Coexistence of A- and B-form DNA in a single crystal lattice

It is well known that DNA can exist in a variety of conformations which can be interconverted by relatively mild changes in conditions. The in vivo conformation of DNA is usually thought to be the B form, but there is recent evidence that other conformations may be important in DNA-protein recognition. Different fragments of DNA crystallized under virtually identical conditions can form A, B or Z helices. A fragment that adopted an A conformation in a crystal was found in the B conformation in solution, whereas NMR spectroscopy of A-DNA films revealed the presence of a substantial amount of disordered B-DNA. Until now, however, a DNA fragment of a given sequence has not been crystallized in more than one global conformation. We report here an X-ray diffraction study of crystals of the DNA octamer dGGBrUABrUACC. In addition to a 'framework' of A-DNA, which gives discrete X-ray reflections, there are partially disordered B-DNA helices, recognized by their diffuse scattering features.     

Ring-opened alkylated guanine is not repaired in Z-DNA

Since the discovery of Z-DNA by X-ray analysis of the alternated hexanucleotide d(C-G)3 crystals, numerous studies have shown that fragments of natural DNAs can adopt the Z conformation, topological constraints being a major factor stabilizing this conformation. Immunochemical assays using antibodies to Z-DNA provide strong evidence for the presence of Z fragments in chromosomes. The biological role of Z-DNA is not yet known, but it might be involved in gene regulation. Proteins which bind specifically to Z-DNA have been isolated and interactions between Z-DNA and several cellular proteins have been studied. The ability of DNA repair enzymes to maintain the genome's integrity is of major importance to the cell. On alkylation of DNA by chemical carcinogens such as dimethyl sulphate, methyl methanesulphonate, methylnitrosourea or methylnitrosoguanidine, the main target is the N7 of the guanosine residue, yielding 7-methylguanine (mG). In alkaline conditions, the imidazole ring of mG opens up, yielding the ring-opened form 2,6-diamino-4-oxo-5-methylformamidopyrimidine (rom7G); this lesion is a block to DNA replication. It occurs in vivo and is enzymatically removed by the DNA glycosylase. Here we report that the lesion is not excised when present in DNA in the left-handed Z conformation.     

Left-handed helical conformation of poly[d(A-m5C).d(G-T)

Poly[d(G-C)] serves as the prototype for the right-to-left (B to Z) transition in he helical sense of DNA, both in solution and inthe crystal form. However, the question remains as to which other synthetic and natural DNAs have the potential to adopt left-handed conformations. One logical candidate is the canonical alternating purine-pyrimidine sequence d(A-C)n.d(G-T)n which seems to be widely disseminated in eukaryotic genomes. Our approach to this question is based on the enzymatic synthesis of poly[d(A-C).d(G-U)] derivatives with systematic methyl and halogen substitutions in the C-5 position of the pyrimidines C and U. Such modifications in poly[d(G-C)] have previously been shown to potentiate the B to Z transition. Here we report a highly cooperative, reversible, salt- and temperature-dependent transition for poly[d(A-m5C).d(G-T)], a repeat of the d(A-m5C) sequence which may occur in natural DNA. Spectroscopic studies and the demonstrated ability to bind anti-Z-DNA antibodies suggest that the new helical conformation is left-handed and shares structural features with known Z-DNA. However, a novel property, not exhibited by poly[d(G-C)], is the profound temperature dependence of the conformational equilibrium.     

Dependence of Z-DNA antibody binding to polytene chromosomes on acid fixation and DNA torsional strain

There is considerable interest in the existence and significance of alternative conformations of DNA to the right-handed B-form described originally by Watson and Crick. The indirect immunofluorescence observations of Nordheim et al., Arndt-Jovin et al. and Lemeunier et al. that antibodies against left-handed Z-DNA bind to polytene chromosomes have thus assumed considerable importance. However, there is a paradox: some workers observe Z-DNA in interbands and others in bands. We report here that binding of Z-DNA antibodies to Drosophila polytene chromosomes prepared without acid fixation is at background level, and that following acid fixation the same antibody treatment leads to intense fluorescence. Depending on the extent of exposure to 45% acetic acid, fluorescence can occur primarily in interbands or in bands. Furthermore, antibody binding is dependent on elastic torsional strain in the DNA molecules.     

Molecular structure of a left-handed double helical DNA fragment at atomic resolution

The DNA fragment d(CpGpCpGpCpG) crystallises as a left-handed double helical molecule with Watson-Crick base pairs and an antiparallel organisation of the sugar phosphate chains. The helix has two nucleotides in the asymmetric unit and contains twelve base pairs per turn. It differs significantly from right-handed B-DNA.     

Use of an 125I-labelled DNA ligand to probe DNA structure

It no longer seems likely that DNA molecules in situ have a uniform conformation, represented by the classical B-form helix. For example, recent structural studies have shown that in certain conditions DNA can have a left-handed (so-called Z-form) helix, and have revealed extensive sequence-dependent variations of B-DNA helical parameters. Such sequence-dependent variations in DNA structure can be investigated in solution with reagents that bind to DNA in a conformation-dependent manner, and cut one or both strands of the double-helix at the site of binding, as, for example, has been shown for the endonuclease DNase I3. We describe here a simple way to endow a DNA-binding ligand with the ability to cleave DNA--labelling with 125I. The radiochemical damage associated with 125I decay induces a double-stranded DNA break. Using this technique we have shown that a sequence of four consecutive A X T base pairs is a necessary, but not sufficient, condition for strong binding to DNA of the bis-benzamide Hoechst 33258--presumably the other important factor is the conformation of the double-helix at the site of the (A/T)4 sequence. We suggest 125I-Hoechst 33258 may be a useful new probe of DNA structure.     

X-ray diffraction from the side-by-side model of DNA

An intriguing topological problem posed by the double-helical Watson-Crick model of DNA is that of unwinding the intertwined strands during replication. Several workers have recently proposed novel side-by-side (SBS) structures for DNA. In all these models the two strands are joined by complementary Watson-Crick base pairs and the antiparallel polynucleotide strands alternate between short segments of right- and left-handed helix, thus both reducing the amount of intertwining and alleviating the unwinding problem. We show here that there are unacceptable discrepancies between the observed diffraction pattern of B-DNA and that calculated for the original SBS structure. We also describe a simple modification of this model which resolves some of the more serious discrepancies. However, the agreement is still markedly inferior to that obtained for a Watson-Crick model of DNA.     

Structure of the nucleosome core particle at 7 A resolution

The crystal structure of the nucleosome core particle has been solved to 7 A resolution. The right-handed B-DNA superhelix on the outside contains several sharp bends and makes numerous interactions with the histone octamer within. The central turn of superhelix and H3 . H4 tetramer have dyad symmetry, but the H2A . H2B dimers show departures due to interparticle associations.     

异优呫吨酮及其衍生物与DNA相互作用的研究

采用紫外光谱、荧光光谱、圆二色谱以及红外差谱法研究了异优呫吨酮(1,6-二羟基吨酮)(A)及其哌啶衍生物(1-羟基-6-(2-(1-哌啶基)乙氧基)呫吨酮)(B)与小牛胸腺DNA(CT-DNA)的作用方式和作用强度。结果表明,这两种化合物的紫外吸收光谱随DNA浓度的增加表现出减色效应;化合物A和B与DNA作用的结合常数分别为1.5×104,2.8×104 L/mol;DNA-EB体系的荧光强度随着两种化合物浓度的增加发生淬灭现象,化合物A和B的淬灭常数Kq分别是1.9×104,3.7×104 L/mol;两种化合物主要以嵌入方式与CT-DNA发生作用,B与DNA的结合能力比A强;并且呫吨酮芳环的嵌入使DNA螺旋变得松散、碱基堆积增强;化合物B插入后使CT-DNA的构象变化更为明显。研究结果对抗肿瘤药物的研发有一定的指导意义。     

Left-handed properties of a composite structure with metallic wires in a homogeneous Lorentz medium

The electromagnetic properties of a composite structure with metallic wires in a Lorentz medium were studied. The results show that the electromagnetic properties of the medium host influence the plasma resonance of metallic wires and the left-handed character of the composite. The plasma frequency of metallic wires reduces with the rise of permittivity or permeability of the medium host. Also, the negative permeability of the medium can destroy the wires' plasma resonance and prevent the realization of left-handed properties. The high loss of medium permittivity or permeability also inhibits the metallic plasma resonance. The negative influence of the media host on the left-handed properties of the composite structure can be effectively reduced by proper structure design, such as introducing a nonmagnetic medium in the host or using an anisotropic medium.     

基于渔网结构的人工电磁材料结构参数研究

提出了一种基于渔网结构的新型低损耗改进型渔网结构左手材料,研究了其传输特性和材料有效电磁参数,计算表明其具有较宽的负折射带宽和左手带宽(介电常数和磁导率同时为负),同时数值研究了提出的结构的几何参数变化对材料电磁参数的影响.结果表明在结构参数相当大范围内保持了材料的左手物理特性.     

实现左、右旋偏振光可调性的新方法

利用斯托克斯矢量法分析了左(右)旋偏振光的产生原理,并由此推导出了将左(右)旋偏振光调为右(左)旋偏振光的新方法.     

DNA主链构象与脱氧鸟苷组分构象相关性的喇曼分析

对小牛胸腺ＤＮＡ纤维样品溶解３ｈ内的空间构象变化过程进行了喇曼光谱分析，结果显示了表征脱氧鸟苷糖环折迭不同类型的６７２ｃｍ＾－１与６８３ｃｍ＾－１峰的同时存在，与ＤＮＡ分子主链不同特征峰的同时出现之间有着明显的联系。     

SBS改性沥青基吸水膨胀防水材料的研制

以改性沥青 (沥青、SBS、基础油的质量比为 1 0 0∶1 4∶6)为基质 ,加入吸水物质A或吸水物质B ,混合均匀后制成吸水膨胀型防水材料。实验结果表明 ,以此改性沥青为基质的材料具有好的形状保持能力和耐热能力。当A物质与改性沥青的质量比由 0 .67增加到 1 .50时 ,所研制的材料不具有吸水膨胀性能 ;当B物质与改性沥青的质量比为 0 .0 5～ 0 .1 1时 ,所研制的材料的吸水能力适中 ,具有好的吸水性能 ;当B物质与改性沥青的质量比为 0 .0 5及A物质与改性沥青的质量比为 0 .1 5～ 0 .30时 ,所研制的材料吸水能力适中 ,性能优良 ,可用作吸水膨胀材料。材料的吸水膨胀能力随着B物质含量的增加而增大。随着A物质含量的增加 ,吸水膨胀材料的断裂延伸值、弹性恢复率都有减小的趋势。当B物质与改性沥青的质量比小于 0 .2 5时 ,B物质的加入对改性沥青的断裂延伸值和弹性恢复率影响不大。A物质和B物质复配的效果好于单独使用A物质和B物质的效果     

Left-handed Z-DNA is induced by supercoiling in physiological ionic conditions

In physiological ionic conditions (200 mM NaCl), the (dC-dG)16 and (dC-dG)13 blocks in plasmid pRW751 are in a left-handed state when the negative superhelical density of the plasmid is greater than 0.972. As the salt concentration decreases or when (dmC-dG) sequences are present, less negative supercoiling is required to induce the right- to left-handed DNA transition. Furthermore, the single strand-specific nuclease, S1, recognizes and cleaves aberrant structural features at the junction between neighbouring right- and left-handed DNA regions.     

Theoretical investigation of negative refraction in metallic and ferromagnetic composites

A new design of LHM (left handed material) is suggested, in which the wave vector k and the energy flow S (the Poyming veclor) are in the opposite direction. Metallic cores or lines are coated with ferromagnetic layers to obtain negative permittivity and permeability. This design may bring some improvements over the binary design, such as higher homogeneity, smaller volume size, lower power loss, higher convenience and economy. The analytical expressions for the permiltivily s and permeability μ are shown to be negative in certain direction and frequency regions. Two specific structures are theoretically discussed and proved to be left-handed.     

威廉环毛蚓Pheretima guillelmi(Michaelsen)神经内分泌系统的研究

以威廉环毛蚓为实验材料,对其神经内分泌系统的显微、亚显微结构及其对体表水交换的影响作了初步研究.通过大量的实验,说明在威廉环毛蚓的中枢神经系统中存在三种不同类型的细胞,即A、B和C三种细胞.A细胞数量最多,位于脑神经节、咽下神经节和腹神经索中.B细胞主要位于咽下神经节.C细胞主要位于咽下神经节和腹神经索中.神经内分泌活动与季节有关,A细胞和C细胞的分泌活动随季节变化而不同,B细胞的内分泌活动受季节的影响不大.蚯蚓体表的水交换受神经内分泌调控,这种因子仅存在于脑神经节内.