NMR studies of an extrahelical cytosine in an A.T rich region of a deoxyribodecanucleotide

Structure of the 1,N(2)-etheno-2'-deoxyguanosine lesion in the 3'-G(epsilon dG)T-5' sequence opposite a one-base deletion

The structure of the 1,N(2)-ethenodeoxyguanosine lesion (1,N(2)-epsilondG) has been characterized in 5'-d(CGCATXGAATCC)-3'.5'-d(GGATTCATGCG)-3' (X = 1,N(2)-epsilondG), in which there is no dC opposite the lesion. This duplex (named the 1-BD duplex) models the product of translesion bypass of 1,N(2)-epsilondG by Sulfolobus solfataricus P2 DNA polymerase IV (Dpo4) [Zang, H., Goodenough, A. K., Choi, J. Y., Irimia, A., Loukachevitch, L. V., Kozekov, I. D., Angel, K. C., Rizzo, C. J., Egli, M., and Guengerich, F. P. (2005) J. Biol. Chem. 280, 29750-29764], leading to a one-base deletion. The T(m) of this duplex is 6 degrees C higher than that of the duplex in which dC is present opposite the 1,N(2)-epsilondG lesion and 8 degrees C higher than that of the unmodified 1-BD duplex. Analysis of NOEs between the 1,N(2)-epsilondG imidazole and deoxyribose H1' protons and between the 1,N(2)-epsilondG etheno H6 and H7 protons and DNA protons establishes that 1,N(2)-epsilondG adopts the anti conformation about the glycosyl bond and that the etheno moiety is accommodated within the helix. The resonances of the 1,N(2)-epsilondG H6 and H7 etheno protons shift upfield relative to the monomer 1,N(2)-epsilondG, attributed to ring current shielding, consistent with their intrahelical location. NMR data reveal that Watson-Crick base pairing is maintained at both the 5' and 3' neighbor base pairs. The structure of the 1-BD duplex has been refined using molecular dynamics calculations restrained by NMR-derived distance and dihedral angle restraints. The increased stability of the 1,N(2)-epsilondG lesion in the absence of the complementary dC correlates with the one-base deletion extension product observed during the bypass of the 1,N(2)-epsilondG lesion by the Dpo4 polymerase, suggesting that stabilization of this bulged intermediate may be significant with regard to the biological processing of the lesion.

Bulge migration of the malondialdehyde OPdG DNA adduct when placed opposite a two-base deletion in the (CpG)3 frameshift hotspot of the Salmonella typhimurium hisD3052 gene

The OPdG adduct N (2)-(3-oxo-1-propenyl)dG, formed in DNA exposed to malondialdehyde, was introduced into 5'-d(ATCGC XCGGCATG)-3'.5'-d(CATGCCGCGAT)-3' at pH 7 (X = OPdG). The OPdG adduct is the base-catalyzed rearrangement product of the M 1dG adduct, 3-(beta- d-ribofuranosyl)pyrimido[1,2- a]purin-10(3 H)-one. This duplex, named the OPdG-2BD oligodeoxynucleotide, was derived from a frameshift hotspot of the Salmonella typhimuium hisD3052 gene and contained a two-base deletion in the complementary strand. NMR spectroscopy revealed that the OPdG-2BD oligodeoxynucleotide underwent rapid bulge migration. This hindered its conversion to the M 1dG-2BD duplex, in which the bulge was localized and consisted of the M 1dG adduct and the 3'-neighbor dC [ Schnetz-Boutaud, N. C. , Saleh, S. , Marnett, L. J. , and Stone, M. P. ( 2001) Biochemistry 40, 15638- 15649 ]. The spectroscopic data suggested that bulge migration transiently positioned OPdG opposite dC in the complementary strand, hindering formation of the M 1dG-2BD duplex, or alternatively, reverting rapidly formed intermediates in the OPdG to M 1dG reaction pathway when dC was placed opposite from OPdG. The approach of initially formed M 1dG-2BD or OPdG-2BD duplexes to an equilibrium mixture of the M 1dG-2BD and OPdG-2BD duplexes was monitored as a function of time, using NMR spectroscopy. Both samples attained equilibrium in approximately 140 days at pH 7 and 25 degrees C.

Sequence context effect on the structure of nitrous acid induced DNA interstrand cross-links

In the preceding paper in this journal, we described the solution structure of the nitrous acid cross-linked dodecamer duplex [d(GCATCCGGATGC)]2 (the cross-linked guanines are underlined). The structure revealed that the cross-linked guanines form a nearly planar covalently linked 'G:G base pair', with the complementary partner cytidines flipped out of the helix. Here we explore the flanking sequence context effect on the structure of nitrous acid cross-links in [d(CG)]2 and the factors allowing the extrahelical cytidines to adopt such fixed positions in the minor groove. We have used NMR spectroscopy to determine the solution structure of a second cross-linked dodecamer duplex, [d(CGCTACGTAGCG)]2, which shows that the identity of the flanking base pairs significantly alters the stacking patterns and phosphate backbone conformations. The cross-linked guanines are now stacked well on adenines preceding the extrahelical cytidines, illustrating the importance of purine- purine base stacking. Observation of an imino proton resonance at 15.6 p.p.m. provides evidence for hydrogen bonding between the two cross-linked guanines. Preliminary structural studies on the cross-linked duplex [d(CGCGACGTCGCG)]2 show that the extrahelical cytidines are very mobile in this sequence context. We suggest that favorable van der Waals interactions between the cytidine and the adenine 2 bp away from the cross-link localize the cytidines in the previous cross-linked structures.

Structural study of DNA duplex containing an N-(2-deoxy-beta-D-erythro-pentofuranosyl) formamide frameshift by NMR and restrained molecular dynamics

The presence of an N-(2-deoxy-beta-D-erythro-pentofuranosyl) formamide (F) residue, a ring fragmentation product of thymine, in a frameshift context in the sequence 5'-d-(AGGACCACG)*d(CGTGGFTCCT) has been studied by 1H and 31P nuclear magnetic resonance (NMR) and molecular dynamics. Two-dimensional NMR studies show that the formamide residue, whether the cis or trans isomer, is rotated out of the helix and that the bases on either side of the formamide residue in the sequence, G14 and T16, are stacked over each other in a way similar to normal B-DNA. The cis and trans isomers were observed in the ratio 3:2 in solution. Information extracted from 31P NMR data reveal a modification of the phosphodiester backbone conformation at the extrahelical site, which is also observed during the molecular dynamics simulations.

New structural and mechanistic insight into the A-rule and the instructional and non-instructional behavior of DNA photoproducts and other lesions

The A-rule in mutagenesis was originally proposed to explain the preponderance of X-->T mutations observed for abasic sites and UV damaged sites. It was deduced that when a polymerase was faced with a non-instructional lesion, typified by an abasic site, it would preferentially incorporate an A. In the absence of any other compelling explanation, any lesion causing an X-->T mutation has often been classified as non-instructional to account for its apparent lack of instructional ability. The A-rule and the classification of lesions as non-instructional were formulated before the active sites of any polymerases or the mechanism by which they synthesized DNA were known. Since then, much structural and kinetic data on DNA polymerases has emerged to suggest mechanistic explanations for the A-rule and the instructive and non-instructive behavior of lesions such as cis-syn dimers. Polymerases involved in the replication of undamaged DNA have highly constrained active sites that evolved to only accommodate the templating base and the complementary nucleotide and as a result are relatively intolerant of modifications that alter the size and shape of the nascent base pair. On the other hand, DNA damage bypass polymerases have much more open and less constrained active sites, which are much more tolerant of modifications. An otherwise instructional lesion would become non-instructional if it were unable to fit into the active site, and thereby behave transiently like an abasic site, leading to the insertion of whichever nucleotide is favored by the polymerase, generally an A. In this review, what is known about the active sites and mechanisms of replicative and DNA damage bypass polymerases will be discussed with regard to the A-rule and non-instructive behavior of lesions, typified by dipyrimidine photoproducts.

The exocyclic 1,N2-deoxyguanosine pyrimidopurinone M1G is a chemically stable DNA adduct when placed opposite a two-base deletion in the (CpG)3 frameshift hotspot of the Salmonella typhimurium hisD3052 gene

The pyrimidopurinone adduct M1G [3-(2'-deoxy-beta-D-erythro-pentofuranosyl)pyrimido[1,2-a]-purin-10(3H)-one], formed in DNA upon exposure to malondialdehyde or base propenals, was incorporated into 5'-d(ATCGCMCGGCATG)-3'-5'-d(CATGCCGCGAT)-3', where M = M1G. This duplex contained a two-nucleotide bulge in the modified strand, and was named the M1G-2BD oligodeoxynucleotide. It provided a model for -2 bp strand slippage deletions associated with the (CpG)3-iterated repeat hotspot for frameshift mutations from the Salmonella typhimurium hisD3052 gene. M1G was chemically stable in the M1G-2BD duplex at neutral pH. The two-base bulge in the M1G-2BD oligodeoxynucleotide was localized and consisted of M1G and the 3'-neighbor deoxycytosine. The intrahelical orientation of M1G was established from a combination of NOE and chemical shift data. M1G was in the anti conformation about the glycosyl bond. The 3'-neighbor deoxycytosine appeared to be extruded toward the major groove. In contrast, when M1G was placed into the corresponding fully complementary (CpG)3-iterated repeat duplex at neutral pH, spontaneous and quantitative ring-opening to N(2)-(3-oxo-1-propenyl)-dG (the OPG adduct) was facilitated [Mao, H., Reddy, G. R., Marnett, L. J., and Stone, M. P. (1999) Biochemistry 38, 13491-13501]. The structure of the M1G-2BD duplex suggested that the bulged sequence lacked a cytosine amino group properly positioned to facilitate opening of M1G and supports the notion that proper positioning of deoxycytosine complementary to M1G is necessary to promote ring-opening of the exocyclic adduct in duplex DNA. The structure of the M1G-2BD duplex was similar to that of the structural analogue 1,N(2)-propanodeoxyguanosine (PdG) in the corresponding PdG-2BD duplex [Weisenseel, J. P., Moe, J. G., Reddy, G. R., Marnett, L. J., and Stone, M. P. (1995) Biochemistry 34, 50-64]. The fixed position of the bulged bases in both instances suggests that these exocyclic adducts do not facilitate transient bulge migration.

DNA structural perturbation induced by the CPI-derived DNA interstrand cross-linker: molecular mechanisms for the sequence specific recognition

The highly potent cytotoxic DNA-DNA cross-linker consists of two cyclopropa[c]pyrrolo[3, 4-3]indol-4(5H)-ones indoles [(+)-CPI-I] joined by a bisamido pyrrole (abbreviated to "Pyrrole"). The Pyrrole is a synthetic analog of Bizelesin, which is currently in phase II clinical trials due to its excellent in vivo antitumor activity. The Pyrrole has 10 times more potent cytotoxicity than Bizelesin and mostly form DNA-DNA interstrand cross-links through the N3 of adenines spaced 7 bp apart. The Pyrrole requires a centrally positioned GC base pair for high cross-linking reactivity (i.e., 5'-T(A/T)2G(A/T)2A*-3'), while Bizelesin prefers purely AT-rich sequences (e.g., 5'-T(A/T)4 or 5A*-3', where T represents the cross-strand adenine alkylation and A* represents an adenine alkylation) (Park et al., 1996). In this study, the high-field 1H-NMR and rMD studies are conducted on the 11-mer DNA duplex adduct of the Pyrrole where the 5'-TTAGTTA*-3' sequence is cross-linked by the drug. A severe structural perturbation is observed in the intervening sequences of cross-linking site, while a normal B-DNA structure is maintained in the region next to the drug-modified adenines. Based upon these observations, we propose that the interplay between the bisamido pyrrole unit of the drug and central G/C base pair (hydrogen-bonding interactions) is involved in the process of cross-linking reaction, and sequence specificity is the outcome of those interactions. This study suggests a mechanism for the sequence specific cross-linking reaction of the Pyrrole, and provides a further insight to develop new DNA sequence selective and distortive cross-linking agents.

Preparation and characterization of DNA containing a site-specific nonadjacent cyclobutane thymine dimer of the type implicated in UV-induced -1 frameshift mutagenesis

One mechanism for the origin of UV-induced -1 deletion mutations involves the bypass of a nonadjacent cis-syn cyclobutane pyrimidine dimer containing a single intervening nucleotide. To begin to investigate this mechanism, we required a method for obtaining a single, site-specific, nonadjacent dimer. One approach to the preparation of a nonadjacent dimer is to irradiate a DNA duplex containing a centrally located TNT sequence in which the two T's are paired to an AA sequence in an otherwise fully complementary strand. Triplet-sensitized irradiation of the duplex formed between the 13-mer d(GAGTATCTATGAG) and the 12-mer d(CTCATAATACTC) on ice gave a major product that could be reverted to the parent 13-mer by 254 nm irradiation. Proton NMR experiments established the major product to be the nonadjacent cis-syn cyclobutane dimer formed between the two T's of the TCT sequence. Melting temperature studies show that the nonadjacent dimer is more destabilizing to DNA duplex structure than a normal cis-syn dimer and is as stable as the parental bulged DNA duplex. The nonadjacent dimer-containing 13-mer was ligated into a 51-mer and used as a template for primer-extension studies by DNA polymerases. The nonadjacent dimer could not be bypassed by Sequenase Version 2.0 and terminated synthesis primarily prior to and opposite the 3'-T of the dimer. In contrast, approximately 30% of the dimer was bypassed by an exonuclease-deficient (exo-) Klenow fragment, and termination occurred primarily opposite the 3'- and 5'-T's of the dimer. Bypass of the nonadjacent dimer by exo(-) Klenow fragment led primarily to a single-nucleotide deletion mutation as well as small amounts of a full-length product and a four-nucleotide deletion that could be explained by a primer misalignment mechanism.

NMR structure and dynamics of an RNA motif common to the spliceosome branch-point helix and the RNA-binding site for phage GA coat protein

The RNA molecules that make up the spliceosome branch-point helix and the binding site for phage GA coat protein share a secondary structure motif in which two consecutive adenine residues occupy the strand opposite a single uridine, creating the potential to form one of two different A.U base pairs while leaving the other adenine unpaired or bulged. During the splicing of introns out of pre-mRNA, the 2'-OH of the bulged adenine participates in the transesterification reaction at the 5'-exon and forms the branch-point residue of the lariat intermediate. Either adenine may act as the branch-point residue in mammals, but the 3'-proximal adenine does so preferentially. When bound to phage GA coat protein, the bulged adenine loops out of the helix and occupies a binding pocket on the surface of the protein, forming a nucleation complex for phage assembly. The coat protein can bind helices with bulged adenines at either position, but the 3'-proximal site binds with greater affinity. We have studied this RNA motif in a 21 nucleotide hairpin containing a GA coat protein-binding site whose four nucleotide loop has been replaced by a more stable loop from the related phage Ms2. Using heteronuclear NMR spectroscopy, we have determined the structure of this hairpin to an overall precision of 2.0 A. Both adenine bases stack into the helix, and while all available NOE and coupling constant data are consistent with both possible A.U base pairs, the base pair involving the 5'-proximal adenine appears to be the major conformation. The 3'-proximal bulged adenine protonates at unusually high pH, and to account for this, we propose a model in which the protonated adenine is stabilized by a hydrogen bond to the uridine O2 of the A.U base pair. The 2'-OH of the bulged adenine adopts a regular A-form helical geometry, suggesting that in order to participate in the splicing reaction, the conformation of the branch-point helix in the active spliceosome may change from the conformation described here. Thus, while the adenine site preferences of the spliceosome and of phage GA may be due to protein factors, the preferred adenine is predisposed in the free RNA to conformational rearrangement involved in formation of the active complexes.

Solution structure of N-(2-deoxy-D-erythro-pentofuranosyl)urea frameshifts, one intrahelical and the other extrahelical, by nuclear magnetic resonance and molecular dynamics

The presence of a N-(2-deoxy-D-erythro pentofuranosyl)urea (henceforth referred to as deoxyribosylurea) residue, ring fragmentation product of a thymine, in a frameshift situation in the sequence 5'd(AGGACCACG).d(CGTGGurTCCT) has been studied by 1H and 31P nuclear magnetic resonance and molecular dynamics. At equilibrium, two species are found in slow exchange. We observe that the deoxyribosylurea residue can be either intra- or extrahelical within structures which otherwise do not deviate strongly from that of a B-DNA as observed by NMR. Our study suggests that this is determined by the nature and number of hydrogen bonds which this residue can form as a function of two possible isomers. There are two possible structures for the urea side chain, either cis or trans for the urido bond which significantly changes the hydrogen bonding geometry of the residue. In the intrahelical species, the cis isomer can form two good hydrogen bonds with the bases on the opposite strand in the intrahelical species, A4 and C5, which is not the case for the trans isomer. This results in a kink in the helical axis. For the major extrahelical species, the situation is reversed. The trans isomer is able to form two good hydrogen bonds, with G13 on the same strand and A7 on the opposite strand. For the extrahelical species, the cis isomer can form only one hydrogen bond. In this major structure the NMR data show that the bases which are on either side of the deoxyribosylurea residue in the sequence, G14 and T16, are stacked over each other in a way similar to a normal B-DNA structure. This requires the formation of a loop for the backbone between these two residues. This loop can belong to one of two families, right- or left-handed. In a previous study of an abasic frameshift [Cuniasse et al. (1989) Biochemistry 28, 2018-2026], a left-handed loop was observed, whereas in this study a right-handed loop is found for the first time in solution. The deoxyribosylurea residue lies in the minor groove and can form both an intra- and an interstrand hydrogen bond.

Creating RNA bulges: cleavage of RNA in RNA/DNA duplexes by metal ion catalysis

The manipulation of a single-stranded RNA target by forming different RNA/antisense hybrids demonstrates the possibility of cleaving the RNA strand within duplexes. This was achieved using the sequence composition of the antisense oligonucleotide, an approach that results in various bulges [unpaired base(s)] in the RNA target, which is then cleavable at these specific bulge sites under free metal ion or metal complex catalysis. RNA cleavages promoted by metal ions were performed under mild conditions and characterized by separating the RNA fragments carrying end label. The observed products result from intramolecular transesterification causing RNA strand scission. No detectable cleavage of the RNA was observed with either a fully complementary RNA/antisense hybrid or a bulged base in the antisense strand. A molecular modeling study of the RNA backbone suggests that the local conformation of the RNA backbone at a bulge in such hybrid duplexes greatly facilitates the metal-assisted catalytic cleavage. Endonucleolytic RNA cleavage within an RNA/antisense hybrid by metal complexes attached to the antisense oligonucleotide might lead to a new approach in antisense technology with artificial ribonucleases which operate with catalytic turnover.

DNA structural forms

In the years that have passed since the publication of Wolfram Saenger's classic book on nucleic acid structure (Saenger, 1984), a considerable amount of new data has been accumulated on the range of conformations which can be adopted by DNA. Many unusual species have joined the DNA zoo, including new varieties of two, three and four stranded helices. Much has been learnt about intrinsic DNA curvature, dynamics and conformational transitions and many types of damaged or deformed DNA have been investigated. In this article, we will try to summarise this progress, pointing out the scope of the various experimental techniques used to study DNA structure, and, where possible, trying to discern the rules which govern the behaviour of this subtle macromolecule. The article is divided into six major sections which begin with a general discussion of DNA structure and then present successively, B-DNA, DNA deformations, A-DNA, Z-DNA and DNARNA hybrids. An extensive set of references is included and should serve the reader who wishes to delve into greater detai.

Modeling large RNAs and ribonucleoprotein particles using molecular mechanics techniques

There is a growing body of low-resolution structural data that can be utilized to devise structural models for large RNAs and ribonucleoproteins. These models are routinely built manually. We introduce an automated refinement protocol to utilize such data for building low-resolution three-dimensional models using the tools of molecular mechanics. In addition to specifying the positions of each nucleotide, the protocol provides quantitative estimates of the uncertainties in those positions, i.e., the resolution of the model. In typical applications, the resolution of the models is about 10-20 A. Our method uses reduced representations and allows us to refine three-dimensional structures of systems as big as the 16S and 23S ribosomal RNAs, which are about one to two orders of magnitude larger than nucleic acids that can be examined by traditional all-atom modeling methods. Nonatomic resolution structural data--secondary structure, chemical cross-links, chemical and enzymatic footprinting patterns, protein positions, solvent accessibility, and so on--are combined with known motifs in RNA structure to predict low-resolution models of large RNAs. These structural constraints are imposed on the RNA chain using molecular mechanics-type potential functions with parameters based on the quality of experimental data. Surface potential functions are used to incorporate shape and positional data from electron microscopy image reconstruction experiments into our models. The structures are optimized using techniques of energy refinement to get RNA folding patterns. In addition to providing a consensus model, the method finds the range of models consistent with the data, which allows quantitative evaluation of the resolution of the model. The method also identifies conflicts in the experimental data. Although our protocol is aimed at much larger RNAs, we illustrate these techniques using the tRNA structure as an example and test-bed.