Thermodynamics of protein-RNA recognition in a highly conserved region of the large-subunit ribosomal RNA

Ribosomal Protein L11 Selectively Stabilizes a Tertiary Structure of the GTPase Center rRNA Domain

The GTPase Center (GAC) RNA domain in bacterial 23S rRNA is directly bound by ribosomal protein L11, and this complex is essential to ribosome function. Previous cocrystal structures of the 58-nucleotide GAC RNA bound to L11 revealed the intricate tertiary fold of the RNA domain, with one monovalent and several divalent ions located in specific sites within the structure. Here, we report a new crystal structure of the free GAC that is essentially identical to the L11-bound structure, which retains many common sites of divalent ion occupation. This new structure demonstrates that RNA alone folds into its tertiary structure with bound divalent ions. In solution, we find that this tertiary structure is not static, but rather is best described as an ensemble of states. While L11 protein cannot bind to the GAC until the RNA has adopted its tertiary structure, new experimental data show that L11 binds to Mg²⁺-dependent folded states, which we suggest lie along the folding pathway of the RNA. We propose that L11 stabilizes a specific GAC RNA tertiary state, corresponding to the crystal structure, and that this structure reflects the functionally critical conformation of the rRNA domain in the fully assembled ribosome.

Formation of Tertiary Interactions during rRNA GTPase Center Folding

The 60-nt GTPase center (GAC) of 23S rRNA has a phylogenetically conserved secondary structure with two hairpin loops and a 3-way junction. It folds into an intricate tertiary structure upon addition of Mg(2+) ions, which is stabilized by the L11 protein in cocrystal structures. Here, we monitor the kinetics of its tertiary folding and Mg(2+)-dependent intermediate states by observing selected nucleobases that contribute specific interactions to the GAC tertiary structure in the cocrystals. The fluorescent nucleobase 2-aminopurine replaced three individual adenines, two of which make long-range stacking interactions and one that also forms hydrogen bonds. Each site reveals a unique response to Mg(2+) addition and temperature, reflecting its environmental change from secondary to tertiary structure. Stopped-flow fluorescence experiments revealed that kinetics of tertiary structure formation upon addition of MgCl2 are also site specific, with local conformational changes occurring from 5 ms to 4s and with global folding from 1 to 5s. Site-specific substitution with (15)N-nucleobases allowed observation of stable hydrogen bond formation by NMR experiments. Equilibrium titration experiments indicate that a stable folding intermediate is present at stoichiometric concentrations of Mg(2+) and suggest that there are two initial sites of Mg(2+) ion association.

A shared RNA-binding site in the Pet54 protein is required for translational activation and group I intron splicing in yeast mitochondria

The Pet54p protein is an archetypical example of a dual functioning ('moonlighting') protein: it is required for translational activation of the COX3 mRNA and splicing of the aI5beta group I intron in the COX1 pre-mRNA in Saccharomyces cerevisiae mitochondria (mt). Genetic and biochemical analyses in yeast are consistent with Pet54p forming a complex with other translational activators that, in an unknown way, associates with the 5' untranslated leader (UTL) of COX3 mRNA. Likewise, genetic analysis suggests that Pet54p along with another distinct set of proteins facilitate splicing of the aI5beta intron, but the function of Pet54 is, also, obscure. In particular, it remains unknown whether Pet54p is a primary RNA-binding protein that specifically recognizes the 5' UTL and intron RNAs or whether its functional specificity is governed in other ways. Using recombinant protein, we show that Pet54p binds with high specificity and affinity to the aI5beta intron and facilitates exon ligation in vitro. In addition, Pet54p binds with similar affinity to the COX3 5' UTL RNA. Competition experiments show that the COX3 5'UTL and aI5beta intron RNAs bind to the same or overlapping surface on Pet54p. Delineation of the Pet54p-binding sites by RNA deletions and RNase footprinting show that Pet54p binds across a similar length sequence in both RNAs. Alignment of the sequences shows significant (56%) similarity and overlap between the binding sites. Given that its role in splicing is likely an acquired function, these data support a model in which Pet54p's splicing function may have resulted from a fortuitous association with the aI5beta intron. This association may have lead to the selection of Pet54p variants that increased the efficiency of aI5beta splicing and provided a possible means to coregulate COX1 and COX3 expression.

Stability of the 'L12 stalk' in ribosomes from mesophilic and (hyper)thermophilic Archaea and Bacteria

The ribosomal stalk complex, consisting of one molecule of L10 and four or six molecules of L12, is attached to 23S rRNA via protein L10. This complex forms the so-called ‘L12 stalk’ on the 50S ribosomal subunit. Ribosomal protein L11 binds to the same region of 23S rRNA and is located at the base of the ‘L12 stalk’. The ‘L12 stalk’ plays a key role in the interaction of the ribosome with translation factors. In this study stalk complexes from mesophilic and (hyper)thermophilic species of the archaeal genus Methanococcus and from the Archaeon Sulfolobus solfataricus, as well as from the Bacteria Escherichia coli, Geobacillus stearothermophilus and Thermus thermophilus, were overproduced in E.coli and purified under non-denaturing conditions. Using filter-binding assays the affinities of the archaeal and bacterial complexes to their specific 23S rRNA target site were analyzed at different pH, ionic strength and temperature. Affinities of both archaeal and bacterial complexes for 23S rRNA vary by more than two orders of magnitude, correlating very well with the growth temperatures of the organisms. A cooperative effect of binding to 23S rRNA of protein L11 and the L10/L12₄ complex from mesophilic and thermophilic Archaea was shown to be temperature-dependent.

Characterization of the metal ion binding site in the anti-terminator protein, HutP, of Bacillus subtilis

HutP is an RNA-binding protein that regulates the expression of the histidine utilization (hut) operon in Bacillus subtilis, by binding to cis-acting regulatory sequences on hut mRNA. It requires L-histidine and an Mg²⁺ ion for binding to the specific sequence within the hut mRNA. In the present study, we show that several divalent cations can mediate the HutP–RNA interactions. The best divalent cations were Mn²⁺, Zn²⁺ and Cd²⁺, followed by Mg²⁺, Co²⁺ and Ni²⁺, while Cu²⁺, Yb²⁺ and Hg²⁺ were ineffective. In the HutP–RNA interactions, divalent cations cannot be replaced by monovalent cations, suggesting that a divalent metal ion is required for mediating the protein–RNA interactions. To clarify their importance, we have crystallized HutP in the presence of three different metal ions (Mg²⁺, Mn²⁺ and Ba²⁺), which revealed the importance of the metal ion binding site. Furthermore, these analyses clearly demonstrated how the metal ions cause the structural rearrangements that are required for the hut mRNA recognition.

Interactions of the N-terminal domain of ribosomal protein L11 with thiostrepton and rRNA

Ribosomal protein L11 has two domains: the C-terminal domain (L11-C76) binds rRNA, whereas the N-terminal domain (L11-NTD) may variously interact with elongation factor G, the antibiotic thiostrepton, and rRNA. To begin to quantitate these interactions, L11 from Bacillus stearothermophilus has been overexpressed and its properties compared with those of L11-C76 alone in a fluorescence assay for protein-rRNA binding. The assay relies on 2'-amino-butyryl-pyrene-uridine incorporated in a 58-nucleotide rRNA fragment, which gives approximately 15-fold enhancement when L11 or L11-C76 is bound. Although the pyrene tag weakens protein binding, unbiased protein-RNA association constants were obtained in competition experiments with untagged RNA. It was found that (i) intact B. stearothermophilus L11 binds rRNA with K approximately 1.2 x 10(9) m(-1) in buffers with 0.2 m KCl, about 100-fold tighter than Escherichia coli L11; (ii) the N-terminal domain makes a small, salt-dependent contribution to the overall L11-RNA binding affinity (approximately 8-fold enhancement at 0.2 m KCl), (iii) L11 stimulates thiostrepton binding by 2.3 +/- 0.6 x 10(3)-fold, predicting an overall thiostrepton affinity for the ribosome of approximately 10(9) m(-1), and (iv) the yeast homolog of L11 shows no stimulation of thiostrepton binding. The latter observation resolves the question of why eukaryotes are insensitive to the antibiotic. These measurements also show that it is plausible for thiostrepton to compete directly with EF-G.GDP for binding to the L11-RNA complex, and provide a quantitative basis for further studies of L11 function and thiostrepton mechanism.

Ionic interactions between PRNA and P protein in Bacillus subtilis RNase P characterized using a magnetocapture-based assay

Ribonuclease P (RNase P) is a ribonucleoprotein complex that catalyzes the cleavage of the 5' end of precursor tRNA. To characterize the interface between the Bacillus subtilis RNA (PRNA) and protein (P protein) components, the intraholoenzyme KD is determined as a function of ionic strength using a magnetocapture-based assay. Three distinct phases are evident. At low ionic strength, the affinity of PRNA for P protein is enhanced as the ionic strength increases mainly due to stabilization of the PRNA structure by cations. Lithium substitution in lieu of potassium enhances the affinity at low ionic strength, whereas the addition of ATP, known to stabilize the structure of P protein, does not affect the affinity. At high ionic strength, the observed affinity decreases as the ionic strength increases, consistent with disruption of ionic interactions. These data indicate that three to four ions are released on formation of holoenzyme, reflecting the number of ion pairs that occur between the P protein and PRNA. At moderate ionic strength, the two effects balance so that the apparent KD is not dependent on the ionic strength. The KD between the catalytic domain (C domain) and P protein has a similar triphasic dependence on ionic strength. Furthermore, the intraholoenzyme KD is identical to or tighter than that of full-length PRNA, demonstrating that the P protein binds solely to the C domain. Finally, pre-tRNAasp (but not tRNAasp) stabilizes the PRNA*P protein complex, as predicted by the direct interaction between the P protein and pre-tRNA leader.

Novel RNA-binding properties of Pop3p support a role for eukaryotic RNase P protein subunits in substrate recognition

Ribonuclease P (RNase P) catalyzes the 5'-end maturation of transfer RNA molecules. Recent evidence suggests that the eukaryotic protein subunits may provide substrate-binding functions (True, H. L., and Celander, D. W. (1998) J. Biol. Chem. 273, 7193-7196). We now report that Pop3p, an essential protein subunit of the holoenzyme in Saccharomyces cerevisiae, displays novel RNA-binding properties. A recombinant form of Pop3p (H6Pop3p) displays a 3-fold greater affinity for binding pre-tRNA substrates relative to tRNA products. The recognition sequence for the H6Pop3p-substrate interaction in vitro was mapped to a 39-nucleotide long sequence that extends from position -21 to +18 surrounding the natural processing site in pre-tRNA substrates. H6Pop3p binds a variety of RNA molecules with high affinity (K(d) = 16-25 nm) and displays a preference for single-stranded RNAs. Removal or modification of basic C-terminal residues attenuates the RNA-binding properties displayed by the protein specifically for a pre-tRNA substrate. These studies support the model that eukaryotic RNase P proteins bind simultaneously to the RNA subunit and RNA substrate.

Interaction of the Neurospora crassa mitochondrial tyrosyl-tRNA synthetase (CYT-18 protein) with the group I intron P4-P6 domain. Thermodynamic analysis and the role of metal ions

The Neurospora crassa mitochondrial tyrosyl-tRNA synthetase (CYT-18 protein) functions in splicing group I introns by promoting the formation of the catalytically active structure of the intron's catalytic core. Previous studies suggested a model in which the protein binds first to the intron's P4-P6 domain, and then makes additional contacts with the P3-P9 domain to stabilize the two domains in the correct relative orientation to form the intron's active site. Here, we analyzed the interaction of CYT-18 with a small RNA (P4-P6 RNA) corresponding to the isolated P4-P6 domain of the N. crassa mitochondrial large subunit ribosomal RNA intron. RNA footprinting and modification-interference experiments showed that CYT-18 binds to this small RNA around the junction of the P4-P6 stacked helices on the side opposite the active-site cleft, as it does to the P4-P6 domain in the intact intron. The binding is inhibited by chemical modifications that disrupt base-pairing in P4, P6, and P6a, indicating that a partially folded structure of the P4-P6 domain is required. The temperature-dependence of binding indicates that the interaction is driven by a favorable enthalpy change, but is accompanied by an unfavorable entropy change. The latter may reflect entropically unfavorable conformational changes or decreased conformational flexibility in the complex. CYT-18 binding is inhibited at > or =125 mM KCl, indicating a strong dependence on phosphodiester-backbone interactions. On the other hand, Mg(2+) is absolutely required for CYT-18 binding, with titration experiments showing approximately 1.5 magnesium ions bound per complex. Metal ion-cleavage experiments identified a divalent cation-binding site near the boundary of P6 and J6/6a, and chemical modification showed that Mg(2+) binding induces RNA conformational changes in this region, as well as elsewhere, particularly in J4/5. Together, these findings suggest a model in which the binding of Mg(2+) near J6/6a and possibly at one additional location in the P4-P6 RNA induces formation of a specific phosphodiester-backbone geometry that is required for CYT-18 binding. The binding of CYT-18 may then establish the correct structure at the junction of the P4/P6 stacked helices for assembly of the P3-P9 domain. The interaction of CYT-18 with the P4-P6 domain appears similar to the TyrRS interaction with the D-/anticodon arm stacked helices of tRNA(Tyr).

The RNA-binding domain of ribosomal protein L11 recognizes an rRNA tertiary structure stabilized by both thiostrepton and magnesium ion

Antibiotics that inhibit ribosomal function may do so by one of several mechanisms, including the induction of incorrect RNA folding or prevention of protein and/or RNA conformational transitions. Thiostrepton, which binds to the 'GTPase center' of the large subunit, has been postulated to prevent conformational changes in either the L11 protein or rRNA to which it binds. Scintillation proximity assays designed to look at the binding of the L11 C-terminal RNA-binding domain to a 23S ribosomal RNA (rRNA) fragment, as well as the ability of thiostrepton to induce that binding, were used to demonstrate the role of Mg(2+), L11 and thio-strepton in the formation and maintenance of the rRNA fragment tertiary structure. Experiments using these assays with both an Escherichia coli rRNA fragment and a thermostable variant of that RNA show that Mg(2+), L11 and thiostrepton all induce the RNA to fold to an essentially identical tertiary structure.

Structure and thermodynamics of RNA-protein binding: using molecular dynamics and free energy analyses to calculate the free energies of binding and conformational change

An adaptive binding mechanism, requiring large conformational rearrangements, occurs commonly with many RNA-protein associations. To explore this process of reorganization, we have investigated the conformational change upon spliceosomal U1A-RNA binding with molecular dynamics (MD) simulations and free energy analyses. We computed the energetic cost of conformational change in U1A-hairpin and U1A-internal loop binding using a hybrid of molecular mechanics and continuum solvent methods. Encouragingly, in all four free energy comparisons (two slightly different proteins, two different RNAs), the free macromolecule was more stable than the bound form by the physically reasonable value of approximately 10 kcal/mol. We calculated the absolute binding free energies for both complexes to be in the same range as that found experimentally.

Contributions of basic residues to ribosomal protein L11 recognition of RNA

The C-terminal domain of ribosomal protein L11, L11-C76, binds in the distorted minor groove of a helix within a 58 nucleotide domain of 23 S rRNA. To study the electrostatic component of RNA recognition in this protein, arginine and lysine residues have been individually mutated to alanine or methionine residues at the nine sequence positions that are conserved as basic residues among bacterial L11 homologs. In measurements of the salt dependence of RNA-binding, five of these mutants have a reduced value of - partial differentiallog(K(obs))/ partial differentiallog[KCl] as compared to the parent L11-C76 sequence, indicating that these residues interact with the RNA electrostatic field. These five residues are located at the perimeter of the RNA-binding surface of the protein; all five of them form salt bridges with phosphates in the crystal structure of the complex. A sixth residue, Lys47, was found to make an electrostatic contribution to binding when measurements were made at pH 6.0, but not at pH 7.0; its pK in the free protein must be <6.5. The unusual behavior of Lys47 is explained by its burial in the hydrophobic core of the free protein, and unburial in the RNA-bound protein, where it forms a salt bridge with a phosphate. The contributions of these six residues to the electrostatic component of binding are not additive; thus the magnitude of the salt dependence cannot be used to count the number of ionic interactions in this complex. By interacting with irregular features of the RNA backbone, including an S-turn, these basic residues contribute to the specificity of L11 for its target site.

Characterization of the Hantaan nucleocapsid protein-ribonucleic acid interaction

The nucleocapsid (N) protein functions in hantavirus replication through its interactions with the viral genomic and antigenomic RNAs. To address the biological functions of the N protein, it was critical to first define this binding interaction. The dissociation constant, K(d), for the interaction of the Hantaan virus (HTNV) N protein and its genomic S segment (vRNA) was measured under several solution conditions. Overall, increasing the NaCl and Mg(2+) in these binding reactions had little impact on the K(d). However, the HTNV N protein showed an enhanced specificity for HTNV vRNA as compared with the S segment open reading frame RNA or a nonviral RNA with increasing ionic strength and the presence of Mg(2+). In contrast, the assembly of Sin Nombre virus N protein-HTNV vRNA complexes was inhibited by the presence of Mg(2+) or an increase in the ionic strength. The K(d) values for HTNV and Sin Nombre virus N proteins were nearly identical for the S segment open reading frame RNA, showing weak affinity over several binding reaction conditions. Our data suggest a model in which specific recognition of the HTNV vRNA by the HTNV N protein resides in the noncoding regions of the HTNV vRNA.

Thiostrepton inhibits the turnover but not the GTPase of elongation factor G on the ribosome

The region around position 1067 in domain II of 23S rRNA frequently is referred to as the GTPase center of the ribosome. The notion is based on the observation that the binding of the antibiotic thiostrepton to this region inhibited GTP hydrolysis by elongation factor G (EF-G) on the ribosome at the conditions of multiple turnover. In the present work, we have reanalyzed the mechanism of action of thiostrepton. Results obtained by biochemical and fast kinetic techniques show that thiostrepton binding to the ribosome does not interfere with factor binding or with single-round GTP hydrolysis. Rather, the antibiotic inhibits the function of EF-G in subsequent steps, including release of inorganic phosphate from EF-G after GTP hydrolysis, tRNA translocation, and the dissociation of the factor from the ribosome, thereby inhibiting the turnover reaction. Structurally, thiostrepton interferes with EF-G footprints in the alpha-sarcin stem loop (A2660, A2662) located in domain VI of 23S rRNA. The results indicate that thiostrepton inhibits a structural transition of the 1067 region of 23S rRNA that is important for functions of EF-G after GTP hydrolysis.

Secondary structure mapping of an RNA ligand that has high affinity for the MetJ repressor protein and interference modification analysis of the protein-RNA complex

The secondary structure of an RNA aptamer, which has a high affinity for the Escherichia coli MetJ repressor protein, has been mapped using ribonucleases and with diethyl pyrocarbonate. The RNA ligand is composed of a stem-loop with a highly structured internal loop. Interference modification showed that the bases within the internal loop, and those directly adjacent to it, are important in the binding of the RNA ligand to MetJ. Most of the terminal stem-loop could be removed with little effect on the binding. Ethylation interference suggests that none of the phosphate groups are absolutely essential for tight binding. The data suggest that the MetJ binding site on the aptamer is distinct from that of the natural DNA target, the 8-base pair Met box.

RNA binding strategies of ribosomal proteins

Structures of a number of ribosomal proteins have now been determined by crystallography and NMR, though the complete structure of a ribosomal protein-rRNA complex has yet to be solved. However, some ribosomal protein structures show strong similarity to well-known families of DNA or RNA binding proteins for which structures in complex with cognate nucleic acids are available. Comparison of the known nucleic acid binding mechanisms of these non-ribosomal proteins with the most highly conserved surfaces of similar ribosomal proteins suggests ways in which the ribosomal proteins may be binding RNA. Three binding motifs, found in four ribosomal proteins so far, are considered here: homeodomain-like alpha-helical proteins (L11), OB fold proteins (S1 and S17) and RNP consensus proteins (S6). These comparisons suggest that ribosomal proteins combine a small number of fundamental strategies to develop highly specific RNA recognition sites.

Ribosomal protein S15 from Thermus thermophilus--cloning, sequencing, overexpression of the gene and RNA-binding properties of the protein

A 6-kb DNA fragment from an extreme thermophile, Thermus thermophilus, carrying the genes for cytochrome oxidase ba3 subunit I (cbaA) and the ribosomal protein S15 (rpsO) was cloned into Escherichia coli. The gene rpsO was sequenced. The deduced amino acid sequence exhibits 59% identity to the corresponding protein from E. coli. Expression of rpsO in E. coli requires the use of a fully repressed inducible promoter because S15 from T. thermophilus is toxic for E. coli cells. When purified without denaturation from either overproducing E. coli strain or from T. thermophilus ribosomes, the S15 protein is stable and binds a cloned T. thermophilus 16S rRNA fragment (nucleotides 559-753), with low identical dissociation constants (2.5 nM), thus demonstrating that the thermophilic protein folds correctly in a mesophilic bacterium. The rRNA fragment bound corresponds in position and structure to the 16S rRNA fragment of E. coli. A similar high affinity was also found for the binding of S15 from T. thermophilus or E. coli to the corresponding E. coli 16S rRNA fragment, whereas a slightly lower affinity was observed in binding experiments between E. coli S15 and T. thermophilus 16S rRNA fragment. These results suggest that S15 from T. thermophilus recognizes similar determinants in both rRNA fragments. Competition experiments support this conclusion.

The RNA binding domain of ribosomal protein L11 is structurally similar to homeodomains

The RNA binding domain of ribosomal protein L11 is strikingly similar to the homeodomain class of eukaryotic DNA binding proteins: it contains three alpha-helices that superimpose with homeodomain alpha-helices, and some conserved residues required for rRNA recognition align with homeodomain helix III residues contacting DNA bases.

Structure of a U.U pair within a conserved ribosomal RNA hairpin

A conserved hairpin corresponding to nt 1057-1081 of large subunit rRNA (Escherichia coli numbering) is part of a domain targeted by antibiotics and ribosomal protein L11. The stem of the hairpin contains a U.U juxtaposition, found as either U.U or U.C in virtually all rRNA sequences. This hairpin has been synthesized and most of the aromatic and sugar protons were assigned by two-dimensional proton NMR. Distances and sugar puckers deduced from the NMR data were combined with restrained molecular dynamics calculations to deduce structural features of the hairpin. The two U residues are stacked in the helix, form one NH3-O4 hydrogen bond and require an extended backbone conformation (trans alpha and gamma) at one of the U nucleotides. The hairpin loop, UAGAAGC closed by a U-A pair, is the same size as tRNA anticodon loops, but not as well ordered.

Messenger RNA recognition by fragments of ribosomal protein S4

Ribosomal protein S4 from Escherichia coli binds a large domain of 16 S ribosomal RNA and also a pseudoknot structure in the alpha operon mRNA, where it represses its own synthesis. No similarity between the two RNA binding sites has been detected. To find out whether separate protein regions are responsible for rRNA and mRNA recognition, proteins with N-terminal or C-terminal deletions have been overexpressed and purified. Protein-mRNA interactions were detected by (i) a nitrocellulose filter binding assay, (ii) inhibition of primer extension by reverse transcriptase, and (iii) a gel shift assay. Circular dichroism spectra were taken to determine whether the proteins adopted stable secondary structures. From these studies it is concluded that amino acids 48-104 make specific contacts with the mRNA, although residues 105-177 (out of 205) are required to observe the same toeprint pattern as full-length protein and may stabilize a specific portion of the mRNA structure. These results parallel ribosomal RNA binding properties of similar fragments (Conrad, R. C., and Craven, G. R. (1987) Nucleic Acids Res. 15, 10331-10343, and references therein). It appears that the same protein domain is responsible for both mRNA and rRNA binding activities.

On the role of rRNA tertiary structure in recognition of ribosomal protein L11 and thiostrepton

Ribosomal protein L11 and an antibiotic, thiostrepton, bind to the same highly conserved region of large subunit ribosomal RNA and stabilize a set of NH4(+)-dependent tertiary interactions within the domain. In vitro selection from partially randomized pools of RNA sequences has been used to ask what aspects of RNA structure are recognized by the ligands. L11-selected RNAs showed little sequence variation over the entire 70 nucleotide randomized region, while thiostrepton required a slightly smaller 58 nucleotide domain. All the selected mutations preserved or stabilized the known secondary and tertiary structure of the RNA. L11-selected RNAs from a pool mutagenized only around a junction structure yielded a very different consensus sequence, in which the RNA tertiary structure was substantially destabilized and L11 binding was no longer dependent on NH4+. We propose that L11 can bind the RNA in two different 'modes', depending on the presence or absence of the NH4(+)-dependent tertiary structure, while thiostrepton can only recognize the RNA tertiary structure. The different RNA recognition mechanisms for the two ligands may be relevant to their different effects on protein synthesis.

Stabilization of a ribosomal RNA tertiary structure by ribosomal protein L11

Interactions between ribosomal protein L11 and a domain of large subunit rRNA have been highly conserved and are essential for efficient protein synthesis. To study the effects of L11 on rRNA folding, a homolog of the Escherichia coli L11 gene has been amplified from Bacillus stearothermophilus DNA and cloned into a phage T7 polymerase-based expression system. The expressed protein is 93% homologous to the L11 homolog from Bacillus subtilis, denatures at temperatures above 72 degrees C, and has nearly identical rRNA binding properties as the Escherichia coli L11 in terms of RNA affinity constants and their dependences on temperature, Mg2+ concentration, monovalent cation, and RNA mutations. Mg2+ and NH4+ are specifically bound by the RNA-protein complex, with apparent ion-RNA affinities of 1.6 mM-1 and 19 M-1, respectively, at 0 degree C. The effect of the thermostable L11 on the unfolding of a 60 nucleotide rRNA fragment containing its binding domain has been examined in melting experiments. The lowest temperature RNA transition, which is attributed to tertiary structure unfolding, is stabilized by approximately 25 degrees C, and the interaction has an intrinsic enthalpy of approximately 13 kcal/mol. The thermal stability of the protein-RNA complex is enhanced by increasing Mg2+ concentration and by NH4+ relative to Na+. Thus L11, NH4+, and Mg2+ all bind and stabilize the same rRNA tertiary interactions, which are conserved and presumably important for ribosome function.

Thermodynamics of RNA unfolding: stabilization of a ribosomal RNA tertiary structure by thiostrepton and ammonium ion

RNAs with interesting secondary and tertiary structures tend to melt in several broad and overlapping transitions over a wide temperature range, and it has been consequently difficult to resolve the thermodynamics of individual unfolding steps. In the case that a ligand selectively binds a single folded state of the RNA, it is possible to obtain reliable thermodynamic parameters for both RNA unfolding and RNA-ligand binding simply from the hyperchromicity of RNA denaturation. The analysis procedure involves fitting a three-dimensional surface to absorbance data collected as a function of both temperature and ligand concentration. Analysis of the unfolding of a fragment of the large subunit ribosomal RNA (Escherichia coli sequence 1051 to 1109) is presented; both an antibiotic (thiostrepton) and ammonium ion specifically stabilize a tertiary structure within this RNA. A consistent set of thermodynamic parameters (delta H and tm) for the first two sequentially linked unfolding transitions is obtained from the experiments, and the binding constants obtained for the two ligands are consistent with other independent measurements. The approach is applicable to a variety of RNAs that specifically bind proteins, antibiotics, ions or other ligands.

Ribosomal protein methylation in Escherichia coli: the gene prmA, encoding the ribosomal protein L11 methyltransferase, is dispensable

The prmA gene, located at 72 min on the Escherichia coli chromosome, is the genetic determinant of ribosomal protein L11-methyltransferase activity. Mutations at this locus, prmA1 and prmA3, result in a severely undermethylated form of L11. No effect, other than the lack of methyl groups on L11, has been ascribed to these mutations. DNA sequence analysis of the mutant alleles prmA1 and prmA3 detected point mutations near the C-terminus of the protein and plasmids overproducing the wild-type and the two mutant proteins have been constructed. The wild-type PrmA protein could be crosslinked to its radiolabelled substrate, S-adenosyl-L-methionine (SAM), by u.v. irradiation indicating that it is the gene for the methyltransferase rather than a regulatory protein. One of the mutant proteins, PrmA3, was also weakly crosslinked to SAM. Both mutant enzymes when expressed from the overproducing plasmids were capable of catalysing the incorporation of 3H-labelled methyl groups from SAM to L11 in vitro. This confirmed the observation that the mutant proteins possess significant residual activity which could account for their lack of growth phenotype. However, a strain carrying an in vitro-constructed null mutation of the prmA gene, transferred to the E. coli chromosome by homologous recombination, was perfectly viable.

Overexpression of the thiostrepton-resistance gene from Streptomyces azureus in Escherichia coli and characterization of recognition sites of the 23S rRNA A1067 2'-methyltransferase in the guanosine triphosphatase center of 23S ribosomal RNA

The thiostrepton-resistance gene encoding the 23S rRNA A1067 methyltransferase from Streptomyces azureus has been overexpressed in Escherichia coli using a T7-RNA-polymerase-dependent expression vector. The protein was efficiently expressed at levels up to 20% of total soluble protein and purified to near homogeneity. Kinetic parameters for S-adenosyl-L-methionine (Km = 0.1 mM) and an RNA fragment containing nucleotides 1029-1122 of the 23S ribosomal RNA from E. coli (Km = 0.001 mM) were determined. S-Adenosyl-L-homocysteine showed competitive product inhibition (Ki = 0.013 mM). Binding of either thiostrepton or protein L11 inhibited methylation. RNA sequence variants of the RNA fragment with mutations in nucleotides 1051-1108 were tested as substrates for the methylase. The experimental data indicate that methylation is dependent on the secondary structure of the hairpin including nucleotide A1067 and the exact sequence U(1066)-A(1067)-G(1068)-A(1069)-A(1070) of the single strand.

Specific ammonium ion requirement for functional ribosomal RNA tertiary structure

In compactly folded RNAs, coordination or hydrogen bonding of cations in specific sites is a potentially important aspect of the tertiary structure. NH4+ specifically stabilizes the tertiary structure of a conserved, 58-nt fragment of the large subunit ribosomal RNA, as judged in two ways: a melting transition associated with tertiary interactions is sharpened and stabilized more effectively by NH4+ than by any alkali metal cation, and the affinity of the RNA fragment for ribosomal protein L11 or the antibiotic thiostrepton is approximately 10-fold stronger when measured in NH4+ than in Na+. The dependence of the melting temperature on NH4+ concentration shows that a single bound ion is responsible for these effects. The requirement of different ribosome functions for NH4+ suggests that other such sites exist in ribosomal RNAs.

Interaction of N-terminal domain of U1A protein with an RNA stem/loop

The U1A protein is a sequence-specific RNA binding protein found in the U1 snRNP particle where it binds to stem/loop II of U1 snRNA. U1A contains two 'RNP' or 'RRM' (RNA Recognition Motif) domains, which are common to many RNA-binding proteins. The N-terminal RRM has been shown to bind specifically to the U1 RNA stem/loop, while the RNA target of the C-terminal domain is unknown. Here, we describe experiments using a 102 amino acid N-terminal RRM of U1A (102A) and a 25-nucleotide RNA stem/loop to measure the binding constants and thermodynamic parameters of this RNA:protein complex. Using nitrocellulose filter binding, we measure a dissociation constant KD = 2 x 10(-11) M in 250 mM NaCl, 2 mM MgC2, and 10 mM sodium cacodylate, pH 6 at room temperature, and a half-life for the complex of 5 minutes. The free energy of association (delta G degrees) of this complex is about -14 kcal/mol in these conditions. Determination of the salt dependence of the binding suggests that at least 8 ion-pairs are formed upon complex formation. A mutation in the RNA loop sequence reduces the affinity 10 x, or about 10% of the total free energy.

Recognition of the highly conserved GTPase center of 23 S ribosomal RNA by ribosomal protein L11 and the antibiotic thiostrepton

The antibiotic thiostrepton, a thiazole-containing peptide, inhibits translation and ribosomal GTPase activity by binding directly to a limited and highly conserved region of the large subunit ribosomal RNA termed the GTPase center. We have previously used a filter binding assay to examine the binding of ribosomal protein L11 to a set of ribosomal RNA fragments encompassing the Escherichia coli GTPase center sequence. We show here that thiostrepton binding to the same RNA fragments can also be detected in a filter binding assay. Binding is relatively independent of monovalent salt concentration and temperature but requires a minimum Mg2+ concentration of about 0.5 mM. To help determine the RNA features recognized by L11 and thiostrepton, a set of over 40 RNA sequence variants was prepared which, taken together, change every nucleotide within the 1051 to 1108 recognition domain while preserving the known secondary structure of the RNA. Binding constants for L11 and thiostrepton interaction with these RNAs were measured. Only a small number of sequence variants had more than fivefold effects on L11 binding affinities, and most of these were clustered around a junction of helical segments. These same mutants had similar effects on thiostrepton binding, but more than half of the other sequence changes substantially reduced thiostrepton binding. On the basis of these data and chemical modification studies of this RNA domain in the literature, we propose that L11 makes few, if any, contacts with RNA bases, but recognizes the three-dimensional conformation of the RNA backbone. We also argue from the data that thiostrepton is probably sensitive to small changes in RNA conformation. The results are discussed in terms of a model in which conformational flexibility of the GTPase center RNA is functionally important during the ribosome elongation cycle.

A chemical interference study on the interaction of ribosomal protein L11 from Escherichia coli with RNA molecules containing its binding site from 23S rRNA

The interaction between ribosomal protein L11 from Escherichia coli and in vitro synthesized RNA containing its binding site from 23S rRNA was characterized by identifying nucleotides that interfered with complex formation when chemically modified by diethylpyrocarbonate or hydrazine. Chemically modified RNA was incubated with L11 under conditions appropriate for specific binding of L11 and the resulting protein-RNA complex was separated from unbound RNA on Mg(2+)-containing polyacrylamide gels. The ability to isolate L11 complexes on such gels was affected by the extent of modification by either reagent. Protein-bound and free RNAs were recovered and treated with aniline to identify their content of modified bases. Exclusion of RNA containing chemically altered bases from L11-associated material occurred for 29 modified nucleotides, located throughout the region corresponding to residues 1055-1105 in 23S rRNA. Ten bases within this region did not reproducibly inhibit binding when modified. Multiple bands of RNA were consistently observed on the nondenaturing gels, suggesting that significant intermolecular RNA-RNA interactions had occurred.

Detection of a key tertiary interaction in the highly conserved GTPase center of large subunit ribosomal RNA

Searches of ribosomal RNA sequences for compensatory base changes preserving Watson-Crick base pairing have led to detailed models of the conserved secondary structures of these RNAs. In principle, tertiary interactions can also be detected by searches for phylogenetically covariant bases. Within a highly conserved region of the large subunit ribosomal RNA termed the "GTPase center," the bases G-1056-U-1082.A-1086 are found in all eubacteria (Escherichia coli numbering), while A-1056.C-1082.G-1086 are found at the homologous positions in eukaryotes; archaebacteria fall into either category with some exceptions. Either sequence can potentially form a similar set of hydrogen bonds connecting the 3 bases. To determine the contribution of these 3 bases to RNA tertiary structure, sequence variants were made in RNA fragments covering the GTPase center. Correct folding of the RNA fragments was assayed by measuring the binding affinities of two different ligands that recognize the RNA tertiary structure: the highly conserved ribosomal protein L11, which is normally associated with the GTPase center RNA, and the peptide antibiotic thiostrepton, which inhibits the GTPase activity of eubacterial and some archaebacterial ribosomes. The results strongly support the existence of a base pair between positions 1082 and 1086: single mutations at either position weaken both L11 and thiostrepton binding by approximately 10-fold or more, while compensatory double mutations bind the ligands nearly as well as the wild-type E. coli sequence. Variants at position 1056 have little effect on either L11 or thiostrepton binding; a 3-base interaction is therefore not supported by these experiments. A base pair between positions 1082 and 1086 strongly constrains the geometry with which three helical segments join in the middle of the GTPase center.

The binding of thiostrepton to 23S ribosomal RNA

The antibiotic, thiostrepton, binds to 23S ribosomal RNA from E coli with a dissociation constant (KD) of 2.4 x 10(-7) M. The specificity of the interaction was established using 16S rRNA and modified or mutationally-altered 23S rRNA. Thus, no binding was detected with rRNA from the 30S subunit nor with rRNA modified in vitro by the thiostrepton resistance methylase. Mutant 23S rRNA, altered at residue 1067 in each of the 3 possible ways, showed reduced binding affinity for thiostrepton. The KD for the G mutation was 3.5 x 10(-6) M; for the C mutation, 2.4 x 10(-5) M; and for the U mutation, 4.8 x 10(-5) M. This reduction in drug binding is compatible with functional analyses; the C or U mutation results in ribosomal particles which are poorly inhibited by the drug compared with wild-type, whereas the G mutation results in an intermediate response to the drug in protein synthesis. The smallest 23S rRNA fragment used here that was capable of binding thiostrepton, in a nitrocellulose filter binding assay, comprised residues 1052-1112 and the dissociation constant was 3.0 x 10(-7) M, ie virtually indistinguishable from that with intact 23S RNA. However, the drug was incapable of binding to the 5'-moiety of this fragment (ie residues 1052-1084) or to an RNA transcript complementary to 1052-1112.

Sites of contact of mRNA with 16S rRNA and 23S rRNA in the Escherichia coli ribosome

The locations of close encounter between ribosomal RNA (rRNA) and messenger RNA (mRNA) were determined by photochemical cross-linking experiments that employ an artificial mRNA, 51 nucleotides long, containing 14 U residues that were randomly substituted by 1-4 4-thiouridine (s4U) residues. The mRNA was bound to 70S ribosomes or 30S subunits and then was irradiated at 366 nm to activate cross-linking between the s4U residues and rRNA. Cross-linking occurred to both 16S rRNA and 23S RNA. The rRNA was then analyzed by a series of reverse transcriptase experiments to determine the locations of cross-linking. Twelve sites in the 16S rRNA and two sites in the 23S rRNA have been detected. In the 16S rRNA, two of the sites (U1381, C1395) are in the middle part of the secondary structure close to position C1400, and the remaining sites (G413, U421, G424; A532; G693; U723; A845; G1131/C1132; G1300; G1338) are distributed between six regions that are peripheral in the secondary structure. In the 23S rRNA, one site (U1065) is located in the GTPase center close to A1067, the site of thiostrepton-resistance methylation in domain II, and the other site (U887) is located a short distance away also in domain II. The distribution of these rRNA sites in the ribosome specifies an mRNA track that is consistent with other information. In addition, some of the contact points represent new constraints for the three-dimensional folding of the rRNA.

Quantitative determination that one of two potential RNA-binding domains of the A protein component of the U1 small nuclear ribonucleoprotein complex binds with high affinity to stem-loop II of U1 RNA

Many RNA-associated proteins contain a ribonucleoprotein (RNP) consensus octamer encompassed by a conserved 80 amino acid sequence, which we have termed an RNA recognition motif (RRM). RRM family members contain either one (class I) or multiple (class II) copies of this motif. We report here that a class II component of the U1 small nuclear RNP (snRNP), the A protein of U1 snRNP (U1snRNP-A), contains two RRMs (RRM1 and -2), yet has only one binding domain (RRM1) that interacts specifically with stem-loop II of U1 RNA. Quantitative analysis of binding affinities of fragments of U1snRNP-A demonstrated that an 86-amino acid polypeptide was competent to bind to U1 RNA with an affinity comparable to that of the full-length protein (Kd approximately 80 nM). The carboxyl-terminal RRM2 of U1snRNP-A did not bind to U1 RNA and may recognize an unidentified heterologous RNA. We propose that class II proteins may function as bridges between RNA components of RNP complexes such as the spliceosome.

Cloning, in vitro transcription, and biological activity of Escherichia coli 23S ribosomal RNA

The 23S rRNA gene was excised from the rrnB operon of pKK3535 and ligated into pUC19 behind the strong class III T7 promoter so that the correct 5' end of mature 23S RNA was produced upon transcription by T7 RNA polymerase. At the 3' end, generation of a restriction site for linearization required the addition of 2 adenosine residues to the mature 23S sequence. In vitro runoff transcripts were indistinguishable from natural 23S RNA in size on denaturing gels and in 5'-terminal sequence. The length and sequence of the 3' terminal T1 fragment was also as expected from the DNA sequence, except that an additional C, A, or U residue was added to 21%, 18%, or 5% of the molecules, respectively. Typical transcription reactions yielded 500-700 moles RNA per mole template. This transcript was used as a substrate for methyl transfer from S-adenosyl methionine catalyzed by Escherichia coli cell extracts. The majority (50-65%) of activity observed in a crude (S30) extract appeared in the post-ribosomal supernatant (S100). Activities catalyzing formation of m5C, m5U, m2G, and m6A residues in the synthetic transcript were observed.