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PubMed1986

[Dynamic and static chromatin].

It was found that the dependence of the viscosity of calf thymus chromatin dispersions and human leukocytes on ethidium bromide concentration had two peaks indicative of domains with circular supercoiled DNA and varying resistance to ultrasound in the cells and isolated chromatin. The hypothesis of V. D. Paponov and P. S. Gromov (Bull. Exp. Biol. Med., N5, 590, 1985) on the transformation of static relations of nucleosome DNA-containing nuclei into dynamic, after chromatin exposure to ultrasound due to DNA linearization in chromatin domains possessing circular supercoiled DNA, has been confirmed.

Animals↗
PubMedDate not supplied

Novel shielded transferrin-polyethylene glycol-polyethylenimine/DNA complexes for systemic tumor-targeted gene transfer.

Tumor-targeting DNA complexes which can readily be generated by the mixing of stable components and freeze-thawed would be very advantageous for their subsequent application as medical products. Complexes were generated by the mixing of plasmid DNA, linear polyethylenimine (PEI22, 22 kDa) as the main DNA condensing agent, PEG-PEI (poly(ethylene glycol)-conjugated PEI) for surface shielding, and Tf-PEG-PEI (transferrin-PEG-PEI) to provide a ligand for receptor-mediated cell uptake. Within the shielding conjugates, PEG chains of varying size (5, 20, or 40 kDa) were conjugated with either linear PEI22 (22 kDa) or branched PEI25 (25 kDa). The three polymer components were mixed together at various ratios with DNA; particle size, surface charge, in vitro transfection activity, and systemic gene delivery to tumors was investigated. In general, increasing the proportion of shielding conjugate in the complex reduced surface charge, particle size, and in vitro transfection efficiency in transferrin receptor-rich K562 cells. The particle size or surface charge of the complexes containing the PEG-PEI conjugate did not significantly change after freeze-thawing, while complexes without the shielding conjugate aggregated. Complexes containing PEG-PEI conjugate efficiently transfected K562 cells after freeze-thawing. Furthermore the systemic application of freeze-thawed complexes exhibited in vivo tumor targeted expression. For complexes containing the luciferase reporter gene the highest expression was found in tumor tissue of mice. An optimum formulation for in vivo application, PEI22/Tf-PEG-PEI/PEI22-PEG5, containing plasmid DNA encoding for the tumor necrosis factor (TNF-alpha), inhibited tumor growth in three different murine tumor models. These new DNA complexes offer simplicity and convenience, with tumor targeting activity in vivo after freeze-thawing.

Animals↗
PubMedDate not supplied

Linearized free maxicircle DNA in Crithidia fasciculata is a product of topoisomerase II-mediated cleavage.

Linearized free maxicircle DNA, present in detergent lysates of Crithidia fasciculata mitochondria, was thought to be a replication intermediate formed during rolling circle replication of maxicircle DNA. Gel electrophoresis of the linearized free maxicircles indicated that they were slightly larger than the maxicircle genome, raising the possibility of the presence of terminal repetitions (Hajduk, S.L., Klein, V.A. and Englund, P.T. (1984) Cell 36, 483-492). We recently found, however, that maxicircles replicate by a theta-mechanism, and not as rolling circles (Carpenter, L.R. and Englund, P.T. (1995) Mol. Cell Biol. 15, 6794-6803). Given that theta-replication does not easily explain the presence of linearized free maxicircles, we investigated alternative explanations for their existence. We present evidence that this DNA species results from the double-strand cleavage of maxicircles due to detergent denaturation of intracellular topoisomerase II cleavable complexes. As expected for a topoisomerase II cleavage product, the linearized free maxicircle DNA is covalently bound to protein at both 5' ends. In addition, the slightly larger apparent size of linearized free maxicircle DNA or maxicircles linearized by a restriction enzyme can be explained by anomalous electrophoretic migration during conventional or pulsed-field agarose gel electrophoresis. This anomalous migration is presumably due to bends or other unusual structures in the DNA.

Animals↗
PubMed1986

Expression of transfected DNA depends on DNA topology.

Supercoiled DNAs, especially those containing enhancers, yield markedly higher levels of expression than linearized DNA when transfected into CV-1 cells or L cells. Different templates, linear or supercoiled, enhancer-containing or not, saturate for expression at 2 micrograms DNA per dish, suggesting that one role for enhancers and supercoiling is to increase the efficiency with which the same limiting component is used. Plasmids containing only enhancers or only promoters do not compete for expression with an enhancer-driven gene. However, plasmids that contain both enhancers and promoters do complete, suggesting that a second role for enhancers is to increase the binding of a limiting transcription factor. Linear and supercoiled enhancer-promoter plasmids compete equivalently. This suggests that supercoiling affects the ability of transcription factors to activate a given promoter, once bound.

Acetyltransferases↗
PubMed1981

Differences among H1 histone subfractions in binding to linear and superhelical DNA. Sedimentation velocity studies.

H1 histone subfractions exhibit differential abilities in aggregating superhelical DNA, as measured by sedimentation velocity analysis. In 0.15 M NaCl, all the calf thymus H1 subfractions bound to superhelical PM2 DNA to produce nonaggregated H1-DNA complexes as well as rapidly sedimenting, aggregated complexes. Notably, the distribution of the complexes between the nonaggregated and aggregated forms was a function of ionic strength and also depended on which H1 subfraction was complexed to the superhelical DNA. All of the H1 subfractions interacted preferentially with superhelical over relaxed PM2 DNA. The cooperative binding of H1 subfractions to linear T7 DNA produced only aggregated H1-DNA complexes in 0.15 M NaCl, while leaving some DNA free. The compositional and structural variation between the subfractions of H1 histone serves as a basis for their differential effectiveness in H1-induced aggregation of superhelical DNA. The observed sensitivity of aggregation to NaCl concentration is interpreted in terms of a dependence on hydrophobic interactions, such as the proper folding of the particular H1 subfraction and intermolecular interactions between neighboring hydrophobic regions, as well as on nonspecific shielding of DNA charge. These sedimentation velocity analyses augment previous studies on the interactions of H1 subfractions with linear and superhelical DNA using circular dichroism, viscosity, and filter-binding analyses. The involvement of particular H1 and DNA conformations in producing the various types of H1-DNA interactions characterized in these studies might also apply to chromatin structure.

Animals↗
PubMed1996

Plasmid recombination by the RecBCD pathway of Escherichia coli.

Previously, we demonstrated that exonuclease I-deficient strains of Escherichia coli accumulate high-molecular-weight linear plasmid concatemers when transformed with plasmids carrying the chi sequence (5'- GCTGGTGG-3') (M. M. Zaman and T. C. Boles, J. Bacteriol. 176:5093-5100, 1994). Since high-molecular weight linear DNA is believed to be the natural substrate for RecBCD-mediated recombination during conjugation (A. J. Clark and K. B. Low, p. 155-215, in K. B. Low, ed., The Recombination of Genetic Material, 1988), we analyzed the recombination frequencies of chi+ and chi0 plasmids in sbcB strains. Here, we report that chi sites stimulate plasmid recombination frequency by 16-fold in sbcB strains. Chi-stimulated plasmid recombination is dependent on RecBCD but is independent of RecF pathway genes. The distribution of recombination products suggests that high-molecular-weight linear plasmid DNA is a substrate for RecBCD-mediated recombination. Surprisingly, our data also suggest that chi+ plasmids also recombine by the RecBCD pathway in rec+ sbcB+ cells.

Base Sequence↗
PubMed1992

Psoralens cleave pBR322 DNA under ultraviolet radiation.

Supercoiled (SC) pBR322 was used to probe the recent claim that 5-geranoxylpsoralen (5-GOP) did not photoreact with DNA. Contrary to expectations, 5-GOP was found to damage DNA in the presence of UV-A through two competing pathways: (a) single strand breaks, identified by the conversion of supercoiled into open circular and linear DNA, and (b) cross-linking, revealed by the fluence-dependent decrease in the extent of denaturation of the double stranded supercoiled DNA to single stranded circular DNA. In addition, a fluence-dependent modification reduced the ability of the restriction enzyme EcoR I to linearize the photosensitized DNA, and alkali-labile lesions were generated. Psoralen, 5-methoxypsoralen, and 8-methoxypsoralen, which are well-known to undergo cycloaddition to DNA, had a more pronounced effect on supercoiled DNA. Single strand breaks occurred more readily than with 5-GOP, and the surviving SC form remaining had reduced electrophoretic mobility in agarose gels. In all cases, the DNA damage was more prominent when oxygen was absent.

DNA, Superhelical↗
PubMed2006

Diffusion of isolated DNA molecules: dependence on length and topology.

The conformation and dynamics of circular polymers is a subject of considerable theoretical and experimental interest. DNA is an important example because it occurs naturally in different topological states, including linear, relaxed circular, and supercoiled circular forms. A fundamental question is how the diffusion coefficients of isolated polymers scale with molecular length and how they vary for different topologies. Here, diffusion coefficients D for relaxed circular, supercoiled, and linear DNA molecules of length L ranging from approximately 6 to 290 kbp were measured by tracking the Brownian motion of single molecules. A topology-independent scaling law D approximately L(-nu) was observed with nu(L) = 0.571 +/- 0.014, nu(C) = 0.589 +/- 0.018, and nu(S) = 0.571 +/- 0.057 for linear, relaxed circular, and supercoiled DNA, respectively, in good agreement with the scaling exponent of nu congruent with 0.588 predicted by renormalization group theory for polymers with significant excluded volume interactions. Our findings thus provide evidence in support of several theories that predict an effective diameter of DNA much greater than the Debye screening length. In addition, the measured ratio D(Circular)/D(Linear) = 1.32 +/- 0.014 was closer to the value of 1.45 predicted by using renormalization group theory than the value of 1.18 predicted by classical Kirkwood hydrodynamic theory and agreed well with a value of 1.31 predicted when incorporating a recently proposed expression for the radius of gyration of circular polymers into the Zimm model.

DNA↗
PubMed1998

Homologous recombination and DNA-end joining reactions in zygotes and early embryos of zebrafish (Danio rerio) and Drosophila melanogaster.

A linear DNA with partial sequence redundancy can be recircularized in cells by either nonhomologous end joining (NEJ) or by homologous recombination (HR). We have studied the relative contributions of these processes in zygotes or early embryos of species that serve as model organisms for developmental genetics. Thus, we have microinjected a linearized plasmid substrate into zygotes of zebrafish (Danio rerio) or into the posterior end of Drosophila melanogaster early embryos before pole cell formation. Similar to the situation observed previously in Xenopus zygotes/early embryos, we detected a large preponderance of DNA-end joining over homologous recombination. A comparison of end-joined junctions revealed that from the three species tested, zebrafish introduced the least number of sequence distortions upon DNA-end joining, while Drosophila produced the largest deletions (average 14 bp) with occasional nucleotide patch insertions, reminiscent of the N nucleotides at V(D)J junctions in mammalian immune receptor genes. Double-strand gap repair by homologous sequences ('homologous recombination') involving a bimolecular reaction was readily detectable in both zebrafish and Drosophila. This involved specifically designed recombination substrates consisting of a mutagenized linear plasmid and DNA fragments carrying the wild-type sequence. Our results show that the basic machinery for homologous recombination is present at early developmental stages of these two genetic model organisms. However, it seems that for any experimental exploitation, such as targeted gene disruption, one would have to inhibit or bypass the overwhelming DNA-end joining activity.

Animals↗
PubMed2001

Isothermal amplification and multimerization of DNA by Bst DNA polymerase.

We have demonstrated the isothermal in vitro amplification and multimerization of several different linear DNA targets using only two primers and the strongly strand-displacing exonuclease-negative Bst DNA polymerase. This reaction has been termed linear target isothermal multimerization and amplification (LIMA). LIMA has been compared with cascade rolling-circle amplification and has been found to be less sensitive but to yield similar variable-length multimeric dsDNA molecules. Products from several different LIMA reactions were characterized by restriction analysis and partial sequence determination. They were found to be multimers of subsets of the target sequence and were not purely primer derived. The sensitivities with respect to target concentration of several different LIMA reactions were determined, and they varied from 0.01 amol to 1 fmol. The sensitivity and specificity of LIMA were further tested using E. coli genomic DNA, and the selective amplification of a transposon fragment was demonstrated. A successful strategy for reducing LIMA-dependent background DNA synthesis in rolling-circle amplification embodiments was devised. This entailed the affinity purification of circular DNA templates before amplification.

Base Sequence↗
PubMed1986

The role of upstream sequences in determining the strength of an rRNA promoter of E. coli.

In vitro transcription experiments were carried out with recombinant plasmids containing the promoters of the rrnB gene of Escherichia coli, and with deletion mutants lacking various lengths of the AT-rich sequence upstream from the P1 promoter of that gene. The main conclusions are as follows: The in vitro transcriptional activity of the P1 and P2 promoters of the rrnB gene are an order of magnitude higher on closed-circular (supercoiled) templates than on linear DNA; the strong P1 and P2 promoters are heparin-sensitive on linear templates, and on circular DNA only P2 is heparin-resistant; removal of the upstream AT-rich region did not decrease the apparent in vitro strength of the P1 promoter under standard conditions (50 mM KCl, high RNA polymerase/DNA ratio); at higher salt concentrations, or with a lower RNA polymerase/DNA ratio, the deletion mutants displayed much lower in vitro transcriptional activity than the wild-type, and the apparent weakening of the P1 promoter was roughly proportional to the length of the deleted AT-rich sequence. The implications of these findings for the possible in vivo role of the AT-rich region are discussed.

Base Sequence↗
PubMed1983

Biologically active recombinant formed through DNA pairing by purified recA protein in vitro.

We have detected in vitro homologous recombination mediated by purified recA protein of Escherichia coli as a recombinant phage produced by using the DNA packaging system of phage lambda. When double-stranded DNA of phage lambda carrying amber mutations is incubated with double-stranded DNA carrying the wild-type genes in the presence of recA protein, Mg++ and ATP, and the DNA packaged, amber+ recombinant phage is produced at a high frequency. This reaction depends completely upon the function of the wild-type recA protein. After incubation of 32P-labeled linear DNA (Form III) with bromouracil-labeled circular DNA (Form I-Form II mixture) in the presence of recA protein, Mg++ and ATP, about 10% of the 32P-counts band at an intermediate density in CsCl equilibrium gradient. This fraction yields a high percentage of the recombinant phage after DNA packaging and shows the alpha-shaped and sigma-shaped joint molecules of linear and circular DNA under the electron microscope. Furthermore, we demonstrate that a non-homologous region inhibits the recombination reaction when it is between the marker concerned and the closer cos end. Our results indicate that recA protein acts directly in the initial step of recombination to join the homologous double-stranded DNA and that the resulting molecule can be matured into the recombinant DNA.

Bacterial Proteins↗
PubMedDate not supplied

Modified linear polyethylenimine-cholesterol conjugates for DNA complexation.

Linear polyethylenimine (LPEI) is an effective nonviral gene carrier with transfection levels equal or above branched polyethylenimine (BPEI) and exhibits a lower cytotoxicity profile than BPEI. High molecular weight LPEI M(w) 25 k was modified with cholesterol in three different geometries: linear shaped (L), T-shaped (T), and a combined linear/T-shaped (LT) forming the LPEI-cholesterol (LPC) conjugates LPC-L, LPC-T, and LPC-LT, respectively. Physical characterization of LPC/pDNA complexes included particle size, zeta potential, DNase protection, mIL-12 p70 expression, and cytotoxicity. The particle size was further confirmed by atomic force microscopy (AFM). The LPC-T/pDNA complexes were optimal at N/P 10/1 that resulted in a particle size of approximately 250 nm, which was confirmed by AFM, and a surface charge of +10 mV. These complexes also effectively protected the pDNA for up to 180 min in the presence of DNase I. B16-F0 cells transfected with LPC-L and LPC-T showed protein expression levels higher than LPEI alone and twice that of BPEI but without any significant loss in cell viability. These results were confirmed with EGFP flow cytometry and transfection of Renca cells. The differences in rates of transfection of the LPC/pDNA complexes is due in part to conformational changes from the point of complex formation to interaction with the plasma membrane. These conformation changes provide protection for unprotonated secondary amines in the LPEI backbone by hydrophobic protection of the cholesterol moiety that we termed "unprotonated reserves". Finally, we show that LPC conjugates exploit receptor-mediated endocytosis via the LDL-R pathway with transgene expression levels decreasing nearly 20% after saturating the LDL-R sites on MCF-7 cells with hLDL-R-Ab.

Animals↗
PubMed1981

Structure of the provirus within NIH 3T3 cells transfected with Harvey sarcoma virus DNA.

NIH 3T3 cells transformed with unintegrated Harvey sarcoma virus (HSV) linear DNA generally acquired a complete HSV provirus. Infection of these transformed cells with Moloney murine leukemia helper virus was followed by release of infectious particles. The HSV provirus within these transfected cells was convalently joined to nonviral DNA sequences and was termed "cell-linked" HSV DNA. The association of this cell-virus DNA sequence with the chromosomal DNA of a transfected cell was unclear. NIH 3T3 cells could also become transformed by transfection with this cell-linked HSV DNA. In this case, the recipient cells generally acquired a donor DNA fragment containing both the HSV provirus and its flanking nonviral sequences. After cells acquired either unintegrated or cell-linked HSV DNA, the newly established provirus and flanking cellular sequences underwent amplifications to between 5 and 100 copies per diploid cell. NIH 3T3 cells transfected with HSV DNA may acquire deleted proviral DNA lacking at least 1.3 kilobase pairs from the right end of full-length HSV 6-kilobase-pair DNA (corresponding to the 3'-proximal portion of wild-type HSV RNA). Cells bearing such deleted HSV genomes were transformed, indicating that the viral transformation gene lies in the middle or 5'-proximal portion of the HSV RNA genome. However, when these cells were infected with Moloney murine leukemia helper virus, only low levels of biologically active sarcoma virus particles were released. Therefore, the 3' end of full-length HSV RNA was required for efficient transmission of the viral genome.

Animals↗
PubMed2006

Oxidative DNA cleavage promoted by multinuclear copper complexes: activity dependence on the complex structure.

Polynuclear copper complexes with two or three Cu(BPA) (BPA, bis(2-pyridylmethyl)amine) motifs, [Cu2(mTPXA)Cl4]3 H2O (1), [Cu2(pTPXA)Cl4]3 H2O (2), [Cu3(HPTAB)Cl5]Cl3 H2O (3) (mTPXA = N,N,N',N'-tetra-(2-pyridylmethyl)-m-xylylene diamine; pTPXA = N,N, N',N'-tetra-(2-pyridylmethyl)-p-xylylenediamine; HPTAB = N,N,N',N',N'',N''-hexakis(2-pyridylmethyl)-1,3,5-tris-(aminomethyl)benzene) have been synthesized and characterized. The crystal structures of compounds 2 and 3 showed each Cu(BPA) motif had a 4+1 square-pyramidal coordination environment with one chloride occupying the apical position and three N atoms from the same BPA moiety together with another Cl atom forming the basal plane. Fluorescence and circular dichroism (CD) spectroscopy studies indicated that the DNA binding followed an order of 3>2>1 in the compounds. These complexes cleave plasmid pUC19 DNA by using an oxidative mechanism with mercaptopropionic acid (MPA) as the reductant under aerobic conditions. Dinuclear Cu2+ complexes 1 and 2 showed much higher cleavage efficiency than their mononuclear analogue [Cu(bpa)Cl2] at the same [Cu2+] concentration, suggesting a synergistic effect of the Cu2+ centers. Moreover, the meta-dicopper centers in complex 1 facilitated the formation of linear DNA. Interestingly, the additional copper center to the meta-dicopper motif in complex 3 decreased the cleavage efficacy of meta-dicopper motif in complex 1, although it is able to cleave DNA to the linear form at higher [Cu2+] concentrations. Therefore, the higher DNA binding ability of complex 3 did not lead to higher cleavage efficiency. These findings have been correlated to the DNA binding mode and the ability of the Cu2+ complexes to activate oxygen (O2). This work is a good example of the rational design of multinuclear Cu2+ artificial nuclease and the activity of which can be manipulated by the geometry and the number of metal centers.

Amines↗
PubMed2003

The redusome hypothesis of aging and the control of biological time during individual development.

The redusome hypothesis of aging and the control of biological time in individual development is proposed. Redusomes are hypothetical perichromosomal particles arising in differentiation events during morphogenesis of an organism. The linear molecule of DNA covered with proteins in the redusome is assumed to be a copy of a segment of chromosomal DNA. Redusomes are located mainly in subtelomeric regions of chromosomes. The redusome does not leave the body of a chromosome even in the course of cellular divisions, being kept in its chromosomal nest. Like telomeric DNA, redusome linear DNA is shortened step by step. Thus, tiny redusomes progressively decrease in size; it is from here their name originates. Together with loss of the length of DNA in a redusome, the number of different genes contained in it also decreases. Shortening of the redusomal DNA molecules (and, coupled to it, changes of the sets of genes in redusomes) is responsible for age-dependent shifts in the level of expression of different chromosomal genes. Owing to this, redusome DNA shortening serves as a key means of measuring biological time in individual development. The main part of DNA of most redusomes is postulated to be occupied by noncoding genes. Low-molecular-weight RNAs (micro RNAs and fountain RNAs, or fRNAs) are assumed to be transcribed from them. These RNAs are involved in regulation of various chromatin repackings that are specific to certain differentiations, while others modulate the levels of expression of chromosomal genes. Hypothetical fountain RNAs can quantitatively regulate the expression levels of chromosomal genes, forming specific complexes with fions. Fions are suggested to be specific sites of a chromosomal DNA which are complementary to different fRNAs. Fions reside in the vicinity of usual chromosomal genes. A complex of the fRNA-fion, specifically interacting with a closed gate of the corresponding ion channel of the internal nuclear membrane, initiates the opening of the gate for a very short time, thus organizing activity of an ion fountain which appears to be automatically aimed at the chromosomal gene nearest to the fion involved. The ion fountain creates, depending on specificity of matching fRNA, fion, and ion channel, a distinctive ionic environment near certain structural genes. Ion fountains exert their action on the configuration of corresponding segments of chromatin and on the transcriptional efficiency of chromosomal genes in a topographically specific manner. Hence, the fountain system of the nucleus is able to regulate the quantitative traits both of cells and organism; it can control dominance of alleles and plays a role in individual development. Significant and escalating truncation of the redusome DNA causes cell aging due to an arising and increasing deficit of fRNAs and, for this reason, the lack of required ions near certain structural genes. Progressive shortening of DNA of redusomes is proposed to result in cellular aging because of a constantly growing shortage of low-molecular-weight RNAs transcribed from redusomal genes. Two types of redusomes are postulated: chronosomes and printosomes. Linear molecules of DNA in these two types of redusomes are called chronomeres and printomeres, respectively. Chronosomes are responsible for measurement of biological time in nondividing cells of the CNS. Printosomes remember positions of cells in the course of interpretation of the positional information in morphogenesis. In accordance with the position of a cell in a morphogenetic field, printomeres do change cellular properties and remember the change made (this is a so-called printomere mechanism of interpretation of positional information). Besides, printomeres participate in maintaining the achieved state of cellular differentiation. Normally, the chronomere is shortened only on the maximum of infradian hormonal rhythm (T-rhythm) which initiates the act of a superhigh velocity of its transcription that is finished with truncation of the end of a chronomere (an effect called scrupting). Theprintomere can be shortened due to the effect of DNA end underreplication and owing to scrupting. The effect of the end underreplication of DNA in doubling cells occurs simultaneously both in printomeres and telomeres. Shortening of telomeres is just a bystander process of aging of cells, whereas the true cause of biological aging is only the shortening of redusome DNA. Processing of certain redusomes in terminally differentiating cells is a cause of a proliferation arrest. Linkage of genes in a eukaryotic chromosome is determined by the distances between genes and redusomes.

Aging↗
PubMed1998

Structure and dynamics of supercoil-stabilized DNA cruciforms.

Understanding DNA function requires knowledge of the structure of local, sequence-dependent conformations that can be dramatically different from the B-form helix. One alternative DNA conformation is the cruciform, which has been shown to have a critical role in the initiation of DNA replication and the regulation of transcription in certain systems. In addition, cruciforms provide a model system for structural studies of Holliday junctions, intermediates in homologous DNA recombination. Cruciforms are not thermodynamically stable in linear DNA due to branch point migration, which makes their study using many biophysical techniques problematic. Atomic Force Microscopy (AFM) was applied to visualize cruciforms in negatively supercoiled plasmid DNA. Cruciforms are seen as clear-cut extrusions on the DNA filament with the lengths of the arms consistent with the size of the hairpins expected from a 106 bp inverted repeat. The cruciform exists in two different conformations, an extended one with the angle of ca. 180 degrees between the hairpin arms and a compact, X-type conformation, with acute angles between the hairpin arms and the main DNA strands. The ratio of molecules with the different conformations of cruciforms depends on ionic conditions. In the presence of high salt or Mg cations, a compact, X-type conformation is highly preferable. Remarkably, the X-conformation was highly mobile allowing the cruciform arms to adopt a parallel orientation. The structure observed is consistent with a model of the Holliday junction with a parallel orientation of the exchanging strands.

DNA, Superhelical↗
PubMed1993

Alteration of superhelical state of DNA by aluminium (Al).

The effect of aluminium (Al) on the supercoiled state of pUC18 DNA was studied by ethidium bromide fluorescence and agarose gel electrophoresis. Al at physiologically relevant concentrations relaxed the intact supercoiled DNA as well as the topoisomers induced by chloroquine. EDTA prevented the unwinding effect of Al on supercoiled DNA. Al did not alter the mobility of linear DNA in agarose gels. The implications of this finding in neurological disorders are discussed.

Aluminum↗