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

The response of Escherichia coli to the alkylating agents chloroacetaldehyde and styrene oxide

DNA damage is ubiquitous and can arise from endogenous or exogenous sources. DNA-damaging alkylating agents are present in environmental toxicants as well as in cancer chemotherapy drugs and are a constant threat, which can lead to mutations or cell death. All organisms have multiple DNA repair and DNA damage tolerance pathways to resist the potentially negative effects of exposure to alkylating agents. In bacteria, many of the genes in these pathways are regulated as part of the SOS reponse or the adaptive response. In this work, we probed the cellular responses to the alkylating agents chloroacetaldehyde (CAA), which is a metabolite of 1,2-dichloroethane used to produce polyvinyl chloride, and styrene oxide (SO), a major metabolite of styrene used in the production of polystyrene and other polymers. Vinyl chloride and styrene are produced on an industrial scale of billions of kilograms annually and thus have a high potential for environmental exposure. To identify stress response genes in E. coli that are responsible for tolerance to the reactive metabolites CAA and SO, we used libraries of transcriptional reporters and gene deletion strains. In response to both alkylating agents, genes associated with several different stress pathways were upregulated, including protein, membrane, and oxidative stress, as well as DNA damage. E. coli strains lacking genes involved in base excision repair and nucleotide excision repair were sensitive to SO, whereas strains lacking recA and the SOS gene ybfE were sensitive to both alkylating agents tested. This work indicates the varied systems involved in cellular responses to alkylating agents, and highlights the specific DNA repair genes involved in the responses.

Observing Translesion Synthesis of an Aromatic Amine DNA Adduct by a High-fidelity DNA Polymerase

Aromatic amines have been studied for more than a half-century as model carcinogens representing a class of chemicals that form bulky adducts to the C8 position of guanine in DNA. Among these guanine adducts, the N-(2'-deoxyguanosin-8-yl)-aminofluorene (G-AF) and N-2-(2'-deoxyguanosin-8-yl)-acetylaminofluorene (G-AAF) derivatives are the best studied. Although G-AF and G-AAF differ by only an acetyl group, they exert different effects on DNA replication by replicative and high-fidelity DNA polymerases. Translesion synthesis of G-AF is achieved with high-fidelity polymerases, whereas replication of G-AAF requires specialized bypass polymerases. Here we have presented structures of G-AF as it undergoes one round of accurate replication by a high-fidelity DNA polymerase. Nucleotide incorporation opposite G-AF is achieved in solution and in the crystal, revealing how the polymerase accommodates and replicates past G-AF, but not G-AAF. Like an unmodified guanine, G-AF adopts a conformation that allows it to form Watson-Crick hydrogen bonds with an opposing cytosine that results in protrusion of the bulky fluorene moiety into the major groove. Although incorporation opposite G-AF is observed, the C:G-AF base pair induces distortions to the polymerase active site that slow translesion synthesis.

Mutational spectrum induced in Saccharomyces cerevisiae by the carcinogen N-2-acetylaminofluorene

The spectrum of mutations induced by the carcinogen N-2-acetylaminofluorene (AAF) was analysed in Saccharomyces cerevisiae using a forward mutation assay, namely the inactivation of the URA3 gene. The URA3 gene, carried on a yeast/bacterial shuttle vector, was randomly modified in vitro using N-acetoxy-N-2-acetylaminofluorene (N-AcO-AAF) as a model reactive metabolite of the carcinogen AAF. The binding spectrum of AAF to the URA3 gene was determined and found to be essentially random, as all guanine residues reacted about equally well with N-AcO-AAF. Independent Ura- mutants were selected in vivo after transformation of the modified plasmid into a ura3 delta yeast strain. Plasmid survival decreased as a function of AAF modification, leading to one lethal hit (37% relative survival) for an average of approximately 50 AAF adducts per plasmid molecule. At this level of modification the mutation frequency was equal to approximately 70 x 10(-4), i.e. approximately 50-fold above the background mutation frequency. UV irradiation of the yeast cells did not further stimulate the mutagenic response, indicating the lack of an SOS-like mutagenic response in yeast. Sequence analysis of the URA3 mutants revealed approximately 48% frameshifts, approximately 44% base substitutions and approximately 8% complex events. While most base substitutions (74%) were found to be targeted at G residues where AAF is known to form covalent C8 adducts, frameshift mutations were observed at GC base pairs in only approximately 24% of cases.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Induction of -2 frameshift mutations within alternating GC sequences by carcinogens that bind to the C8 position of guanine residues: development of a specific mutation assay

Using a forward mutation assay we have previously found that N-2-acetylaminofluorene (AAF), a strong chemical carcinogen, induces a majority of frameshift mutations located at specific sequences called mutation hot spots. Among these hot spot sequences, the NarI sequence (GGCGCC), is specific for -2 frameshifts (GGCGCC)----GGCC). Interestingly, these frameshift mutations occur independently of a functional umuDC locus. Being interested in elucidating this mutation pathway we have developed a reversion assay that is specific for this class of mutations. The assay is based on the reversion of a +2 frameshift mutant of plasmid pBR322 from tetracycline sensitivity to tetracycline resistance. It is shown that only "true" reversion events lead to tetracycline resistance. The carcinogen AAF induces this reversion event at a frequency that is increased four- to fivefold over the background frequency. A series of chemical carcinogens which, like AAF, bind covalently to the C8 position of guanine, are compared for their efficiency to induce this specific mutation event. Large variations in the mutagenic efficiency of these chemicals are observed and discussed in terms of the anti/syn conformation of the carcinogen-modified guanine residue. Based on this test, we describe a convenient spot assay that this presently used in our laboratory to isolate Escherichia coli mutants affected in this mutation pathway.

Nucleotide excision repair in Escherichia coli

One of the best-studied DNA repair pathways is nucleotide excision repair, a process consisting of DNA damage recognition, incision, excision, repair resynthesis, and DNA ligation. Escherichia coli has served as a model organism for the study of this process. Recently, many of the proteins that mediate E. coli nucleotide excision have been purified to homogeneity; this had led to a molecular description of this repair pathway. One of the key repair enzymes of this pathway is the UvrABC nuclease complex. The individual subunits of this enzyme cooperate in a complex series of partial reactions to bind to and incise the DNA near a damaged nucleotide. The UvrABC complex displays a remarkable substrate diversity. Defining the structural features of DNA lesions that provide the specificity for damage recognition by the UvrABC complex is of great importance, since it represents a unique form of protein-DNA interaction. Using a number of in vitro assays, researchers have been able to elucidate the action mechanism of the UvrABC nuclease complex. Current research is devoted to understanding how these complex events are mediated within the living cell.

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.

uvrC gene function has no specific role in repair of N-2-aminofluorene adducts

In Escherichia coli, plasmid DNA modified with N-2-aminofluorene adducts survived equally well in wild-type, uvrA, or uvrB strains. Increased sensitivity was found in uvrC and uvrD strains. Moreover, N-2-aminofluorene-mediated toxicity in the uvrC background was reversed when an additional uvrA mutation was introduced into the strain.

Base pair substitution and frameshift mutagenesis induced by apurinic sites and two fluorene derivatives in a recA441 lexA (Def) strain

One of the consequences of the induction of the Escherichia coli SOS system is the increased ability of the cells to perform mutagenesis. Induction of the SOS system is the result of derepression of a set of genes through a regulatory mechanism controlled by LexA and RecA. In response to an inducing signal, RecA is activated in a form that facilitates the proteolytic cleavage of LexA repressor. Previous works have shown that activated RecA plays a second role, i.e. it is required for the establishment of base pair substitution mutations promoted by UV irradiation. Using a forward mutational assay and recA441 lexA (Def) host bacteria, we show that the result can be extended not only to other mutagens promoting base pair substitution mutations (Apurinic sites, Ap sites and N-hydroxy-N-2-aminofluorene, N-OH-AF) but also mutagens promoting frameshift mutations (N-Acetoxy-N-2-acetylaminofluorene, N-AcO-AAF). In the recA441 lexA (Def) strain all the genes which are part of the lexA regulon, including recA itself, are expressed constitutively. The recA441 mutation allows RecA to acquire its activated form when the bacteria are grown at 42 degrees C. We show that in such strains Ap sites or N-OH-AF induce a high level of mutations only when the bacteria are grown at 42 degrees C. On the other hand, we show that N-AcO-AAF can promote mutations even at 30 degrees C; the number of mutations being increased when the bacteria were grown at 42 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

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).

RECA immunological assay as a tool to analyze the SOS response

The content of RECA protein, one of the SOS genes product, was determined in a bacterial extract by a two site-radioimmunometric assay. The variation of the RECA concentration after induction by physical or chemical treatments was used as a probe to analyze the SOS response. Relationships between either the number or the nature of DNA lesions and the level of the relative amplification of RECA have been established. The modulation of the recA gene expression is discussed.

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.

Measurement of in vivo expression of the recA gene of Escherichia coli by using lacZ gene fusions

A recA-lacZ protein fusion was constructed in vivo by using bacteriophage Mu dII301(Ap lac). The fusion contained the promoter and first 47 codons of the recA mutant, as determined by DNA sequence analysis. The fusion was cloned and used to construct a recA-lacZ operon fusion at the same site within the recA gene. These fusions were introduced into the Escherichia coli chromosome at the lambda attachment site either as complete or cryptic lambda prophages. Synthesis of beta-galactosidase from these fusions was inducible by UV radiation. As the UV dose was increased, induction became slower and persisted for a longer period of time. At low doses of UV radiation, more beta-galactosidase was produced in a uvrA mutant than in a wild-type strain; however, at high doses, no induced synthesis of beta-galactosidase occurred in a uvrA mutant. recA+ strains carrying either the protein or operon fusion on a multicopy plasmid showed reduced survival after UV irradiation. This UV sensitivity was not exhibited by strains containing a single copy of either fusion, however; hence, the fusions provide a reliable measure of recA expression.

Carcinogen-induced mutation spectrum in wild-type, uvrA and umuC strains of Escherichia coli. Strain specificity and mutation-prone sequences

Forward mutations induced by the ultimate carcinogen N-acetoxy-N-2-acetylaminofluorene (N-Aco-AAF) in the tetracycline resistance gene carried on plasmid pBR322 are shown to be dependent upon the induction of the host SOS functions in wild-type and umuC Escherichia coli cells. The mutation frequency in the umuC strain is equal to about 40% of the mutation frequency observed in the umu+ background. In the excision-repair-deficient uvrA mutant strain the mutagenic response is the same as in SOS-induced wild-type cells whether or not the uvrA bacteria are SOS-induced. Equal mutation frequencies are obtained in both the wild-type and the uvrA strains for equal modification levels although the survival of AAF-modified plasmid DNA is greatly reduced in the uvrA strain as compared to the wild-type strain. Sequence analysis of the mutations reveals that more than 90% of the N-Aco-AAF-induced mutations are frameshift mutations. Two types of mutational hotspots are observed occurring either at repetitive sequences or at non-repetitive sequences. Both types of mutants appear at similar locations and frequencies in both the wild-type and the uvrA strains. On the other hand, only the non-repetitive sequence mutants are obtained in the umuC background. These non-repetitive sequence mutants preferentially occur within the sequence 5' G-G-C-G-C-C 3' (the NarI restriction enzyme recognition sequence). The analysis of the -AAF binding spectrum to the same DNA fragment shows that there is no direct correlation between the modification spectrum and the mutation spectrum. We suggest that certain sequences are "mutation-prone" in the sense that only these sequences can be efficiently mutated as the result of an active processing mediated by specific proteins. When a sequence is said to be mutation-prone it probably corresponds to a particular structure that is induced within this sequence as a result of the binding to the DNA of the mutagen. This sequence-specific conformational change is the substrate for the protein(s) that fixes the mutation. The mutagenic processing pathway(s) is part of the cellular response to DNA-damaging agents (the so-called SOS response). Two pathways for frameshift mutagenesis are suggested by the data: an umuC-dependent pathway, which is involved in the mutagenic processing of lesions within repetitive sequences; an umuC-independent pathway responsible for the fixation of mutations within specific non-repetitive sequences.