Conformation of acetylaminofluorene and aminofluorene modified guanosine and guanosine derivatives

Enhanced expression from the human cytomegalovirus immediate-early promoter in a non-replicating adenovirus encoded reporter gene following cellular exposure to chemical DNA damaging agents

We have examined expression from the human cytomegalovirus (CMV) promoter of a reporter gene encoded in a replication-deficient adenovirus following cellular exposure to heat shock and chemical DNA damaging agents. Expression of the reporter gene was enhanced following prior treatment of cells with cisplatin and N-acetoxy-acetylaminofluorine, but not heat shock. This enhancement was more pronounced and induced by lower chemical concentrations in xeroderma pigmentosum (XP) and Cockayne syndrome fibroblasts that are deficient in the transcription-coupled repair (TCR) pathway of nucleotide excision repair (NER) compared to that in TCR-proficient XP-C and normal strains. This is consistent with an induction of expression from the CMV promoter mediated by persistent (unrepaired) DNA damage in active genes. We show also that expression of the CMV-driven reporter is enhanced following treatment of several human tumour cell lines. This later finding has implications for combined chemotherapy and gene therapy using CMV-driven expression vectors.

A single N-2-acetylaminofluorene adduct alters the footprint of T7 (exo-) DNA polymerase bound to a model primer-template junction

Bovine pancreatic deoxyribonuclease I (DNaseI) has been used to footprint T7 (exo-) DNA polymerase bound to a model primer-template junction. The polymerase was blocked at a specific position either by the omission of dCTP from the reaction mix or by the presence of a N-(deoxyguanosin-8-yl)-2-acetylaminofluorene (dGuo-AAF) adduct. This lesion has been shown to be a severe block for several DNA polymerases, both in in vitro primer elongation experiments, and during the in vivo replication of AAF-monomodified single-stranded vectors. The footprints obtained with unmodified primer-template DNA define two protected domains separated by an inter-region that remains sensitive to DNaseI, and several hypersensitive sites located on both strands. Binding of the polymerase to AAF monomodified duplexes results in the same protection pattern as that obtained with the unmodified duplexes. However, the hypersensitive sites either disappear or are dramatically reduced. The results suggest that the AAF lesion alters the correct positioning of the duplex DNA within the polymerase cleft.

The sensitivity of human fibroblasts to N-acetoxy-2-acetylaminofluorene is determined by the extent of transcription-coupled repair, and/or their capability to counteract RNA synthesis inhibition

Nucleotide excision repair (NER) mechanism is the major pathway responsible for the removal of a large variety of bulky lesions from the genome. Two different NER subpathways have been identified, i.e. the transcription-coupled and the global genome repair pathways. For DNA-damage induced by ultraviolet light both transcription-coupled repair and global genome repair are essential to confer resistance to cytotoxic effects. To gain further insight into the contribution of NER subpathways in the repair of bulky lesions and in their prevention of biological effects we measured the rate of repair of dG-C8-AF in active and inactive genes in normal human cells, XP-C cells (only transcription-coupled repair) and XP-A cells (completely NER-deficient) exposed to NA-AAF. XP-C cells are only slightly more sensitive to NA-AAF than normal cells and, like normal cells, they are able to recover RNA synthesis repressed by the treatment. In contrast, XP-A cells are sensitive to NA-AAF and unable to recover from RNA synthesis inhibition. Repair of dG-C8-AF in the active ADA gene proceeds in a biphasic way and without strand specificity, with a subclass of lesions quickly repaired during the first 8 h after treatment. Repair in the inactive 754 gene occurs more slowly than in the ADA gene. In XP-C cells, repair of dG-C8-AF in the ADA gene is confined to the transcribed strand and occurs at about half the rate of repair seen in normal cells. Repair in the inactive 754 gene in XP-C cells is virtually absent. Consistent with these results we found that repair replication in XP-C is drastically reduced when compared with normal cells and abolished by alpha-amanitin indicating that the repair in XP-C cells is mediated by transcription-coupled repair only. Our data suggest that dG-C8-AF is a target for transcription-coupled repair and that this repair pathway is the main pathway or recovery of RNA synthesis inhibition conferring resistance to cytotoxic effects of NA-AAF. In spite of this, repair of dG-C8-AF in active genes in normal cells by transcription-coupled repair and global genome repair is not additive, but dominated by global genome repair. This indicates that the subset of lesions which are capable of stalling RNA polymerase II, and are, therefore, a substrate for TCR, are also the lesions which are very efficiently recognized by the global genome repair system.

The sensitivity of Cockayne's syndrome cells to DNA-damaging agents is not due to defective transcription-coupled repair of active genes

Two of the hallmarks of Cockayne's syndrome (CS) are the hypersensitivity of cells to UV light and the lack of recovery of the ability to synthesize RNA following exposure of cells to UV light, in spite of the normal repair capacity at the overall genome level. The prolonged repressed RNA synthesis has been attributed to a defect in transcription-coupled repair, resulting in slow removal of DNA lesions from the transcribed strand of active genes. This model predicts that the sensitivity of CS cells to another DNA-damaging agent, i.e., the UV-mimetic agent N-acetoxy-2-acetylaminofluorene (NA-AAF), should also be associated with a lack of resumption of RNA synthesis and defective transcription-coupled repair of NA-AAF-induced DNA adducts. We tested this by measuring the rate of excision of DNA adducts in the adenosine deaminase gene of primary normal human fibroblasts and two CS (complementation group A and B) fibroblast strains. High-performance liquid chromatography analysis of DNA adducts revealed that N-(deoxyguanosin-8-yl)-2-aminofluorene (dG-C8-AF) was the main adduct induced by NA-AAF in both normal and CS cells. No differences were found between normal and CS cells with respect to induction of this lesion either at the level of the genome overall or at the gene level. Moreover, repair of dG-C8-AF in the active adenosine deaminase gene occurred at similar rates and without strand specificity in normal and CS cells, indicating that transcription-coupled repair does not contribute significantly to repair of dG-C8-AF in active genes. Yet CS cells are threefold more sensitive to NA-AAF than are normal cells and are unable to recover the ability to synthesize RNA. Our data rule out defective transcription-coupled repair as the cause of the increased sensitivity of CS cells to DNA-damaging agents and suggest that the cellular sensitivity and the prolonged repressed RNA synthesis are primarily due to a transcription defect. We hypothesize that upon treatment of cells with either UV or NA-AAF, the basal transcription factor TFIIH becomes involved in nucleotide excision repair and that the CS gene products are involved in the conversion of TFIIH back to the transcription function. In this view, the CS proteins act as repair-transcription uncoupling factors. If the uncoupling process is defective, RNA synthesis will stay repressed, causing cellular sensitivity. Since transcription is essential for transcription-coupled repair, the CS defect will affect those lesions whose repair is predominantly transcription coupled, i.e., UV-induced cyclobutane pyrimidine dimers.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

2-Aminofluorene modified DNA duplex exists in two interchangeable conformations

One- and two-dimensional NMR shows that the carcinogen 2-aminofluorene exists in two unique, interchangeable conformations when covalently bound to a model human c-H-ras1 proto-oncogene codon 61 oligomer duplex. In one conformation the 2-aminofluorene moiety protrudes out of the major groove leaving the Watson-Crick base pairing of the cytosine and 2-aminofluorene-guanine bases intact, consistent with the ability of replicating enzymes to bypass the lesion and correctly incorporate cytosine. The second form of the modified oligomer duplex may be representative of a pre-mutagenic conformation in that the 2-aminofluorene moiety is stacked within the DNA helix, disrupting base pairing between the 2-aminofluorene-modified guanine and its complementary cytosine.

Use of single-turnover kinetics to study bulky adduct bypass by T7 DNA polymerase

The mechanism by which T7 DNA polymerase (exo-) bypasses N-2-acetylaminofluorene (AAF) and N-2-aminofluorene (AF) adducts was studied by single-turnover kinetics. These adducts are known to be mutagenic in several cell types, and their bypass was studied in the framework of understanding how they promote mutations. Synthetic primer/templates were made from a template sequence containing a single guanine, to which the adducts were covalently attached, and one of three primers whose 3' ends were various distances from the adduct in the annealed substrates. Upon approaching the site of either adduct, the polymerase was found to add nucleotides as rapidly as to unmodified primer/templates, until just opposite the lesion. The incorporation rate of dCTP (at 100 microM) opposite AF-dG or AAF-dG was approximately 5 x 10(4)- and 4 x 10(6)-fold slower, respectively, than incorporation at the same position into an unmodified primer/template. The polymerase dissociated from the sites of the adducts at approximately the same rate that it dissociated from unmodified DNA. Correct nucleotide incorporation was favored both opposite and immediately after AF-dG. However, at both positions, dATP was the most rapidly misincorporated nucleotide. Misincorporation of dATP was more rapid than correct nucleotide incorporation both opposite and immediately after AAF-dG. These results are discussed in terms of the effects of AF and AAF adducts in vivo.

Fifty years of research on N-acetyl-2-aminofluorene, one of the most versatile compounds in experimental cancer research

It is just about 50 years since the publication of the report on the toxicity and carcinogenicity of the potent carcinogen N-acetyl-2-aminofluorene (AAF). In 1940 very few reports on the carcinogenic activity of chemical compounds in experimental animals were available. The discovery of pure chemicals as carcinogens, such as AAF, azo dyes and benzo[a]pyrene, provided cancer researchers with a number of tools whereby the progressive changes involved in the induction of cancer could be studied in experimental systems. Contrary to the results with other carcinogens then known, AAF induced numerous types of tumors, but not at the site of application. This finding stimulated a great deal of interest in its use as an experimental carcinogen to study its metabolic fate and mechanism of action. During the following years an ever increasing number of reports appeared on the carcinogenicity of AAF in various species, on its metabolic fate, on the interaction of reactive metabolites with nucleic acids and proteins, and on its mutagenic activity. Particularly studies on the metabolism of AAF and the interaction with nucleic acids have contributed appreciably to our understanding of the mechanism of action of aromatic amines and also of other chemical carcinogens. It can be expected that AAF and its derivatives will continue to be used for specific applications in experimental cancer research. One of the most recent achievements is the preparation of site-specific AAF- and aminofluorene-modified DNA sequences for mutagenesis studies.

A circular dichroism study on the conformation of d(CGT) modified with N-acetyl-2-aminofluorene or 2-aminofluorene

The trinucleotide d(CGT) was modified by covalent binding of the carcinogen N-acetyl-2-aminofluorene (AAF) or 2-aminofluorene (AF) at the C8 position of the guanine base. The conformations of d(CGT)-AAF and -AF were studied by comparing the absorption and circular dichroism properties with those of dCMP + dGMP-AAF or -AF + dTMP in a molar ratio of 1:1:1 and AAF- and AF-containing dGMP. For both AAF- and AF-d(CGT) complexes the results show significant stacking interactions between the fluorene residue and the base(s) and are discussed in terms of the conformation of d(CGT)-AAF and -AF. In d(CGT)-AF we observe a clear interaction between AF and thymine, whereas the C-G stack is still intact. In the case of d(CGT)-AAF the C-G stack is weakened and the glycosidic rotation angle of dGuo-C8-AAF is most probably syn. The specific fluorene-base interactions persist at elevated temperatures. The carcinogen-base interactions are stronger in the AAF-carrying d(CGT) than in the case of the deacetylated complex. This is consistent with the higher mobility of the AF-adduct and its conformationally heterogeneous appearance in DNA.

Minor-groove binding models for acetylaminofluorene modified DNA

Minimized potential energy calculations have been employed to locate and evaluate energetically a number of different models for DNA modified at carbon-8 of guanine by acetylaminofluorene (AAF). Three different duplex nonamer sequences were investigated. In addition to syn guanine models which have some denaturation and a Z-DNA model, we have found two new types of structures in which guanine remains syn and the AAF is placed in the minor groove of a B-DNA helix. One type features Hoogsteen base pairing between the modified guanine and protonated cytosine, with a sharply bent helix. The other (here termed the "wedge" model because the aromatic amine is wedged into the minor groove) maintains a single hydrogen bond between O6 of the modified guanine and N3 of protonated cytosine, with much less deformation of the helix, and close Van der Waals contacts between the AAF and the walls of the minor groove. Both types of structures (as well as the related forms produced by deprotonation of cytosine) are energetically important in all three sequences examined. The wedge-type model, which is most favored except in alternating G-C sequences, has been previously observed in a combined NMR and computational characterization of an aminofluorene (AF) modified guanine opposite adenine in a DNA duplex undecamer (D. Norman, P. Abuaf, B.E. Hingerty, D. Live, D. Grunberger, S. Broyde and D.J. Patel, Biochemistry 28, 7462 (1989)).

Mutational spectrum and recombinogenic effects induced by aminofluorene adducts in bacteriophage M13

Double-stranded replicative form (RFI) DNA of bacteriophage M13 strain M13mp10 which carries partial lacZ gene has been modified in vitro to various extents with N-hydroxy-2-amino-fluorene (N-OH-AF) and then transfected into E. coli cells. High-performance liquid chromatography (HPLC) analysis results demonstrate that the sole adduct (95%) formed in modified DNA is N-(deoxyguanosine-8-yl)-2-aminofluorene (dG-C8-AF). Approximately 20 adducts per RFI molecule constitute 1 lethal event when plaque-forming ability is assayed on E. coli cells which have received no prior SOS induction. The mutagenicity of dG-C8-AF adducts was assayed by measuring loss of beta-galactosidase activity as a function of adducts per molecule. A dose-dependent increase in Lac- mutants was observed, with a 4-fold increase in mutants per survivor at 30 adducts/molecule. The mutations produced, characterized by DNA sequencing, occur predominantly at either G or C positions different from those observed in the spontaneous mutant spectrum. Restriction-mapping results show that in our assay system, dG-C8-AF adducts induce a previously unreported recombinogenic activity.

Energy minimized structures of carcinogen-DNA adducts: 2-acetylaminofluorene and 2-aminofluorene

Energy minimized structures of DNA modified by the aromatic amines 2-acetylaminofluorene (AAF) and 2-aminofluorene (AF), for which no experimental atomic resolution data exist, are presented. These have been computed with a new molecular mechanics program specifically designed to define distortions imposed by such adducts, and employing a rational strategy for searching the conformation space of a DNA molecule with covalently linked carcinogen. In alternating G-C sequences, the AAF adduct prefers to reside at the exterior of an undeformed Z-helix. It can also induce base displacement with attendant denaturation and helix bending in sequences that disfavor the Z form, but undeformed B helices are excluded. The AF adduct, by contrast, prefers the major groove of an unperturbed B-helix, but can also induce carcinogen-base stacking in single stranded regions of the DNA, such as at the replication fork. The different biological properties of these two adducts may be related to their distinct

Conformation and configuration at the central amine nitrogen of a nucleotide adduct of the carcinogen 2-(acetylamino)fluorene as studied by 13C and 15N NMR spectroscopy

The conformation and configuration at the central nitrogen of the adduct 8-(N-fluoren-2-ylamino)-2'-deoxyguanosine 5'-monophosphate has been investigated by high-field 13C and 15N NMR spectroscopy. One-bond nitrogen-hydrogen coupling constants and 13C chemical shifts for the adduct as well as for the model compounds diphenylamine, 4-nitrodiphenylamine and 2-aminofluorene have been measured in nonaqueous solutions. The data indicate a near planar configuration at the amine nitrogen that links the guanine and fluorene rings of the adduct. The orientations about the guanyl-nitrogen and fluorenyl-nitrogen bonds place the two ring systems in either perpendicular (Type A) or helical (Type B) conformations. It is suggested, based on structural similarities to diarylamines, that the G-N-C bond angle of the adduct is greater than 120 degrees in order to reduce unfavorable steric interactions between the two ring systems. Space-filling molecular models of the adduct in duplex DNA show that the aminofluorene moiety can be oriented into both Type A and Type B conformations within the major groove. The configuration at nitrogen of diphenylamine, 4-nitrodiphenylamine and 2-aminofluorene has also been examined.

DNA binding and mutation spectra of the carcinogen N-2-aminofluorene in Escherichia coli. A correlation between the conformation of the premutagenic lesion and the mutation specificity

When the chemical carcinogen N-2-acetylaminofluorene binds to DNA in vivo, two major adducts are formed, both at position C-8 of the guanine residue. One of these (the acetylaminofluorene adduct) retains the acetyl group, while the other (the aminofluorene adduct) is the corresponding deacetylated form. Unlike -AAF adducts, which trigger important structural changes of the DNA secondary structure (either the insertion-denaturation model or the induction of a Z-DNA structure, depending upon the local nucleotide sequence), -AF adducts bind to the C-8 of guanine residues without causing any major conformational change of the B-DNA structure. Well-defined adducts (either -AF or -AAF) can be formed in vitro by reacting DNA with either N-hydroxy-N-2-aminofluorene or N-acetoxy-N-2-acetylaminofluorene. Specific cleavage of the phosphodiester backbone at -AF adducts can be achieved by treating -AF-modified DNA in 1 M-piperidine at 90 degrees C. This observation led us to construct the spectrum for -AF binding to a defined DNA restriction fragment. It is found that only guanine residues react to form alkali-labile lesions and that the reactivity among the different guanines is similar. In a forward mutation assay, namely the inactivation of the tetracycline resistance gene, we found previously that more than 90% of mutations induced by -AAF adducts are frameshift mutations. Using the same assay, we show here that -AF adducts induce primarily base substitution mutations (85%), mainly of the G to T transversion type. There is therefore a strong correlation between the nature of the carcinogen-induced conformational change of the DNA structure and the corresponding mutation specificity. The -AF-induced base substitution mutations depend upon the umuC gene function(s). The data obtained in our forward mutation assay are compared to the data previously obtained in the histidine reversion assay (Ames test).

In vitro enzymatic methylation of DNA substituted by N-2-aminofluorene

Both the initial velocity and the overall methylation of DNA substituted by aminofluorene, by a rat liver DNA(cytosine-5-)-methyltransferase, are increased as compared to native DNA. The Km and Vmax of the modified DNA for the enzyme increase as a function of the extent of modification. The carcinogen may induce a secondary structure favouring the 'walking' of the enzyme along the DNA. The hypermethylation caused by this carcinogen could have a significance in gene activity, cellular differentiation and cancer induction.

Studies on the conformation and dynamics of the C8-substituted guanine adduct of the carcinogen acetylaminofluorene; model for a possible Z-DNA modified structure

The structure of an adduct between guanine and the carcinogen acetylaminofluorene has been examined in the solid state by X-ray crystallography, and in solution by NMR techniques. The observed conformations have been compared with predictions from energy calculations and their relevance to models of adducts with DNA has been examined.

Conformations of poly(dG-dC).poly(dG-dC) modified by the O-acetyl derivative of the carcinogen 4-hydroxyaminoquinoline 1-oxide

Poly(dG-dC).poly(dG-dC) has been modified by reaction with 4-acetoxyaminoquinoline 1-oxide (Ac-4 HAQO), the ultimate carcinogen of 4-nitroquinoline 1-oxide. The circular dichroism (CD) spectra of the modified and unmodified polymers have been compared under various experimental conditions. The CD spectra were recorded in 1 mM phosphate, 50% (v/v) ethanol, 3.8 M LiCl and 95% (v/v) ethanol, conditions in which poly(dG-dC).poly(dG-dC) adopts the B-, Z-, C- and A-form respectively. In 1 mM phosphate buffer, poly(dG-dC).poly(dG-dC) modified by Ac-4 HAQO seems not to contain regions in the Z-form. Z-form induction could be progressively obtained by the addition of ethanol as follows: in the buffer with about 30% ethanol the modified polymer started to adopt the Z structure, while 40% of ethanol in the buffer was necessary for the unmodified polymer. In the 50% ethanol-1 mM phosphate buffer mixture (v/v), poly(dG-dC).poly(dG-dC) was entirely in the Z-form while poly(dG-dC).poly(dG-dC) modified by Ac-4 HAQO remained partially in the B-form. Enzymatic digestions with the nuclease S1 which is specific of the single-stranded DNA were carried out in order to support the modified poly(dG-dC).poly(dG-dC) CD study conclusions. The role played by the two major adducts on the conformational characteristics of modified polymer is discussed.

8-Chloroguanosine: solid-state and solution conformations and their biological implications

The three-dimensional structure of 8-chloroguanosine dihydrate was determined by X-ray crystallography. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), and the cell dimensions are a = 4.871 (1) A, b = 12.040 (1) A, and c = 24.506 (1) A. The structure was determined by direct methods, and least-squares refinement, which included all hydrogen atoms, converged at R = 0.031 for 1599 observed reflections. The conformation about the glycosidic bond is syn with chi CN = -131.1 degrees. The ribose ring has a C(2')-endo/C-(1')-exo (2T1) pucker, and the gauche+ conformation of the -CH2OH side chain is stabilized by an intramolecular O-(5')-H...N(3) hydrogen bond. Conformational analysis by means of 1H NMR spectroscopy showed that, in dimethyl sulfoxide, the sugar ring exhibits a marked preference for the C(2')-endo conformation (approximately 70%) and a conformation about the glycosidic bond predominantly syn (approximately 90%), hence similar to that in the solid state. However, the conformation of the exocyclic 5'-CH2OH group exhibits only a moderate preference for the gauche+ rotamer (approximately 40%), presumably due to the inability to form the intramolecular hydrogen bond to N(3) in a polar medium. The conformational features are examined in relation to the behavior of 8-substituted purine nucleosides in several enzymatic systems, with due account taken of the steric bulk and electronegativities of the 8-substituents.

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.

Comparative mutagenesis by aminofluorene derivatives. A possible effect of DNA configuration

The mutagenicity of N-acetoxy-N-2-acetylaminofluorene (N-acetoxy- 2AAF ) for Salmonella typhimuricum TA98 is greatly reduced when compared to that of N-hydroxy-2-aminofluorene. This decrease in mutagenic response is accompanied by the formation of a deoxyguanosine-2-acetylaminofluorene adduct. The deoxyguanosine-2-aminofluorene adduct, characteristic of cells exposed to N - hydroxy-2-aminofluorene, was not detected in N-acetoxy- 2AAF -treated cells. Enzymic deacetylation of N - acetoxy- 2AAF results in restoration of potent mutagenicity. N-Acetoxy-2-acetylamino-7- iodofluorene is also more mutagenic than N-acetoxy- 2AAF . Because the acetylated and unacetylated guanine adducts induce greatly different configurational changes, the results may be indicative that the introduction of the syn configuration and a possible shift to the Z-conformation at the mutational hot spot of Salmonella typhimurium TA98 [(dG-dC)8] results in reduced mutagenic potency.

13C-NMR studies of the effects of the carcinogen acetylaminofluorene on the conformation of dinucleoside monophosphate

13C-NMR spectra are obtained in aqueous solution of dinucleoside monophosphates (ApG and GpA) and of their adducts formed by the addition of the carcinogen acetylaminofluorene (AAF) to the C8 position of the guanine. The base and sugar carbons of all dimers and adducts are assigned. The task of assigning base and carbohydrate resonances was accomplished using a series of reference compounds. Significant changes in many of the carbon resonances of the adducts are observed suggesting three general conformational changes, namely: (1) chemical shift changes are noted in base carbon atom resonances as a function of temperature and adduct formation which are indicative of stacking effects; (2) large upfield shifts of the furanose C2' resonance of the guanosine-adduct indicate a shift to higher populations of the syn conformation. Other shifts of carbohydrate resonances are indicative of a change in conformation of the carbohydrate itself. (3) Large temperature effects on linewidth of several fluorine and furanose resonances indicate interconversion of various conformers in the dimer adduct.

Different levels of induction of RecA protein in E. coli (PQ 10) after treatment with two related carcinogens

By means of an immunoradiometric assay the induction of protein RecA in E. coli PQ 10 was measured after treatment by two related carcinogens. On an adduct basis N-Acetoxy-N-2-acetylaminofluorene was shown to induce the protein RecA at a similar level as U.V. On the other hand, N-hydroxy-N-2-aminofluorene shows only a poor induction capacity of the RecA protein. The difference in the SOS inducing potential of the aminofluorene and acetylaminofluorene adducts is discussed in relation to the major difference in the local conformational change the two adducts induce in DNA.

Preferential binding of the chemical carcinogen N-hydroxy-2-aminofluorene to B-DNA as compared to Z-DNA

The reaction between the chemical carcinogen N-hydroxy-2-aminofluorene and poly (dG-dC) . poly (dG-dC) (B-form), poly (dG-m5dC) . poly (dG-m5dC) (B-or Z-form), poly(dG-br5dC) . poly (dG-br5dC) (Z-form) has been studied. The carcinogen binds covalently to B-DNA but does not bind significantly to Z-DNA. These results are discussed as related to the accessibility, the electrostatic potential and the dynamic structure of DNA. The accessibility and the electrostatic potential of DNA do not explain the difference in reactivity of the carcinogen since a related carcinogen N-acetoxy-N-acetyl-2-aminofluorene binds equally well to both B and Z-DNA. On the other hand, poly (dG-dC) . poly(dG-dC) and poly (dG-br5dC) . poly(dG-br5dC), in presence of ethidium bromide binds equally well to N-hydroxy-2-aminofluorene. It is suggested that the very low binding of this carcinogen to Z-DNA as compared to B-DNA is due to differences in the dynamic structures of these two forms of DNA.

Conformation of the deoxydinucleoside monophosphate dCpdG modified at carbon 8 of guanine with 2-(acetylamino)fluorene

Minimized conformational potential energy calculations were performed for dCpdG modified with the carcinogen 2-(acetylamino)fluorene (AAF). The major adduct, linked via a covalent bond between guanine C-8 and N-2 of AAF, was investigated. The 12 variable torsion angles and both deoxyribose puckers were independent flexible parameters in the energy minimizations. Three categories of low-energy conformers were calculated in which the guanine was syn and nearly perpendicular to the plane of the fluorene: (1) forms in which fluorene is stacked with cytidine (included among these is the global minimum energy conformation); (2) conformers which preserve guanine-cytidine stacking while placing the fluorene in a base-pair obstructing position; (3) conformers which maintain guanine-cytidine stacking and place the fluorene at the helix exterior, without interfering with base pairing. The Z form is important in this group. In addition, a low-energy conformation with guanine anti, but still nearly perpendicular to fluorene, was computed. Molecular models were constructed for the most important conformations incorporated into larger polymers. These indicated that the fluorene-cytidine stacked forms induce a severe kink in the B helix. Conformers with guanine-cytidine stacking and AAF in a base-pair obstructing position place the AAF at the B-type helix interior with little distortion in the helix direction. Conformers with the guanine-cytidine stack in which AAF does not affect base pairing place the fluorene at the Z or alternate helix exterior. It is suggested that base sequence, extent of modification, and external conditions such as salt concentration determine which of a number of possible conformational effects is actually induced by AAF. The variety of observed experimental results with AAF-modified DNA may reflect there various conformational possibilities.

Force field conformational analysis of aminofluorene and acetylaminofluorene substituted deoxyguanosine

The hepatocarcinogen N-hydroxy-2-acetylaminofluorene forms two C8-substituted deoxyguanosine adducts in vivo. The conformation of these adducts, as well as 2'-deoxyguanosine and 8-amino-2'-deoxyguanosine has been studied with Allinger's force field. Using the glycoside rotation as a reaction coordinate, multidimensional potential energy surface were determined by relaxing all internal degrees of freedom. The calculations indicate the 2'-deoxyguanosine should exist as a mixture of syn and anti forms, that the syn form is slightly favored for 8-amino-2'-deoxyguanosine, that N-(deoxyguanosin-8-yl)-2-acetylaminofluorene (dG-C8-AAF) will only be found in the syn conformation and that, although the syn form is also more stable for N-(deoxyguanosin-8-yl)-2-aminofluorene (dG-C8-AF), it will have a substantially greater proportion of the anti-conformer than is found with dG-C8-AAF. The results of the force field calculations are discussed in relation to the effects these adducts may have on DNA structure.

Conformational changes of poly(dG-dC) . poly(dG-dC) modified by the carcinogen N-acetoxy-N-acetyl-2-aminofluorene

Poly (dG-dC) . poly(dG-dC) was modified by the reaction with N-acetoxy-N-acetyl-2-aminofluorene. The conformations of poly(dG-dC) . poly(dG-dC) and of poly d(G-C)AAF were studied by circular dichroism under various experimental conditions. In 95% ethanol, the two polynucleotides adopt the A-form. In 3.9 M LiCl, the transition B-form-C-form is observed with poly(dG-dC) . poly (dG-dC) but not with poly d(G-C)AAF. In 1 mM phosphate buffer, poly d(G-C)AAF behaves as a mixture of B- and Z-form, the relative percentages depending upon the amounts of modified bases. The percentage of Z-form is decreased by addition of EDTA and is increased by addition of Mg++. Spermine favors the Z-form in modified and unmodified polynucleotides. No defect in the double helix of poly d(G-C)AAF is detected by SI endonuclease.