Study of the antiradical and radioprotective properties of S-containing compounds using the complex of in vitro test-systems

Capability of S-radioprotectors (AET, 2-AT, 2-ADT) to react with OH-radicals and to protect various molecular biotest systems against radiation damage was compared with that of 5-methoxytryptamine, some amino acids and t-butanol. A method of competing acceptors was used to determine rate constants in reactions of the radioprotectors with OH-radicals. A complex of biotest systems (protein, DNA, protein-lipid complex) was applied to estimate the radioprotective activity in vitro. It was found that the studied S-compounds are capable of modifying the protective effect as compared to the expectation from the competitive kinetics approach. Both enhancing and lessening of the effect was observed depending on the test system used. The obtained results can be explained by the impact of secondary radicals on the bio-target and/or by the interaction of the S-compounds with the bio-target that altered its radiosensitivity.     

Low doses of selenium specifically stimulate the repair of oxidative DNA damage in LNCaP prostate cancer cells

Epidemiological studies have demonstrated an inverse relationship between selenium (Se) intake and cancer incidence and/or mortality. However, the molecular mechanisms underlying the cancer chemopreventive activity of Se compounds remain largely unknown. The objective of this study was to investigate the effect of low doses of Se on the stimulation of DNA repair systems in response to four different qualities of DNA damage. P53-proficient LNCaP human prostate adenocarcinoma cells were grown either untreated or in the presence of low concentrations of two Se compounds (30° nM sodium selenite, or 10 μM selenomethionine) and exposed to UVA, H2O2, methylmethane sulfonate (MMS) or UVC. Cell viability as well as DNA damage induction and repair were evaluated by the alkaline Comet assay. Overall, Se was shown to be a very potent protector against cell toxicity and genotoxicity induced by oxidative stress (UVA or H2O2) but not from the agents that induce other types of deleterious lesions (MMS or UVC). Furthermore, Se-treated cells exhibited increased oxidative DNA repair activity, indicating a novel mechanism of Se action. Therefore, the benefits of Se could be explained by a combination of antioxidant activity, the reduction in DNA damage and the enhancement of oxidative DNA repair capacity.     

DNA lesions in hyperthermic cell killing: effects of thermotolerance, procaine, and erythritol

When HeLa S3 cells were subjected to 45 degrees C hyperthermia, DNA lesions were detected by the use of the alkaline unwinding/hydroxylapatite method. The number of lesions formed was not affected when the cells were made thermotolerant by either an acute (15 min 44 degrees C + 5 h 37 degrees C) or a chronic (5 h 42 degrees C) pretreatment before 45 degrees C hyperthermia. The presence of 10 mM procaine (heat sensitizer) or 0.5 M erythritol (heat protector) during hyperthermia also had no effect on the rate of formation of heat-induced alkali labile DNA lesions. These observations do not support a concept where DNA lesions are considered to be the ultimate cause of hyperthermic cell killing. Both drugs, however, influenced the rate of repair of radiation-induced strand breaks when present during preirradiation heat treatment. We conclude that the initial number of heat-induced alkali labile DNA lesions is not directly related to cell survival. It cannot be excluded, however, that differences in posthyperthermic repair of these lesions may lead to a positive correlation between residual DNA damage and survival after the different experimental conditions.     

Resistance of Spiroplasma citri Lines to the Virus SVTS2 Is Associated with Integration of Viral DNA Sequences into Host Chromosomal and Extrachromosomal DNA

Spiroplasmavirus SVTS2, isolated from Spiroplasma melliferum TS2, produces plaques when inoculated onto lawns of Spiroplasma citri M200H, a derivative of the type strain Maroc R8A2. S. citri strains MR2 and MR3, originally selected as colonies growing within plaques on a lawn of M200H inoculated with SVTS2, were resistant to SVTS2. Genomic DNA fingerprints and electrophoretic protein profiles of M200H, MR2, and MR3 were similar, but three proteins present in M200H were missing or significantly reduced in both resistant lines. None of these three polypeptides reacted with antiserum against S. citri membrane proteins, indicating that they probably are not surface-located virus receptors. Electroporation with SVTS2 DNA produced 1.5 x 10(sup5) transfectants per (mu)g of DNA in M200H but none in MR2 or MR3, suggesting that resistance may result from inhibition of viral replication. The digestion patterns of the extrachromosomal double-stranded (ds) DNA of these lines were similar. Three TaqI fragments of MR2 extrachromosomal DNA that were not present in M200H extrachromosomal DNA hybridized strongly to an SVTS2 probe, and two of these fragments plus an additional one hybridized with the MR3 extrachromosomal DNA, indicating that a fragment of SVTS2 DNA was present in the extrachromosomal ds DNA of MR2 and MR3 but not of M200H. When the restricted genomes of all three lines were probed with SVTS2 DNA, strong hybridization to two EcoRI fragments of chromosomal MR2 and MR3 DNA but not M200H DNA indicated that SVTS2 DNA had integrated into the genomes of MR2 and MR3 but not of M200H. When MR3 extrachromosomal ds DNA containing a 2.1-kb SVTS2 DNA fragment was transfected into M200H, the transformed spiroplasmas were resistant to SVTS2. These results suggest that SVTS2 DNA fragments, possibly integrated into the chromosomal or extrachromosomal DNA of a previously susceptible spiroplasma, may function as viral incompatibility elements, providing resistance to superinfection by SVTS2.     

Phenylalanine biosynthesis in Arabidopsis thaliana. Identification and characterization of arogenate dehydratases

There is much uncertainty as to whether plants use arogenate, phenylpyruvate, or both as obligatory intermediates in Phe biosynthesis, an essential dietary amino acid for humans. This is because both prephenate and arogenate have been reported to undergo decarboxylative dehydration in plants via the action of either arogenate (ADT) or prephenate (PDT) dehydratases; however, neither enzyme(s) nor encoding gene(s) have been isolated and/or functionally characterized. An in silico data mining approach was thus undertaken to attempt to identify the dehydratase(s) involved in Phe formation in Arabidopsis, based on sequence similarity of PDT-like and ACT-like domains in bacteria. This data mining approach suggested that there are six PDT-like homologues in Arabidopsis, whose phylogenetic analyses separated them into three distinct subgroups. All six genes were cloned and subsequently established to be expressed in all tissues examined. Each was then expressed as a Nus fusion recombinant protein in Escherichia coli, with their substrate specificities measured in vitro. Three of the resulting recombinant proteins, encoded by ADT1 (At1g11790), ADT2 (At3g07630), and ADT6 (At1g08250), more efficiently utilized arogenate than prephenate, whereas the remaining three, ADT3 (At2g27820), ADT4 (At3g44720), and ADT5 (At5g22630) essentially only employed arogenate. ADT1, ADT2, and ADT6 had k(cat)/Km values of 1050, 7650, and 1560 M(-1) S(-1) for arogenate versus 38, 240, and 16 M(-1) S(-1) for prephenate, respectively. By contrast, the remaining three, ADT3, ADT4, and ADT5, had k(cat)/Km values of 1140, 490, and 620 M(-1) S(-1), with prephenate not serving as a substrate unless excess recombinant protein (>150 microg/assay) was used. All six genes, and their corresponding proteins, are thus provisionally classified as arogenate dehydratases and designated ADT1-ADT6.     

The effect of chemical radiation protectors on cell cycle progression after gamma or neutron irradiation

Abstract. The effects of two chemical radiation protectors, WR-1065 and WR-151326, were characterized in V79 Chinese hamster cells after either cobalt-60 (⁶⁰Co) gamma or fission spectrum neutron irradiation. Each protector was administered at a concentration of 4 mM to exponentially growing cultures for 30 min prior to and during irradiation with either ⁶⁰Co gamma or JANUS fission spectrum neutrons. After irradiation the cells were either plated immediately for survival or returned to the incubator and assayed for cell progression. Aliquots of cells were removed at selected times, counted, fixed and stained with 4'6-diamidino-2-phenylindole (DAPI). Analysis of DNA histograms indicate that the presence of the protector during irradiation reduced the division delay experienced at the G₂-M interface. Implications of these effects are discussed.     

Distribution of Three Auxin Protector Substances in Seeds and Shoots of the Japanese Morning Glory (Pharbitis nil)

The existence of substances which inhibit the enzymatic destruction of auxin in shoots of the Japanese morning glory (Pharbitis nil Choisy) has been confirmed, as has the fact that these substances are distributed in a gradient diminishing from apex to base in a manner indicating a regulatory role in internode elongation and tissue maturation. In addition to the 2 auxin protector substances reported previously (protectors I and II) which appear to account for most of the inhibition of the enzymatic destruction of auxin in young, elongating stem tissue, a third substance, designated as protector A, has been found to be highly active in seeds, and shoot tips of mature plants: In germinating seeds, no protector I or II activity was observed; in stem tips, no protector II and only slight protector I activity was observed. In contrast, old tissue contained no detectable amounts of protector A, but did contain protectors I and II. Between these extremes along the shoot axis, mixtures of the 3 substances were found. The evidence can be interpreted to mean that protector A is degraded into protectors I and II and perhaps translocated in this form. Gel filtration studies indicate that protector A has a molecular weight exceeding 200,000 gm/mole.     

Salt-resistant association of simian virus 40 T antigen with simian virus 40 DNA in nucleoprotein complexes.

Treatment of nucleoprotein complexes (NPCs) from simian virus 40 (SV40)-infected TC7 cells with NaCl (1 or 2 M) or guanidine-hydrochloride (1 or 2 M) resulted in a significant fraction of T antigen still associated with SV40 (I) DNA. Immunoprecipitation of the salt-treated NPCs with SV40 anti-T serum indicated that T antigen is preferentially associated with SV40 (I) DNA rather than with SV40 (II) DNA. Treatment of the NPCs with 4 M guanidine-hydrochloride, however, resulted in a substantial decrease in the amount of SV40 (I) and (II) DNA associated with T antigen. As the temperature was increased to 37 degrees C during incubation of NPCs with NaCl or guanidine-hydrochloride, there was a decrease in the amount of SV40 (I) and (II) DNA immunoprecipitated with SV40 anti-T serum. In the absence of salt, temperature had no effect on the association of T antigen with the SV40 DNA in the NPCs. Treatment of NPCs from SV40 wildtype or tsA58-infected cells grown at the permissive temperature with 1 or 2 M NaCl indicated that tsA T antigen has the same sensitivities as wild-type T antigen to high salt treatment when bound to DNA in NPCs. Characterization of the proteins associated with SV40 (I) DNA after high salt treatment revealed that, in addition to T antigen, a certain amount of viral capsid proteins VP1 and VP3 remained associated with the DNA. Complexes containing SV40 (I) DNA had a sedimentation value of 53S after 1 M NaCl treatment and 43S after 2 M NaCl treatment.     

竹红菌乙素修饰物（－乙醇胺）对于腹水肝癌细胞光敏损伤的研究

通过竹红菌乙素修饰物（－乙醇胺,简称ＨＢ－Ｅ）对小鼠腹水肝癌细胞光敏损伤以及ＨＢＥ被ＡＨ细胞摄取过程的研究,结果表明,ＨＢ－Ｅ在６４０ｎｍ光照的条件下对ＡＨ细胞产生很强的光动力作用,细胞对ＨＢ－Ｅ的摄取速度快（室温下２ｍｉｎ,达到平衡）在饱和时每个细胞可摄取ＨＢ－Ｅ分子５×１０＾９个;经多种竹红菌素修饰物光敏能力的比较中ＨＢ－Ｅ光敏作用明显要大于其它的光敏剂,光敏机理的研究中确定了活性氧的作用是存     

In vitro bypass of malondialdehyde-deoxyguanosine adducts: differential base selection during extension by the Klenow fragment of DNA polymerase I is the critical determinant of replication outcome

The major malondialdehyde-derived adduct in DNA is 3-(2'-deoxy-beta-D-erythro-pentofuranosyl)pyrimido[1,2-alpha]purin-10(3H)-one (M(1)dG). M(1)dG undergoes hydrolytic ring opening in duplex DNA to 9-(2'-deoxy-beta-D-erythro-pentofuranosyl)-N(2)-(3-oxo-1-propenyl)guanine (N(2)OPdG). Template-primers were constructed containing M(1)dG or N(2)OPdG in a (CpG)(4) repeat sequence and replicated with the Klenow fragment of DNA polymerase I (Kf). Incorporation opposite the lesion and replication beyond the adduct sites by Kf was reduced compared to unadducted controls. The amount of bypass to full-length products was significantly greater with the acyclic adduct, N(2)OPdG, than with the cyclic adduct, M(1)dG. Sequence analysis indicated that the fully extended primers contained dC opposite both adducts when replication was conducted with Kf exo(+). In contrast, with Kf exo(-), primers extended past M(1)dG contained T opposite the adduct, but primers extended past N(2)OPdG contained dC opposite the adduct. Single nucleotide incorporation experiments indicated that Kf exo(-) incorporates all four nucleotides opposite M(1)dG or N(2)OPdG. Kf exo(+) removed dA, dG, and T opposite M(1)dG and N(2)OPdG but was much less active when dC was opposite the adduct. NMR studies on duplex DNA indicated that N(2)OPdG hydrogen bonds with dC in the complementary strand. The fact that base pairing can occur for the acyclic adduct may explain why N(2)OPdG is less blocking than M(1)dG. These results support in vivo findings that the ring-closed adduct, M(1)dG, is more mutagenic than the ring-opened adduct, N(2)OPdG. They also provide a detailed picture of in vitro replication in which the outcome is determined primarily by the selectivity of template-primer extension beyond rather than insertion opposite the adducts.     

The (+)- and (−)-gossypols potently inhibit human and rat 11β-hydroxysteroid dehydrogenase type 2

Gossypol has been proven to be a very effective male contraceptive. However, clinical trials showed that the major side effect of gossypol was hypokalemia. Gossypol occurs naturally as enantiomeric mixtures of (+)-gossypol and (−)-gossypol. The (−)-gossypol is found to be the active component of antifertility. 11β-Hydroxysteroid dehydrogenase 2 (11βHSD2) has been demonstrated to be a mineralocorticoid receptor (MR) protector by inactivating active glucocorticoids including corticosterone (CORT) in rats, and therefore mutation or suppression of 11βHSD2 causes hypokalemia and hypertension. In the present study, the potency of gossypol enantiomers was tested for the inhibition of 11βHSD1 and 2 in rat and human. Both (+) and (−)-gossypols showed a potent inhibition of 11βHSD2 with the half maximal inhibitory concentration (IC₅₀) of 0.61 and 1.33 μM for (+) and (−)-gossypols, respectively in rats and 1.05 and 1.90 μM for (+) and (−)-gossypols, respectively in human. The potency of gossypol to inhibit 11βHSD1 was far less; the IC₅₀ was ≥100 μM for racemic gossypol. The gossypol-induced hypokalemia is likely associated with its potent inhibition of kidney 11βHSD2.     

Inducible COX-2 dominates over COX-1 in prostacyclin biosynthesis: mechanisms of COX-2 inhibitor risk to heart disease

AimOur aim is to understand the molecular mechanisms of the selective nonsteroidal anti-inflammatory drugs (NSAID), cyclooxygenase-2 (COX-2) inhibitors', higher “priority” to reduce synthesis of the vascular protector, prostacyclin (PGI2), compared to that of nonselective NSAIDs.Main methodsCOX-1 or COX-2 was co-expressed with PGI2 synthase (PGIS) in COS-7 cells. The Km and initial velocity (½t Vmax) of the coupling reaction between COX-1 and COX-2 to PGIS were established. The experiment was further confirmed by a kinetics study using hybrid enzymes linking COX-1 or COX-2 to PGIS. Finally, COX-1 or COX-2 and PGIS were respectively fused to red (RFP) and cyanic (CFP) fluorescence proteins, and co-expressed in cells. The distances between COXs and PGIS were compared by FRET.Key findingsThe Km for converting arachidonic acid (AA) to PGI2 by COX-2 coupled to PGIS is ~ 2.0 μM; however, it was 3-fold more (~ 6.0 μM) for COX-1 coupled to PGIS. The Km and ½t Vmax for COX-2 linked to PGIS were ~ 2.0 μM and 20 s, respectively, which were 2–5 folds faster than that of COX-1 linked to PGIS. The FRET study found that the distance between COX-2-RFP and PGIS–CFP is shorter than that between COX-1-RFP and PGIS–CFP.SignificanceThe study provided strong evidence suggesting that the low Km, faster ½t Vmax, and closer distance are the basis for COX-2 dominance over COX-1 (coupled to PGIS) in PGI2 synthesis, and further demonstrated the mechanisms of selective COX-2 inhibitors with higher potential to reduce synthesis of the vascular protector, PGI2.     

Different effects of DNA adducts induced by carcinogenic and noncarcinogenic azo dyes on in vitro DNA synthesis

M13mp10 phage DNA modified with the carcinogen 3-methoxy-4-aminoazobenzene (3-MeO-AAB) or the noncarcinogen 2-methoxy-4-aminoazobenzene (2-MeO-AAB) was used as a template for E.coli DNA polymerase I. Analysis of the reaction products on DNA sequencing gels showed that with both types of compound the induced lesions blocked DNA synthesis, mainly at one base prior to guanine adducts, but that the inhibition by 3-MeO-AAB-adducts was substantially greater than that by 2-MeO-AAB-adducts. Thus different effects on DNA replication between 3-MeO-AAB- and 2-MeO-AAB-adducts might be a reflection of differences in their carcinogenic potency.     

Evaluation of breast carcinoma chemosensitivity by flow cytometric DNA analysis and computer assisted image analysis.

Flow cytometric (FCM) DNA and S-Phase (S%) analyses were compared to computerized image analysis (SAMBA 2005) in 27 breast carcinomas (T3, N0-N1, M0) treated by 3 cycles of preoperative Adriamycin, vincristine, cyclophosphamide, methotrexate, 5-fluorouracil (AVCMF) chemotherapy (CT). Twelve carcinomas had shown objective regression and 15 no regression. Samples studied were obtained by sequential fine-needle cytopunctures. Comparing DNA profiles obtained by both methods before and after the first cycle, it appears that tumors can be divided into 3 groups. In the first group (10 cases), no changes were observed after the first cycle of CT. These tumors before treatment had either single DNA peak without cells in S% and G2M or a major peak with a small S% and G2M peak. The second group (9 cases) showed some changes in DNA profiles with an increased G2M peak but no additional values; these tumors before treatment had a small S% and a G2M peak. In the third group (8 cases), before treatment, all were non-diploid with high S% and high G2M. After the first cycle, all showed obvious changes in DNA profiles with a decrease of the G0/G1 peak and an increased S% and G2M with dispersed additional values along the scale in (G2M) x 2 and (G2M) x 4 regions. When changes were compared to tumor regression in the 1st and 2nd groups, 1/10 and 3/9 cases, respectively, were evaluated as objective regression. In the third group, all had objective regression (p less than 0.001). In most cases, a good correlation was observed with both methods.(ABSTRACT TRUNCATED AT 250 WORDS)     

Homoserine kinase from Escherichia coli K-12: properties, inhibition by L-threonine, and regulation of biosynthesis

We have partially purified homoserine kinase from a genetically derepressed strain of Escherichia coli K-12. The optimum pH of the enzyme-substrate reaction was 7.8 and the K(m) values for l-homoserine and adenosine 5'-triphosphate were both 3 x 10(-4) M. K(+) (or NH(4) (+)) as well as Mg(2+) were required for its activity. The sedimentation coefficient determined by ultracentrifugation in a sucrose density gradient was 5.0 +/- 0.25S. l-Homoserine was an excellent protector against heat inactivation of homoserine kinase. l-Threonine was a competitive inhibitor of homoserine kinase, suggesting that end-product inhibition of this enzyme plays a role in vivo in the overall regulation of threonine biosynthesis. The specific activity of aspartokinase I-homoserine dehydrogenase I and of homoserine kinase showed a strong positive correlation in extracts from strains under widely varying conditions of genetic or physiological derepression; it was concluded that these two enzymes are coordinately regulated in E. coli K-12.     

1 alpha, 25-dihydroxyvitamin D3 modulates the growth of 3T3 cells and human skin fibroblasts stimulated by platelet-derived growth factor

We investigated the effect of 1 alpha,25-dihydroxyvitamin D3 (1,25 (OH)2 vit D3) on the 3H-thymidine uptake by Balb/c 3T3 cells and by human skin fibroblasts stimulated by normal human serum or by purified PDGF. We found an inhibitory effect of 1,25 (OH)2 vit D3 on the DNA synthesis of Balb/c 3T3 cells grown in the presence of human serum as well as in the presence of PDGF. At 5% human serum this effect is minimal at 10(-12) M 1,25 (OH)2 vit D3 and is maximal at 10(-9) M. On the DNA synthesis of human fibroblasts stimulated by human serum or by PDGF a modulatory effect of 1,25 (OH)2 vit D3 was shown. On these cells the vitamin had a stimulatory effect between 10(-11) and 10(-9) M and an inhibitory effect at very high concentrations (10(-7) M). Our results suggested that the effect of 1,25 (OH)2 vit D3 on fibroblast DNA synthesis could be mediated by interactions with its specific intracellular receptor. 1,25 (OH)2 vit D3 had no any action on the growth of human fibroblasts stimulated by fibroblast growth factor.     

Hematopoietic Recovery and Amelioration of Radiation-Induced Lethality by the Vitamin E Isoform δ-Tocotrienol

δ-Tocotrienol (DT3), a vitamin E isoform, is associated with strong antioxidant and immunomodulatory properties. We confirmed the potent antioxidant activity in membrane systems and showed that DT3 is an effective radiation protector and mitigator. DT3 (4 μM, P < 0.001) inhibited lipid peroxidation in mouse liver microsomes and nitric oxide (NO) formation (20 μM DT3, P < 0.01) in RAW264.7 cells, a murine alveolar macrophage line. In CD2F1 mice exposed to lethal total-body radiation from a (60)Co γ-radiation source, a single subcutaneous (s.c.) injection of DT3 before or after irradiation produced a significant increase in 30-day survival. DT3 was effective from 18.75 to 300 mg/kg (--24 h, P < 0.001). A single dose of 150 or 300 mg/kg DT3 given 24 h before irradiation (radioprotection) resulted in dose reduction factors (DRFs) of 1.19 and 1.27, respectively (P < 0.001). Further, DT3 reduced radiation lethality when administered 2, 6 or 12 h after irradiation, and 150 mg/kg DT3 administered 2 h after exposure conferred a DRF of 1.1 (mitigation). The optimum schedule of 300 mg/kg DT3 24 h prior to 7 Gy significantly reduced pancytopenia compared to irradiated controls (P < 0.05). The large therapeutic potential of and multi-lineage hematopoietic recovery for DT3 warrants further studies.     

Eukaryotic DNA repair is blocked at different steps by inhibitors of DNA topoisomerases and of DNA polymerases α and β

Inhibitors of (a) DNA topoisomerases (novobiocin and nalidixic acid) and of (b) eukaryotic DNA polymerases α (cytosine arabinoside) and β (dideoxythymidine) blocked different steps of DNA repair, demonstrated by the effects of the inhibitors on the relaxation of supercoiled DNA nucleoids following treatment of human cell cultures with ultraviolet light (1–3 J/m²) or MNNG (5 or 20 μM) and the subsequent restoration of the supercoiled nucleoids during repair incubation. Changes in the supercoiling of nucleoid DNA were assayed by analysis of their sedimentation profiles in 15–30% neutral sucrose gradients. Inhibition of repair by novobiocin was partially reversible; upon its removal from the culture medium, the nucleoid DNA of repairing cells became relaxed. The DNA polymerase inhibitors allowed the initial relaxation of DNA after treatment of the cells with ultraviolet or MNNG but delayed the regeneration of rapidly-sedimenting (supercoiled) nucleoid DNA for 2–4 h. Dideoxythymidine (1 mM) was more effective than cytosine arabinoside (1 μM) in producing this delay, but neither inhibitor by itself blocked repair permanently. Incubation of ultraviolet-irradiated cells with 1 μM cytosine arabinoside plus 1 mM dideoxythymidine blocked the completion of repair for 24 h, whereas incubation with 10 μM cytosine arabinoside or 5 mM dideoxythymidine produced only temporary repair delays of 2–4 h. Thus, it is likely that the two DNA polymerase inhibitors act upon separate targets and that both targets are involved in repair. It is concluded from these and from previous studies that (1) the DNA repair-sensitive target of novobiocin and nalidixic acid in vivo is not a DNA polymerase, but, rather, a DNA topoisomerase; (2) this target affects an initial step of DNA repair leading to the relaxation of supercoiled DNA; (3) the DNA polymerization step of repair may involve both α- and β-type DNA polymerases; and (4) in repair, one type of DNA polymerase may substitute for another.     

Evidence for in vitro translesion DNA synthesis past a site-specific aminofluorene adduct

The ability of Escherichia coli DNA polymerase I and T7 DNA polymerase to bypass bulky C-8 guanyl-2-aminofluorene adducts in DNA was studied by in vitro DNA synthesis reactions on a site-specific aminofluorene-modified M13mp9 template. This site-specifically modified DNA was prepared by ligating an oligonucleotide containing a single aminofluorene adduct into a gapped heteroduplex of M13mp9 DNA (Johnson, D. L., Reid, T. M., Lee, M.-S., King, C. M., and Romano, L. J. (1986) Biochemistry 25, 449-456). The resulting covalently closed duplex DNA molecule was then cleaved with a restriction endonuclease, denatured, and annealed to a primer on the 3' side of the adduct to form a template specifically designed to study bypass. In this system, any synthesis that was not blocked by the bulky aminofluorene adduct would proceed to the 5' terminus of the single-stranded template, while synthesis interrupted by the adduct would terminate at or near the adduct location. We have measured DNA synthesis on this template and find that the amount of radiolabeled nucleotide incorporated by either E. coli DNA polymerase I (large fragment) or T7 DNA polymerase was much greater than would be predicted if the aminofluorene adduct were an absolute block to DNA synthesis. Furthermore, the products of similar reactions electrophoresed on polyacrylamide gels showed conclusively that the majority of the DNA synthesized by either the T7 DNA polymerase or E. coli DNA polymerase I bypassed the aminofluorene lesion. Substitution of Mn2+ for Mg2+ as the divalent cation resulted in even higher levels of translesion synthesis.