Kinetics of the interaction between echinomycin and deoxyribonucleic acid

Biolayer interferometry provides a robust method for detecting DNA binding small molecules in microbial extracts

DNA replication is an exceptional point of therapeutic intervention for many cancer types and several small molecules targeting DNA have been developed into clinically used antitumor agents. Many of these molecules are naturally occurring metabolites from plants and microorganisms, such as the widely used chemotherapeutic doxorubicin. While natural product sources contain a vast number of DNA binding small molecules, isolating and identifying these molecules is challenging. Typical screening campaigns utilize time-consuming bioactivity-guided fractionation approaches, which use sequential rounds of cell-based assays to guide the isolation of active compounds. In this study, we explore the use of biolayer interferometry (BLI) as a tool for rapidly screening natural product sources for DNA targeting small molecules. We first verified that BLI robustly detected DNA binding using designed GC- and AT-rich DNA oligonucleotides with known DNA intercalating, groove binding, and covalent binding agents including actinomycin D (1), doxorubicin (2), ethidium bromide (3), propidium iodide (4), Hoechst 33342 (5), and netropsin (6). Although binding varied with the properties of the oligonucleotides, measured binding affinities agreed with previously reported values. We next utilized BLI to screen over 100 bacterial extracts from our microbial library for DNA binding activity and found three highly active extracts. Binding-guided isolation was used to isolate the active principle component from each extract, which were identified as echinomycin (8), actinomycin V (9), and chartreusin (10). This biosensor-based DNA binding screen is a novel, low-cost, easy to use, and sensitive approach for medium-throughput screening of complex chemical libraries. Graphical abstract.

Modulation of Hoogsteen dynamics on DNA recognition

In naked duplex DNA, G–C and A–T Watson-Crick base pairs exist in dynamic equilibrium with their Hoogsteen counterparts. Here, we used nuclear magnetic resonance (NMR) relaxation dispersion and molecular dynamics (MD) simulations to examine how Watson-Crick/Hoogsteen dynamics are modulated upon recognition of duplex DNA by the bisintercalator echinomycin and monointercalator actinomycin D. In both cases, DNA recognition results in the quenching of Hoogsteen dynamics at base pairs involved in intermolecular base-specific hydrogen bonds. In the case of echinomycin, the Hoogsteen population increased 10-fold for base pairs flanking the chromophore most likely due to intermolecular stacking interactions, whereas actinomycin D minimally affected Hoogsteen dynamics at other sites. Modulation of Hoogsteen dynamics at binding interfaces may be a general phenomenon with important implications for DNA–ligand and DNA–protein recognition.

DNA is found in a dynamic equilibrium between standard Watson-Crick (WC) base pairs and non-standard Hoogsteen (HG) base pairs. Here the authors describe the influence of echinomycin and actinomycin D ligands binding on the HG-WC base pair dynamics in DNA.

Single molecule high-throughput footprinting of small and large DNA ligands

Most DNA processes are governed by molecular interactions that take place in a sequence-specific manner. Determining the sequence selectivity of DNA ligands is still a challenge, particularly for small drugs where labeling or sequencing methods do not perform well. Here, we present a fast and accurate method based on parallelized single molecule magnetic tweezers to detect the sequence selectivity and characterize the thermodynamics and kinetics of binding in a single assay. Mechanical manipulation of DNA hairpins with an engineered sequence is used to detect ligand binding as blocking events during DNA unzipping, allowing determination of ligand selectivity both for small drugs and large proteins with nearly base-pair resolution in an unbiased fashion. The assay allows investigation of subtle details such as the effect of flanking sequences or binding cooperativity. Unzipping assays on hairpin substrates with an optimized flat free energy landscape containing all binding motifs allows determination of the ligand mechanical footprint, recognition site, and binding orientation.

Mapping the sequence specificity of DNA ligands remains a challenge, particularly for small drugs. Here the authors develop a parallelized single molecule magnetic tweezers approach using engineered DNA hairpins that can detect sequence selectivity, thermodynamics and kinetics of binding for small drugs and large proteins.

Photophysics of aminoxanthone derivatives and their application as binding probes for DNA

Xanthones with amino substituents were synthesized to diminish the photoreactivity of the xanthone chromophore with DNA, with the objective of using these molecules to study their binding dynamics with DNA. The aminoxanthones showed a strong solvatochromic effect on their singlet and triplet excited-state photophysics, where polar solvents led to a decrease of the energies for the excited states. Quenching of the triplet excited states by nitrite anions was used to determine the binding dynamics, and a residence time in the microsecond time domain was estimated for the bound 2-aminoxanthone with DNA. The quenching experiments performed showed that this methodology will not be applicable to study the binding dynamics of a wide variety of guests with DNA.

Visualising DNA: Footprinting and 1-2D Gels

The study of molecular recognition of DNA by natural and synthetic ligands has made enormous progress due in large part to the discovery and development of methods for separating DNA fragments by gel electrophoresis in one and two dimensions, and for characterizing DNA-ligand complexes by footprinting techniques.

Echinomycin inhibits chromosomal DNA replication and embryonic development in vertebrates

Echinomycin, a member of the quinoxaline family of antibiotics, is known to be a strong inhibitor of RNA synthesis which has been attributed to its ability to bind to double-helical DNA. Here we study the effect of echinomycin upon DNA replication using egg extracts and embryos from Xenopus laevis as well as cultured human cells. Evidence is presented that echinomycin interferes with chromatin decondensation, nuclear assembly and DNA replication. In the absence of transcription and translation, the drug specifically blocks DNA replication in both Xenopus sperm chromatin and HeLa cell nuclei in vitro. By contrast, replication of single-stranded DNA is not inhibited indicating that echinomycin acts by interacting with the DNA and not the replication elongation proteins of chromatin. The addition of the antibiotic to HeLa cells and X.laevis embryos results in anaphase bridges and cell death. Importantly, in X.laevis embryos injected with echinomycin at the two-cell stage the drug specifically inhibits the cell cycle prior to the onset of transcription, suggesting that quinoxaline antibiotics could exert anti- proliferative effects by inhibition of chromosomal DNA replication.

Energetics of echinomycin binding to DNA

Differential scanning calorimetry and UV thermal denaturation have been used to determine a complete thermodynamic profile for the bis-intercalative interaction of the peptide antibiotic echinomycin with DNA. The new calorimetric data are consistent with all previously published binding data, and afford the most rigorous and direct determination of the binding enthalpy possible. For the association of echinomycin with DNA, we found DeltaG degrees = -7.6 kcal mol(-1), DeltaH = +3.8 kcal mol(-1) and DeltaS = +38.9 cal mol(-1) K(-1) at 20 degrees C. The binding reaction is clearly entropically driven, a hallmark of a process that is predominantly stabilized by hydrophobic interactions, though a deeper analysis of the free energy contributions suggests that direct molecular recognition between echinomycin and DNA, mediated by hydrogen bonding and van der Waals contacts, also plays an important role in stabilizing the complex.

On the kinetics of distamycin binding to its target sites on duplex DNA

Distamycin A is a well known polyamide antibiotic that can bind in the minor groove of duplex DNA primarily at AT-rich sequences both as a monomer or as a side-by-side antiparallel dimer. The association phase of the distamycin binding reaction has not been studied in either of its binding modes, because of the lack of an adequate UV or CD signal at the low concentrations needed to monitor the fast bimolecular reaction. We report a significant increase in fluorescence amplitude, accompanied by a small red shift, on binding distamycin to its specific target sites. This signal can be used to monitor drug binding in steady-state and time-resolved processes. Distamycin shows extremely fast association with the 1:1 binding site, with a bimolecular rate of 7 x 10(7) M(-1) small middle dots(-1) and also fairly rapid dissociation ( approximately 3 s(-1)). When DNA is in excess, there is a slow component in the association reaction whose rate decreases strongly with increasing DNA concentration. Binding of the drug to the 2:1 site occurs in two distinct steps: fast, sequential binding of each drug molecule to the DNA with a bimolecular rate comparable to that at the 1:1 site, followed by a slow ( approximately 4 s(-1)) equilibration to the final population. Dissociation from the 2:1 site is approximately 40-fold slower than from the 1:1 site. This study provides the groundwork for analysis of the binding kinetics of longer polyamides and covalently linked polyamides that have recently been shown to inhibit transcription in vivo.

Dissociation kinetics of actinomycin D from individual GpC sites in DNA

We have examined the kinetics of dissociation of actinomycin from GpC sites in several DNA fragments containing synthetic DNA inserts, by a variation of the footprinting technique. Complexes of the ligand with radiolabelled DNA fragments were dissociated by adding a large excess of unlabelled calf thymus DNA. Samples were removed from this mixture at subsequent time intervals and subjected to DNase I footprinting. The rate of disappearance of the footprints varied considerably between the GpC sites located in different sequence environments. Actinomycin dissociates more slowly from GpC sites flanked by (AT)n than An.Tn. Within regions of alternating AT, TGCA represents a better binding site than AGCT, and CGCA is a better binding site than GGCA. GpC sites flanked by (AC)n.(GT)n present good binding sites; in this context, dissociation from CGCG is faster than from TGCA.

Visualising the dissociation of sequence selective ligands from individual binding sites on DNA

We have used a modification of the footprinting technique to measure the dissociation of mithramycin, echinomycin and nogalamycin from their binding sites in a natural DNA fragment. Complexes with radiolabelled DNA were dissociated by addition of unlabelled DNA. Samples were removed at various times and subjected to DNase I digestion, and the rate of dissociation from each site was estimated from the time-dependent disappearance of the footprints. For echinomycin the slowest rate of dissociation is from ACGT, while the slowest site for mithramycin contains four contiguous guanines. The dissociation of nogalamycin is extremely slow, even from its weaker sites; the slowest rate was from ACGTA, which took longer than 4 h, even at 37 degrees C.

Dissociation of the AT-specific bifunctional intercalator [N-MeCys3,N-MeCys7]TANDEM from TpA sites in DNA

We have examined the dissociation of [N-MeCys3,N-MeCys7]TANDEM, an AT-selective bifunctional intercalator, from TpA sites in mixed-sequence DNAs by a modification of the footprinting technique. Dissociation of complexes between the ligand and radiolabelled DNA fragments was initiated by adding a vast excess of unlabelled calf thymus DNA. Portions of this mixture were subjected to DNAse I footprinting at various times after adding the competitor DNA. Dissociation of the ligand from each site was seen by the time-dependent disappearance of the footprinting pattern. Within a natural DNA fragment (tyrT) the ligand dissociates from TTAT faster than from ATAT. We found that the stability of complexes with isolated TpA steps decreases in the order ATAT > TTAA > TATA. Dissociation from each of these sites is much faster than from longer regions of (AT)n. These results confirm the requirement for A and T base-pairs surrounding the TpA step and suggest that the interaction is strongest with regions of alternating AT, possibly as a result of its unusual structure. The ligand dissociates more slowly from the centre of (AT)n tracts than from the edges, suggesting that variations in dissociation rate arise from sequence-dependent variations in local DNA structure.

Visualising the kinetics of dissociation of actinomycin from individual sites in mixed sequence DNA by DNase I footprinting

We have investigated the kinetics of dissociation of actinomycin D from DNA by a variation of the footprinting technique. Complexes of actinomycin with a radiolabelled DNA fragment (tyrT) were dissociated by addition of a large excess of unlabelled calf thymus DNA and the mixture subjected to DNase I footprinting at subsequent intervals. The rates at which the footprints disappeared varied between the different binding sites. The dissociation was temperature dependent with average time constants of 30 s, 10 mins and 2 hours at temperatures of 37 degrees C, 20 degrees C and 4 degrees C respectively. The dissociation from a DNA fragment containing the synthetic insert T9GCA9 was significantly faster, with a half-life of about 1 min at 20 degrees C. In contrast, the dissociation of distamycin was too fast to measure (< 5 s) even at 4 degrees C.

Proton exchange in DNA-luzopeptin and DNA-echinomycin bisintercalation complexes: rates and processes of base-pair opening

Imino proton exchange studies are reported on the complexes formed by bisintercalation of luzopeptin around the two central A.T pairs of the d(CCCATGGG) and d(AGCATGCT) duplexes and of echinomycin around the two central C.G pairs of the d(AAACGTTT) and d(CCAAACGTTTGG) duplexes. The depsipeptide backbone of the drugs occupies the minor groove of the complexes at the bisintercalation site. The exchange time of the amide protons of the depsipeptide rings provides a lower estimate of the complex lifetime: 20 min at 15 degrees C for the echinomycin complexes and 4 days at 45 degrees C for the luzopeptin complexes. The exchange time of imino protons is always shorter than the complex lifetime. Hence, base pairs open even within the complexed oligomers. For the two base pairs sandwiched between the aromatic rings of the drug, the base-pair lifetime is strongly increased, and the dissociation constant is correspondingly reduced. Hence, the lifetime of the open state is unchanged. This suggests similar open states in the free duplex and in the complex. In contrast to the sandwiched base pairs, the base pairs flanking the intercalation site are not stabilized in the complex. Thus, the action of the bisintercalating drug may be compared to a vise clamping the inner base pairs. Analysis suggests that base-pair opening may require prior unwinding or bending of the DNA duplex.

Interaction of echinomycin with An.Tn. and (AT)n regions flanking its CG binding site

We have prepared DNA fragments containing the sequences A15CGT15, T15CGA15 and T(AT)8CG(AT)15 cloned within the SmaI site of the pUC19 polylinker. These have been used as substrates in footprinting experiments with DNase I and diethylpyrocarbonate probing the effects of echinomycin, binding to the central CG, on the structure of the surrounding sequences. No clear DNase I footprints are seen with T15CGA15 though alterations in the nuclease susceptibility of surrounding regions suggest that the ligand is binding, albeit weakly at this site. All the other fragments show the expected footprints around the CG site. Regions of An and Tn are rendered much more reactive to DNase I and adenines on the 3'-side of the CG become hyperreactive to diethylpyrocarbonate. Regions of alternating AT show unusual changes in the presence of the ligand. At low concentrations (5 microM) cleavage of TpA is enhanced, whereas at higher concentrations a cleavage pattern with a four base pair repeat is evident. A similar pattern is seen with micrococcal nuclease. Modification by diethylpyrocarbonate is strongest at alternate adenines which are staggered in the 5'-direction across the two strands. We interpret these changes by suggesting secondary drug binding within regions of alternating AT, possibly to the dinucleotide ApT. DNase I footprinting experiments performed at 4 degrees C revealed neither enhancements nor footprints for flanking regions of homopolymeric A and T suggesting that the conformational changes are necessary consequence of drug binding.

Echinomycin binding to alternating AT

We have studied the binding of echinomycin to DNA fragments containing GC-rich regions flanked by blocks of alternating AT by DNase I footprinting and diethylpyrocarbonate modification. Regions of alternating AT flanking the sequences CCCG, CCGC, CGGC and GG show a four base pair DNase I cleavage pattern and reaction of alternate adenines with diethylpyrocarbonate. This pattern is strongest when the AT-block is immediately adjacent to the CpG ligand binding site. We explain these phenomena by suggesting that echinomycin binds to the dinucleotide step ApT in a cooperative fashion. The cooperative effects can be transmitted through the dinucleotide step GC but not CC or AA. No such repetitive patterns are seen with surrounding regions of (ATT).(AAT). Evidence is presented for secondary drug binding sites at CpC and TpG with weaker interaction at the CpG site within the hexanucleotide TTCGAA.

Conformational heterogeneity of quinoxaline peptides in solution

The conformational heterogeneity of several quinoxaline antibiotics, a class of naturally occurring quinoxaline peptides with antitumor properties, and their synthetic analogues was investigated in polar and nonpolar solvents by high performance liquid chromatography (HPLC) with uv photodiode array detection, uv-absorbance, low-temperature phosphorescence, and nmr techniques. Multiple peak formation and interconversion in the HPLC and 1H-nmr analysis of triostin A, its under-N-methylated synthetic analogues (des-N-tetramethyltriostin A [TANDEM] and [N-MeCys3, N-MeCys7]-TANDEM [MCTAN-DEM]), and echinomycin were examined as a function of temperature, solvent polarity, and residence time in solution prior to analysis. Slow interconversion between HPLC peaks, ascribed to the presence of multiple solution conformers, was exhibited by these peptides although at very different interconversion rates. Among the triostins, the rate of interconversion appeared to vary with the degree of N-methylation of the residues in the cyclic depsipeptide chain. Interconversion of the n and p conformers of triostin A in chloroform occurred on a chromatographic timescale (a few minutes with kn----p calculated to be 0.02 s-1 at 25 degrees C) while the solution conformers of TANDEM in methanol equilibrated very slowly to one preferred conformer over a period of several weeks at ambient temperature. MCTANDEM, a synthetic analogue of triostin A with an intermediate degree of N-methylation of the residues in the peptide ring, consisted of an equilibrium mixture of n and p conformers in methanol that interconverted on a chromatographic time scale. Two additional conformers of MCTANDEM developed within a few weeks' residence time in methanol at ambient temperature. Echinomycin was found to exist in methanol as an interconverting mixture of at least four minor conformers in addition to the major isoform (95% by peak area) of the peptide. The solution conformers of the quinoxaline peptides investigated in this report are most likely a consequence of hindered rotation about the N-methylated peptide bonds in the depsipeptide ring and/or intramolecular hydrogen bonding.

Phosphorescence and optically detected magnetic resonance studies of echinomycin-DNA complexes

Echinomycin complexes with polymeric DNAs and model duplex oligonucleotides have been studied by low-temperature phosphorescence and optical detection of triplet-state magnetic resonance (ODMR) spectroscopy, with the quinoxaline chromophores of the drug used as intrinsic probes. Although not optically resolved, plots of ODMR transition frequencies versus monitored wavelength revealed heterogeneity in the phosphorescence emission of echinomycin, which was ascribed to the presence of two distinct quinoxaline triplet-state environments (referred to as the blue and red triplet states of echinomycin in this report). We think that a likely origin of the two triplet states of echinomycin is the occurrence of two or more distinct conformations of the drug in aqueous solutions. Spectroscopically observed perturbations of the triplet-state properties of echinomycin such as the phosphorescence emission spectrum, phosphorescence lifetime, ODMR spectrum, and zero-field splitting (zfs) energies were investigated upon drug binding to the double-stranded alternating copolymers poly(dG-dC).poly(dG-dC) [abbreviated as poly[d(G-C)2]] and poly(dA-dT).poly(dA-dT) [abbreviated as poly[d(A-T)2]], the homopolymer duplexes poly(dG).poly(dC) [abbreviated as poly(dG.dC)] and poly(dA).poly(dT) [abbreviated as poly(dA.dT)], and the natural DNAs from Escherichia coli, Micrococcus lysodeikticus, and calf thymus. Echinomycin bisintercalation complexes with the self-complementary oligonucleotides d(ACGT), d(CGTACG), and d(ACGTACGT), which are thought to model drug binding sites, were also investigated. Phosphorescence and ODMR spectroscopic results indicate that the quinoxaline chromophores of the drug are involved in aromatic stacking interactions in complexes with the natural DNAs as evidenced by red shifts in the phosphorescence 0,0 band of the drug, a small but significant reduction in the phosphorescence lifetime of the red triplet state, and reduction of the zfs D-value of both the blue and red triplet states upon drug complexation. These changes in the triplet-state properties of echinomycin are consistent with stacking interactions that increase the polarizability of the quinoxaline environment. The extent of the reduction of the D parameter for the red triplet state upon complexation with the polymeric DNAs was found to correlate with the binding affinities measured for these targets [Wakelin, L. P. G., & Waring, M. J. (1976) Biochem. J. 157, 721-740], with the single exception of the drug-poly[d(G-C)2] complex, for which an increase in the D-value was noted. In addition, upon drug binding to the natural DNAs, there is a reversal of signal polarity in the ODMR spectra of the red triplet state. Among the synthetic DNA polymers investigated, a reversal of ODMR signal polarity was found only with the echinomycin-poly[d(A-T)2] complex.(ABSTRACT TRUNCATED AT 400 WORDS)

The use of micrococcal nuclease as a probe for drug-binding sites on DNA

The cutting pattern produced by micrococcal nuclease on three DNA fragments has been determined in the absence and presence of various DNA-binding drugs. The enzyme itself cuts almost exclusively at pA and pT bonds, showing a greater activity at (A-T)n than in homopolymeric runs of A and T. Each drug produces distinct changes in the cleavage pattern. The protected regions can not be pinpointed with sufficient precision to assess the exact drug-binding sites on account of the sequence selectivity of the enzyme, although where a direct comparison is possible these include most of those seen as DNAase I footprints. The enzyme is most useful for assessing the selectivity of drugs which bind to AT-rich regions. Several drugs protect the DNA from micrococcal nuclease attack in regions which do not contain their acknowledged best binding sites. It appears that micrococcal nuclease is sensitive to the existence of secondary drug-binding sites which are not evident with other footprinting techniques.

Effect of echinomycin on DNA methylation

Although echinomycin is reported to intercalate and to bind to DNA at CG dinucleotides, the effects of the drug on DNA methylation in vitro and in vivo are much less apparent than are the effects on DNA synthesis and cell growth.

Stopped-flow kinetic analysis of the interaction of anthraquinone anticancer drugs with calf thymus DNA, poly[d(G-C)].poly[d(G-C)], and poly[d(A-T)].poly[d(A-T)]

The sodium dodecyl sulfate driven dissociation reactions of daunorubicin (1), mitoxantrone (2), ametantrone (3), and a related anthraquinone without hydroxyl groups on the ring or side chain (4) from calf thymus DNA, poly[d(G-C)]2, and poly[d(A-T)]2 have been investigated by stopped-flow kinetic methods. All four compounds exhibit biphasic dissociation reactions from their DNA complexes. Daunorubicin and mitoxantrone have similar dissociation rate constants that are lower than those for ametantrone and 4. The effect of temperature and ionic strength on both rate constants for each compound is similar. An analysis of the effects of salt on the two rate constants for daunorubicin and mitoxantrone suggests that both of these compounds bind to DNA through a mechanism that involves formation of an initial outside complex followed by intercalation. The daunorubicin dissociation results from both poly[d(G-C)]2 and poly[d(A-T)]2 can be fitted with a single exponential function, and the rate constants are quite close. The ametantrone and 4 polymer dissociation results can also be fitted with single exponential curves, but with these compounds the dissociation rate constants for the poly[d(G-C)]2 complexes are approximately 10 times lower than for the poly[d(A-T)]2 complexes. Mitoxantrone also has a much slower dissociation rate from poly[d(G-C)]2 than from poly[d(A-T)]2, but its dissociation from both polymers exhibits biphasic kinetics. Possible reasons for the biphasic behavior with the polymers, which is unique to mitoxantrone, are selective binding and dissociation from the alternating polymer intercalation sites and/or dual binding modes of the intercalator with both side chains in the same groove or with one side chain in each groove.

Kinetic evidence that echinomycin migrates between potential DNA binding sites

The hypothesis that echinomycin locates its preferred nucleotide sequences in DNA by a process of "shuffling" between potential binding sites has been tested. Immediately after reacting with calf thymus DNA the antibiotic is relatively weakly bound inasmuch as the complex dissociates quite rapidly when detergent is added. If the complex is allowed to equilibrate for various periods of time after mixing, an increasing proportion of the bound antibiotic dissociates slowly on addition of detergent. The kinetics of appearance of the slowly-dissociating form, and its dependence upon ionic strength, are fully consistent with the shuffling model. In contrast the dissociation profiles from poly(dG-dC) and poly(dA-dT) are independent of mixing time.

DNA structural variations produced by actinomycin and distamycin as revealed by DNAase I footprinting

The technique of DNAase I footprinting has been used to investigate preferred binding sites for actinomycin D and distamycin on a 160-base-pair DNA fragment from E. coli containing the tyr T promoter sequence. Only sites containing the dinucleotide step GpC are protected by binding of actinomycin, and all such sites are protected. Distamycin recognizes four major regions rich in A + T residues. Both antibiotics induce enhanced rates of cleavage at certain regions flanking their binding sites. These effects are not restricted to any particular base sequence since they are produced in runs of A and T by actinomycin and in GC-rich sequences by distamycin. The observed increases in susceptibility to nuclease attack are attributed to DNA structural variations induced in the vicinity of the ligand binding site, most probably involving changes in the width of the helical minor groove.

Kinetic evidence for redistribution of actinomycin molecules between potential DNA-binding sites

The kinetics of interaction between actinomycin D and DNA have been measured by stopped-flow and detergent-dissociation methods. The results are consistent with a model in which the antibiotic initially binds to many sequences on the heterogeneous DNA lattice and subsequently 'shuffles' between the available sites until a thermodynamically determined optimal state of binding is attained. The amplitudes of the two slowest components in the reaction with calf thymus DNA do not vary in parallel as the total level of antibiotic binding is increased; they appear to reflect directly the redistribution of antibiotic molecules along the DNA lattice. The dissociation profile is shown to depend upon the time for which the antibiotic and DNA are premixed, so that for short mixing times a higher proportion of the decay is represented by faster-dissociating species. The rate of appearance of the slowest-dissociating species correlates well with the slowest optical change in the association reaction. Stopped-flow experiments indicate that the antibiotic first binds to sites on natural DNA with an average association constant of 4 X 10(3) M-1 and that it subsequently migrates to sites with higher affinity. Similar experiments performed with poly(dG-dC) are less easily interpreted and seem to indicate that conformational changes or cooperative effects can also occur.

Interaction of phenylthiolato-(2,2',2"-terpyridine)platinum(II) cation with DNA

The interaction between a novel aromatic thiolato derivative from the family of DNA-intercalating platinum complexes, phenylthiolato-(2,2',2"-terpyridine)platinum(II)-[PhS(ter py)Pt+], and nucleic acids was studied by using viscosity, equilibrium-dialysis and kinetic measurements. Viscosity measurements with sonicated DNA provide direct evidence for intercalation, and show that at binding ratios below 0.2 molecules per base-pair PhS(terpy)Pt+ causes an increase in contour length of 0.2 nm per bound molecule. However, helix extension diminishes at greater extents of binding, indicating the existence of additional, non-intercalated, externally bound forms of the ligand. The ability of PhS(terpy)Pt+ to aggregate in neutral aqueous buffers at a range of ionic strengths and temperatures was assessed by using optical-absorption methods. Scatchard plots for binding to calf thymus DNA at ionic strength 0.01 (corrected for dimerization) are curvilinear, concave upward, providing further evidence for two modes of binding. The association constant decreases at higher ionic strengths, in accord with the expectations of polyelectrolyte theory, although the number of cations released per bound unipositive ligand molecule is substantially greater than 1. Stopped-flow kinetic measurements confirm the complexity of the binding reaction by revealing multiple bound forms of the ligand whose kinetic processes are both fast and closely coupled. Thermal denaturation of DNA radically alters the shapes of binding isotherms and either has little effect on, or enhances, the affinity of potential binding sites, depending on experimental conditions. Scatchard plots for binding to natural DNA species with differing nucleotide composition show that the ligand has a requirement for a single G X C base-pair at the highest-affinity intercalation sites.

Sequence-specific binding of echinomycin to DNA: evidence for conformational changes affecting flanking sequences

The technique of DNAase I footprinting has been used to investigate preferred binding sites for echinomycin on a 160-base-pair DNA fragment from E. coli containing the tyr T promoter sequence. Six binding sites have been precisely located in the sequence; a seventh has been partially identified. The minimum site-size is six base pairs. All the binding sites contain the dinucleotide sequence CpG but no other regularities can be discerned. When the protected regions on each complementary strand are compared it is evident that they are staggered by 2-3 base-pairs towards the 3' end at each site. Footprinting with DNAase II reports a similar, though less precise, pattern of protection. Cutting by both enzymes is markedly enhanced at AT-rich regions flanking the antibiotic-binding sites. This increased susceptibility to nuclease attack can be attributed to an altered helix conformation in the vicinity of the bis-intercalated echinomycin molecule. It seems that certain sequences, mainly runs of A or runs of T, switch from a nuclease-resistant to a nuclease-sensitive form when echinomycin binds nearby.

Stopped-flow kinetic studies on the interaction between echinomycin and DNA

The kinetics of association between the quinoxaline antitumor antibiotic echinomycin and DNA have been studied by stopped-flow methods. With natural DNAs, the reaction profile is completely described by a single exponential, the time constant for which varies linearly with the DNA concentration. This bimolecular rate constant is similar for both calf thymus and Micrococcus lysodeikticus DNA (k = 6 X 10(4) M-1 s-1 at 25 degrees C, I = 0.01) and is probably dominated by interaction with relatively weak but abundant binding sites from which the antibiotic dissociates fairly quickly. The observed single exponential suggests a molecular mechanism of binding in which both chromophores of the antibiotic become intercalated simultaneously rather than sequentially; no transition from a mono-intercalated state to a bis-intercalated state could be detected. The reaction is slowed by a factor of about 3 on raising the salt concentration from I = 0.01 to I = 0.5. Binding to poly(dA-dT) is also described by a single exponential, the time constant for which is about 3 times faster than that seen with natural DNAs. By contrast, the interaction with poly-(dG-dC) requires two exponentials for a proper description, the faster of which is similar to that seen with natural DNAs. This may reflect the initial interaction of the antibiotic with two types of sequences, tentatively identified as GpC and CpG, from which it dissociates at very different rates. The differences in kinetic behavior may be explicable on the basis of an alternating B structure for poly(dA-dT) and a more classical B form for poly(dG-dC).

Preferential reaction of the carcinogen N-acetoxy-2-acetylaminofluorene with satellite DNA

The carcinogens N-acetoxy-2-acetylaminofluorene (N-acetoxy-AAF) and N-hydroxy-2-aminofluorene (N-hydroxy-AF) were incubated with calf thymus DNA to determine if reaction occurred preferentially with discrete regions within the DNA. Derivative melting profiles indicated that both compounds decreased satellite transitions and that N-acetoxy-AAF depressed the melting of higher temperature regions. These data suggest that N-acetoxy-AAF reacted to a greater extent with G + C-rich regions and, because the resulting adduct disrupted the helix, the cooperativity of melting decreased. Reaction of N-acetoxy-AAF with purified satellite III DNA confirmed the preferential interaction of this carcinogen with G + C-rich regions as compared to main component DNA. The derivative melting profile of lambda DNA in the presence of actinomycin D further demonstrated that this type of analysis can detect preferential interactions with specific DNA sequences.

The use of radiolabelled triostin antibiotics to measure low levels of binding to deoxyribonucleic acid

Triostin antibiotics, which contain a cyclic peptide with a disulphide bridge, have been prepared by growing Streptomyces triostinicus in the presence of inorganic [35S]-sulphate. The labelled triostin A has been shown to behave in all respects similarly to the authentic natural product and to enable a much more sensitive radiochemical adaptation of the solvent-partition method for determining antibiotic binding to DNA. By this means, binding isotherms at low, biologically relevant levels (down to one antibiotic molecule per gene) have been measured. The results indicate the existence of some tight binding sites in natural DNA species that are preferentially occupied at low concentrations. No evidence has been found for any allosteric transitions provoked by interaction between these antibiotics and natural DNA species, though there is evidence for co-operativity in the binding of triostin A to poly(dA-dT). For the first time accurate isotherms have been determined for the binding of triostin C to DNA; its binding constants for a variety of polydeoxynucleotides are uniformly tighter than those of triostin A but fall into the same ranking order when different species of natural DNA are compared.

Equilibrium and kinetic studies on the binding of des-N-tetramethyltriostin A to DNA

The interaction between TANDEM (a des-methyl analogue of triostin A) and poly(dA-dT) results in extension of the helix by 6.8 A for each ligand molecule bound, exactly as predicted for a bis-intercalation reaction. Cooperativity is evident in Scatchard plots for the interaction at ionic strengths of 0.2 and 1.0, where the binding constant is diminished compared to that which pertains at low salt concentrations. Binding to a natural DNA (calf thymus), already considerably weaker than binding to poly(dA-dT), is also sensitive to increased ionic strength. With a self-complementary octanucleotide d(G-G-T-A-T-A-C-C) the binding curve indicates the presence of a single des-N-tetramethyltriostin A binding site per helical fragment with a non-cooperative association constant about 6 . 10(6) M-1. Detergent-induced dissociation of des-N-tetramethyltriostin A-poly(dA-dT) complexes results in a simple exponential decay at all levels of binding, but the time constant of decay is dependent upon the initial binding ratio. This behavior cannot directly explain the cooperativity of equilibrium binding isotherms but suggests the occurrence of relatively long-lived perturbations of the helical structure by binding of the ligand. [Ala3, Ala7]des-N-tetramethyltriostin A, which has a more flexible octapeptide ring lacking the disulphide cross-bridge, dissociates from poly(dA-dT) much faster than des-N-tetramethyltriostin A. Dissociation of des-N-tetramethyltriostin A from calf thymus DNA is more rapid than dissociation of triostin A or other quinoxaline antibiotics, which may account for its low antimicrobial activity.