 10(6)-fold at 0.2 M). Replacement of NaCl by NaOAc does not significantly affect Ska (Ska = -12.0 +/- 0.7 in NaOAc) and (by extrapolation) increased ka by approximately 80-fold at any fixed [Na+]. In the absence of Mg2+, replacement of NaCl by NaOAc is found to increase the half-life of the open complex by approximately 560-fold at fixed [Na+] without affecting Skd [Skd = 7.6 +/- 0.1 in NaOAc; in NaCl, Skd = 7.7 +/- 0.2 (Roe & Record, 1985)]. Replacement of NaCl by MgCl2 drastically reduces both Skd and the half-life of the open complex at any salt concentration below approximately 0.2 M. Strikingly, Skd = 0.4 +/- 0.1 in MgCl2, indicating that the net uptake of Mg2+ ions in the kinetically significant steps in dissociation of the open complex is much smaller than that expected by analogy with the uptake of approximately 8 Na+ ions in the corresponding steps in NaCl. In NaCl/MgCl2 mixtures, at a constant [NaCl] in the range 0.1-0.2 M, initial addition of MgCl2 (0.5 mM less than or equal to [MgCl2] less than or equal to 1 mM) increases the half-life of the open complex; further addition of MgCl2 causes the half-life to decrease, though the effect of [MgCl2] on kd is always less than that predicted by a simple competitive model. The observed effects of MgCl2 on Skd and kd differ profoundly from those expected from the behavior of kd and Skd in NaCl and NaOAc and indicate that the role of Mg2+ in dissociation is not merely that of a nonspecific divalent competitor with RNAP for interactions with DNA phosphates and of a DNA helix-stabilizer, both of which should cause kd to increase monotonically with increasing [Mg2+].(ABSTRACT TRUNCATED AT 400 WORDS)     

RNA tertiary interactions mediate native collapse of a bacterial group I ribozyme

Large RNAs collapse into compact intermediates in the presence of counterions before folding to the native state. We previously found that collapse of a bacterial group I ribozyme correlates with the formation of helices within the ribozyme core, but occurs at Mg2+ concentrations too low to support stable tertiary structure and catalytic activity. Here, using small-angle X-ray scattering, we show that Mg2+-induced collapse is a cooperative folding transition that can be fit by a two-state model. The Mg2+ dependence of collapse is similar to the Mg2+ dependence of helix assembly measured by partial ribonuclease T1 digestion and of an unfolding transition measured by UV hypochromicity. The correspondence between multiple probes of RNA structure further supports a two-state model. A mutation that disrupts tertiary contacts between the L9 tetraloop and its helical receptor destabilized the compact state by 0.8 kcal/mol, while mutations in the central triplex were less destabilizing. These results show that native tertiary interactions stabilize the compact folding intermediates under conditions in which the RNA backbone remains accessible to solvent.     

Mechanistic characterization of the HDV genomic ribozyme: classifying the catalytic and structural metal ion sites within a multichannel reaction mechanism

Prior studies of the metal ion dependence of the self-cleavage reaction of the HDV genomic ribozyme led to a mechanistic framework in which the ribozyme can self-cleave by multiple Mg2+ ion-independent and -dependent channels [Nakano et al. (2001) Biochemistry 40, 12022]. In particular, channel 2 involves cleavage in the presence of a structural Mg2+ ion without participation of a catalytic divalent metal ion, while channel 3 involves both structural and catalytic Mg2+ ions. In the present study, experiments were performed to probe the nature of the various divalent ion sites and any specificity for Mg2+. A series of alkaline earth metal ions was tested for the ability to catalyze self-cleavage of the ribozyme under conditions that favor either channel 2 or channel 3. Under conditions that populate primarily channel 3, nearly identical K(d)s were obtained for Mg2+, Ca2+, Ba2+, and Sr2+, with a slight discrimination against Ca2+. In contrast, under conditions that populate primarily channel 2, tighter binding was observed as ion size decreases. Moreover, [Co(NH3)6]3+ was found to be a strong competitive inhibitor of Mg2+ for channel 3 but not for channel 2. The thermal unfolding of the cleaved ribozyme was also examined, and two transitions were found. Urea-dependent studies gave m-values that allowed the lower temperature transition to be assigned to tertiary structure unfolding. The effects of high concentrations of Na+ on the melting temperature for RNA unfolding and the reaction rate revealed ion binding to the folded RNA, with significant competition of Na+ (Hill coefficient of 1.5-1.7) for a structural Mg2+ ion and an unusually high intrinsic affinity of the structural ion for the RNA. Taken together, these data support the existence of two different classes of metal ion sites on the ribozyme: a structural site that is inner sphere with a major electrostatic component and a preference for Mg2+, and a weak catalytic site that is outer sphere with little preference for a particular divalent ion.     

Extraordinary rates of transition metal ion-mediated ribozyme catalysis

In pre-steady-state, fast-quench kinetic analysis, the tertiary-stabilized hammerhead ribozyme "RzB" cleaves its substrate RNA with maximal measured k (obs) values of approximately 3000 min(-1) in 1 mM Mn(2+) and approximately 780 min(-1) in 1 mM Mg(2+) at 37 degrees C (pH 7.4). Apparent pKa for the catalytic general base is approximately 7.8-8.5, independent of the corresponding metal hydrate pKa, suggesting potential involvement of a nucleobase as general base as suggested previously from nucleobase substitution studies. The pH-rate profile is bell-shaped for Cd(2+), for which the general catalytic acid has a pKa of 7.3 +/- 0.1. Simulations of the pH-rate relation suggest a pKa for the general catalytic acid to be approximately 9.5 in Mn(2+) and >9.5 in Mg(2+). The acid pKa's follow the trend in the pKa of the hydrated metal ions but are displaced by approximately 1-2 pH units in the presence of Cd(2+) and Mn(2+). One possible explanation for this trend is direct metal ion coordination with a nucleobase, which then acts as general acid.     

Stability and cooperativity of individual tertiary contacts in RNA revealed through chemical denaturation

For proteins, understanding tertiary interactions involved in local versus global unfolding has become increasingly important for understanding the nature of the native state ensemble, the mechanisms of unfolding, and the stability of both the native and intermediate states in folding. In this work we have addressed related questions with respect to RNA structure by combining chemical denaturation and hydroxyl radical footprinting methods. We have determined unfolding isotherms for each of 26 discrete sites of protection located throughout the Tetrahymena thermophila group I ribozyme. The cooperativity of folding, m-value, and the free energy, DeltaG degrees N-U, associated with formation of each tertiary contact was determined by analysis of the isotherms. The DeltaG degrees N-U values measured in this study vary from 1.7 +/- 0.2 to 7. 6 +/- 1.2 kcal mol-1. Thus, the stability of these discrete tertiary contacts vary by almost 104. In addition, an intradomain contact and three interdomain contacts show high cooperativity (m-values of 1.1 +/- 0.2 to 1.7 +/- 0.3 kcal mol-1 M-1) indicating that these contacts exhibit global cooperatively in their folding behavior. This new approach to examining RNA stability provides an exciting comparison to our understanding of protein structure and folding mechanisms.     

An alternative route for the folding of large RNAs: apparent two-state folding by a group II intron ribozyme

Despite a growing literature on the folding of RNA, our understanding of tertiary folding in large RNAs derives from studies on a small set of molecular examples, with primary focus on group I introns and RNase P RNA. To broaden the scope of RNA folding models and to better understand group II intron function, we have examined the tertiary folding of a ribozyme (D135) that is derived from the self-splicing ai5gamma intron from yeast mitochondria. The D135 ribozyme folds homogeneously and cooperatively into a compact, well-defined tertiary structure that includes all regions critical for active-site organization and substrate recognition. When D135 was treated with increasing concentrations of Mg(2+) and then subjected to hydroxyl radical footprinting, similar Mg(2+) dependencies were seen for internalization of all regions of the molecule, suggesting a highly cooperative folding behavior. In this work, we show that global folding and compaction of the molecule have the same magnesium dependence as the local folding previously observed. Furthermore, urea denaturation studies indicate highly cooperative unfolding of the ribozyme that is governed by thermodynamic parameters similar to those for forward folding. In fact, D135 folds homogeneously and cooperatively from the unfolded state to its native, active structure, thereby demonstrating functional reversibility in RNA folding. Taken together, the data are consistent with two-state folding of the D135 ribozyme, which is surprising given the size and multi-domain structure of the RNA. The findings establish that the accumulation of stable intermediates prior to formation of the native state is not a universal feature of RNA folding and that there is an alternative paradigm in which the folding landscape is relatively smooth, lacking rugged features that obstruct folding to the native state.     

Disparate efficacy of tobramycin on Ca(2+)-, Mg(2+)-, and HEPES-treated Pseudomonas aeruginosa biofilms.

Mucoid exopolysaccharide (MEP) obtained from Pseudomonas aeruginosa 579 was suspended in 10 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) pH 7.2 containing 0.1-10.0 mM of CaCl2.2H2O or MgCl2.4H2O. MEP treated with HEPES or < 5.0 mM of the Ca2+ or Mg2+ salts remained soluble and bound tobramycin in an equilibrium dialysis bioassay. MEP treated with 5.0 or 10.0 mM of the Ca2+ or Mg2+ salts did not bind tobramycin. Five and 10 mM Ca(2+)-treated MEP precipitated but Mg(2+)-treated MEP did not. Pseudomonas aeruginosa 579 biofilms formed using a defined growth medium having < 1 mM Ca2+ or Mg2+ were treated for 1 h with 10 mM HEPES +/- 5.0 mM CaCl2.2H2O or MgCl2.4H2O, prior to an 8-h exposure to HEPES, or the defined growth medium, +/- 125 micrograms/mL of tobramycin. The tobramycin kill kinetics for the HEPES-, Mg(2+)-, and Ca(2+)-treated biofilms were similar and gradual from T = 0-6 h. The viability of the HEPES- and Mg(2+)-treated populations declined sharply (from 6 to 8 h). Bacteria dispersed from the MEP in control biofilms at 0 and 8 h did not grow in the presence of 7.81 micrograms/mL of tobramycin. Thus, binding of tobramycin of P. aeruginosa 579 MEP may not be as influential to the impediment of tobramycin diffusion as is the steric hindrance imposed by the Ca2+ condensation of the polymer.