Orthorhombic crystal structure of the A-DNA octamer d(GTACGTAC). Comparison with the tetragonal structure

Base pairing and structural insights into the 5-formylcytosine in RNA duplex

5-Formylcytidine (f⁵C), a previously discovered natural nucleotide in the mitochondrial tRNA of many species including human, has been recently detected as the oxidative product of 5-methylcytidine (m⁵C) through 5-hydroxymethylcytidine (hm⁵C) in total RNA of mammalian cells. The discovery indicated that these cytosine derivatives in RNA might also play important epigenetic roles similar as in DNA, which has been intensively investigated in the past few years. In this paper, we studied the base pairing specificity of f⁵C in different RNA duplex contexts. We found that the 5-formyl group could increase duplex thermal stability and enhance base pairing specificity. We present three high-resolution crystal structures of an octamer RNA duplex [5′-GUA(f⁵C)GUAC-3′]₂ that have been solved under three crystallization conditions with different buffers and pH values. Our results showed that the 5-formyl group is located in the same plane as the cytosine base and forms an intra-residue hydrogen bond with the amino group in the N4 position. In addition, this modification increases the base stacking between the f⁵C and the neighboring bases while not causing significant global and local structure perturbations. This work provides insights into the effects of 5-formylcytosine on RNA duplex.

Optimization of the CHARMM additive force field for DNA: Improved treatment of the BI/BII conformational equilibrium

The B-form of DNA can populate two different backbone conformations: BI and BII, defined by the difference between the torsion angles ε and ζ (BI = ε-ζ < 0 and BII = ε-ζ > 0). BI is the most populated state, but the population of the BII state, which is sequence dependent, is significant and accumulating evidence shows that BII affects the overall structure of DNA, and thus influences protein-DNA recognition. This work presents a reparametrization of the CHARMM27 additive nucleic acid force field to increase the sampling of the BII form in MD simulations of DNA. In addition, minor modifications of sugar puckering were introduced to facilitate sampling of the A form of DNA under the appropriate environmental conditions. Parameter optimization was guided by quantum mechanical data on model compounds, followed by calculations on several DNA duplexes in the condensed phase. The selected optimized parameters were then validated against a number of DNA duplexes, with the most extensive tests performed on the EcoRI dodecamer, including comparative calculations using the Amber Parm99bsc0 force field. The new CHARMM model better reproduces experimentally observed sampling of the BII conformation, including sampling as a function of sequence. In addition, the model reproduces the A form of the 1ZF1 duplex in 75 % ethanol, and yields a stable Z-DNA conformation of duplex (GTACGTAC) in its crystal environment. The resulting model, in combination with a recent reoptimization of the CHARMM27 force field for RNA, will be referred to as CHARMM36.

Selenium derivatization of nucleic acids for crystallography

The high-resolution structure of the DNA (5'-GTGTACA-C-3') with the selenium derivatization at the 2'-position of T2 was determined via MAD and SAD phasing. The selenium-derivatized structure (1.28 A resolution) with the 2'-Se modification in the minor groove is isomorphorous to the native structure (2.0 A). To directly compare with the conventional bromine derivatization, we incorporated bromine into the 5-postion of T4, determined the bromine-derivatized DNA structure at 1.5 A resolution, and found that the local backbone torsion angles and solvent hydration patterns were altered in the structure with the Br incorporation in the major groove. Furthermore, while the native and Br-derivatized DNAs needed over a week to form reasonable-size crystals, we observed that the Se-derivatized DNAs grew crystals overnight with high-diffraction quality, suggesting that the Se derivatization facilitated the crystal formation. In addition, the Se-derivatized DNA sequences crystallized under a broader range of buffer conditions, and generally had a faster crystal growth rate. Our experimental results indicate that the selenium derivatization of DNAs may facilitate the determination of nucleic acid X-ray crystal structures in phasing and high-quality crystal growth. In addition, our results suggest that the Se derivatization can be an alternative to the conventional Br derivatization.

Free energy and structural pathways of base flipping in a DNA GCGC containing sequence

Structural distortions of DNA are essential for its biological function due to the genetic information of DNA not being physically accessible in the duplex state. Base flipping is one of the simplest structural distortions of DNA and may represent an initial event in strand separation required to access the genetic code. Flipping is also utilized by DNA-modifying and repair enzymes to access specific bases. It is typically thought that base flipping (or base-pair opening) occurs via the major groove whereas minor groove flipping is only possible when mediated by DNA-binding proteins. Here, umbrella sampling with a novel center-of-mass pseudodihedral reaction coordinate was used to calculate the individual potentials of mean force (PMF) for flipping of the Watson-Crick (WC) paired C and G bases in the CCATGCGCTGAC DNA dodecamer. The novel reaction coordinate allowed explicit investigation of the complete flipping process via both the minor and major groove pathways. The minor and major groove barriers to flipping are similar for C base flipping while the major groove barrier is slightly lower for G base flipping. Minor groove flipping requires distortion of the WC partner while the flipping base pulls away from its partner during major groove flipping. The flipped states are represented by relatively flat free energy surfaces, with a small, local minimum observed for the flipped G base. Conserved patterns of phosphodiester backbone dihedral distortions during flipping indicate their essential role in the flipping process. During flipping, the target base tracks along the respective grooves, leading to hydrogen-bonding interactions with neighboring base-pairs. Such hydrogen-bonding interactions with the neighboring sequence suggest a novel mechanism of sequence dependence in DNA dynamics.

Structural variability of A-DNA in crystals of the octamer d(pCpCpCpGpCpGpGpG)

We have determined the structure of the synthetic DNA octamer d(pCpCpCpGpCpGpGpG) in five different crystal forms by single crystal X-ray diffraction. One crystal belongs to the space group P4(3)2(1)2 with a = b = 41.77, c = 25.15 A, whereas all others have the space group P2(1)2(1)2(1) with progressively decreasing unit cell volumes. In all crystals the octamer forms duplexes of A-DNA and all crystals display a similar packing mode, typical for A-DNA. The structure of the duplex varies from loose to very compact when going from one crystal form to another. The most compact form exhibits a volume of 995 A3 per base pair. Such a high density has never been found in A-DNA, being more characteristic of Z-DNA crystals. A comparison of the most with the least compact forms gives a RMS value of 1.7 A, with the distance between the phosphate centers through the major groove being almost twice shorter in the compact form. The phosphate-phosphate separation across the major groove in the compact form is extremely small, 0.7 A. The helical parameters also vary significantly in the various crystal forms. Differences in the helical twist can reach 13 degrees in the same step of the octamer in different crystal forms. The results prove that A-DNA is structurally very variable and demonstrate that the local structure of the same DNA fragment can strongly depend on the crystal environment.

Methylation of the Z-DNA decamer d(GC)5 potentiates the formation of A-DNA: crystal structure of d(Gm5CGm5CGCGCGC)

It is well known that methylation of alternating Py x Pu sequences potentiates the formation of Z-DNA. However, we have now observed that methylation of the alternating Z-DNA oligomer d(GCGCGCGCGC), which starts with a 5'-purine, unexpectedly stabilizes the A-DNA conformation. The double methyl derivative d(Gm5CGm5CGCGCGC), which crystallizes as duplex A-DNA in the hexagonal space group P6(1)22, a = b = 39.33 A and c = 77.93 A with one strand per asymmetric unit and six duplexes in the unit cell, refined to an R factor of 19.1 for 204 DNA atoms and 43 solvent molecules. This is the first report of a DNA sequence crystallized in both right and left-handed conformations, allowing structural comparisons not previously possible and, more importantly, this is the first time that methylation has been shown to potentiate the formation of A-DNA from a sequence known to crystallize as Z-DNA. From this study, ten base-pairs appear to be the critical length in determining the handedness of d(GC)n-type sequences in the crystalline state. Because methylation of nuclear DNA is linked to a number of cellular processes, including transcriptional inactivation, this study has important implications for the role of A-DNA in methylated regions of genomic DNA and, thus, in the regulation of gene expression. In this context, the structure of d(Gm5Cm5CGCGCGC) will be compared with that of the alternating A-DNA decamer d(GCACGCGTGC) and the alternating Z-DNA decamer d(GCGCGCGCGC) and discussed in terms of the forces that govern the handedness of duplex DNA oligomers.

Crystal structure of the self-complementary 5'-purine start decamer d(GCGCGCGCGC) in the Z-DNA conformation. I

Alternating self-complementary oligonucleotides starting with a 5'-pyrimidine usually form left-handed Z-DNA; however, with a 5'-purine start sequence they form the right-handed A-DNA. Here we report the crystal structure of the decamer d(GCGCGCGCGC) with a 5'-purine start in the Z-DNA form. The decamer crystallizes in the hexagonal space group P6(5)22, unit cell dimensions a = b = 18.08 and c = 43.10 A, with one of the following four dinucleotide diphosphates in the asymmetric unit: d(pGpC)/d(GpCp)/d(pCpG)/d(CpGp). The molecular replacement method, starting with d(pGpC) of the isomorphous Z-DNA hexamer d(araC-dG)3 without the 2'-OH group of arabinose, was used in the structure analysis. The method gave the solution only after the sugar-phosphate conformation of the GpC step was manipulated. The refinement converged to a final R value of 18.6% for 340 unique reflections in the resolution range 8.0-1.9 A. A result of the sequence alternation is the alternation in the nucleotide conformation; guanosine is C3'-endo, syn, and cytidine is C2'-endo, anti. The CpG step phosphodiester conformation is the same as ZI or ZII, whereas that of the GpC step phosphodiester is "intermediate" in the sense that zeta (O3'-P bond) is the same as ZII but alpha (P-O5' bond) is the same as ZI. The duplexes generated from the dinucleotide asymmetric unit are stacked one on top of the other in the crystal to form an infinite pseudocontinuous helix. This renders it a quasi-polymerlike structure that has assumed the Z-DNA conformation further strengthened by the long inner Z-forming stretch d(CG)4. An interesting feature of the structure is the presence of water strings in both the major and the minor grooves. In the minor groove the cytosine carbonyl oxygen atoms of the GpC and CpG steps are cross-bridged by water molecules that are not themselves hydrogen bonded but are enclosed by the water rings in the mouth of the minor groove. In the major groove three independent water molecules form a zigzagging continuous water string that runs throughout the duplex.