Structure of a DNA analog of the primer for HIV-1 RT second strand synthesis.

The non-self-complementary DNA decamer C-A-A-A-G-A-A-A-A-G/C-T-T-T-T-C-T-T-T-G is a DNA/DNA analogue of a portion of the polypurine tract or PPT, which is a RNA/DNA hybrid that serves as a primer for synthesis of the (+) DNA strand by HIV reverse transcriptase (RT), and which is not digested by the RNase H domain of reverse transcriptase following (-) strand synthesis. The same unusual conformation that eludes RNase H, thought to be a change in width of minor groove, may also be responsible for the inhibition of HIV RT by minor groove binding drugs such as distamycin and their bis-linked derivatives. The present X-ray crystal structure of this DNA decamer exhibits the usual properties of A-tract B-DNA under biologically relevant conditions: large propeller twist of base-pairs, narrowed minor groove, and a straight helix axis. Groove narrowing is fully developed in the A-A-A-A region, but not in the A-A-A region, which previous investigators have proposed as being too short to exhibit typical A-tract properties. The RNA/DNA hybrid produced by HIV reverse transcriptase during (-) strand synthesis presumably forms a "heteromerous" or H-helix with narrower minor groove than an A-helical RNA/RNA duplex. If the narrowing of minor groove in A-tract H-helices is comparable to that seen in A-tract B-helices, then the narrowed minor groove of the polypurine tract could make the second primer site both (1) impervious to RNase H digestion, and (2) susceptible to inhibition by minor groove binding drugs.

Sequence-dependent structural variation in B-DNA.

Though fiber diffraction originally led to the belief that the structure of DNA would be a simple regular helix, X-ray crystallography of synthetic oligomers has shown that both deformability and structure depend on sequence. But the rules that determine these factors remain mysterious.

Structure of the Escherichia coli response regulator NarL.

The crystal structure analysis of the NarL protein provides a first look at interactions between receiver and effector domains of a full-length bacterial response regulator. The N-terminal receiver domain, with 131 amino acids, is folded into a 5-strand beta sheet flanked by 5 alpha helices, as seen in CheY and in the N-terminal domain of NTRC. The C-terminal DNA-binding domain, with 62 amino acids, is a compact bundle of 4 alpha helices, of which the middle 2 form a helix-turn-helix motif closely related to that of Drosophila paired protein and other H-T-H DNA-binding proteins. The 2 domains are connected by an alpha helix of 10 amino acids and a 13-residue flexible tether that is not visible and presumably disordered in the X-ray structure. In this unphosphorylated form of NarL, the C-terminal domain is turned against the receiver domain in a manner that would preclude DNA binding. Activation of NarL via phosphorylation of Asp59 must involve transfer of information to the interdomain interface and either rotation or displacement of the DNA-binding C-terminal domain. Docking of a B-DNA duplex against the isolated C-terminal domain in the manner observed in paired protein and other H-T-H proteins suggests a stereochemical basis for DNA sequence preference: T-R-C-C-Y (high affinity) or T-R-C-T-N (low affinity), which is close to the experimentally observed consensus sequence: T-A-C-Y-N. The NarL structure is a model for other members of the FixJ or LuxR family of bacterial transcriptional activators, and possibly to the more distant OmpR and NtrC families as well.

Crystal structure of C-T-C-T-C-G-A-G-A-G. Implications for the structure of the Holliday junction.

The structure of the B-DNA decamer of sequence C-T-C-T-C-G-A-G-A-G shows a crossed arrangement of helices in the C2 crystal lattice. This is the fourth example of a crossed arrangement of B-DNA oligomers in a crystal, and in spite of the fact that each of these four crystallizes in a different lattice, all have nearly identical structures at the crossing contact. This ubiquitous crossing arrangement may be used to generate a structure for the Holliday junction that is fully consistent with the available physical data.

Crystal structure of a covalent DNA-drug adduct: anthramycin bound to C-C-A-A-C-G-T-T-G-G and a molecular explanation of specificity.

A 2.3-A X-ray crystal structure analysis has been carried out on the antitumor drug anthramycin, covalently bound to a ten base pair DNA double helix of sequence C-C-A-A-C-G-T-T-G-G. One drug molecule sits within the minor groove at each end of the helix, covalently bound through its C11 position to the N2 amine of the penultimate guanine of the chain. The stereochemical conformation is C11S, C11aS. The natural twist of the anthramycin molecule in the C11aS conformation matches the twist of the minor groove as it winds along the helix; a C11aR drug would only fit into a left-handed helix. The C11S attachment is roughly equatorial to the overall plane of the molecule, whereas a C11R attachment would be axial and would obstruct the fitting of the drug into the groove. The six-membered ring of anthramycin points toward the 3' end of the chain to which it is covalently attached or toward the end of the helix. The acrylamide tail attached to the five-membered ring extends back along the minor groove toward the center of the helix, binding in a manner reminiscent of netropsin or distamycin. The drug-DNA complex is stabilized by hydrogen bonds from C9-OH, N10, and the end of the acrylamide tail to base pair edges on the floor of the minor groove. The origin of anthramycin specificity for three successive purines arises not from specific hydrogen bonds but from the low twist angles adopted by purine-purine steps in a B-DNA helix. Binding of anthramycin induces a low twist at T-G in the T-G-G sequence of this DNA-drug complex, by comparison with the structure of the free DNA. The origin of anthramycin's preference for adenines flanking the alkylated guanine arises from a netropsin-like fitting of the acrylamide tail into the minor groove.

Crystallographic analysis of C-C-A-A-G-C-T-T-G-G and its implications for bending in B-DNA.

Stacked B-DNA double helices of sequence C-C-A-A-G-C-T-T-G-G exhibit the same 23 degrees bend at -T-G-G C-C-A- across the nonbonded junction between helices that is observed in the middle of the decamer helix of sequence C-A-T-G-G-C-C-A-T-G, even though the space group (hexagonal vs orthorhombic), crystal packing, and connectedness at the center of the bent segment are quite different. An identical bend occurs across the interhelix junction of every monoclinic crystal structure of sequence C-C-A-x-x-x-x-T-G-G, suggesting that T-G-G-C-C-A constitutes a natural bending element in B-DNA. The bend occurs by rolling stacked base pairs about their long axes; there is no "tilt" component. Of the three possible models for A-tract bending--bent-A-tract, junction bends, or bent-non-A--which cannot be distinguished by solution measurements, all crystallographic evidence over the past 10 years unanimously supports the non-A regions as the actual bending loci.

The crystal structure of the trigonal decamer C-G-A-T-C-G-6meA-T-C-G: a B-DNA helix with 10.6 base-pairs per turn.

The B-DNA decanucleotide C-G-A-T-C-G-6meA-T-C-G has been crystallized under the same conditions used earlier for C-G-A-T-C-G-A-T-C-G, but is found to adopt a new trigonal P3(2)21 packing mode instead of the expected orthorhombic P2(1)2(1)2(1) form. Unit cell dimensions a = b = 33.38 A, c = 98.30 A, gamma = 120 degrees, imply ten base-pairs or one complete decamer double helix per asymmetric unit. The 2282 two-sigma data to 2.0 A refine to R = 17.2% with 45 water molecules, 1.5 hexavalent hydrated magnesium complexes, and 0.5 chloride ion per asymmetric unit. Neighboring helices interlock backbone chains and major grooves, crossing at an angle of 120 degrees in a manner that yields an excellent model for a Holliday junction. Local helix parameters differ markedly in the trigonal and orthorhombic structures, with the trigonal helix exhibiting behavior closer to that expected of B-DNA in solution. The trigonal form has an average of 10.6 base-pairs per turn, in contrast to 9.7 base-pairs per turn in the orthorhombic cell. A comparison of all known B-DNA decamer and dodecamer crystal structure analyses indicates that, the greater the cell volume per base-pair (and hence the more open the crystal structure), the closer the mean helix twist approaches an expected 10.6 base-pairs per turn.

Structure of a B-DNA decamer with a central T-A step: C-G-A-T-T-A-A-T-C-G.

The X-ray crystal structure analysis of the decamer C-G-A-T-T-A-A-T-C-G has been carried out to a resolution of 1.5 A. The crystals are space group P2(1)2(1)2(1), cell dimensions a = 38.60 A, b = 39.10 A, c = 33.07 A. The structure was solved by molecular replacement and refined with X-PLOR and NUCLSQ. The final R factor for a model with 404 DNA atoms, 108 water molecules and one magnesium hexahydrate cation is 15.7%. The double helix is essentially isostructural with C-G-A-T-C-G-A-T-C-G, with closely similar local helix parameters. The structure of the T-T-A-A center differs from that found in C-G-C-G-T-T-A-A-C-G-C-G in that the minor groove in our decamer is wide at the central T-A step rather than narrow, and the twist angle of the T-A step is small (31.1 degrees) rather than large. Whereas the tetrad model provides a convenient framework for discussing local DNA helix structure, it cannot be the entire story. The articulated helix model of DNA structure proposes that certain sequence regions of DNA show preferential twisting or bending properties, whereas other regions are less capable of deformation, in a manner that may be useful in sequence recognition by drugs and protein. Further crystal structure analyses should help to delineate the precise nature of sequence-dependent articulation in the DNA double helix.

The structure of B-helical C-G-A-T-C-G-A-T-C-G and comparison with C-C-A-A-C-G-T-T-G-G. The effect of base pair reversals.

The crystal structure of the DNA decamer C-G-A-T-C-G-A-T-C-G has been solved to a resolution of 1.5 A, with a final R-factor of 16.1% for 5,107 two-sigma reflections. Crystals are orthorhombic space group P2(1)2(1)2(1), with cell dimensions a = 38.93 A, b = 39.63 A, c = 33.30 A, and 10 base pairs/asymmetric unit. The final structure contains 404 DNA atoms, 142 water molecules treated as oxygen atoms, and two Mg(H2O)6(2+) complexes. Decamers stack atop one another to simulate continuous helical columns through the crystal, as with three previously solved monoclinic decamers, but the lateral contacts between columns are quite different in the orthorhombic and monoclinic cells. Narrow and wide regions of the minor groove exhibit a single spine or two ribbons of hydration, respectively, and the minor groove is widest when BII phosphate conformations are opposed diagonally across the groove. Phosphate conformation, in turn, appears to have a base sequence dependence. Twist, rise, cup, and roll are linked as has been observed in the three monoclinic decamers and can be characterized by high or low twist profiles. In all five known decamer crystal structures and eight representative dodecamers, a high twist profile is observed with G-C and G-A steps whereas all other R-R steps are low twist profiles (R = purine). A-T and A-C steps are intermediate in character whereas C-A and C-G exhibit behavior that is strongly influenced by the profiles of the preceding and following steps. When sufficient data are in hand, sequence/structure relationships for all helix parameters probably should be considered in a 4-base pair context. At this stage of limited information the problem is compounded because there are 136 unique 4-base steps x-A-B-y in a double helix as compared with only 10 2-base steps A-B.

Structure of a T4 hairpin loop on a Z-DNA stem and comparison with A-RNA and B-DNA loops.

The synthetic DNA oligomer C-G-C-G-C-G-T-T-T-T-C-G-C-G-C-G crystallizes as a Z-DNA hexamer, capped at one end by a T4 loop. The crystals are monoclinic, space group C2, with a = 57.18 A, b = 21.63 A, c = 36.40 A, beta = 95.22 degrees, and one hairpin molecule per asymmetric unit. The structure of the z-hexamer stem was determined by molecular replacement, and the T4 loop was positioned by difference map methods. The final R factor at 2.1 A resolution for hairpin plus 70 water molecules is 20% for 2 sigma data, with a root-mean-square error of 0.26 A. The (C-G)3 stem resembles the free Z-DNA hexamer with minor crystal packing effects. The T4 loop differs from that observed on a B-DNA stem in solution, or in longer loops in tRNA, in that it shows intraloop and intermolecular interactions rather than base stacking on the final base-pair of the stem. Bases T7, T8 and T9 stack with one another and with the sugar of T7. Two T10 bases from different molecules stack between the C1-G12 terminal base-pairs of a third and fourth molecule, to simulate a T.T "base-pair". Distances between thymine N and O atoms suggest that the two thymine bases are hydrogen bonded, and a keto-enol tautomer pair is favored over disordered keto-keto wobble pairs. The hairpin molecules pack in the crystal in herringbone columns in a manner that accounts well for the observed relative crystal growth rates in a, b and c directions. Hydration seems to be most extensive around the phosphate groups, with lesser hydration within the grooves.

The structure of DAPI bound to DNA.

The structure of the DNA fluorochrome 4'-6-diamidine-2-phenyl indole (DAPI) bound to the synthetic B-DNA oligonucleotide C-G-C-G-A-A-T-T-C-G-C-G has been solved by single crystal x-ray diffraction methods, at a resolution of 2.4 A. The structure is nearly isomorphous with that of the native DNA molecule alone. With one DAPI and 25 waters per DNA double helix, the residual error is 21.5% for the 2428 reflections above the 2-sigma level. DAPI inserts itself edgewise into the narrow minor groove, displacing the ordered spine of hydration. DAPI and a single water molecule together span the four AT base pairs at the center of the duplex. The indole nitrogen forms a bifurcated hydrogen bond with the thymine O2 atoms of the two central base pairs, as with netropsin and Hoechst 33258. The preference of all three of these drugs for AT regions of B-DNA is a consequence of three factors: (1) The intrinsically narrower minor groove in AT regions than in GC regions of B-DNA, leading to a snug fit of the flat aromatic drug rings between the walls of the groove. (2) The more negative electrostatic potential within the minor groove in AT regions, attributable in part to the absence of electropositive-NH2 groups along the floor of the groove, and (3) The steric advantage of the absence of those same guanine-NH2 groups, thus permitting the drug molecule to sink deeper into the groove. Groove width and electrostatic factors are regional, and define the relative receptiveness of a section of DNA since they operate over several contiguous base pairs. The steric factor is local, varying from one base pair to the next, and hence is the means of fine-tuning sequence specificity.

X-ray structure of a DNA hairpin molecule.

We have solved the crystal structure of a synthetic DNA hexadecanucleotide of sequence: C-G-C-G-C-G-T-T-T-T-C-G-C-G-C-G, at 2.1 A resolution, and observed that it adopts a monomeric hairpin configuration with a Z-DNA hexamer stem. In the T4 loop the bases stack with one another and with neighbouring molecules of the crystal, and not with base pairs of their own hexamer stem. Two thymine T10 rings from different molecules stack between the C1-G16 ends of a third and a fourth hairpin helix, in a manner that suggests T-T base 'pairing' and simulates a long, 13-base-pair helix. Although such T-T interactions would not be present in solution, they illustrate a remarkable tendency of thymines for self-association. Purine-purine G-A base pairs are known to exist in the anti-anti conformation with an increase in local helix width; it may be that more serious consideration should be given to the possible existence of pyrimidine-pyrimidine C-T base pairs with decreased local helix width, particularly where several such base pairs occur sequentially.

Binding of Hoechst 33258 to the minor groove of B-DNA.

An X-ray crystallographic structure analysis has been carried out on the complex between the antibiotic and DNA fluorochrome Hoechst 33258 and a synthetic B-DNA dodecamer of sequence C-G-C-G-A-A-T-T-C-G-C-G. The drug molecule, which can be schematized as: phenol-benzimidazole-benzimidazole-piperazine, sits within the minor groove in the A-T-T-C region of the DNA double helix, displacing the spine of hydration that is found in drug-free DNA. The NH groups of the benzimidazoles make bridging three-center hydrogen bonds between adenine N-3 and thymine O-2 atoms on the edges of base-pairs, in a manner both mimicking the spine of hydration and calling to mind the binding of the auti-tumor drug netropsin. Two conformers of Hoechst are seen in roughly equal populations, related by 180 degrees rotation about the central benzimidazole-benzimidazole bond: one form in which the piperazine ring extends out from the surface of the double helix, and another in which it is buried deep within the minor groove. Steric clash between the drug and DNA dictates that the phenol-benzimidazole-benzimidazole portion of Hoechst 33258 binds only to A.T regions of DNA, whereas the piperazine ring demands the wider groove characteristic of G.C regions. Hence, the piperazine ring suggests a possible G.C-reading element for synthetic DNA sequence-reading drug analogs.

Template-directed synthesis on short oligoribocytidylates.

The oligonucleotides C(pC)n, n = 4, 5, 6, 7, are efficient templates for the oligomerization of guanosine-5'-phospho-2-methylimidazole (2-MeImpG). They yield oligomeric products that are substantially less regiospecific than those obtained on polycytidylate [poly(C)]. The overall distributions of products obtained on oligo(C)s are generally similar to those of products obtained on oligodeoxycytidylates [oligo(dC)s], but there are substantial differences in the ratios of isomers. The 3'-5'-linked dinucleoside monophosphate GpG efficiently initiates oligomer formation with 2-MeImpG on oligo(C) templates. The pattern of products obtained by chain extension parallels closely that of products obtained directly from 2-MeImpG, except that the former products lack the 5'-terminal phosphate group.

Template-directed synthesis on the pentanucleotide CpCpGpCpC.

The pentanucleotide CpCpGpCpC facilitates the synthesis of oligomers containing G and C from a mixture of the two activated mononucleotides (guanosine 5'-phosphor)-2-methylimidazolide and (cytidine 5'-phosphor)-2-methylimidazolide. The major pentameric product of the template-directed reaction is all 3' to 5'-linked and has the sequence pGpGpCpGpG, which is complementary to that of the template. It can be obtained in a yield of up to 17%, based on the input of the template. The 3' to 5' isomer of GpG is elongated on the template to give GpGpC, GpGpCpG and GpGpCpGpG, while the 2' to 5' isomer does not initiate the synthesis of detectable amounts of longer oligomers.

Template-directed synthesis with 2-aminoadenosine.

A random copolymer, poly(CA), containing approximately equal amounts of cytidine (C) and adenosine (A), when incubated with a mixture of guanosine-5'-phosphoro-(2-methylimidazole) (2-MeImpG) and uridine-5'-phosphoro-(2-methylimidazole) (2-MeImpU), facilitates the incorporation of uridine (U) into oligomeric products with low efficiency. If 2-aminoadenosine (aA) is substituted for adenosine in the template, U is incorporated into the products with much higher efficiency. Random copolymers of C and U act as templates for the efficient synthesis of oligomers from 2-MeImpG and 2-MeImpA only if the concentration of substrates is relatively high (0.1 M). The substitution of 2-MeImpA permits the reaction to occur with much lower substrate concentrations. This effect is most prominent for template containing large amounts of U.

Chromatography on Sephadex LH20 as an efficient purification step after removal of internucleotide 2,2,2-trichloroethyl protective groups from oligoribonucleotide phosphotriesters.

Chromatography on Sephadex LH20, in a linear gradient of methanol in 0.02M TEAB buffer pH 7.5, is proposed as a fast and efficient method for the isolation and purification of protected oligoribonucleotide phosphodiesters obtained by deprotection of internucleotide phosphotriesters, and for the monitoring of the deprotection step itself. Its utility is shown on the example of removal of 2,2,2-trichloroethyl groups from oligoribonucleotide phosphotriester I of sequence CCCAUAA by two methods: /1/ reductive elimination with zinc in the presence of acetylacetone modified as presented here, and /2/ hydrogenolytic dehalogenation over palladium in pyridine. This method of chromatography on Sephadex LH20 is used as a key purification step during the removal of 2,2,2-trichloroethyl groups from I by method /1/ and allows to raise the yield of III during fianl deprotection step from 5 to 65%.

Further studies on oligoribonucleotide synthesis.

Recent results concerning the synthesis of oligoribonucleotides via the phosphotriester method, such as functionalization of ribonucleosides, new phosphorylating agents, 5'-O-sulfonylation and chromatography on Sephadex LH-20 for monitoring the removal of internucleotide phosphotriester groups, are presented. To show that efficiency of a new approach to the synthesis of oligoribonucleotides the pentamer /Up/4U was obtained.

Benzo(a)pyrene-7,8-dihydrodiol 9,10-oxide adenosine and deoxyadenosine adducts: structure and stereochemistry.

The structure and absolute stereoconfigurations of four adenosine adducts with (+/-)-7 alpha,8 beta-dihydroxy-9 beta, 10 beta-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene (BPDE) and their deoxyadenosine analogs have been determined. They result from both cis and trans addition of the N6 amino group of ademine to the 10 position of both enantiomers of BDPE. This was determined from studies of the nuclear magnetic resonance spectra, mass spectra, and circular dichroism spectra, as well as from their pKa values and chemical reactivities.

The chemical synthesis of the anticodon loop of an eukaryotic initiator tRNA containing the hypermodified nucleoside N6-/N-threonylcarbonyl/-adenosine/t6A/1.

In this work, the first example of chemical synthesis of oligoribonucleotide containing the hypermodified nucleoside N6-/N-threonylcarbonyl/-adenosine /t6A/ is presented. Synthesis of the heptamer C-C-C-A-U-t6A-A IX, the sequence of which is related to the anticodon loop of the initiator tRNA from yellow lupine, was achieved by: /i/ phosphotriester block synthesis of suitably protected heptamer VI containing an adenosine unit with a free exo-NH2 group, /ii/ highly effective "one-flask" procedure for the transformation of the free exo-NH2 group of adenosine unit of heptamer VI into a N,N'-disubstituted urea system of t6A of heptamer VII /hypermodification/, and /iii/ final deprotection of VIII /32% total yield/ with the use of a new approach for simultaneous hydrogenolysis /PdO-hydrogen-pyridine/ of the p-nitrobenzyl group and 2,2,2-trichloroethyl groups from carboxyl function of t6A and internucleotide phosphates respectively.