Role of DNA repair by (A)BC excinuclease and Ogt alkyltransferase in the final distribution of LacI-d mutations induced by N-butyl-N-nitrosourea in Escherichia coli

In the absence of nucleotide excision repair, the additional deficiency of the DNA alkyltransferase (ATase) encoded by the constitutive ogt gene of Escherichia coli caused a marked increase in mutation induction by N-butyl-N-nitrosourea (BNU). Irrespective of the presence or absence of the Ogt ATase, little mutagenic response was detected in Uvr+ bacteria in the concentration range 0-8 mM BNU, indicating that most premutagenic DNA lesions induced at these concentrations are efficiently recognized and repaired by the nucleotide excision repair system. Increased susceptibility to mutagenesis by BNU was detected in Uvr- Ogt+ bacteria, but the Uvr- Ogt- double mutant exhibited much higher sensitivity. These data suggest that the Ogt ATase can replace to a great extent the repair capacity of the (A)BC excinuclease. Forward mutations induced by 6 mM BNU within the initial part of the lacI gene of E.coli were recovered from Uvr+ Ogt-, Uvr- Ogt+ and Uvr- Ogt- bacteria. A total of 454 independent mutations were characterized by DNA sequence analysis. The BNU-induced spectra were dominated by G:C-->A:T transitions, consistent with the major role of the O6-alkylguanine miscoding lesion in mutagenesis by alkylating agents. Specific sites for G:C-->A:T transitions were recovered more or less frequently in one genetic background versus the others, giving statistically significant differences among the spectra (P < 10(-6)). We examined the influence of DNA repair by (A)BC excinuclease and Ogt ATase on the 5'-flanking base associated with the BNU-induced G:C-->A:T transitions; preferences different from those previously reported for other alkylnitrosoureas were detected. We discuss how these differences might be caused by BNU producing branched chain derivatives, in addition to the expected linear chain adducts, and by possible preferences with respect to both the initial distribution of O6-butylguanine lesions and their repairability.

Spontaneous and ENU-induced mutation spectra at the cII locus in Big Blue Rat2 embryonic fibroblasts

Big Blue Rat2 embryonic fibroblasts carry the lambda-Liz shuttle vector which is also present in the Big Blue mouse and rat. Mutations in the Big Blue systems have most often been measured at the lacI locus. However, a method for positive selection of mutations at the lambda cII locus was recently described. This assay appears to have many advantages over the use of lacI as a mutational target, but it has yet to be well characterized in mammalian mutagenesis studies. The objective of these studies was to determine the spontaneous and ethylnitrosourea (ENU)-induced mutant frequencies (MFs) and mutational spectra at cII using Big Blue Rat2 embryonic fibroblasts. The average spontaneous MF was 13 +/- 1.4 x 10(-5). The average induced MF was 60 +/- 10 x 10(-5) 10 days following a 30 min treatment with 0.1 mg/ml ENU. Eighty four independent spontaneous mutants were sequenced: 23 (27.4%) were frameshift mutations and 61 (72.6%) were base substitutions. Two spontaneous frameshift hotspots were detected, both in mononucleotide runs. G:C-->A:T transitions were the most common type of base substitution in cII; of these 71% occurred at CpG sites. The ENU-induced mutational spectrum at cII (44 mutants) consisted of 42 base substitutions (95.5%) and two -1 frameshift mutations (4.5%). Compared with the spontaneous spectrum, the ENU-induced spectrum had significantly fewer frameshift mutations (4.5 versus 27%) and base substitutions occurred predominantly at A:T base pairs (71 versus 34%). Overall, the spontaneous cII mutational spectrum reported here differs slightly from spontaneous spectra reported at the Big Blue lacI locus, but the mutational spectra and base substitution MFs following treatment with ENU were comparable at both loci. These data support the continued use of cII as a selectable marker in mutagenesis studies involving cells or tissues that carry a lambda transgene.

Aryl hydrocarbon receptor activation in genital tubercle, palate, and other embryonic tissues in 2,3,7, 8-tetrachlorodibenzo-p-dioxin-responsive lacZ mice

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the toxicity of 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons. Although the normal function and endogenous ligand for this receptor are not known, it is thought to have a role in growth regulation processes. The AhR has been found in both adult and certain developing tissues, and AhR agonists like the environmental contaminant TCDD cause a number of developmental anomalies. We sought to determine whether the AhR is directly activated to a transcriptionally functional form in tissues known to be adversely affected by AhR agonist exposure. To this end, a transgenic mouse model was developed that could be used to indicate the temporal and spatial context of transcriptionally active AhR following agonist exposure in vivo. A synthetic promoter containing two dioxin-responsive elements (DREs) and a minimal TATA box was strongly induced by TCDD in transfected cells when linked to the lacZ or luciferase reporter gene. Transgenic mice harboring the lacZ construct had TCDD-inducible beta-galactosidase activity in tissues following adult and in utero exposure. Embryonic lacZ expression was induced in hard and soft palates, genital tubercle, certain facial regions, shoulder, as well as other tissues by in utero exposure to 30 microg TCDD/kg at Gestational Day 13. The most intense reporter response was observed in the genital tubercle. Histopathology of the palate and tubercle demonstrated the reporter gene activity to be both cell- and region-specific. This is the first publication to correlate reported TCDD-elicited toxicity (e.g., cleft palate in mice) with TCDD-dependent AhR activation. These data indicate the ability of TCDD to initiate a signal transduction process leading to a transcriptionally active AhR in these tissues, thereby identifying potential targets of dioxin-induced toxicity during development. Weak activation of the reporter gene was consistently observed only in the genital tubercle in the absence of exogenous inducer. This indicates minimal or no endogenous AhR activators at the developmental stage examined. This mouse model will prove useful for both the examination of the endogenous role of the AhR in proliferation or differentiation and of the developmental targets of dioxin-like compounds.

Statistical analysis of lacZ mutant frequency data from MutaMouse mutagenicity assays

Transgenic mouse assays have provided an unprecedented opportunity to study mutagenesis in diverse rodent tissues. In this article data from MutaMouse mutagenicity assays based on the Escherichia coli gene lacZ were analyzed systematically using liver and bone marrow as potential target tissues. Sources of variation, including plates (within packaging reactions), packaging reactions (within animals) and animals, were evaluated for extra-binomial variation. Although hardly any evidence of overdispersion was detected at the plate level, limited evidence of extra-binomial variation was observed at the packaging reaction level. There was, however, much stronger evidence of overdispersion at the animal level. Statistical tests for increasing trend in mutant frequency with increasing dose were also performed at the animal level. A significant increasing trend following exposure to N-nitrosodibenzylamine was detected in liver but not in bone marrow. A logistical model was used to further describe the dose-response relationship observed in N-nitrosodibenzylamine-treated liver tissue.

Development of new tester strains derived from E. coli WP2uvrA for the determination of mutational specificity

We have developed a set of multipurpose tester strains (WP3101 to WP3106) derived from E. coli WP2uvrA for the detection and classification of mutagens. Six kinds of F' plasmid (lacI, lacZ, proAB+) in strains CC101-CC106, each of which carried a different lacZ allele, were transferred to a delta(lac-pro) derivative of WP2uvrA. Assays for transitions and transversions are based upon Lac+ reversion of a specific mutation located within the lacZ gene on an F' plasmid in strains WP3101-WP3106. In addition, the trpE65(ochre) allele in the same strains is available for Trp+ reversion assays. Using the new tester strains, we investigated the mutational specificities of various chemical mutagens. Base analog mutagens and alkylating mutagens induced specific types of base substitutions. G:C-->A:T transitions and G:C-->T:A transversions predominated in mutagenesis induced by 4-nitroquinoline 1-oxide. Only a slight increase in G:C-->T:A transversions was observed in cells treated with 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide (AF-2), although the potent mutagenicity of AF-2 was detected in a concurrent Trp+ reversion assay in the same strain. Sodium azide, on the other hand, was negative in the Trp+ reversion assay but specifically induced G:C-->A:T transitions. Present finding suggested that target sites for AF-2- and azide-induced lesions may largely depend on sequence context.

Mutagenicity of ethyl carbamate to lacZ- transgenic mice

The mutagenicity of the rodent carcinogen ethyl carbamate (EC, urethane) has been assessed using the lacZ- transgenic mouse mutation assay (Muta Mouse). In two separate experiments a single i.p. dose of 900 mg/kg urethane followed by a 14-16 day expression period yielded statistically significant (approximately 2-fold) increases in mutation frequency (MF) in the lung and liver of the mice. In the second experiment MF in the spleen and bone marrow was also assessed and statistically significant increases in MF were observed in each case. Concomitantly conducted bone marrow micronucleus assays gave 15.8- and 8-fold increases respectively in the incidence of micronucleated polychromatic erythrocytes in the two studies.

In vivo mutagenesis by the hepatocarcinogen quinoline in the lacZ transgenic mouse: evidence for its in vivo genotoxicity

Quinoline is carcinogenic to the liver of rats and mice and mutagenic to bacterial tester strains in the presence of rat liver microsomal enzymes. The unscheduled DNA synthesis (UDS) study suggested that quinoline might be a non-genotoxic carcinogen because of the lack of UDS-inducing capacity. In order to determine whether or not cancer induction is initiated by mutagenic DNA lesions, the present study was undertaken to evaluate the mutagenicity of quinoline in an in vivo mutation assay system using the lac Z transgenic mouse (Muta Mouse). Mutation was only induced in the liver, the target organ of carcinogenesis by quinoline, but not in the other organs examined, i.e. lung, kidney and spleen. Mutant frequency in the liver was 4-fold higher than in the untreated control animals. Dimethylnitrosamine, used as a positive control, induced mutation at a frequency 5-fold higher in the liver and 3-fold higher in the spleen than in their respective control organs. It can be concluded that the genotoxicity of quinoline is responsible for its hepatocarcinogenesis, although UDS was not induced under the conditions previously reported.

Lack of mutagenicity of levofloxacin in lacZ transgenic mice

The mutagenic effects of levofloxacin in lacZ transgenic mice (Muta Mouse) have been investigated. Male Muta-Mouse mice were administered levofloxacin i.p. at a dose of 300 or 600 mg/kg. The higher dose corresponded to half the LD50 of the compound in ddY strain mice. The mutant frequencies in the bone marrow, liver (day 10 only), testis and sperm were examined by the positive selection method for lacZ mutations on days 1 and 10 after treatment. Levofloxacin did not induce any statistically significant increase in mutant frequency in any of the examined tissues at either dose level or at either sampling time. The mutant frequency increases over the spontaneous values in the bone marrow, liver, testis and sperm were 1.0- to 1.2-fold, 0.9-fold, 0.5- to 1.0-fold and 0.9- to 1.3-fold respectively. N-Ethyl-N-nitrosourea (100 mg/kg as a positive control), on the other hand, induced significant increases in mutant frequencies in both somatic cells (bone marrow and liver) and germ cells (testis and sperm) on day 10 after treatment. The mutagenic potency for ENU was bone marrow >> liver approximately testis approximately sperm (50.1-fold, 3.4-fold, 2.9-fold and 2.3-fold respectively over the spontaneous values). Levofloxacin was not mutagenic in the lacZ transgenic mice under the present experimental conditions.

Long-term mutagenicity studies with chloroform and dimethylnitrosamine in female lacI transgenic B6C3F1 mice

The weight of evidence indicates that chloroform induces cancer in the female B6C3F1 mouse liver via a nongenotoxic-cytotoxic mode of action. However, it is probable that DNA damage occurs secondary to events associated with cytolethality and regenerative cell proliferation. The purpose of the present study was to evaluate the potential mutagenic activity of chloroform in the B6C3F1 lacI transgenic mouse liver mutagenesis assay including mutagenic events that might occur secondary to cytolethality. The positive control, dimethylnitrosamine (DMN) is a DNA-reactive mutagen and carcinogen. DMN-induced mutations were anticipated to require only a brief exposure and without further treatment were predicted to remain unchanged over time at those frequencies. Chloroform-induced mutations secondary to toxicity were anticipated to require longer exposure periods and to occur only under conditions that produced sustained cytolethality and regenerative cell proliferation. Female B6C3F1 lacI transgenic mice were treated with daily doses of 2, 4, or 8 mg/kg of DMN by gavage for 4 days and then held until analysis 10, 30, 90, and 180 days postexposure. Livers from DMN-treated mice exhibited a dose-related 2- to 5-fold increase over control mutant frequencies and remained at those levels for 10 through 180 days postexposure. Thus, following the initial induction by DMN no selective mutation amplification or loss was seen for this extended period of time. Female B6C3F1 lacI mice were exposed daily for 6 hr/day 7 days/week to 0, 10, 30, or 90 ppm chloroform by inhalation, representing nonhepatotoxic, borderline, or overtly hepatotoxic chloroform exposures. Timepoints for determination of lacI mutant frequency were 10, 30, 90, and 180 days of exposure. No increase in lacI mutant frequency in the liver was observed at any dose or timepoint with chloroform, indicating a lack of DNA reactivity. DNA alterations secondary to toxicity either did not occur or were of a type not detectable by lacI mutant frequency analysis, such as large deletions.

Photoreaction of new psoralen analogs with DNA: sequence and mutation specificity in the Escherichia coli lacZ gene

New thio- and seleno-analogs of psoralen were synthesized and analyzed for their photoreactivity toward DNA. Using oligonucleotides of defined sequence, we first showed that these derivatives predominantly generated interstrand crosslinks at 5'-TpA sites. We also observed a surprisingly high reactivity of 7H-thiopyrano[3,2-f][1]benzofuran-7-one (PSO[O-S]) with the BamHI and PstI oligomers, giving rise to the formation of crosslinks at 5'-ApT sites and of the thymidine-psoralen-cytosine type. Next, the sequence specificity in the photochemical binding of all the compounds was investigated in two DNA fragments encompassing the lacZ gene of Escherichia coli, using the T4 DNA polymerase sequencing methodology. Resulting maps demonstrated that thio- and seleno-analogs of psoralen preferentially photoreacted with thymine and cytosine residues. The AT-rich sequences proved to be particularly reactive sites as did adjacent thymines, especially at C-surrounding residues. Likewise, photoaddition at cytosines in CA/AC context was observed. It was highly significant that all of the derivatives exhibited similar sequence specificities with only minor differences. However, PSO(O-S) differed from the other heteropsoralens. Photoadducts occurred with a higher frequency at AC and CA dinucleotides, and new sites were detected. A comparison with 8-methoxypsoralen photobinding is also reported. Finally, the mutagenic consequences of photoadducts induced in M13mp19 DNA by PSO(O-S) were determined in a forward system that detects all classes of mutagenic events. The high phototoxicity exhibited by PSO(O-S) could be attributed to crosslinks, and the comparison of the observed mutational specificity with the photoadduct distribution within the same gene showed that mutations were targeted at potential monoadduct sites where photolesions were detected in our footprinting experiments.

Sodium phenobarbital-enhanced mutation frequency in the liver DNA of lacZ transgenic mice treated with diethylnitrosamine

To investigate how a carcinogenic promoter acts on cells mutated by an initiator, we used as a model, lacZ transgenic mouse and a positive selection system. Preliminary data for the mutational events in liver DNA of the mice was generated using diethylnitrosamine (DEN) and sodium phenobarbital (S-PB) as initiator and promoter, respectively. In our first experiment, male MutaMice received a single i.p. injection of saline or 100 mg/kg DEN and were fed a normal diet for 7 days and 500 p.p.m. S-PB in the diet for 21 days. Liver DNA was harvested after a 1 night fast on days 7 and 28 post-DEN treatment. In our second experiment, male mice received a single i.p. injection of phosphate buffered saline or 50 mg/kg DEN and were fed a normal diet for 7 days, a diet with S-PB for 14 days and then a normal diet for 7 days. Liver DNA was harvested after a 1 night fast on days 7, 21 and 28 post-DEN treatment. The S-PB diet enhanced absolute and relative liver weights in all groups. The single intraperitoneal dose of 50 or 100 mg/kg DEN induced high mutation frequencies (MF) in liver, lacZ genes on days 7, 21 and 28. There were no remarkable differences of the MF among any sampling days for animals receiving DEN and a normal diet. S-PB feeding at 500 p.p.m. for 21 days failed to affect the MF in groups given saline or 100 mg/kg DEN. On the other hand, when 50 mg/kg DEN was given, S-PB feeding at 500 p.p.m. for 14 days elevated the MF in liver DNA on days 21 and 28 to approximately 1.8 and 4.0 times the MF, respectively, of the mice fed the normal diet. Consequently, S-PB might preferentially promote certain initiated cells participating in a balance between cell death and proliferation.

o,p'-DDT and its metabolites inhibit progesterone-dependent responses in yeast and human cells

Using a combination of in vitro assays we have evaluated whether DDT metabolites can interact with the progesterone receptor pathway in yeast expressing human progesterone receptor (hPR) and in T47D human breast cancer cells which express endogenous hPR. In transactivation assays using both yeast and T47D cells, o,p'-DDT and the metabolites p,p'-DDT, o,p'-DDD, p,p'-DDD, o,p'-DDE, p,p'-DDE, p,p'-DDA, and DDOH inhibited progesterone-induced reporter gene activity in a dose-dependent manner. None of the DDT metabolites functioned as hPR agonists. Whole cell competition binding assays using T47D cells indicated that the inhibitory effects of DDT metabolites on progesterone-dependent activites may occur through both hPR-dependent and hPR-independent pathways. Our results and previous reports of DDT metabolites interacting with estrogen and androgen receptors suggests that this class of environmental chemicals may interact with numerous hormone receptor signaling pathways.

Escherichia coli lac repressor-lac operator interaction and the influence of allosteric effectors

The wild type E. coli lac operator is embedded in a 35 base-pair DNA sequence containing extensive 2-fold symmetry, suggesting a symmetric repressor operator complex. However, deviations from strict 2-fold symmetry occur at the central base-pair and at three additional base-pairs. Using an operator fragment binding analysis we have determined: (a) a relative contribution each pair provides to the lac repressor-lac operator DNA complex, (b) the operator DNA length necessary for maximum binding to lac repressor; and (c) the contribution of the several non-symmetric base in the wild-type operator to the binding affinity. Since lac repressor-lac operator DNA interaction is reduced upon binding of the gratuitous inducer, isopropyl-beta-D-galactoside (IPTG), the same DNA fragment binding analysis was performed with the low affinity form of lac repressor. In the presence of inducer, the affinity for the left half site of the wild-type lac operator is reduced without significant reduction on the right half of the operator. Conversely, the anti-inducer orthonitrophenylfucoside (ONPF) which stabilizes the lac repressor-lac operator complex increases the binding affinity, particularly to the right half of the operator.

Comparison of the induction of SOS repair in Escherichia coli PQ37 and PQ243 by antineoplastic agents

Six alkylating antineoplastic drugs (Cyclophosphamide, Chlorambacil, Busulfan, Melphalan. Streptozotocin and Lomustine) and two reference compounds (methyl methanesulphonate and N-methyl-N'-nitro-N-nitrosoguanidine) were investigated in the SOS Chromotest using the Escherichia coli strain PQ37 (wilde-type) and derived strain (PQ243), which carries the same markers as PQ37 and additionally tagA alkA. As a measure of the SOS induced activity induction factors of sflA::lacZ expression were determined. The strain PQ243 was more sensitive towards all compounds inducing SOS DNA repair then the strain PQ37 Cyclophosphamide was detected as negative in the strain PQ243 in the presence of an exogenous metabolic activation system. Lomustine was inactive both in the mutant strain and in the wild-type strain in the presence of S9 mix fraction as well as in the absence of it. Melphalan and Busulfan (without or with S9 mix) were shown to be positive exclusively in the strain PQ243. Based on these results, we discuss the usefulness of the strain PQ243 in the monitoring of the genotoxicity of drugs and in the genetic analysis of their mode of action.

A transgenic mouse model to study transsynaptic regulation of tyrosine hydroxylase gene expression

Previous studies demonstrated that 9 kb of the rat tyrosine hydroxylase (TH) 5' flanking sequence directed appropriate spatiotemporal expression of a lacZ reporter gene to catecholaminergic cells in the CNS of transgenic mice. In the present study, specificity of transgene expression was further extended to demonstrate cell type-specific functional regulation of lacZ expression using manipulations known to alter endogenous TH expression. Alterations in lacZ reporter expression should parallel changes in endogenous TH levels if the DNA elements mediating these functional changes of TH expression in vivo reside within the 9 kb of the TH promoter region. Naris closure induced an activity-dependent decrease of TH expression in dopaminergic periglomerular cells in the olfactory bulb that was paralleled by down-regulation of lacZ expression in the transgenic mice. Densitometry and image analysis were used to quantify lacZ expression following acute reserpine administration (5 mg/kg s.c.), which up-regulates endogenous TH. At 48 h postinjection, analysis of OD values indicated a significant increase of X-gal staining in the locus coeruleus and ventral tegmental area but not in the substantia nigra or olfactory bulb of reserpine-treated transgenic animals. These data showed that the 9-kb sequence also mediates cell type-specific transsynaptic regulation of reporter gene expression. Analysis of this transgenic animal offers a useful model system to study in vivo regulation of TH gene expression.

Mutational specificity of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea in the Escherichia coli lacl gene of O6-alkylguanine-DNA alkyltransferase-proficient and -deficient strains

Forward mutations induced by 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) in the lacl gene of Escherichia coli were recovered from bacteria proficient (Ogt+ Ada+) and deficient (Ogt- Ada-) in O6-alkylguanine-DNA alkyltransferase activity. A CCNU dose of 1 mM was selected for DNA sequence analysis. A total of 245 induced mutations were characterized. The mutations were almost exclusively (95%) GC-->AT transitions, indicating that CCNU-induced mutations arose in bacteria primarily from misreplication of O6-chloroethylguanine, in total agreement with results obtained for monofunctional alkylating agents. The distribution of CCNU-induced GC-->AT mutations was significantly altered by the presence of DNA alkyltransferase activity (P = 0.01). In the Ogt+ Ada+ mutational spectrum, guanines flanked on both sides by A:T base-pairs were on average 2.8 times more likely to mutate than those flanked by G:C base-pairs on at least one side. This bias disappeared in the Ogt- Ada- genetic background, thereby providing evidence that O6-chloroethylated guanines adjacent to G:C base-pairs are better targets for bacterial alkyltransferase than those not adjacent to G:C base-pairs. We recently reported a similar bias for ethyl methanesulfonate, strengthening the idea that CCNU is acting as a simple ethylating compound. In summary, this paper presents for the first time evidence that DNA repair by O6-alkylguanine-DNA alkyltransferases plays a major role in removing lesions responsible for GC-->AT transitions induced by CCNU, influencing their ultimate distribution with respect to sequence context.

Effects of oncogenes on the resistance to cis-diamminedichloroplatinum(II) and metallothionein gene expression

Transformation of NIH3T3 cells with the ras, the sis, or the neu oncogene rendered cells less susceptible to cis-diamminedichloroplatinum(II). Since resistance to cis-diamminedichloroplatinum(II) is reported to be associated with increased levels of metallothionein, we examined effects of these oncogenes on metallothionein gene expression. NIH3T3 cells were first transfected with the lacZ gene whose transcription is under the control of mouse metallothionein I promoter and then with the ras, the sis, or the neu oncogene. The ras and the sis oncogenes increased beta-galactosidase activities which were induced either by metal (cadmium and zinc) or by glucocorticoid (dexamethasone), whereas the neu oncogene repressed its activity. When SV40 early promoter was used instead of metallothionein I promoter for the lacZ gene transcription, the beta-galactosidase activities were not affected by metal, dexamethasone, or any of these oncogenes. This result was coincident with that of reverse transcription polymerase chain reaction that metal-induced MT I mRNA was only detected in the sis- or the ras-transformed cells, whereas any of these oncogenes did not affect the metal-induced transcription of the MT II gene. These results demonstrate that the ras and the sis oncogenes upregulate the metal- or glucocorticoid-induced transcription from metallothionein I promoter, but the neu oncogene negatively regulates it. Thus, resistance to the chemotherapeutic agent by oncogenic transformation is partly associated with the metallothionein gene expression, and MT I and MT II gene expressions are differently controlled by different oncogenes.

Simple aromatics identified with a NFAT-lacZ transcription assay for the detection of immunosuppressants

Determination of the mechanism of action of FK506 and cyclosporin A has yielded new molecular targets involved in signal transduction during T cell activation. A common target of FK506 and cyclosporin A is inhibition of activation of the NFAT transcription factor, for which a specific binding region is present in the promoter of the IL-2 gene. A reporter gene assay has been used to screen for agents that interfere with this early step in T cell activation. Simple aromatic compounds that block NFAT-dependent transcription and show in vitro immunosuppressive activity were isolated from the broth and mycelia of two Streptomyces sp. fermentations. The compounds were active at concentrations that were not directly cytotoxic.

DNA repair by Ogt alkyltransferase influences EMS mutational specificity

Forward mutations induced by ethylmethane sulfonate (EMS) in the lacI gene of Escherichia coli were recovered from bacteria proficient or deficient in the alkyltransferase encoded by the constitutive ogt gene. EMS doses of 100 or 200 mM (Ogt+) and of 50 mM (Ogt-) were selected from the corresponding dose-response curves for DNA sequence analysis. A total of 239 induced mutations affecting the N-terminal region of the lacI gene were characterized. All mutations were G:C-->A:T transitions, consistent with the predominant role of the O6-ethylguanine miscoding lesion in mutagenesis by EMS. In the Ogt+ spectrum at the lowest tested dose of 100 mM EMS, guanines preceded by an A or T base at the 5' side were on average 3.2 times more likely to mutate than those preceded by a G or C base. This bias diminished at the higher EMS dose (200 mM) and disappeared in the Ogt- genetic background. Previously reported data for Ogt+ bacteria in a Uvr-proficient background show an opposite bias in favor of mutations at guanines preceded by a G or C base. The overall 5' flanking base influence was estimated as 8-fold. These data suggest that DNA repair by Ogt alkyltransferase plays an important role in the processing of ethylation-induced lesions responsible for GC-->AT transitions, influencing their ultimate distribution with respect to sequence context. The data further suggest that Ogt and UvrABC excision repair, the two major mechanisms of protection against the biological consequences of long-chain alkylating agents, show different DNA sequence specificity and that the relative importance of these two systems is highly dependent upon the chemical dose.

Evidence for in vivo non-mutagenicity of the carcinogen hydrazine sulfate in target tissues of lacZ transgenic mice

Transgenic mouse models permit the confirmation of in vitro mutagenicity in vivo without the constraints in the selection of tissues imposed by other in vivo assays. This feature is of particular importance in the determination of mutagenicity in the target tissues of carcinogens, especially those that are in vitro mutagens. Such information is critical in the determination of whether a chemical is carcinogenic via a genotoxic or non-genotoxic mechanism. Hydrazine sulfate is an in vitro mutagen that induces lung and liver tumours in mice. Transgenic mice from strain 40.6 (Mutamouse) were administered single oral doses up to a toxic concentration (400 mg/kg). No dose induced any lacZ mutations in lung, liver or bone marrow. Since the highest single dose used is higher than the cumulative dose that induced tumours in previous studies, it may be that either hydrazine sulfate is genotoxic in target tissues in vivo only when given in multiple doses or that it is a non-genotoxic carcinogen.

A basic tail increases repression by dimeric lac repressor

Tetrameric Lac repressor achieves cooperative repression by binding simultaneously to O1 and to one of the auxiliary operators O2 or O3, thereby forcing the intervening DNA into a loop. Dimeric Lac repressor is not able to form DNA loops and consequently shows no cooperative repression. We constructed a dimeric Lac repressor mutant which exhibits increased repression to the lac operon that does not depend on specific operator-repressor-operator loops. This Lac repressor carries a synthetic tail of basic residues attached to its C terminus. With this construct, we observe an increase of the in vivo repression upon addition of auxiliary lac operators to a chromosomal lac operon controlled by O1. This suggests that the basic tail enables dimeric Lac repressor to enhance its repression by additional non-specific DNA contacts.

A preliminary assessment of the toxic and mutagenic potential of steroidal alkaloids in transgenic mice

Impregnated CD2 transgenic mice, which contain multiple copies of a lambda gt10lacZ construct integrated into the genome of each cell, were given a predetermined estimated maximum tolerated dose of several steroidal alkaloids: Solanum glycoalkaloids from potato, alpha-chaconine and alpha-solanine; aglycones, solanidine and solasodine, and a Veratrum alkaloid, jervine. Observations were made of dams and foetuses for indications of toxicity and/or terata; some dam livers and foetuses were assayed for mutagenicity using the lacZ gene. Other dams were gavaged with a single dose of 75 mg all-trans-retinol/kg to serve as a reference teratogen. Unexpectedly, this level of retinol was not clearly teratogenic. The results of both positive and non-positive selection systems showed that the mutation frequencies in the livers of the dams dosed with alpha-chaconine, alpha-solanine and solanidine were three to four times higher than historically normal in the livers of this transgenic mouse strain.

Mutagenicity of furfural in plasmid DNA

Furfural is recognized as a dietary mutagen and is present in various foods and beverages. We have examined the mutagenic effect of furfural induced lesions in plasmid pBluescript SK(+/-). There was a progressive decrease in the transformation capacity of the plasmid as a function of furfural concentration with a concomitant increase in the number of mutant plasmids. Several mutant plasmids with reduced transformation capacity and a molecular size similar to the parental plasmid were isolated. A stretch of DNA of 108 basepairs within the multiple cloning region was sequenced. It was observed that the number of mutagenic events in the case of furfural damaged plasmid was not significantly greater than in spontaneously arisen mutants. These results were interpreted to indicate that furfural mediated DNA damage is efficiently repaired.

Spectra of superoxide-induced mutations in the lacI gene of a wild-type and a mutM strain of Escherichia coli K-12

We have analyzed the spectra of superoxide-induced mutations in the chromosomal lacI gene of a wild-type and a mutM strain of Escherichia coli K-12. The mutM strain is known to be deficient in removing 8-hydroxyguanine from DNA. An intracellular superoxide-generating agent, menadione, was used to cause the mutation. Analysis of the mutated DNA showed marked differences between the mutants from the wild type and those from the mutM strain. In the mutants from the wild type, all possible base-pair substitutions were present and their proportions were similar to each other, whereas in those from the mutM bacteria there was a 90% bias in favor of transversion. Furthermore, in the mutM strain GC-to-CG transversion rather than GC-to-TA was predominantly induced. 64% of the GC-to-CG transversions in the mutM strain occurred at the site of (CT/GC)GGC (mutated base underlined). The favorable mutation site, CTGGC, was the same as that of the UV- and sunlight-induced mutations previously reported: the mutations observed there were also G-to-C transversions. We speculate from these results that the superoxide in the cells may lead to production of a modified guanine that can pair with guanine and is subject to removal by the MutM protein.

Effect of isopropyl-beta-D-thiogalactopyranosid induction of the lac operon on the specificity of spontaneous and doxorubicin-induced mutations in Escherichia coli

Previous studies of doxorubicin-induced mutations employing F' lacl/lacO as an endogenous gene target have focused on properties of large deletions with 3' endpoints residing in the lacO region of the target gene. This study considers the influence of Lac repressor binding on the distribution of these deletions. Results of the DNA sequence level analysis of spontaneous and doxorubicin-induced i-d and lacO mutations in Escherichia coli uvrB- are reported for mutants isolated under conditions where Lac repression is relieved by isopropyl-beta-D-thiogalactopyranosid (IPTG; an inducer that prevents repressor binding to lacO). The location of deletions isolated from doxorubicin-treated cultures in the presence and absence of IPTG suggests that doxorubicin preferentially focuses deletion endpoints adjacent to its binding sites in lacO and that the distribution of these deletion endpoints is not modulated by Lac repressor binding. In contrast, spontaneous deletion endpoints are preferentially clustered in the loop away from the palindromic sequences under conditions of repression. However, when the Lac repressor/lacO binding complex is dissociated by IPTG, the spontaneous 3'-deletion endpoints distribute proportionally between the putative stem and loop of the lacO palindrome. The single most striking effect of IPTG induction of the Lac operon was elimination of a "hot spot" for T:A-->C:G transitions at position +6 in lacO. This base substitution "hot spot," which accounted for 17.6% of total doxorubicin-induced mutants and 16.4% of spontaneous mutants in repressed bacterial cultures, accounted for approximately 1% of total mutations in similar experiments carried out in the presence of IPTG. A large number of mutations at the +6 position are induced only by doxorubicin in the absence of IPTG, however, suggesting that both doxorubicin-induced and spontaneous mutation at this transition "hot spot" are mediated by Lac repressor binding to lacO.