Refined crystal structure of an octanucleotide duplex with I.T. mismatched base pairs.

The structure of the synthetic deoxyoctamer d(GGIGCTCC) has been determined by single crystal X-ray diffraction techniques to a resolution of 1.7A. The sequence crystallises in space group P6(1), with unit cell dimensions a = b = 45.07, c = 45.49A. The refinement converged with a crystallographic residual R = 0.14 and the location of 81 solvent molecules. The octamer forms an A-DNA duplex with 6 Watson-Crick (G.C) base pairs and 2 inosine-thymine (I.T) pairs. Refinement of the structure shows it to be essentially isomorphous with that reported for d(GGGGCTCC) with the mispairs adopting a "wobble" conformation. Conformational parameters and base stacking interactions are compared to those for the native duplex d(GGGGCCCC) and other similar sequences. A rationale for the apparent increased crystal packing efficiency and lattice stability of the I.T octamer is given.     

The crystal structure at 1.5 angstroms resolution of an RNA octamer duplex containing tandem G.U basepairs

The crystal structure of the RNA octamer, 5'-GGCGUGCC-3' has been determined from x-ray diffraction data to 1.5 angstroms resolution. In the crystal, this oligonucleotide forms five self-complementary double-helices in the asymmetric unit. Tandem 5'GU/3'UG basepairs comprise an internal loop in the middle of each duplex. The NMR structure of this octameric RNA sequence is also known, allowing comparison of the variation among the five crystallographic duplexes and the solution structure. The G.U pairs in the five duplexes of the crystal form two direct hydrogen bonds and are stabilized by water molecules that bridge between the base of guanine (N2) and the sugar (O2') of uracil. This contrasts with the NMR structure in which only one direct hydrogen bond is observed for the G.U pairs. The reduced stability of the r(CGUG)2 motif relative to the r(GGUC)2 motif may be explained by the lack of stacking of the uracil bases between the Watson-Crick and G.U pairs as observed in the crystal structure.     

Crystal structure and stability of a DNA duplex containing A(anti).G(syn) base-pairs

The synthetic dodecanucleotide d(CGCAAATTGGCG) has been analysed by single-crystal X-ray diffraction techniques and the structure refined to R = 0.16 and 2.25 A resolution, with the location of 94 solvent molecules. The sequence crystallizes as a full turn of a B-DNA helix with ten Watson-Crick base-pairs and two adenine-guanine mispairs. The analysis clearly shows that the mismatches are of the form A(anti).G(syn). Thermal denaturation studies indicate that the stability of the duplex is strongly pH dependent, with a maximum at pH 5.0, suggesting that the base-pair is stabilized by protonation. Three different arrangements have been observed for base-pairs between guanine and adenine and it is likely that A.G mismatch conformation is strongly influenced by dipole-dipole interactions with adjacent base-pairs.     

Refined crystal structure of deoxyhemoglobin S. II. Molecular interactions in the crystal

The refined crystal structure of deoxyhemoglobin S (Padlan, E. A., and Love, W. E. (1985) J. Biol. Chem. 260, 8272-8279) was used to analyze in detail the molecular interactions between hemoglobin tetramers in the crystal. The analysis confirms the close similarity and also the nonequivalence of the molecular interactions involving the two independent tetramers in the asymmetric unit of the crystal. The residue at the site of the hemoglobin S mutation, beta 6, is intimately involved in the lateral contacts between adjacent molecules. The molecular contacts in the crystals of deoxyhemoglobin S, deoxyhemoglobin A, and deoxyhemoglobin F were compared; some contacts involve the same regions of the molecule although the details of the interactions are very different. The effect of introducing an R state tetramer into the deoxyhemoglobin S strands was investigated using the known structure of carbon monoxyhemoglobin A. It was found that substituting a molecule of carbon monoxyhemoglobin A for one of the deoxyhemoglobin S tetramers results in extensive molecular interpenetration.     

Solution structure of an oncogenic DNA duplex containing a G.A mismatch

The DNA duplex 5'-d(GCCACAAGCTC).d(GAGCTGGTGGC), which contains a central G.A mismatch has been studied by one and two-dimensional NMR techniques. The duplex corresponds to the sequence 29-39 of the K-ras gene. The mismatch position is that of the first base of the Gly12 codon, a hot spot for mutations. The observed NOEs of the nonexchangeable protons show that both of the bases of the mismatched pair are intrahelical over a wide range of pH. However, the structure of the G.A mispair and the conformation of the central part of the duplex change with pH. This structural change shows a pK of 6.0. At low pH, the G.A bases are base paired with hydrogen bonds between the keto group of the G residue and the amino group of the A residue and, secondly, between the N7 of the G and a proton on N1 of A. This causes the G residue to adopt a syn conformation. On raising the pH, the N1-H proton of the protonated A residue is removed, and the base pair rearranges. In the neutral G.A base pair both residues adopt an anti conformation, and the mismatch is stabilized by hydrogen bonds. Our results on the exchangeable and A(H2) protons of the mismatched pair indicate a shift from a classical face-to-face two hydrogen-bonded structure to a slipped structure stabilized by bifurcated hydrogen bonds. This may be a particular characteristics of this oncogenic sequence in which the G.A error is poorly repaired.     

Genetic conversion of G.C base-pairs to A.U base-pairs in a transfer RNA

The cloverleaf stem segments of the suppressor gene of bacteriophage T4 tRNA(Gln) contain ten G.C and ten A.U base-pairs. To gain a better appreciation of the G.C base-pair requirement, we isolated multiple mutants of this suppressor gene in which base-pairs of G.C were replaced by A.U. One active suppressor gene contained only A.U base-pairs on the anticodon stem, indicating that G.C base-pairs in this region of tRNA(Gln) are not essential for function. In contrast, replacement was not possible at two base-pairs on the D stem and at one base-pair on the T stem.     

Improved free energies for G.C base-pairs

Thermodynamic parameters of helix formation are reported for seven oligoribonucleotides containing only G.C pairs. These data are used with the nearest-neighbor model to calculate enthalpies and free energies of base-pair formation for G.C pairs. For helix initiation, the free energy change at 37 degrees C, delta G(0)37, is +3.9 kcal/mol; for helix propagation, the delta G(0)37 values are -2.3, -3.2 and -3.3 kcal/mol for C-G, G-G and G-C neighbors, respectively.     

Refined crystal structure of ascorbate oxidase at 1.9 A resolution.

The crystal structure of the fully oxidized form of ascorbate oxidase (EC 1.10.3.3) from Zucchini has been refined at 1.90 A (1 A = 0.1 nm) resolution, using an energy-restrained least-squares refinement procedure. The refined model, which includes 8764 protein atoms, 9 copper atoms and 970 solvent molecules, has a crystallographic R-factor of 20.3% for 85,252 reflections between 8 and 1.90 A resolution. The root-mean-square deviation in bond lengths and bond angles from ideal values is 0.011 A and 2.99 degrees, respectively. The subunits of 552 residues (70,000 Mr) are arranged as tetramers with D2 symmetry. One of the dyads is realized by the crystallographic axis parallel to the c-axis giving one dimer in the asymmetric unit. The dimer related about this crystallographic axis is suggested as the dimer present in solution. Asn92 is the attachment site for one of the two N-linked sugar moieties, which has defined electron density for the N-linked N-acetyl-glucosamine ring. Each subunit is built up by three domains arranged sequentially on the polypeptide chain and tightly associated in space. The folding of all three domains is of a similar beta-barrel type and related to plastocyanin and azurin. An analysis of intra- and intertetramer hydrogen bond and van der Waals interactions is presented. Each subunit has four copper atoms bound as mononuclear and trinuclear species. The mononuclear copper has two histidine, a cysteine and a methionine ligand and represents the type-1 copper. It is located in domain 3. The bond lengths of the type-1 copper centre are comparable to the values for oxidized plastocyanin. The trinuclear cluster has eight histidine ligands symmetrically supplied from domain 1 and 3. It may be subdivided into a pair of copper atoms with histidine ligands whose ligating N-atoms (5 NE2 atoms and one ND1 atom) are arranged trigonal prismatic. The pair is the putative type-3 copper. The remaining copper has two histidine ligands and is the putative spectroscopic type-2 copper. Two oxygen atoms are bound to the trinuclear species as OH- or O2- and bridging the putative type-3 copper pair and as OH- or H2O bound to the putative type-2 copper trans to the copper pair. The bond lengths within the trinuclear copper site are similar to comparable binuclear model compounds. The putative binding site for the reducing substrate is close to the type-1 copper.(ABSTRACT TRUNCATED AT 400 WORDS)     

Refined 1.8 A crystal structure of the lambda repressor-operator complex.

The crystal structure of the lambda repressor-operator complex has been refined to an R-factor of 18.9% at 1.8 A resolution. This refinement, using data collected at low temperature, has revealed the structure of the N-terminal arm and shows that the interactions of repressor with the two halves of the pseudo-symmetric operator site are significantly different. The two halves of the complex are most similar near the outer edge of the operator site (in a region where the lambda and 434 repressors make similar contacts), but they become increasingly different toward the center of the operator. There are striking differences near the center of the site where it appears that the arm makes significant contacts to only one half of the DNA site. This suggested a new way of aligning the operator sites in phage lambda. The high resolution structure confirms many of the previously noted features of the complex, but also reveals a number of new protein-DNA contacts. It also gives a better view of the extensive H-bonding networks that couple contacts made by different residues and different regions of the protein, and reveals important new details about the helix-turn-helix (HTH) region, and the positions of many water molecules in the complex.     

Refined crystal structure of the phosphorylase-heptulose 2-phosphate-oligosaccharide-AMP complex

The crystal structure of phosphorylase b-heptulose 2-phosphate complex with oligosaccharide and AMP bound has been refined by molecular dynamics and crystallographic least-squares with the program XPLOR. Shifts in atomic positions of up to 4 A from the native enzyme structure were correctly determined by the program without manual intervention. The final crystallographic R value for data between 8 and 2.86 A resolution is 0.201, and the overall root-mean-square difference between the native and complexed structure is 0.58 A for all protein atoms. The results confirm the previous observation that there is a direct hydrogen bond between the phosphate of heptulose 2-phosphate and the pyridoxal phosphate 5'-phosphate group. The close proximity of the two phosphates is stabilized by an arginine residue, Arg569, which shifts from a site buried in the protein to a position where it can make contact with the product phosphate. There is a mutual interchange in position between the arginine and an acidic group, Asp283. These movements represent the first stage of the allosteric response which converts the catalytic site from a low to a high-affinity binding site. Communication of these changes to other sites is prevented in the crystal by the lattice forces, which also form the subunit interface. The constellation of groups in the phosphorylase transition state analogue complex provides a structural basis for understanding the catalytic mechanism in which the cofactor pyridoxal phosphate 5'-phosphate group functions as a general acid to promote attack by the substrate phosphate on the glycosidic bond when the reaction proceeds in the direction of glycogen degradation. In the direction of glycogen synthesis, stereoelectronic effects contribute to the cleavage of the C-1-O-1 bond. In both reactions the substrate phosphate plays a key role in transition state stabilization. The details of the oligosaccharide, maltoheptaose, interactions with the enzyme at the glycogen storage site are also described.     

Crystal structure of an RNA duplex r(G GCGC CC)2 with non-adjacent G*U base pairs

The crystal structure of a self-complementary RNA duplex r(GGGCGCUCC)2with non-adjacent G*U and U*G wobble pairs separated by four Watson-Crick base pairs has been determined to 2.5 A resolution. Crystals belong to the space group R3; a = 33.09 A,alpha = 87.30 degrees with a pseudodyad related duplex in the asymmetric unit. The structure was refined to a final Rworkof 17.5% and Rfreeof 24.0%. The duplexes stack head-to-tail forming infinite columns with virtually no twist at the junction steps. The 3'-terminal cytosine nucleosides are disordered and there are no electron densities, but the 3' penultimate phosphates are observed. As expected, the wobble pairs are displaced with guanine towards the minor groove and uracil towards the major groove. The largest twist angles (37.70 and 40.57 degrees ) are at steps G1*C17/G2*U16 and U7*G11/C8*G10, while the smallest twist angles (28.24 and 27.27 degrees ) are at G2*U16/G3*C15 and C6*G12/U7*G11 and conform to the pseudo-dyad symmetry of the duplex. The molecule has two unequal kinks (17 and 11 degrees ) at the wobble sites and a third kink at the central G5 site which may be attributed to trans alpha (O5'-P), trans gamma (C4'-C5') backbone conformations. The 2'-hydroxyl groups in the minor groove form inter-column hydrogen bonding, either directly or through water molecules.     

Effects of N2,N2-dimethylguanosine on RNA structure and stability: crystal structure of an RNA duplex with tandem m2 2G:A pairs

Methylation of the exocyclic amino group of guanine is a relatively common modification in rRNA and tRNA. Single methylation (N²-methylguanosine, m²G) is the second most frequently encountered nucleoside analog in Escherichia coli rRNAs. The most prominent case of dual methylation (N²,N²-dimethylguanosine, m²₂G) is found in the majority of eukaryotic tRNAs at base pair m²₂G26:A44. The latter modification eliminates the ability of the N² function to donate in hydrogen bonds and alters its pairing behavior, notably vis-à-vis C. Perhaps a less obvious consequence of the N²,N²-dimethyl modification is its role in controlling the pairing modes between G and A. We have determined the crystal structure of a 13-mer RNA duplex with central tandem m²₂G:A pairs. In the structure both pairs adopt an imino-hydrogen bonded, pseudo-Watson–Crick conformation. Thus, the sheared conformation frequently seen in tandem G:A pairs is avoided due to a potential steric clash between an N²-methyl group and the major groove edge of A. Additionally, for a series of G:A containing self-complementary RNAs we investigated how methylation affects competitive hairpin versus duplex formation based on UV melting profile analysis.     

An analysis of mismatched duplex DNA unzipping through a bacterial nanopore.

A 50-base Guide strand was synthesized that consisted of a central 10-base probe sequence flanked by two tracts of 20 adenine residues. Target sequences of 10 bases containing up to three mismatches were prepared and hybridized to the Guide strand in 1 M KCl. The transport of these constructs through single alpha-hemolysin pores was analysed by measuring the current blockade as a function of time. Complementary dsDNA takes significantly longer (840 +/- 60 micro s) to pass through the pore than a sequence of the same length containing a single (590 +/- 45 micro s) and a double (270 +/- 50 micro s) mismatch. Constructs involving three mismatches were indistinguishable from Guide ssDNA transport (120 +/- 30 micro s). The results suggest that dsDNA must unzip as it is transported through the nanopore. Duplexes containing mismatches unzip more quickly and can be distinguished from those with perfect complementarity.     

Refined crystal structure of the influenza virus N9 neuraminidase-NC41 Fab complex.

The crystal structure of the complex between neuraminidase from influenza virus (subtype N9 and isolated from an avian source) and the antigen-binding fragment (Fab) of monoclonal antibody NC41 has been refined by both least-squares and simulated annealing methods to an R-factor of 0.191 using 31,846 diffraction data in the resolution range 8.0 to 2.5 A. The resulting model has a root-mean-square deviation from ideal bond-length of 0.016 A. One fourth of the tetrameric complex comprises the crystallographic model, which has 6577 non-hydrogen atoms and consists of 389 protein residues and eight carbohydrate residues in the neuraminidase, 214 residues in the Fab light chain, and 221 residues in the heavy chain. One putative Ca ion buried in the neuraminidase, and 73 water molecules, are also included. A remarkable shape complementarity exists between the interacting surfaces of the antigen and the antibody, although the packing density of atoms at the interface is somewhat looser than in the interior of a protein. Similarly, there is a high degree of chemical complementarity between the antigen and antibody, mediated by one buried salt-link, two solvated salt-links and 12 hydrogen bonds. The antibody-binding site on neuraminidase is discontinuous and comprises five chain segments and 19 residues in contact, whilst 33 neuraminidase residues in eight segments have 899 A2 of surface area buried by the interaction (to a 1.7 A probe), including two hexose units. Seventeen residues in NC41 Fab lying in five of the six complementarity determining regions (CDRs) make contact with the neuraminidase and 36 antibody residues in seven segments have 916 A2 of buried surface area. The interface is more extensive than those of the three lysozyme-Fab complexes whose crystal structures have been determined, as judged by buried surface area and numbers of contact residues. There are only small differences (less than 1.5 A) between the complexed and uncomplexed neuraminidase structures and, at this resolution and accuracy, those differences are not unequivocal. The main-chain conformations of five of the CDRs follow the predicted canonical structures. The interface between the variable domains of the light and heavy chains is not as extensive as in other Fabs, due to less CDR-CDR interaction in NC41. The first CDR on the NC41 Fab light chain is positioned so that it could sterically hinder the approach of small as well as large substrates to the neuraminidase active-site pocket, suggesting a possible mechanism for the observed inhibition of enzyme activity by the antibody.(ABSTRACT TRUNCATED AT 400 WORDS)     

Repair of thymine.guanine and uracil.guanine mismatched base-pairs in bacteriophage M13mp18 DNA heteroduplexes

Repair of thymine.guanine (T.G) and uracil.guanine (U.G) mismatched base-pairs in bacteriophage M13mp18 replicative form (RF) DNA was compared upon transfection into repair-proficient or repair-deficient Escherichia coli strains. Oligonucleotide-directed mutagenesis was used to prepare covalently closed circular heteroduplexes that contained the mismatched base-pair at a restriction recognition site. The heteroduplexes were unmethylated at dam (5'-GATC-3') sites to avoid methylation-directed biasing of repair. In an E. coli host containing uracil-DNA glycosylase (ung+), about 97% of the transfecting U.G-containing heteroduplexes had the U residue excised by the uracil-excision repair system. With the analogous T.G mispair, mismatch repair operated on almost all of the transfecting heteroduplexes and removed the T residue in about 75% of them when the mismatched T was on the minus strand of the RF DNA. Similar preferential excision of the minus-strand's mismatched base was observed whether the heteroduplex RF DNA molecules had only one or both strands unmethylated at dcm (5'-CC(A/T)GG-3') sites and whether the RF DNA was prepared by primer extension in vitro or by reannealing mutant and non-mutant DNA strands. Also, the extent and directionality of repair was the same at a U.G mispair in ung- host cells as at the analogous T.G mispair in ung- or ung+ cells. Only in a mismatch repair-deficient (mutH-) host was the plus strand of the transfecting M13mp18 heteroduplex DNA preferentially repaired. It is suggested that the plus strand nick made by the M13-encoded gene II protein might be employed by a mutH- host to initiate repair on that strand.     

Molecular and crystal structure of d(CGCGmo4CG): N4-methoxycytosine.guanine base-pairs in Z-DNA

The base analogue N4-methoxycytosine (mo4C) is ambivalent in its hydrogen-bonding potential, since it forms stable base-pairs with both adenine and guanine in oligomer duplexes. To investigate the base-pair geometry, the structure of d(CGCGmo4CG) has been determined by single-crystal X-ray diffraction techniques. The d(CGCGmo4CG)2 crystallized in a left-handed double helical structure (Z-type). Refinement using 2559 reflections between 10 and 1.7 A converged with a final R = 0.181 (Rw = 0.130) including 68 solvent molecules. The orthorhombic crystals are in the space group P2(1)2(1)2(1), with cell dimensions a = 18.17 A, b = 30.36 A, c = 43.93 A. The mo4C.G base-pair is of the wobble type, with mo4C in the imino form, and the methoxy group in the syn configuration.     

Refined crystal structure of deoxyhemoglobin S. I. Restrained least-squares refinement at 3.0-A resolution

The crystal structure of deoxyhemoglobin S has been refined at 3.0-A resolution using the Hendrickson-Konnert restrained least-squares method. Comparison with the structure of deoxyhemoglobin A reveals a hingelike movement of the beta-chain A helices, which are involved in molecular contacts, toward the EF corners of their respective subunits. This movement brings the amino termini of the beta-chains closer to the molecular dyad. The A helices remain alpha-helical throughout their entire lengths. No other major structural difference is found between deoxyhemoglobin A and deoxyhemoglobin S.     

Refined crystal structure of lipoamide dehydrogenase from Azotobacter vinelandii at 2.2 A resolution. A comparison with the structure of glutathione reductase

The structure of lipoamide dehydrogenase from Azotobacter vinelandii has been refined by the molecular dynamics technique to an R-factor of 19.8% at 2.2 A resolution. In the final model, the root-mean-square deviation from ideality is 0.02 A for bond lengths and 3.2 degrees for bond angles. The asymmetric unit comprises two subunits, each consisting of 466 amino acid residues and the prosthetic group FAD, plus 512 solvent molecules. The last ten amino acid residues of both chains are not visible in the electron density distribution and they are probably disordered. The operation required to superimpose the two chains forming the dimer is a rotation of exactly 180 degrees with no translation component. The final model shows the two independently refined subunits to be very similar, except for six loops located at the surface of the molecule. The structure of each subunit of the enzyme consists of four domains with the catalytic centre located at the subunit interface. The reactive disulphide bridge, 48-53, is oxidized with S gamma of Cys53 located 3.5 A away from carbon C-4a of the isoalloxazine ring. The side-chain of His450' points its N epsilon 2 towards S gamma of Cys48 and is hydrogen bonded to the carboxylate of Glu455'. The FAD is bound in an extended conformation and the isoalloxazine ring is not completely planar with an angle between the pteridine and the benzene ring of 7.3 degrees in the first subunit and of 12.1 degrees in the second one. The overall folding of lipoamide dehydrogenase is very similar to that of glutathione reductase. However, a comparison of the two enzymes, which have only 26% sequence identity, reveals significant conformational differences. These concern the tertiary as well as the quaternary structure of the two molecules. In each subunit of lipoamide dehydrogenase the NAD-binding domain and the interface domain appear to be differently oriented with respect to the FAD-binding domain by 7.1 degrees and 7.8 degrees, respectively. The interface domain contains, in addition, major changes in tertiary structure. Furthermore, the two subunits forming the dimer appear to be shifted with respect to each other by more than 4 A, when the lipoamide dehydrogenase dimer is compared with that of glutathione reductase. In spite of all these changes at the tertiary and quaternary level the active sites of the enzymes, which occur at the dimer interface, appear to be remarkably similar.(ABSTRACT TRUNCATED AT 400 WORDS)     

The crystal structure of N4-methylcytosine.guanosine base-pairs in the synthetic hexanucleotide d(CGCGm4CG).

The structure of d(CGCGm4CG) were m4C = N4-methylcytosine has been determined by crystallographic methods. The crystals are multifaced prisms, with orthorhombic space group P2(1)2(1)2(1) and unit cell dimensions of a = 17.98, b = 30.77 and c = 44.75A. The asymmetric unit consists of one duplex of hexanucleotide and 49 waters. The R-factor is 0.189 for 1495 reflections with F > or = sigma(F) to a resolution limit of 1.8A. The double helix has a Z-DNA type structure which appears to be intermediate in structure to the two previously characterised structure types for Z-DNA hexamers. The two m4C.G base-pairs adopt structures that are very similar to those of the equivalent base-pairs in the structure of the native sequence d(CGCGCG) except for the presence of the methyl groups which are trans to the N3 atoms of their parent nucleotides and protrude into the solvent region. The introduction of the modified base-pairs into the d(CGCGCG) duplex appears to have a minimal effect on the overall base-pair morphology of the Z-DNA duplex.     

Imidazole-imidazole pair as a minor groove recognition motif for T:G mismatched base pairs.

The T:G mismatched base pair is associated with many genetic mutations. Understanding its biological consequences may be aided by studying the structural perturbation of DNA caused by a T:G base pair and by specific probing of the mismatch using small molecular ligands. We have shown previously that AR-1-144, a tri-imidazole (Im-Im-Im) minor groove binder, recognizes the sequence CCGG. NMR structural analysis of the symmetric 2:1 complex of AR-1-144 and GAACCGGTTC revealed that each AR-1-144 binds to four base pairs with the guanine N2 amino group forming a bifurcated hydrogen bond to a side-by-side Im/Im pair. We predicted that the free G-N2 amino group in a T:G wobble base pair can form two individual hydrogen bonds to a side-by-side Im/Im pair. Thus an Im/Im pair may be a good recognition motif for a T:G base pair in DNA. Cooperative and tight binding of an AR-1-144 homodimer to GAACTGGTTC permits a detailed structural analysis by 2D NOE NMR refinement and the refined structure confirms our prediction. Surprisingly, AR-1-144 does not bind to GAATCGGTTC. We further show that both the Im-Im-Im/Im-Py-Im heterodimer and the Im-Im-Im/Im-Im-Im homodimer bind strongly to the CACGGGTC + GACTCGTG duplex. These results together suggest that an Im/Im pair can specifically recognize a single T:G mismatch. Our results may be useful in future design of molecules (e.g. linked dimers) that can recognize a single T:G mismatch with specificity.