Time-resolved synchrotron X-ray "footprinting", a new approach to the study of nucleic acid structure and function: application to protein-DNA interactions and RNA folding

Mg2+-dependent compaction and folding of yeast tRNAPhe and the catalytic domain of the B. subtilis RNase P RNA determined by small-angle X-ray scattering

We apply synchrotron-based small-angle X-ray scattering to investigate the relationship between compaction, metal binding, and structure formation of two RNAs at 37 degrees C: the 76 nucleotide yeast tRNA(Phe) and the 255 nucleotide catalytic domain of the Bacillus subtilis RNase P RNA. For both RNAs, this method provides direct evidence for the population of a distinct folding intermediate. The relative compaction between the intermediate and the native state does not correlate with the size of the RNA but does correlate well with the amount of surface burial as quantified previously by the urea-dependent m-value. The total compaction process can be described in two major stages. Starting from a completely unfolded state (4-8 M urea, no Mg(2+)), the major amount of compaction occurs upon the dilution of the denaturant and the addition of micromolar amounts of Mg(2+) to form the intermediate. The native state forms in a single transition from the intermediate state upon cooperative binding of three to four Mg(2+) ions. The characterization of this intermediate by small-angle X-ray scattering lends strong support for the cooperative Mg(2+)-binding model to describe the stability of a tertiary 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.

Studies of RNA cleavage by photolysis of N-hydroxypyridine-2(1H)-thione. A new photochemical footprinting method

N-Hydroxypyridine-2(1H)-thione (N-HPT) has been studied as a photochemical source of hydroxyl radicals for use in photoinitiated nucleic acid footprinting experiments. Steady-state photolysis of dilute aqueous solutions of N-HPT at 350 nm in the presence of a 385 nucleotide (32)P-labeled RNA, the Tetrahymena L-21 ribozyme, resulted in cleavage of the RNA at nucleotide resolution. No cleavage of the RNA occurred in the absence of light or in the absence of N-HPT. Photolysis of the analogous pyridine lacking the N-hydroxyl group did not result in detectable amounts of RNA cleavage. The addition of RNA to preirradiated solutions of N-HPT gave no apparent RNA cleavage products, suggesting that the photoproducts of N-HPT do not result in RNA modification. Cleavage of RNA, upon photolysis in the presence of N-HPT, occurred in a sequence-independent fashion with double-stranded RNA being cleaved as efficiently as single-stranded RNA. Based on these observations, we conclude that photochemically generated diffusable hydroxyl radicals are responsible for the RNA cleavage. Experiments involving the photolysis of N-HPT in the presence of the Tetrahymena ribozyme and magnesium showed a magnesium-dependent protection from RNA cleavage due to formation of a folded RNA tertiary structure. The locations and amount of protection were identical to those observed in footprinting experiments performed with other hydroxyl radical sources. The presence of N-HPT had no effect on either the rate of folding or the catalytic activity of the folded RNA, indicating that this reagent does not disrupt RNA tertiary structure or otherwise affect activity. Thus, N-HPT is established as a new reagent for use in photoinitiated RNA footprinting experiments.

Characterization of the Tetrahymena ribozyme folding pathway using the kinetic footprinting reagent peroxynitrous acid

Large RNAs fold into complex structures which determine their biological activities. A full understanding of both RNA structure and dynamics will include the description of the pathways by which these structures are formed. Kinetic footprinting [Sclavi, B., et al. (1997) J. Mol. Biol. 266, 144-159] has been shown to be a powerful method for the study of dynamic processes involving RNA. Here we describe the use of a readily available reagent, peroxynitrous acid, as a kinetic footprinting tool for the study of RNA folding. Hydroxyl radicals generated from this reagent were used to footprint the Tetrahymena ribozyme during its magnesium-dependent folding-in agreement with synchroton X-ray footprinting [Sclavi, B., et al. (1998) Science 279, 1940-1943] and oligonucleotide/hybridization cleavage experiments [Zarrinkar, P. P., and Williamson, J. R. (1994) Science 265, 918-924], this work suggests an ordered, hierarchical folding pathway for the ribozyme. Several slow steps in the folding pathway were observed in the peroxynitrous acid footprinting, but none of these corresponded to the rate-determining step of folding. This suggests that the formation of the global, protected structure is followed by one or more slow local rearrangements to yield the final active structure. These studies illustrate the utility of peroxynitrous acid as a reagent for the elucidation of RNA folding pathways and the study of RNA dynamics.

Applicability of urea in the thermodynamic analysis of secondary and tertiary RNA folding

The equilibrium folding of a series of self-complementary RNA duplexes and the unmodified yeast tRNA(Phe) is studied as a function of urea and Mg(2+) concentration with optical spectroscopies and chemical modification under isothermal conditions. Via application of standard methodologies from protein folding, the folding free energy and its dependence on urea concentration, the m value, are determined. The free energies of the RNA duplexes obtained from the urea titrations are in good agreement with those calculated from thermal melting studies [Freier, S. I., et al. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 9373]. The m value correlates with the length of the RNA duplex and is not sensitive to ionic conditions and temperature. The folding of the unmodified yeast tRNA(Phe) can be described by two Mg(2+)-dependent transitions, the second of which corresponds to the formation of the native tertiary structure as confirmed by hydroxyl radical protection and partial nuclease digestion. Both transitions are sensitive to urea and have m values of 0.94 and 1.70 kcal mol(-)(1) M(-)(1), respectively. Although the precise chemical basis of urea denaturation of RNA is uncertain, the m values for the duplexes and tRNA(Phe) are proportional to the amount of the surface area buried in the folding transition. This proportionality, 0.099 cal mol(-)(1) M(-)(1) A(-)(2), is very similar to that observed for proteins, 0.11 cal mol(-)(1) M(-)(1) A(-)(2) [Myers, J., Pace, N., and Scholtz, M. (1995) Protein Sci. 4, 2138]. These results indicate that urea titration can be used to measure both the free energy and the magnitude of an RNA folding transition.

An optimal Mg(2+) concentration for kinetic folding of the tetrahymena ribozyme

Divalent metal ions, such as Mg(2+), are generally required for tertiary structure formation in RNA. Although the role of Mg(2+) binding in RNA-folding equilibria has been studied extensively, little is known about the role of Mg(2+) in RNA-folding kinetics. In this paper, we explore the effect of Mg(2+) on the rate-limiting step in the kinetic folding pathway of the Tetrahymena ribozyme. Analysis of these data reveals the presence of a Mg(2+)-stabilized kinetic trap that slows folding at higher Mg(2+) concentrations. Thus, the Tetrahymena ribozyme folds with an optimal rate at 2 mM Mg(2+), just above the concentration required for stable structure formation. These results suggest that thermodynamic and kinetic folding of RNA are cooptimized at a Mg(2+) concentration that is sufficient to stabilize the folded form but low enough to avoid kinetic traps and misfolding.

A survey of the sequence-specific interaction of damaging agents with DNA: emphasis on antitumor agents

This article reviews the literature concerning the sequence specificity of DNA-damaging agents. DNA-damaging agents are widely used in cancer chemotherapy. It is important to understand fully the determinants of DNA sequence specificity so that more effective DNA-damaging agents can be developed as antitumor drugs. There are five main methods of DNA sequence specificity analysis: cleavage of end-labeled fragments, linear amplification with Taq DNA polymerase, ligation-mediated polymerase chain reaction (PCR), single-strand ligation PCR, and footprinting. The DNA sequence specificity in purified DNA and in intact mammalian cells is reviewed for several classes of DNA-damaging agent. These include agents that form covalent adducts with DNA, free radical generators, topoisomerase inhibitors, intercalators and minor groove binders, enzymes, and electromagnetic radiation. The main sites of adduct formation are at the N-7 of guanine in the major groove of DNA and the N-3 of adenine in the minor groove, whereas free radical generators abstract hydrogen from the deoxyribose sugar and topoisomerase inhibitors cause enzyme-DNA cross-links to form. Several issues involved in the determination of the DNA sequence specificity are discussed. The future directions of the field, with respect to cancer chemotherapy, are also examined.

Millisecond radiolytic modification of peptides by synchrotron X-rays identified by mass spectrometry

Radiolysis of peptide and protein solutions with high-energy X-ray beams induces stable, covalent modifications of amino acid residues that are useful for synchrotron protein footprinting. A series of 5-14 amino acid residue peptides of varied sequences were selected to study their synchrotron radiolysis chemistry. Radiolyzed peptide products were detected within 10 ms of exposure to a white light synchrotron X-ray beam. Mass spectrometry techniques were used to characterize radiolytic modification to amino acids cysteine (Cys), methionine (Met), phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), proline (Pro), histidine (His), and leucine (Leu). A reactivity order of Cys, Met >> Phe, Tyr, > Trp > Pro > His, Leu was determined under aerobic reaction conditions from MS/MS analysis of the radiolyzed peptide products. Radiolysis of peptides in 18O-labeled water under aerobic conditions revealed that oxygenated radical species from air and water both contribute to the modification of amino acid side chains. Cysteine and methionine side chains reacted with hydroxyl radicals generated from radiolysis of water as well as molecular oxygen. Phenylalanine and tyrosine residues were modified predominantly by hydroxyl radicals, and the source of modification of proline was exclusively through molecular oxygen.

New pathways in folding of the Tetrahymena group I RNA enzyme

Previous studies have shown that the earliest detectable step in folding of the Tetrahymena ribozyme is tertiary structure formation of the peripheral element P5abc. This, along with other results, has suggested that P5abc may serve as a scaffold upon which additional tertiary structure is built. Herein we use the onset of oligonucleotide cleavage activity as a readout for native state formation and investigate the effect of P5abc on the rate of folding to the native structure. Despite the early folding of P5abc, its removal to give the E delta P5abc variant decreases the rate of attainment of an active structure less than fivefold (20-100 mM Mg2+, 15-50 degrees C). Furthermore, P5abc added in trans is able to bind the folded E delta P5abc ribozyme and promote oligonucleotide cleavage at least tenfold more rapidly than folding of the wild-type ribozyme, indicating that E delta P5abc does not have to first unfold before productively binding P5abc to form the true native state. This suggests that a state with the overall tertiary structure formed but with P5abc unfolded represents a viable on-pathway intermediate for the wild-type ribozyme. These results provide strong evidence for the existence of two pathways to the native state: in one pathway P5abc forms tertiary structure first, and in another it forms late. The pathway in which P5abc forms first is favored because P5abc can fold quickly and because its tertiary structure is stable in the absence of additional structured elements, not because P5abc formation is required for subsequent folding steps. In the course of these experiments, we also found that most of the ribozyme population does not reach the native state directly under standard conditions in vitro, but instead forms an inactive structure that is stable for hours. Finally, the fraction that does fold to the native state folds with a single rate constant of 1 min-1, suggesting that there are no significantly populated "fast-track" pathways that reach the native state directly by avoiding slow folding steps.

The reaction mechanism of ribonuclease II and its interaction with nucleic acid secondary structures

Ribonuclease II is a processive 3'- to 5'-exoribonuclease in Escherichia coli with two binding sites: a catalytic site associated with the first few 3'-nucleotides and an anchor site binding nucleotides approximately 15 to 25 from the 3'-end. When RNase II degrades single-stranded helical poly(C), the enzyme-substrate complex dissociates at discrete intervals of 12 nucleotides. RNase II stalled at the last rC of single-stranded 3'-(rC)(n)(dC)(m) oligonucleotides. The more residues released, the faster the stalled complex dissociated and the less it inhibited RNase II activity, i.e. the enzyme-substrate association weakened progressively. Using phosphodiesterase I (PDE I) as a probe, a method was developed to identify cytidine residues in (32)P-oligonucleotides interacting with a protein. PAGE bands corresponding to nucleotides 1-6 from the 3'-end were consistent with interaction at the catalytic site, and following a gap, bands approximately 15 to 25 from the 3'-end, with anchor site association. Both 3' and 5' binding were necessary to maintain the complex. Of most significance, the original anchor site nucleotides remained fixed at the anchor site while the 3'-end was pulled, or threaded, through the catalytic site, i.e. the substrate did not 'slide' through the enzyme. DNA oligonucleotides with double-stranded stem-loops were good competitive inhibitors of RNase II. A 3'-single-stranded arm was essential, while optimal binding required both 5'- and 3'-arms. PDE I probing indicated that the nucleotides at the anchor site were specified by the spatial distance from the catalytic site, and on only one of the duplex strands. When degradation of a structured RNA paused or stopped, the RNase II-product commenced cycles of dissociation-reassociation. Duplex strand binding by RNase II made complex DNA or RNA structures accessible to degradation by other nucleases and further verified the PDE I footprinting method.

RNA folds: insights from recent crystal structures

An RNA fold is the result of packing together two or more coaxial helical stacks. To date, four RNA folds have been determined at near-atomic resolution by X-ray crystallography: transfer RNA, the hammerhead ribozyme, the P4-P6 domain of the Tetrahymena group I intron, and the hepatitis delta virus ribozyme. All four folds result in RNAs that are considerably more compact than isolated A-form duplexes. These structures illustrate, to varying degrees, three modes of fold stabilization: association of complementary molecular surfaces, stabilization of close RNA packing by binding of cations, and stabilization through pseudoknotting.

Exposing the kinetic traps in RNA folding

Large ribozymes fold on a 'glacial' timescale compared to the folding of their protein counterparts. The sluggish folding exhibited by large RNAs results from the formation of kinetically trapped, misfolded intermediates, which are nonessential features of the folding mechanism. Newly developed mutant ribozymes that avoid kinetic traps should facilitate the study of the RNA folding problem.

Following the folding of RNA with time-resolved synchrotron X-ray footprinting

The rapid mixing synchrotron X-ray footprinting technique described in this article allows nucleic acid folding and ligand binding reactions to be followed on a millisecond time resolution with single nucleotide resolution. In principle, the change in .OH protection of every nucleotide in a nucleic acid hundreds of nucleotides long can be monitored separately. In addition, a wide range of solution conditions are compatible with the radiolytic generation of .OH. These characteristics of synchrotron X-ray footprinting create opportunities for conducting thermodynamic and kinetic studies of nucleic acids that are both comprehensive and detailed. Kinetic footprinting studies of a number of systems have been initiated by the Center for Synchrotron Biosciences using this technique.

Fast folding mutants of the Tetrahymena group I ribozyme reveal a rugged folding energy landscape

A model for the kinetic folding pathway of the Tetrahymena ribozyme has been proposed where the two main structural domains, P4-P6 and P3-P7, form in a hierarchical manner with P4-P6 forming first and P3-P7 folding on the minute timescale. Recent studies in our laboratory identified a set of mutations that accelerate P3-P7 formation, and all of these mutations appear to destabilize a native-like kinetic trap. To better understand the microscopic details of this slow step in the Tetrahymena ribozyme folding pathway, we have used a previously developed kinetic oligonucleotide hybridization assay to characterize the folding of several fast folding mutants. A comparison of the temperature dependence of P3-P7 folding between the mutant and wild-type ribozymes demonstrates that a majority of the mutations act by decreasing the activation enthalpy required to reach the transition state and supports the existence of the native-like kinetic trap. In several mutant ribozymes, P3-P7 folds with biphasic kinetics, indicating that only a subpopulation of molecules can evade the kinetic barrier. The rate of folding of the wild-type increases in the presence of urea, while for the mutants urea merely shifts the distribution between the two folding populations. Small structural changes or changes in solvent can accelerate folding, but these changes lead to complex folding behavior, and do not give rise to rapid two-state folding transitions. These results support the recent view of folding as an ensemble of molecules traversing a rugged energy landscape to reach the lowest energy state.

Thermodynamic stability of the P4-P6 domain RNA tertiary structure measured by temperature gradient gel electrophoresis

The P4-P6 domain RNA from the Tetrahymena self-splicing group I intron is an independent unit of tertiary structure that, in the kinetic folding pathway, folds before the rest of the intron and then stabilizes the remainder of the intron's tertiary structure. We have employed temperature gradient gel electrophoresis (TGGE) to examine the unfolding of the tertiary structure of P4-P6. In 0.9 mM Mg2+, the global tertiary fold of the molecule has a melting temperature of approximately 40 degreesC and is completely unfolded by 60 degreesC. Calculated thermodynamic parameters for folding of P4-P6 are DeltaH degrees' = -28 +/- 3 kcal/mol and DeltaS degrees' = -91 +/- 8 eu under these conditions. Chemical probing of the P4-P6 tertiary structure using dimethyl sulfate and CMCT confirms that these TGGE experiments monitor the unfolding of the global tertiary fold of the domain and that the secondary structure is largely unaffected over this temperature range. Thus, unlike the entropically driven P1 docking and guanosine binding steps of Tetrahymenagroup I intron self-splicing, which have positive or zero DeltaH terms, P4-P6 tertiary structure formation is stabilized by a negative DeltaH term. This implies that enthalpically favorable hydrogen bond formation, nucleotide base stacking, and/or binding of Mg2+ within the folded structure are responsible for stabilizing the P4-P6 domain.

RNA structure

New information concerning RNA structure is accumulating at an ever increasing rate-from short helices with mismatched bases of 5S rRNA and complex RNA aptamers. The importance of recurring structural motifs, ion binding, and the kinetics and energetics of folding in RNA structure and function is now being recognized and addressed.

The parallel universe of RNA folding

How do large RNA molecules find their active conformations among a universe of possible structures? Two recent studies reveal that RNA folding is a rapid and ordered process, with surprising similarities to protein folding mechanisms.

Tertiary structure formation in the hairpin ribozyme monitored by fluorescence resonance energy transfer

The complex formed by the hairpin ribozyme and its substrate consists of two independently folding domains which interact to form a catalytic structure. Fluorescence resonance energy transfer methods permit us to study reversible transitions of the complex between open and closed forms. Results indicate that docking of the domains is required for both the cleavage and ligation reactions. Docking is rate-limiting for ligation (2 min-1) but not for cleavage, where docking (0.5 min-1) precedes a rate-limiting conformational transition or slow-reaction chemistry. Strikingly, most modifications to the RNA (such as a G+1A mutation in the substrate) or reaction conditions (such as omission of divalent metal ion cofactors) which inhibit catalysis do so by preventing docking. This demonstrates directly that mutations and modifications which inhibit a step following substrate binding are not necessarily involved in catalysis. An improved kinetic description of the catalytic cycle is derived, including specific structural transitions.

RNA folding at millisecond intervals by synchrotron hydroxyl radical footprinting

Radiolysis of water with a synchrotron x-ray beam permits the hydroxyl radical-accessible surface of an RNA to be mapped with nucleotide resolution in 10 milliseconds. Application of this method to folding of the Tetrahymena ribozyme revealed that the most stable domain of the tertiary structure, P4-P6, formed cooperatively within 3 seconds. Exterior helices became protected from hydroxyl radicals in 10 seconds, whereas the catalytic center required minutes to be completely folded. The results show that rapid collapse to a partially disordered state is followed by a slow search for the active structure.

Kinetic intermediates trapped by native interactions in RNA folding

In the magnesium ion-dependent folding of the Tetrahymena ribozyme, a kinetic intermediate accumulates in which the P4-P6 domain is formed, but the P3-P7 domain is not. The kinetic barriers to P3-P7 formation were investigated with the use of in vitro selection to identify mutant RNA molecules in which the folding rate of the P3-P7 domain was increased. The critical mutations disrupt native tertiary interactions within the P4-P6 domain and increase the rate of P3-P7 formation by destabilizing a kinetically trapped intermediate. Hence, kinetic traps stabilized by native interactions, and not simply by mispaired nonnative structures, can present a substantial barrier to RNA folding.

Emerging themes in RNA folding

RNAs, like proteins, readily form specific structures adapted for ligand binding and catalysis. Since they are composed of completely different chemical building blocks, however, RNAs and proteins necessarily use distinct strategies to assemble complex architectures. While burial of hydrophobic residues drives protein folding, the hydrophobic effect in RNA contributes primarily to the formation of secondary structure. To form tertiary structure, RNA must overcome electrostatic repulsions from the phosphate backbone. How do negatively charged double helices pack together to produce catalytic centers and ligand binding surfaces? Here, we review our understanding of the principles that underlie RNA folding based on the structural information currently available.

Intermediates and kinetic traps in the folding of a large ribozyme revealed by circular dichroism and UV absorbance spectroscopies and catalytic activity

The folding thermodynamics and kinetics for the ribozyme from Bacillus subtilis RNase P are analyzed using circular dichroism and UV absorbance spectroscopies and catalytic activity. At 37 degrees C, the addition of Mg2+ (Kd approximately 50 microM) to the unfolded state produces an intermediate state within 1 ms which contains a comparable amount of secondary structure as the native ribozyme. The subsequent transition to the native state (Kd[Mg] approximately 0.8 mM, Hill coefficient approximately 3.5) has a half-life of hundreds of seconds as measured by circular dichroism at 278 nm and by a ribozyme activity assay. Surprisingly, the formation of the native structure is accelerated strongly by the addition of a denaturant; approximately 30-fold at 4.5 M urea. Thus, the rate-limiting step entails the disruption of a considerable number of interactions. The folding of this, and presumably other large RNAs, is slow due to the structural rearrangement of kinetically trapped species. Taken together with previous submillisecond relaxation kinetics of tRNA tertiary structure, we suggest that error-free RNA folding can be on the order of milliseconds.

Folding of RNA involves parallel pathways

Folding kinetics of large RNAs are just beginning to be investigated. We show that the Tetrahymena self-splicing RNA partitions into a population that rapidly reaches the native state, and a slowly folding population that is trapped in metastable misfolded structures. Transitions from the misfolded structures to the native state involve partial unfolding. The total yield of native RNA is increased by iterative annealing of the inactive population, and mildly denaturing conditions increase the rate of folding at physiological temperatures. These results provide the first evidence that an RNA can fold by multiple parallel paths.

Guanosine binds to the Tetrahymena ribozyme in more than one step, and its 2'-OH and the nonbridging pro-Sp phosphoryl oxygen at the cleavage site are required for productive docking

The dynamics of binding of various guanosine, or G, substrates to the Tetrahymena thermophila L-21 ScaI ribozyme have been investigated by fluorescence-detected stopped-flow experiments. Upon rapid mixing of various G substrates with a preformed complex of the ribozyme and the fluorescent 5' splice site analogue CCUCUepsilonA, fluorescence transients that provide rates for binding of G substrates and the rate-limiting step for transesterification are observed. The measured apparent bimolecular rate constant for binding of pG is 10(3) M-1 s-1, much slower than expected for diffusion. pG appears to bind to the preformed complex of the ribozyme and CCUCUepsilonA in at least two steps, a bimolecular step followed by at least one conformational change. This two-step binding of pG, involving a rapid pre-equilibrium, leads to the slow apparent rate constant for binding of pG. Furthermore, the 2'-OH of pG and of the 3' terminal G of the G substrate GUCG and the nonbridging pro-Sp phosphoryl oxygen atom at the site of phosphoryl transfer on CCUCUepsilonA appear to mediate formation of a properly conformed docked ternary complex of the G substrate, 5' splice site, and ribozyme which may represent an intermediate required for initiation of transesterification. It is possible that the 2'-OH of pG and this nonbridging pro-Sp phosphoryl oxygen interact, directly or indirectly, with one another.

Effects of Mg2+ and the 2' OH of guanosine on steps required for substrate binding and reactivity with the Tetrahymena ribozyme reveal several local folding transitions

Transient kinetic studies with fluorescence detection were used to determine the effects of Mg2+ concentration and the 2' OH group of guanosine monophosphate, prG, substrate on various steps in the transesterification reaction of prG with 5' pyrene-labeled oligonucleotides as catalyzed by the L-21 ScaI ribozyme. The effect of increasing Mg2+ from 5 to 10 mM on the rate constants of association and dissociation of 5' pyrene-labeled CUCUA at 15 degrees C was measured. The rate constant of association increases about 3-fold to (8.7 +/- 0.7) x 10(6) M-1 s-1 at 10 mM Mg2+. The rate constant for dissociation is 25 +/- 4 s-1 at 10 mM Mg2+, within experimental error of the rate constant of 17 +/- 5 s-1 measured at 5 mM Mg2+. This Mg2+ dependence is attributed to nonspecific binding of Mg2+ to the duplex helix. In the absence of prG, no docking of substrate is observed. The effect of Mg2+ concentration on rates for docking of 5' pyrene-labeled substrate, pyrCCUCUA, were measured at [Mg2+] >/= 2 mM and at temperatures </= 20 degrees C, where optical melting curves indicate global folding is complete. Thus the rates monitor local folding steps important for catalytic function. Three and possibly four local cooperative transitions were induced by Mg2+. The fastest fluorescence transient, which is associated with substrate docking, changes from a quenching to an enhancement between 2 and 4 mM Mg2+, and its observed rate constant at pH 6.5 and 7.5 is about 1 s-1, independent of [Mg2+] when 4 </= [Mg2+] </= 15 mM. The slowest fluorescence transient, which is apparently associated with transesterification, has an observed rate constant that continues to increase when [Mg2+] >/= 4 mM. In the presence of Ca2+, such that [Ca2+] + [Mg2+] = 15 mM, the observed rate constants of both transients are constant when 4 mM </= [Mg2+] </= 7 mM but double between 7 and 11 mM Mg2+. At pH 6.5 when 4 mM </= [Mg2+] </= 7 mM in the absence of Ca2+, there is also evidence for a third transient with a Mg2+-dependent observed rate constant that is intermediate between the observed rate constants of docking and transesterification. Thus these experiments reveal several separable, local folding transitions that are dependent on Mg2+ in a very cooperative manner and are important for function. When pdG is substituted for prG, no transesterification is observed, and fluorescence quenching is observed for 1-15 mM Mg2+. The switch from fluorescence enhancements with prG to quenching with pdG suggests the 2' OH of prG is important for proper positioning of substrate in the catalytic site.

Comparison of the DNA association kinetics of the Lac repressor tetramer, its dimeric mutant LacIadi, and the native dimeric Gal repressor

The rates of association of the tetrameric Lac repressor (LacI), dimeric LacIadi (a deletion mutant of LacI), and the native dimeric Gal repressor (GalR) to DNA restriction fragments containing a single specific site were investigated using a quench-flow DNase I "footprinting" technique. The dimeric proteins, LacIadi and GalR, and tetrameric LacI possess one and two DNA binding sites, respectively. The nanomolar protein concentrations used in these studies ensured that the state of oligomerization of each protein was predominantly either dimeric or tetrameric, respectively. The bimolecular association rate constants (ka) determined for the LacI tetramer exceed those of the dimeric proteins. The values of ka obtained for LacI, LacIadi, and GalR display different dependences on [KCl]. For LacIadi and GalR, they diminish as [KCl] increases from 25 mM to 200 mM, approaching rates predicted for three-dimensional diffusion. In contrast, the ka values determined for the tetrameric LacI remain constant up to 300 mM [KCl], the highest salt concentration that could be investigated by quench-flow footprinting. The enhanced rate of association of the tetramer relative to the dimeric proteins can be modeled by enhanced "sliding" (Berg, O. G., Winter, R. B., and von Hippel, P. H. (1981) Biochemistry 20, 6929-6948) of the LacI tetramer relative to the LacIadi dimer or a combination of enhanced sliding and the superimposition of "direct transfer" mediated by the bidentate DNA interactions of the tetramer.

A new method to monitor the rate of conformational transitions in RNA

Many RNAs need Mg2+to produce stable tertiary structures. Here we describe a simple method to measure the rate and activation parameters of tertiary structure unfolding that exploits this Mg2+dependence. Our approach is based on mixing an RNA solution with excess EDTA in a stopped-flow instrument equipped with an absorbance detector, under conditions of temperature and ionic strength where, after chelation of Mg2+, tertiary structure unfolds. We have demonstrated the utility of this method by studying phenylalanine-specific transfer RNA from yeast (tRNAPhe) because the unfolding rates and the corresponding activation parameters have been determined previously and provide a benchmark for our technique. We find that within error, our stopped-flow method reproduces both the rate and activation enthalpy for tertiary unfolding of yeast tRNAPhe measured previously by temperature-jump relaxation kinetics. Since many different RNAs require divalent magnesium for tertiary structure stabilization, this technique should be applicable to study the folding of other RNAs.

Quantitative nucleic acids footprinting: thermodynamic and kinetic approaches

Quantitative footprinting techniques allow a detailed analysis of the thermodynamic forces that characterize nucleic acid-ligand interactions and ligand-induced changes in nucleic acid structure by separately resolving the intrinsic and cooperative Gibbs free energy changes describing the reactions being investigated. A new implementation of the quantitative footprinting technique is the application of stopped-flow techniques to the study of kinetic reactions.