The flexibility of low molecular weight double-stranded dna as a function of length: I. Isolation and physical characterization of seven fractions

Sonicated calf thymus DNA was fractionated by rate zonal centrifugation into seven fractions with weight average molecular weights ranging from 0.28 to 1.3 × 10⁶ daltons, as determined by sedimentation equilibrium and light scattering measurements (the latter are described in the accompanying paper). Electron microscopy and sedimentation equilibrium analysis revealed these fractions to be narrowly disperse with M_w/M_n ratios averaging about 1.06. Intrinsic viscosities and sedimentation rates were measured and found to vary linearly with molecular weight in double-logarithmic plots in fair agreement with previously published functions relating these parameters for low molecular weight DNA. The average value for β from the Mandelkern— Flory equation was 2.59 × l0⁶, also agreeing with reported estimates of this parameter for short DNA. These data will be used in the second paper of this series to calculate the persistence length of the DNA fragments in each of the seven fractions by light scattering and hydrodynamic theories for the Kratky—Porod worm-like coil.     

The flexibility of low molecular weight double-stranded dna as a function of length: II. Light scattering measurements and the estimation of persistence lengths from light scattering,sedimentation and viscosity

In the preceding paper are described the isolation and physical characterization of seven narrowly disperse fractions of calf thymus DNA in the molecular weight range 0.3 to 1.3 × 10⁶ daltons. Herein, we have determined by light scattering the molecular weights and root mean square radii of these fractions in a solvent comprising 0.2 M NaCl, 2 mM EDTA, 2m MNa-PO₄, pH 7. Measurements were made in a modified Wippler—Scheibling photometer to a 20° lower limit of scattering angle on solutions rendered virtually dust-free by procedures described. The optical aniso tropics of the DNA fractions were measured permitting the experimental molecular weights and root mean square radii to be corrected to their true values. From these values, with appropriate polydispersity corrections, we calculate a Kratky—Porod persistence length, a, of 54.0 ± 5.6 nm which is invariant over the molecular weight range examined. From the sedimentation coefficients (preceding paper) and the theory of Yamakawa and Fujii, we calculate a to be 66 nm, a value found to apply equally well to several DNA samples of various origins whose sedimentation rates are known in the molecular weight range from about 4 × 10⁴ to 10⁸ daltons. Similarly, from the intrinsic viscosities and the theory of Yamakawa and Fujii, we calculate a to be 59 nm, which again adequately applies to a number of DNA samples whose viscosities have been measured by other workers in the molecular weight range 3 × 10⁵ to 10⁸ daltons. The Flory—Mandelkern parameter, β, was found to vary with molecular weight in the manner predicted by the theory of Yamakawa and Fujii. The average value of a from the three sets of measurements is 60 ± 6 nm, which we believe applies to double-stranded DNA molecules, independent of chain length, over the whole range of molecular weights for which reliable data exist.     

A new method to characterize saturation functions by their first four moments

The calculation of the first four moments of saturation functions is proposed as a method to describe the properties of enzymes or receptors models. The values of these moments in the case of the Langmuir or Michaelis-Menten equation and the Hill equation are reviewed. They have been calculated for the second degree Adair equation and in the case of binding site heterogeneity. A method for generalization to cases of greater complexity is also proposed. The advantage of this method over the classical ones—graphical representations and derivation of coefficients like n_H, $\text{[L]0.9}\text{[L]0.1}$…—is essentially that the moments are defined by one single value independently of any particular model for the whole of the saturation curve.     

A technique for mapping transfer RNA genes by electron microscopy of hybrids of ferritin-labeled transfer RNA and DNA: the phi-80hpsu+3-system

A method for mapping transfer RNA genes on single strands of DNA is described. tRNA is covalently coupled to the electron-opaque label, ferritin. The ferritinlabeled tRNA, Fer-tRNA, is hybridized to a single strand of DNA, or to a single- strand region of a DNA in a heteroduplex. The sites where the Fer-RNA binds to the complementary sequence on the DNA are then mapped by electron microscopy. Several alternative coupling procedures are described (see Fig. 1). In HzI a — COCH₂Br group is attached to ferritin by acylation. 3'-Oxidized tRNA is joined to HSRCONHNH₂ by hydrazone formation. Ferritin is then coupled to tRNA by reaction of the CBr and SH bonds. In the BI procedure a lysine amino group of ferritin is coupled by Schiff base formation with 3'-oxidized RNA. The conjugate is stabilized by borohydride reduction. In the BII procedure, a —COCH₂Br group is attached to ferritin. (H₂NCH₂CH₂S—)₂ is coupled to oxidized tRNA by Schiff base formation and borohydride reduction. An SH group is exposed by reduction. This HS-tRNA is coupled to a —COCH₂Br group attached to ferritin. All the procedures work but BII is recommended. Methods for purifying the Fer-tRNA and the Fer-tRNA-DNA hybrid are described. For the transducing phages, φ80hpsu^+,?_III and φ80hpsu^?_III, the DNA molecules each carry a piece of bacterial DNA of length 0·066±0·007 λ unit (3100 nucleotide pairs; we find the length of λ is 8·99 φX174 units) replacing a piece of phage DNA of φ80h of length 0·045±0·005 λ unit. The left junction of this bacterial DNA with phage DNA (referred to as P-B′) is at or close to the att site. The two tandem tyrosine genes of φ80hpsu^+,?_III and the single tRNA gene of φ80hpsu^?_III have been mapped at a position 1100 nucleotides to the right of the left (P·B′) junction of phage DNA and bacterial DNA, by hybridizing Escherichia coli Fer-tRNA to φ80hpsu_III/φ80h heteroduplexes. The separation of the two ferritin labels in φ80hpsu^+,?_III hybrids gives 140±20 nucleotides as the size of a single tyrosine tRNA gene.     

Augmented sparse reconstruction of protein signaling networks

The problem of reconstructing and identifying intracellular protein signaling and biochemical networks is of critical importance in biology. We propose a mathematical approach called augmented sparse reconstruction for the identification of links among nodes of ordinary differential equation (ODE) networks, given a small set of observed trajectories with various initial conditions. As a test case, the method is applied to the epidermal growth factor receptor (EGFR) driven signaling cascade, a well-studied and clinically important signaling network. Our method builds a system of representation from a collection of trajectory integrals, selectively attenuating blocks of terms in the representation. The system of representation is then augmented with random vectors, and l₁ minimization is used to find sparse representations for the dynamical interactions of each node. After showing the performance of our method on a model of the EGFR protein network, we sketch briefly the potential future therapeutic applications of this approach.     

Differences in TLR9-dependent inhibitory effects of H2O2-induced IL-8 secretion and NF-kappa B/I kappa B-alpha system activation by genomic DNA from five Lactobacillus species

Lactic acid bacteria (LAB) show anti-inflammatory effects, and their genomic DNA was identified as one of the anti-inflammatory components. Despite the differences in anti-inflammatory effects between live LAB dependent not only on genus but also species, this effect has not been compared at the genomic DNA level. We compared the anti-inflammatory effects of the genomic DNA from five Lactobacillus species—Lactobacillus acidophilus, Lactobacillus casei, Lactobacillus gasseri, Lactobacillus plantarum, and Lactobacillus reuteri—using Caco-2 cells. To evaluate anti-inflammatory effects, decreases in H₂O₂-induced IL-8 secretion and inhibition of H₂O₂-induced NF-κB/IκB-α system activation were examined. All LAB genomic DNAs dose-dependently decreased H₂O₂-induced IL-8 secretion and inhibited H₂O₂-induced NF-κB/IκB-α system activation. Comparison of these effects between Lactobacillus species showed that the anti-inflammatory effects of L. acidophilus genomic DNA are lower than those of the other species. Furthermore, suppression of Toll-like receptor 9 (TLR9), a specific receptor of bacterial DNA, expression by RNAi abolished the decrease of H₂O₂-induced IL-8 secretion and inhibition of H₂O₂-induced NF-κB/IκB-α system activation by LAB genomic DNA. Our results demonstrated that the anti-inflammatory effects of genomic DNA differ between Lactobacillus species and TLR9 is one of the major pathways responsible for the anti-inflammatory effect of LAB genomic DNA.     

New method for global alignment of 2 DNA sequences by the tree data structure

We introduce a new approach to investigate problem of DNA sequence alignment. The method consists of three parts: (i) simple alignment algorithm, (ii) extension algorithm for largest common substring, (iii) graphical simple alignment tree (GSA tree). The approach firstly obtains a graphical representation of scores of DNA sequences by the scoring equation R₀*R−S₀*S−T₀*(a+bk). Then a GSA tree is constructed to facilitate solving the problem for global alignment of 2 DNA sequences. Finally we give several practical examples to illustrate the utility and practicality of the approach.     

Prospect of Bioflavonoid Fisetin as a Quadruplex DNA Ligand: A Biophysical Approach

Quadruplex (G₄) forming sequences in telomeric DNA and c-myc promoter regions of human DNA are associated with tumorogenesis. Ligands that can facilitate or stabilize the formation and increase the stabilization of G₄ can prevent tumor cell proliferation and have been regarded as potential anti-cancer drugs. In the present study, steady state and time-resolved fluorescence measurements provide important structural and dynamical insights into the free and bound states of the therapeutically potent plant flavonoid fisetin (3,3′,4′,7-tetrahydroxyflavone) in a G₄ DNA matrix. The excited state intra-molecular proton transfer (ESPT) of fisetin plays an important role in observing and understanding the binding of fisetin with the G₄ DNA. Differential absorption spectra, thermal melting, and circular dichroism spectroscopic studies provide evidences for the formation of G₄ DNA and size exclusion chromatography (SEC) proves the binding and 1∶1 stoichiometry of fisetin in the DNA matrix. Comparative analysis of binding in the presence of EtBr proves that fisetin favors binding at the face of the G-quartet, mostly along the diagonal loop. Time resolved fluorescence anisotropy decay analysis indicates the increase in the restrictions in motion from the free to bound fisetin. We have also investigated the fingerprints of the binding of fisetin in the antiparallel quadruplex using Raman spectroscopy. Preliminary results indicate fisetin to be a prospective candidate as a G₄ ligand.     

Antirestriction and antimodification activities of the ArdA protein encoded by the IncI1 transmissive plasmids R-64 and ColIb-P9

Proteins of the Ard family are specific inhibitors of type I restriction-modification enzymes. The ArdA of R64 is highly homologous to ColIb-P9 ArdA, differing only by four amino acid residues of the overall 166. However, unlike ColIb-P9 ArdA, which inhibits both the endonuclease and the methylase activities of EcoKI, the R64 ArdA protein inhibits only the endonuclease activity of this enzyme. The mutant forms of R64 ArdA—A29T, S43A, and Y75W, capable of partially reversing the protein to ColIb-P9 ArdA form—were produced by directed mutagenesis. It was demonstrated that only Y75W mutation of these three variants essentially influenced the functional activity of ArdA: the antimodification activity was restored to approximately 90%. It is assumed that R64 ArdA inhibits formation of the complex between unmodified DNA and the R subunit of the type I restriction-modification enzyme EcoKI (R₂M₂S), which translocates and cleaves DNA. ColIb-P9 ArdA protein is capable of forming the DNA complex not only with the R subunit, but also with the S subunit, which contacts sK site (containing modified adenine residues) in DNA. ArdA bound to the specific sK site inhibits concurrently the endonuclease and methylase activities of EcoKI (R₂M₂S), while ArdA bound to the nonspecific site in the R subunit blocks only its endonuclease activity.     

Indirect readout in drug-DNA recognition: role of sequence-dependent DNA conformation

DNA-binding drugs have numerous applications in the engineered gene regulation. However, the drug-DNA recognition mechanism is poorly understood. Drugs can recognize specific DNA sequences not only through direct contacts but also indirectly through sequence-dependent conformation, in a similar manner to the indirect readout mechanism in protein-DNA recognition. We used a knowledge-based technique that takes advantage of known DNA structures to evaluate the conformational energies. We built a dataset of non-redundant free B-DNA crystal structures to calculate the distributions of adjacent base-step and base-pair conformations, and estimated the effective harmonic potentials of mean force (PMF). These PMFs were used to calculate the conformational energy of drug-DNA complexes, and the Z-score as a measure of the binding specificity. Comparing the Z-scores for drug-DNA complexes with those for free DNA structures with the same sequence, we observed that in several cases the Z-scores became more negative upon drug binding. Furthermore, the specificity is position-dependent within the drug-bound region of DNA. These results suggest that DNA conformation plays an important role in the drug-DNA recognition. The presented method provides a tool for the analysis of drug-DNA recognition and can facilitate the development of drugs for targeting a specific DNA sequence.     

Inverse PCR for subtyping of <Emphasis Type="Italic">Acinetobacter baumannii</Emphasis> carrying IS<Emphasis Type="Italic">Aba</Emphasis>1

Acinetobacter baumannii has been prevalent in nosocomial infections, often causing outbreaks in intensive care units. ISAba1 is an insertion sequence that has been identified only in A. baumannii and its copy number varies among strains. It has been reported that ISAba1 provides a promoter for bla_OXA-51-like, bla_OXA-23-like, and bla_ampC, which are associated with the resistance of A. baumannii to carbapenems and cephalosporins. The main purpose of this study was to develop a novel inverse PCR method capable of typing A. baumannii strains. The method involves three major steps: cutting of genomic DNA with a restriction enzyme, ligation, and PCR. In the first step, bacterial genomic DNA was digested with DpnI. In the second step, the digested genomic DNAs were ligated to form intramolecular circular DNAs. In the last step, the ligated circular DNAs were amplified by PCR with primers specific for ISAba1 and the amplified PCR products were electrophoresed. Twenty-two clinical isolates of A. baumannii were used for the evaluation of the inverse PCR (iPCR) typing method. Dendrogram analysis revealed two major clusters, similar to pulsed-field gel electrophoresis (PFGE) results. Three ISAba1-associated genes — bla_ampC, bla_OXA-66-like, and csuD — were amplified and detected in the clinical isolates. This novel iPCR typing method is comparable to PFGE in its ability to discriminate A. baumannii strains, and is a promising molecular epidemiological tool for investigating A. baumannii carrying ISAba1.     

Resolution of α, β and γ DNA of Saccharomyces cerevisiae with the antitumor drug cis-Pt(NH3)2Cl2. Evidence for preferential drug binding by GpG sequences of DNA

This paper reports further studies on the separation of DNAs with the antitumor drug cis-Pt(NH₃)₂Cl₂. cis-Pt(NH₃)₂Cl₂ permits resolution of the three DNA components from whole Saccharomyces cerevisiae in CsCl gradients, avoids pelleting of mitochondrial (β) DNA and does not require a critical molar ratio of platinum drug to DNA-P. However, the difficulty in removing all of the DNA-bound platinum may limit its preparative use. The linear relationship between the increase in buoyant density of platinized double-stranded DNA and its G + C content is employed to calculate a G + C content of 41.2% and 45.8% for α and γ DNA, respectively, using a value of 20% G + C for β DNA. In parallel experiments, we find that poly(dG)·poly(dC), which contains sequential guanine bases, exhibits an unexpectedly large buoyant density increase with cis-Pt(NH₃)₂ Cl₂, while the buoyant density increase of poly[d(G-C)]is markedly retarded, indicating an effect of nucleotide base sequence on DNA separation. The trans platinum compound, which has no antitumor properties, separates DNAs on the basis of G + C content in a similar fashion, but does not preferentially increase the buoyant density of poly(dG)·poly(dC).     

Interaction of Phospholipase A of the E. coli Outer Membrane with the Inhibitors of Eucaryotic Phospholipases A2 and Their Effect on the Ca2+-Induced Permeabilization of the Bacterial Membrane

Phospholipase A of the bacterial outer membrane (OMPLA) is a β-barrel membrane protein which is activated under various stress conditions. The current study examines interaction of inhibitors of eucaryotic phospholipases A₂—palmitoyl trifluoromethyl ketone (PACOCF₃) and aristolochic acid (AA)—with OMPLA and considers a possible involvement of the enzyme in the Ca²⁺-dependent permeabilization of the outer membrane of Escherichia coli. Using the method of molecular docking, it has been predicted that PACOCF₃ and AA bind to OMPLA at the same site and with the same affinity as the OMPLA inhibitors, hexadecanesulfonylfluoride and bromophenacyl bromide, and the substrate of the enzyme palmitoyl oleoyl phosphatidylethanolamine. It has also been shown that PACOCF₃, AA, and bromophenacyl bromide inhibit the Ca²⁺-induced temperature-dependent changes in the permeability of the bacterial membrane for the fluorescent probe propidium iodide and suppressed the transformation of E. coli cells with plasmid DNA induced by Ca²⁺ and heat shock. The cell viability was not affected by the eucaryotic phospholipases A₂ inhibitors. The study discusses a possible involvement of OMPLA in the mechanisms of bacterial transmembrane transport based on the permeabilization of the bacterial outer membrane.     

Competitive reactions in solutions of DNA and water-soluble interpolyelectrolyte complexes

The reaction of competitive binding of two polyanions—DNA and synthetic fluorescence-tagged poly(methacrylate) (PMA*)—with the polycation-quencher poly(N-ethyl-4-vinyl-pyridinium) (PEVP) was studied by fluorescence quenching technique. It was found that ability of DNA to displace PMA *from the water-soluble nonstoichiometric interpolyelectrolyte complex (NPEC) formed by PMA* and PEVP—NPEC(PMA*-PEVP)—and to form water-soluble NPEC(DNA-PEVP) can be determined by the parameter Ψ = P _PMA*/P _PEVP where P _PMA* and P _PEVP are the degrees of polymerization of PMA* and PEVP, respectively. In the case of Ψ < 1 the decrease of Ψ leads to the shift of the reaction equilibrium to the right, which can be explained by the gain of entropy due to the increase of the total number of polymeric particles in the solution. Introduction of alkali metal cations into the reaction mixture results in the shift of the reaction equilibrium, and according to their ability to shift the equilibrium to the right the cations can be arranged in the series Na⁺ > K⁺ > Li⁺. The substitution of native DNA by denatured DNA practically does not affect the reaction equilibrium in solutions of NaCl and KCl but considerably shifts it to the right in solutions of LiCl. The data obtained are in accordance with the differences in the selectivity of alkali cations binding with competitive polyanions. © 1995 John Wiley & Sons, Inc.     

Kinetics of proflavin binding to bacteriophages T2L and T4D

We have measured the kinetics of proflavin binding to T-even bacteriophages—the 700 S and 1000 S forms of T2L, T4D, and T4D os41—by difference spectroscopy at 430 nm. Measurements were carried out from 22° to 37°C. Binding is very slow to encapsulated DNA compared to free DNA, requiring hours to reach equilibrium. The kinetic data are compatible with the two-step mechanism where P is proflavin, N is nucleotide, and PN₁ and PN₂ are complexes. Computer integration of the rate equations allows evaluation of the rate constants; previous equilibrium measurements gave thermodynamic parameters. For all phage studied, the bimolecular step is endothermic with high positive entropy; the second, unimolecular step is highly exothermic with small negative entropy change. Both forms of T2L bind proflavin with essentially the same rate, as do T4D and the osmotic shock resistant mutant T4D os41. This suggests that the encapsulated DNA is equally accessible to proflavin in both forms of each phage. However, T4D binds dye appreciably faster than T2L, indicating that capsid permeability or DNA environment (glucosylation or packing) is different in the two species.     

What determines water-bridge lifetimes at the surface of DNA? Insight from systematic molecular dynamics analysis of water kinetics for various DNA sequences

The lifetime during which a water molecule resides at the surface of a biomolecule varies according to the hydration site. What determines this variety of lifetimes? Despite many previous studies, there is still no uniform picture quantitatively explaining this phenomenon. Here we calculate the lifetime for a particular hydration pattern in the DNA minor groove, the water bridge, for various DNA sequences to show that the water-bridge lifetime varies from 1 to ~ 300 ps in a sequence-dependent manner. We find that it follows 1/k(V_step)P_m, where P_m and V_step are two crucial factors, namely the probability of forming a specific hydrogen bond in which more than one donor atom participates, and the structural fluctuation of DNA, respectively. This relationship provides a picture of the water kinetics with atomistic detail and shows that water dissociation occurs when a particular hydrogen-bonding pattern appears. The rate constant of water dissociation k can be described as a function of the structural fluctuations of DNA. This picture is consistent with the model of Laage and Hynes proposing that hydrogen-bond switching occurs when an unusual number of hydrogen bonds are formed. The two new factors suggested here are discussed in the context of the surface's geometry and electrostatic nature, which were previously proposed as the determinants of water lifetimes.     

DNA with damage in both strands as affinity probes and nucleotide excision repair substrates

Nucleotide excision repair (NER) is a multistep process of recognition and elimination of a wide spectrum of damages that cause significant distortions in DNA structure, such as UV-induced damage and bulky chemical adducts. A series of model DNAs containing new bulky fluoro-azidobenzoyl photoactive lesion dC^FAB and well-recognized nonnucleoside lesions nFlu and nAnt have been designed and their interaction with repair proteins investigated. We demonstrate that modified DNA duplexes dC^FAB/dG (probe I), dC^FAB/nFlu₊₄ (probe II), and dC^FAB/nFlu_?3 (probe III) have increased (as compared to unmodified DNA, umDNA) structure-dependent affinity for XPC—HR23B (Kd_um > Kd_I > Kd_II ≈ Kd_III) and differentially crosslink to XPC and proteins of NER-competent extracts. The presence of dC^FAB results in (i) decreased melting temperature (ΔT_m = ?3°C) and (ii) 12° DNA bending. The extended dC^FAB/dG-DNA (137 bp) was demonstrated to be an effective NER substrate. Lack of correlation between the affinity to XPC—HR23B and substrate properties of the model DNA suggests a high impact of the verification stage on the overall NER process. In addition, DNAs containing closely positioned, well-recognized lesions in the complementary strands represent hardly repairable (dC^FAB/nFlu₊₄, dC^FAB/nFlu_?3) or irreparable (nFlu/nFlu₊₄, nFlu/nFlu_?3, nAnt/nFlu₊₄, nAnt/nFlu_?3) structures. Our data provide evidence that the NER system of higher eukaryotes recognizes and eliminates damaged DNA fragments on a multi-criterion basis.