DNA-binding affinities and sequence selectivity of quaternary benzophenanthridine alkaloids sanguinarine, chelerythrine, and nitidine

A comparative study on the intercalating binding of sanguinarine, chelerythrine, and nitidine with CT DNA, poly(dG-dC).poly(dG-dC), poly(dA-dT).poly(dA-dT), and seven sequence-designed double-stranded oligodeoxynucleotides has been performed using fluorometric and spectrophotometric techniques, aiming at providing insights into their sequence selectivity for DNA-binding. The results show that both sanguinarine and nitidine bind preferentially to DNA containing alternating GC base pairs [d(TGCGCA)(2)], while chelerythrine exhibits quite distinct sequence selectivity from sanguinarine, which shows a high specificity for DNA containing contiguous GC base pairs [5'-TGGGGA-3'/3'-ACCCCT-5'].     

Vibrational circular dichroism studies of the A-to-B conformational transition in DNA.

The vibrational circular dichroism (VCD) spectra of several natural DNAs as well as tRNA, poly(dG-dC).poly(dG-dC), and poly(dA-dT).poly(dA-dT) are reported for the base deformation modes in the IR region from 1700 to 1550 cm-1 for the polymers in D2O as well as in high alcohol dehydrating conditions. Spectra of both the B- and A-forms were identified. The A-form DNA VCD, not previously reported, has characteristics that can be found in the VCD spectra of RNAs as would be expected from the similarity of their structures. The VCD is sequence-dependent. Under the dehydrating conditions studied, poly(dA-dT)poly(dA-dT),poly(dA).poly(dT), and a high-A-T fraction natural DNA had a different bandshape from the other DNAs, which was similar to that of poly(rA).poly(rU). Poly(dG-dC).poly-(dG-dC) did not form an A-form in high-alcohol conditions but instead had a VCD spectrum much like that of its high-salt-induced Z-form. Qualitative differences seen experimentally between A- and B-form DNA VCD were suggested by the differences in the coupled oscillator VCD calculated for the two forms.     

Interaction of ellipticine and an indolo[2,3b]-quinoxaline derivative with DNA and synthetic polynucleotides

The non-covalent DNA interaction of the anticancer drug ellipticine (Scheme I, 1a) as well as an indolo[2,3-b]-quinoxaline derivative (Scheme I, 3b) with a dimethylaminoethyl side chain has been studied by light absorption, linear dichroism (LD) and fluorescence. Compound 3b (Scheme I) has antitumorigenic as well as antiviral activity. Both compounds bind to DNA or synthetic polynucleotides such as poly(dA-dT).(dA-dT) and poly(dG-dC).(dG-dC) by intercalation. In contrast to ellipticine, compound 3b (Scheme I) exhibits a significant binding specificity for alternating AT sequences. Its fluorescence is strongly enhanced in AT sequences and quenched in GC sequences. Fluorescence titrations evaluated as Scatchard plots show that both ellipticine and compound 3b (Scheme I) bind to the nucleic acids according to a non-cooperative neighbor exclusion model.     

Studies on binding of toromycin, an antitumor antibiotic, to DNA

Toromycin, an antitumor, bactericidal and antiviral compound, was found to bind to DNA in such a way as to interfere with the dissociation of double helix at an elevated temperature. The antibiotic did not introduce strand scission into DNA. Single-strand-specific nuclease S1-susceptibility of negatively supercoiled DNA was not influenced by its binding. The antibiotic was shown to bind to both of the alternating purine-pyrimidine copolymers, poly(dG-dC):poly(dG-dC) and poly(dA-dT):poly(dA-dT). The unique C-glycoside molecule of toromycin interacted with single-stranded DNA, but was found to have no affinity for RNA.     

Sequence-dependent mechanics of single DNA molecules

Atomic force microscope-based single-molecule force spectroscopy was employed to measure sequence-dependent mechanical properties of DNA by stretching individual DNA double strands attached between a gold surface and an AFM tip. We discovered that in lambda-phage DNA the previously reported B-S transition, where 'S' represents an overstretched conformation, at 65 pN is followed by a nonequilibrium melting transition at 150 pN. During this transition the DNA is split into single strands that fully recombine upon relaxation. The sequence dependence was investigated in comparative studies with poly(dG-dC) and poly(dA-dT) DNA. Both the B-S and the melting transition occur at significantly lower forces in poly(dA-dT) compared to poly(dG-dC). We made use of the melting transition to prepare single poly(dG-dC) and poly(dA-dT) DNA strands that upon relaxation reannealed into hairpins as a result of their self-complementary sequence. The unzipping of these hairpins directly revealed the base pair-unbinding forces for G-C to be 20 +/- 3 pN and for A-T to be 9 +/- 3 pN.     

Photoelectron transfer processes with ruthenium(II) polypyridyl complexes and Cu/Zn superoxide dismutase

The processes that are photoinduced by [Ru(bpz)(3)](2+) (bpz = 2,2'-bipyrazyl) in the presence of Cu/Zn superoxide dismutase (Cu/Zn SOD) are investigated by laser flash photolysis and electron paramagnetic resonance (EPR) spectroscopy; they are compared to those of the system [Ru(bpy)(3)(2+)-Cu/Zn SOD]. Although the mechanism is complicated, primary and secondary reactions can be evidenced. First, the excited [Ru(bpz)(3)](2+) complex is quenched reductively by Cu/Zn SOD with the production of a reduced complex and an oxidized enzyme. The oxidation site of Cu/Zn SOD is proposed to correspond to amino acids located on the surface of the protein. Afterward and only when this reductive electron transfer to the excited complex has produced enough oxidized protein, another electron-transfer process can be evidenced. In this case, however, the charge-transfer process takes place in the other direction, i.e., from the excited complex to the Cu(II) center of the SOD with the formation of Ru(III) and Cu(I) species. This proposed mechanism is supported by the fact that [Ru(bpy)(3)](2+), which is less photo-oxidizing than [Ru(bpz)(3)](2+), exhibits no photoreaction with Cu/Zn SOD. Because Ru(III) species are generated as intermediates with [Ru(bpz)(3)](2+), they are proposed to be responsible for the enhancement of [poly(dG-dC)](2) and [poly(dA-dT)](2) oxidation observed when Cu/Zn SOD is added to the [Ru(bpz)(3)](2+)-DNA system.     

Kinetics of the proton-deuteron exchange at position H8 of adenine and guanine in DNA.

Proton-NMR has been used to determine the activation energies and pre-exponential factors for the deuterium exchange of AH8 in poly(dA-dT).poly(dA-dT), and for GH8 in poly(dG-dC).poly(dG-dC). No simple relationship between the kinetic parameters and molecular conformation was found. By addition of 4.5 M NaCl a transition from the B to the Z conformation was induced for poly(dG-dC).poly(dG-dC), and an increased exchange rate was observed. The exchange rate for poly(dA-dT).poly(dA-dT) also increased below 64 degrees C, and a significant decrease in activation energy on addition of 4.5 M NaCl was observed. The exchange rates at T = 55.8 degrees C were also measured for the AH8 and GH8 in random sequence calf thymus DNA. From the difference in exchange rates, a method of preferential labeling of either the AH8 or the GH8 in high molecular weight DNA is evaluated.     

Mechanical stability of single DNA molecules

Using a modified atomic force microscope (AFM), individual double-stranded (ds) DNA molecules attached to an AFM tip and a gold surface were overstretched, and the mechanical stability of the DNA double helix was investigated. In lambda-phage DNA the previously reported B-S transition at 65 piconewtons (pN) is followed by a second conformational transition, during which the DNA double helix melts into two single strands. Unlike the B-S transition, the melting transition exhibits a pronounced force-loading-rate dependence and a marked hysteresis, characteristic of a nonequilibrium conformational transition. The kinetics of force-induced melting of the double helix, its reannealing kinetics, as well as the influence of ionic strength, temperature, and DNA sequence on the mechanical stability of the double helix were investigated. As expected, the DNA double helix is considerably destabilized under low salt buffer conditions (相似文献   

Conformational Variability of Poly(dA-dT)·Poly(dA-dT) and Some Other Deoxyribonucleic Acids Includes a Novel Type of Double Helix

Abstract

The article reviews data indicating that poly(dA-dT)?poly (dA-dT) is able of adopting three distinct double helical structures in solution, of which only the A form conforms to classical notions. The other two structures have dinucleotides as double helical repeats. At low salt concentrations poly(dA-dT)?poly(dA-dT) adopts a B-type alternating conformation which is exceptionally variable. Its architecture can gradually move in the limits demarcated by the CD spectra with inverted long wavelength CD bands and the ³¹P NMR spectra with a very low and a 0.6 ppm separation of two resonances. Contrary to Z-DNA, the ³¹P NMR spectrum of the limiting alternating B conformation of poly(dA-dT)?poly(dA-dT) is characterized by an upfield shift of one resonance. We attribute the exceptional conformational flexibility of the alternating B conformation to the unequal tendency of bases in the dA-dT and dT-dA steps to stack.

However, by assuming the limiting alternating B conformation, the variability of the synthetic DNA is not exhausted. Specific agents make it isomerize into another conformation by a fast, two-state mechanism, which is reflected by a further deepening of the negative long wavelength CD band and a downfield shift of the ³¹P NMR resonance of poly (dA-dT)?poly(dA-dT) that was constant in the course of the gradual alterations of the alternating B conformation. These changes are, however, qualitatively different from the way poly(dG-dC)?poly(dG-dC) behaves in the course of the B-Z isomerization. Poly(dG-dC) ?poly(dG-dC) displays purine-pyrimidine (dGpdC) resonance in the characteristic downfield position, while the downfield resonance of poly(dA-dT)?poly(dA-dT) belongs to the pyrimidine-purine (dTpdA) phosphodiester linkages. Consequently, phosphodiester linkages in the purine-pyrimidine steps play a similar role in the appearance of the Z form to the pyrimidine-purine phosphodiesters in the course of the isomerization of poly(dA-dT)?poly(dA-dT). This excludes that the high-salt structures of poly(dA-dT)?poly(dA-dT) and poly(dG-dC)?poly(dG-dC) are members of the same conformational family. We call the high-salt conformation of poly(dA-dT)?poly(dA-dT) X-DNA.

It furthermore follows from the review that synthetic molecules of DNA with alternating purine-pyrimidine sequences of bases can adopt either the Z form or the X form, or even both, depending on the environmental conditions. This introduces a new dimension into the DNA double helix conformational variability. The possible biological relevance of the X form is suggested by experiments with linear molecules of natural DNA. These indicate that Arich regions in natural DNAs can isomerize into the X form while the bulk of the molecule remains in the B form. The coexistence of both structures in a single DNA molecule may be understood in view of the favourable kinetic and thermodynamic properties with which the X form appears.     

Hexammineruthenium (III) chloride: a highly efficient promoter of the B-DNA to Z-DNA transition of poly-(dG-m5dC).poly(dG-m5dC) and poly(dG-dC).poly(dG-dC)

The effects of Ru(NH3)(3+)6 on the conformation of poly(dG-m5dC).poly(dG-m5dC) and poly(dG-dC).poly(dG-dC) were studied by circular dichroism (CD) spectroscopy. Ru(NH3)(3+)6 at very low concentrations provokes the Z-DNA conformation in both polynucleotides. In the presence of 50 mM NaCl, the concentration of Ru(NH3)(3+)6 at the midpoint of B to Z transition of poly(dG-m5dC).poly(dG-m5dC) is 4 microM compared to 5 microM for Co(NH3)(3+)6. The half-lives of B to Z transition of poly(dG-m5dC).poly(dG-m5dC) in the presence of 10 microM Ru(NH3)(3+)6 and Co(NHG3)(3+)6 are at 23 and 30 min, respectively. The concentration of Ru(NH3)(3+)6 at the midpoint of B to Z transition of poly(dG-dC).poly(dG-dC) is 50 microM. These results demonstrate that Ru(NH3)(3+)6 is a highly efficient trivalent cation for the induction of B to Z transition in poly(dG-m5dC).poly(dG-m5dC) and poly(dG-dC).poly(dG-dC). In contrast, Ru(NH3)(3+)6 has no significant effect on the conformation of calf thymus DNA, poly(dA-dT).poly(dA-dT) and poly(dA-dC).poly(dG-dT).     

Conformational variability of poly(dA-dT).poly(dA-dT) and some other deoxyribonucleic acids includes a novel type of double helix

The article reviews data indicating that poly(dA-dT).poly(dA-dT) is able of adopting three distinct double helical structures in solution, of which only the A form conforms to classical notions. The other two structures have dinucleotides as double helical repeats. At low salt concentrations poly(dA-dT).poly(dA-dT) adopts a B-type alternating conformation which is exceptionally variable. Its architecture can gradually move in the limits demarcated by the CD spectra with inverted long wavelength CD bands and the 31P NMR spectra with a very low and a 0.6 ppm separation of two resonances. Contrary to Z-DNA, the 31P NMR spectrum of the limiting alternating B conformation of poly(dA-dT).poly(dA-dT) is characterized by an upfield shift of one resonance. We attribute the exceptional conformational flexibility of the alternating B conformation to the unequal tendency of bases in the dA-dT and dT-dA steps to stack. However, by assuming the limiting alternating B conformation, the variability of the synthetic DNA is not exhausted. Specific agents make it isomerize into another conformation by a fast, two-state mechanism, which is reflected by a further deepening of the negative long wavelength CD band and a downfield shift of the 31P NMR resonance of poly(dA-dT).poly(dA-dT) that was constant in the course of the gradual alterations of the alternating B conformation. These changes are, however, qualitatively different from the way poly(dG-dC).poly(dG-dC) behaves in the course of the B-Z isomerization. Poly(dG-dC).poly(dG-dC) displays purine-pyrimidine (dGpdC) resonance in the characteristic downfield position, while the downfield resonance of poly(dA-dT).poly(dA-dT) belongs to the pyrimidine-purine (dTpdA) phosphodiester linkages. Consequently, phosphodiester linkages in the purine-pyrimidine steps play a similar role in the appearance of the Z form to the pyrimidine-purine phosphodiesters in the course of the isomerization of poly(dA-dT).poly(dA-dT). This excludes that the high-salt structures of poly(dA-dT).poly(dA-dT) and poly(dG-dC).poly(dG-dC) are members of the same conformational family. We call the high-salt conformation of poly(dA-dT).poly(dA-dT) X-DNA. It furthermore follows from the review that synthetic molecules of DNA with alternating purine-pyrimidine sequences of bases can adopt either the Z form or the X form, or even both, depending on the environmental conditions. This introduces a new dimension into the DNA double helix conformational variability. The possible biological relevance of the X form is suggested by experiments with linear molecules of natural DNA.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inactivation of de novo DNA methyltransferase activity by high concentrations of double-stranded DNA

The activity of eukaryotic DNA methyltransferase diminishes with time when the enzyme is incubated with high concentrations (200-300 micrograms/ml) of unmethylated double-stranded Micrococcus luteus DNA. Under similar conditions, single-stranded DNA induces only a limited decrease of enzyme activity. The inactivation process is apparently due to a slowly progressive interaction of the enzyme with double-stranded DNA that is independent of the presence of S-adenosyl-L-methionine. The inhibited enzyme cannot be reactivated either by high salt dissociation of the DNA-enzyme complex or by extensive digestion of the DNA. Among synthetic polydeoxyribonucleotides both poly(dG-dC).poly(dG-dC) and poly(dA-dT).poly(dA-dT), but not poly(dI-dC).poly(dI-dC), cause inactivation of DNA methyltransferase. This inactivation process may be of interest in regulating the 'de novo' activity of the enzyme.     

DNA solution conformation via infrared circular dichroism: experimental and theoretical results for B-family polymers

Infrared (vibrational) circular dichroism (VCD) has been observed for the DNA models d(CG)5, poly(dG-dC).poly(dG-dC), poly(dG).poly(dC), poly(dA-dT).poly(dA-dT), and poly(dA).poly(dT) in the B-conformation in buffered, aqueous solution. The observed results are quantitatively interpreted in terms of the exciton model for coupled carbonyl stretching vibrational states.     

23Na NMR relaxation study of the effects of conformation and base composition on the interactions of counterions with double-helical DNA

NMR relaxation rates (T1(-1) and T2(-1)) have been determined for 23Na in aqueous salt solutions containing various types of helical double-stranded deoxyribonucleic acids. These measurements were performed on three synthetic polynucleotides having different overall conformations, poly-(dA-dT).poly(dA-dT) (alternating B-DNA), poly(dG-dC).poly(dG-dC) at low salt (B-DNA), and Br-poly(dG-dC).Br-poly(dG-dC) (left-handed Z-DNA), and on four types of natural DNA differing in base composition, Clostridium perfringens (26% GC), calf thymus (40% GC), Escherichia coli (50% GC), and Micrococcus lysodeikticus (72% GC). For all types of DNA investigated, except poly(dA-dT).poly(dA-dT), the 23Na NMR spectra measured at 21 degrees C and an applied field of 4.7 T are non-Lorentzian. These non-Lorentzian spectra were analyzed on the basis of the two-state model and the standard theory of nonexponential quadrupolar relaxation processes in order to obtain estimates of the correlation times (tau c) characteristic of the sodium nuclei associated with the various nucleic acids. All of the correlation times estimated in this way are in the range of nanoseconds. The magnitudes of these correlation times show a significant dependence on the overall conformation of the nucleic acid (B vs. Z) but not on its base composition. To investigate the concentration dependence of tau c, sodium or magnesium salts were added to solutions of Br-poly(dG-dC).Br-poly(dG-dC) (Z-DNA).(ABSTRACT TRUNCATED AT 250 WORDS)     

DNA sequence has an effect on the extent and kinds of alkylation of DNA by a potent carcinogen

A system has been developed to study the effects of base sequence (neighboring bases) upon the alkylation of guanine (G) and adenine (A) bases in DNA. The study was performed on the synthetic polydeoxyribonucleotides, poly(dG).poly(dC), poly(dG-dC).poly(dG-dC), poly(dA).poly(dT), poly(dA-dT).poly(dA-dT), poly(dA-dC).poly(dG-dT), poly(dA-dG).poly(dC-dT), as well as calf thymus DNA. Each polynucleotide was treated with N-[3H]methyl-N-nitrosourea (MNU), depurinated, and the freed alkylpurines separated by HPLC and quantitated by liquid scintillation counting. The amounts of 3-methylguanine (3-MG), 7-MG, and O6-MG relative to guanine, and 3-methyladenine (3-MA) and 1-MA plus 7-MA relative to adenine, and also the O6-MG/7-MG ratios were highly reproducible for a given polynucleotide. Significant differences were found in the amounts of each of the methylpurines formed when compared among the six synthetic polynucleotides and DNA. This evidence is interpreted as an effect upon alkylation which is ultimately dependent upon the base sequence. These findings may have significance in defining the specificity of chemical carcinogens in terms of the susceptability to modification of nucleotide sequences such as those found in certain oncogenes.     

Quinacrine: Spectroscopic properties and interactions with polynucleotides

The acridine dye quinacrine and its interactions with calf thymus DNA, poly(dA-dT) · poly (dA-dT), and poly (dG-dC) · poly(dG-dC) were studied by light absorption, linear dichroism, and fluorescence spectroscopy. The transition moments of quinacrine give rise to absorption bands polarized along the short axis (400–480-nm band), and the long axis (345-nm and 290-nm bands) of the molecule, respectively. Linear dichroism studies show that quinacrine intercalates into calf thymus DNA as well as into the polynucleotides, displaying fairly homogeneous binding to poly (dA-dT) · poly (dA-dT), but more than one type of intercalation site for calf thymus DNA and poly (dG-dC) · poly(dG-dC). Fluorescence spectroscopy shows that for free quinacrine the pK = 8.1 between the mono- and diprotonated states also remains unchanged in the excited state. Quinacrine bound to calf thymus DNA and polynucleotides exhibits light absorption typical for the intercalated diprotonated form. The fluorescence enhancement of quinacrine bound to poly (dA-dT) · poly(dA-dT) may be due to shielding from water interactions involving transient H-bond formation. The fluorescence quenching in poly(dG-dC) · poly(dG-dC) may be due to excited state electron transfer from guanine to quinacrine. © 1993 John Wiley & Sons, Inc.     

Interaction of topotecan,DNA topoisomerase I inhibitor,with double-stranded polydeoxyribonucleotides. 4. Topotecan binds preferably to the GC base pairs of DNA

Interaction of topotecan (TPT) with synthetic double-stranded polydeoxyribonucleotides has been studied in solutions of low ionic strength at pH = 6.8 by linear flow dichroism (LD), circular dichroism (CD), UV-Vis absorption and Raman spectroscopy. The complexes of TPT with poly(dG-dC).poly(dG-dC), poly(dG).poly(dC), poly(dA-dC).poly(dG-dT), poly(dA).poly(dT) and previously studied by us complexes of TPT with calf thymus DNA and coliphage T4 DNA have been shown to have negative LD in the long-wavelength absorption band of TPT, whereas the complex of TPT with poly(dA-dT).poly(dA-dT) has positive LD in this absorption band of TPT. Thus, there are two different types of TPT complexes with the polymers. TPT has been established to bind preferably to GC base pairs because its affinity to the polymers of different GC composition decreases in the following order: poly(dG-dC).poly(dG-dC) > poly(dG).poly(dC) > poly(dA-dC).poly(dG-dT) > poly(dA).poly(dT). The presence of DNA has been shown to shift monomer-dimer equilibrium in TPT solutions toward dimer formation. Several duplexes of the synthetic polynucleotides bound together by the bridges of TPT dimers may participate in the formation of the studied type of TPT-polynucleotide complexes. Molecular models of TPT complex with linear and ring supercoiled DNAs and with deoxyguanosine have been considered. TPT (and presumably all camptothecin family) proved to be a representative of a new class of DNA-specific ligands whose biological action is associated with formation of dimeric bridges between two DNA duplexes.     

Molecular recognition of DNA by small molecules: AT base pair specific intercalative binding of cytotoxic plant alkaloid palmatine

The base dependent binding of the cytotoxic alkaloid palmatine to four synthetic polynucleotides, poly(dA).poly(dT), poly(dA-dT).poly(dA-dT), poly(dG).poly(dC) and poly(dG-dC).poly(dG-dC) was examined by competition dialysis, spectrophotometric, spectrofluorimetric, thermal melting, circular dichroic, viscometric and isothermal titration calorimetric (ITC) studies. Binding of the alkaloid to various polynucleotides was dependent upon sequences of base pairs. Binding data obtained from absorbance measurements according to neighbour exclusion model indicated that the intrinsic binding constants decreased in the order poly(dA).poly(dT)>poly(dA-dT).poly(dA-dT)>poly(dG-dC).poly(dG-dC)>poly(dG).poly(dC). This affinity was also revealed by the competition dialysis, increase of steady state fluorescence intensity, increase in fluorescence quantum yield, stabilization against thermal denaturation and perturbations in circular dichroic spectrum. Among the polynucleotides, poly(dA).poly(dT) showed positive cooperativity at binding values lower than r=0.05. Viscosity studies revealed that in the strong binding region, the increase of contour length of DNA depended strongly on the sequence of base pairs being higher for AT polymers and induction of unwinding-rewinding process of covalently closed superhelical DNA. Isothermal titration calorimetric data showed a single entropy driven binding event in the AT homo polymer while that with the hetero polymer involved two binding modes, an entropy driven strong binding followed by an enthalpy driven weak binding. These results unequivocally established that the alkaloid palmatine binds strongly to AT homo and hetero polymers by mechanism of intercalation.     

Comparison of the helix–coil conformational changes of poly(deoxyadenylate-deoxytymidylate) and poly(deoxyadenylate-deoxythymidylate) induced by variations of hydrogen-ion concentration

Double-helical poly(dG-dC) and poly(dA-dT) are DNA analogs in which the interactions between the two strands of the helix are, respectively, either the stronger G/C type or the weaker A/T type along the entire length of macromolecules. Thus, these synthetic polynucleotides can be considered as representatives of the most stable and the least stable DNA. In the investigations presented here, potentiometric titrations and stopped-flow kinetic experiments were carried out in order to compare the pH-induced helix–coil conformations (10°C and 150mM [Na⁺]) the pH of the helix–coil transition (pH_m) is 12.81 for poly(dG-dC) and 11.76 for poly(dA-dT). The unwinding of double-helical poly(dG-dC) initiated by a sudden change in pH was found to be a simple exponential process with rate constants in the range of 200–600 sec^?1, depending on the final value of the pH jump. The intramolecular double-helix formation of poly(dG-dC) was studied by lowering the pH of the solutions from a value above pH_m to that below pH_m in dilute solutions (15.5 ug/ml [polymer]). Under these conditions, the observed rewinding reactions displayed a major and two exponential phases, all of which were independent of polymer concentration. From the comparison of the results of poly(dA-dT) and poly(dG-dT) would unwind faster than poly(dG-dC). However, if the pH jumps are such that they present the same perturbation of these polymers relative to their pH_m values, no significant differences exist between the rates of helix–coil conformation changes of poly(dA-dT) and poly(dG-dC).     

Inactivation of de novo DNA methyltransferase activity by high concentrations of double-stranded DNA

The activity of eukaryotic DNA methyltransferase diminishes with time when the enzyme is incubated with high concentrations (200–300 μg/ml) of unmethylated double-stranded Micrococcus luteus DNA. Under similar conditions, single-stranded DNA induces only a limited decrease of enzyme activity. The inactivation process is apparently due to a slowly progressive interaction of the enzyme with double-stranded DNA that is independent of the presence of S-adenosyl-l-methionine. The inhibited enzyme cannot be reactivated either by high salt dissociation of the DNA-enzyme complex or by extensive digestion of the DNA. Among synthetic polydeoxyribonucleotides both poly(dG-dC) · poly(dG-dC) and poly(dA-dT) · poly(dA-dT), but not poly(dI-dC) · poly(dI-dC), cause inactivation of DNA methyltransferase. This inactivation process may be of interest in regulating the ‘de novo’ activity of the enzyme.