Mutagenesis, Vol. 14, No. 2, 249-253,
March 1999
© 1999 UK Environmental Mutagen Society/Oxford University Press
Cobaltous chloride-induced mutagenesis in the supF tRNA gene of Escherichia coli
1 Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, Tobata-ku, Kitakyushu 804-8550, 2 Department of Biochemical Engineering and Science, Faculty of Computer Science and Systems Engineering, Kyushu Institute of Technology, Kawazu, Iizuka 820-8502 and 3 Biological Institute, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and methodsResults and discussionReferences |
|---|
The spectrum of mutations induced by cobalt(II) chloride (CoCl2) was examined using plasmid pUB3 DNA, which was propagated after transfection into Escherichia coli SY1032/pKY241 host cells. The vector plasmid carried an E.coli supF suppressor tRNA gene as a target for mutations. After CoCl2 treatment, 64 independent nalidixic acid-resistant, ampicillin-resistant and Lac+ (SupF–) clones were obtained and the altered sequences of the mutated supF– genes were determined. Deletions and frameshifts were the predominant mutational event (61%) induced by CoCl2 and base substitutions were induced to a lesser degree (29%). Analysis of sequence alterations at all the sites of mutation revealed that: (i) 18 of 19 base substitutions and eight of 10 frameshifts occurred at G:C sites, suggesting that the formation of N7G–Co(II) adducts may be responsible for premutagenic lesions of these mutations; (ii) short sequence repeats were mostly found at the sites of deletions and frameshifts. Slippage–misalignment is also suggested to be a mechanism for the induction of mutations at these sites.
|
Introduction |
|---|
|
TopAbstractIntroductionMaterials and methodsResults and discussionReferences |
|---|
The cytotoxicity and carcinogenicity of metals and their compounds have attracted attention and been studied for a long time. Of special interest among metal compounds is cobalt. In previous studies, cobalt(II) chloride (CoCl2) was mutagenic in gene mutation assays with Ames Salmonella strains and cultured mouse FM3A cells (Ogawa et al., 1986
, 1988; Morita et al., 1991) and induced somatic cell mutations in the Drosophila wing spot test (Ogawa et al., 1994). The mutagenic activity of cobalt(II) salts was demonstrated to be caused by the Co(II) ion (cation). On the other hand, certain cobalt compounds have shown carcinogenic effects in animals (Heath, 1956; Heath and Webb, 1967). It has also been reported that CoCl2 exhibited DNA binding activity (Eichorn and Shin, 1968; Zimmer et al., 1974), caused DNA strand breaks (Robinson et al., 1982; Hamilton-Koch et al., 1986; Hartwig et al., 1990) and reduced the fidelity of DNA synthesis (Sirover and Loeb, 1976).
We have previously established the supF system for studying forward mutation in Escherichia coli (Uematsu et al., 1997). Using this system, the spectrum of mutations induced by CoCl2 in an E.coli supF suppressor tRNA gene was determined, as part of a program devoted to studies on metal ion mutagenesis (Ogawa et al., 1986, 1987a, 1987b, 1988, 1994; Morita et al., 1991). In the present report, we found that CoCl2-induced mutations in the supF gene were primarily deletions and frameshifts, which occurred mostly at the sites of short direct repeats; the others were base substitutions and guanine residues are the suggested target for 90% of CoCl2-induced frameshifts and base substitutions.
|
Materials and methods |
|---|
|
TopAbstractIntroductionMaterials and methodsResults and discussionReferences |
|---|
Plasmids, bacterial strains and bacteriophage
Vector pUB3, described by Rodriguez et al. (1992), contains the tyrosine amber suppressor tRNA (supF) gene, lacZ
gene and ampicillin resistance (Ampr) gene used for mutagenesis studies. Escherichia coli K12 strains DL16 [leu64 lacZ53(am) recA1 rpsL151 ilvE720::Tn5 trxA1 metE70 rha5 malB45 deoC2] and KS40 [lacZ(am) CA7020 gyrA lacY1 hsdR hsdM araD139 
(araABC-leu)7679 galU galK rpsL thi] were used to prepare plasmid pUB3 DNA and allow processing and replication of CoCl2-treated plasmid pUB3 (Akasaka et al., 1992). The indicator host bacterial strain used to distinguish wild-type and mutant pUB3 was SY1032/pKY241 [gyrA96 (pro-lac) sup0 (F' pro+ lacIq lacIam26 lacZM15)/gyrA(am) cml] (Uematsu et al., 1997). Strain XL1-BlueMRF' (Stratagene) was used as a host for bacteriophage M13KO7, which was used to prepare single-stranded DNA for DNA sequencing.
Reagents and media
CoCl2 and other chemicals were purchased from Wako Pure Chemical Industries Ltd (Osaka, Japan). Enzymes and reagents used for DNA manipulation and DNA sequencing were purchased from Takara Shuzo Co. Ltd (Kyoto, Japan) and PE Applied Biosystems Inc. (Foster City, CA). Compositions of L broth, L agar plates and M buffer were as described previously (Otsuji et al., 1974). Minimal (M) lactose agar medium consisted of M buffer containing 10 g/l lactose, 15 g/l agar and 2 mg/l thiamine.
Mutagenesis by CoCl2
The mutagenesis was carried out by the method of Uematsu et al. (1997) with a slight modification. Plasmid pUB3 DNA (700 ng/reaction mixture), isolated from strain DL16/pUB3 according to the `midi' protocol of the Qiagen kit (Qiagen Inc., Chatsworth, CA) (Lutze and Winegar, 1990; Kimura et al., 1993), was exposed to freshly prepared solutions of 20 µM CoCl2 for 2 h at 37°C in 0.1 M Tris–HCl buffer (pH 7.2). CoCl2-treated pUB3 DNA was immediately transfected into competent KS40 cells which were induced by the CaCl2/RbCl method (Maniatis et al., 1982). They were incubated overnight to attain plasmid replication and then the progeny plasmids were extracted from the cells by the alkaline lysis method (Birnboim and Doly, 1979). Progeny plasmids from individual transfections were assayed separately for the mutant supF gene to distinguish mutants that were derived from siblings. Competent SY1032/pKY241 cells were transfected with progeny plasmids and the transformants with supF mutant plasmids were plated on M lactose agar plates containing 50 µg/ml nalidixic acid (Nal), 50 µg/ml ampicillin (Amp) and 30 µg/ml chloramphenicol (Cml). To collect supF mutants, only one nalidixic acid-resistant (Nalr) and Lac+ colony was chosen from each transformation experiment. This approach ensured that all mutants analyzed were of independent origin.
DNA sequencing
Single-strand DNA was prepared by infecting putative mutants [SY1032/pKY241/pUB3(supF–)] with M13KO7 phage and sequenced by the dideoxy chain termination method (Sanger et al., 1977) using an ABI automated sequencer model 373A, performed in exactly the same way as described previously (Uematsu et al., 1997). The polymerization reaction was primed by a primer (5'-GTCGATTTTTGTGATGCTCGTC-3') which is 237 bp upstream of the supF gene and is 45 bp upstream of the pUB3 replication origin (Uematsu et al., 1997).
Statistical analysis
To decide if there was a difference in the mutation frequency between CoCl2-induced mutation and spontaneous mutation, the 
2 test was used. P < 0.01 was regarded as significant.
|
Results and discussion |
|---|
|
TopAbstractIntroductionMaterials and methodsResults and discussionReferences |
|---|
CoCl2-induced SupF– mutations
Using a vector pUB3 carrying the E.coli supF gene and the E.coli SY1032/pKY241 host cells, we analyzed the spectrum of CoCl2-induced mutations. Initially, 700 ng of plasmid pUB3 DNA was exposed to 20 µM CoCl2 for 2 h and then transfected into competent SY1032/pKY241 cells. The transformation frequency was ~9.7x10–1, which was the same as that of untreated DNA. The mutation frequency of induced Nalr Lac+ (SupF–) transformants on the selective medium comprised of M lactose agar plates supplemented with Nal, Amp and Cml, averaged over eight experiments, was 2.5x10–6. This value was ~8-fold higher than that of spontaneous mutations (3.1x10–7). This fact indicated that 90% of the isolated mutant clones were probably induced by CoCl2. Furthermore, from the treated cells, 64 Nalr Lac+ (Ampr) mutant clones of presumably independent origin were isolated from selective medium and vector DNA molecules containing the expected mutated supF gene were recovered from all mutant cell clones. DNA sequences of the mutant supF genes were determined as ~150 bp from the Pre-tRNA to the End-tRNA region, by the dideoxy chain termination method, and altered sequences were found in all 64 mutant genes. While 63 mutants carried one mutation each, one mutant, CC19, carried two base substitutions 6 bases apart. Thus we identified 65 mutations. The distribution of mutants in the supF gene is shown in Figure 1
.
|
, base substitution; 
, frameshift; 
, insertion sequence; 
, duplication.Spectrum of CoCl2-induced mutations
Table I
lists CoCl2-induced DNA sequence alterations in the supF gene. Forty six percent of mutational events were deletions involving loss of 8–85 bp, whereas 29% were base substitutions. They differed from the spontaneous mutations, which occurred rather non-specifically (Akasaka et al., 1992). The remaining mutations were frameshifts (15%), insertion sequences (8%) and duplications (2%). Each class of mutations is hereby described in detail.
|
Deletions and duplication. The location and DNA sequence context at the sites of duplication and deletion mutations are presented in Table II
. One duplication was recovered at site 66–73; this mutation was caused by the addition of 8 bp. Mutation at five of seven sites (55–111, 60–112, 68–129, 77–119 and 124–212) were large deletions (38–85 bp) and those at the remaining two sites (95–109 and 153–166) were deletions of 13 and 8 bp, respectively. One of these sites, 68–129, was found to be a hot-spot for CoCl2 mutagenesis, since 20 mutations were recovered at this site. There were direct repeats of 10 bases (AAGGGAGCAG) flanking the fragment of 52 nucleotides to be deleted and one of the repeats was lost in the mutant sequences. This mutation was similar to spontaneous mutations in the same gene, as previously reported (Akasaka et al., 1992). A common sequence resulting from the deletion of 8–85 bp was also seen at the sites of other deletions. They had two short direct repeats of 2–7 bases and one of the repeats was missing in the mutant sequences. This unique sequence feature at the sites of deletions was frequently observed in spontaneous and topB mutant-induced mutations (Uematsu et al., 1997; Akasaka et al., 1992). We suggest slippage–misalignment (Ikehata et al., 1989; Akagi et al., 1990; Ogawa et al., 1993) to be the mechanism for CoCl2 mutagenesis at the supF system.
|
Eichhorn and Shin (1968) and Zimmer et al. (1974) reported that CoCl2 reacts with DNA producing two major adducts [the N7G–Co(II) and -O3PO–Co(II) adducts] and a chelate [N7G–Co(II)–OPO3] at the N7 position of guanine (N7G) or/and the phosphate residue (-O3PO–). It has also been shown that induction of DNA strand breaks generated by oxygen free radicals arose in vivo by reaction of Co(II) ions with superoxide or hydrogen peroxide (Moorhouse et al., 1985
; Yamamoto et al., 1989; Kadiiska et al., 1989). These studies suggest that the premutagenic lesions of the deletions (Table II
) may be due to cleavage at either the 5'- or 3'-position of phosphoester bonding of the DNA strand by the action of oxygen free radicals, which will probably be generated in cells by the reaction of the -O3PO–Co(II) adduct or/and the N7G–Co(II)NOPO3- chelate with superoxide or hydrogen peroxide.
Base substitutions.
Base substitution mutations were found in 19 of 65 mutations (29%) in the supF gene, of which four were transitions (G:C
A:T) and 15 were transversions (eight G:CT:A, six G:CC:G and one A:TC:G) (Tables I and III). CoCl2-induced base substitution mutation frequency was 7.3x10–7 (= 2.5x10–6x29%), which is 3.6-fold higher than the frequency of spontaneous base substitution, i.e. 3.1x10–7x63% = 2.0x10–7. Thus, CoCl2 can induce base substitution weakly but significantly (Table I). The distribution of CoCl2-induced base substitution sites was similar to that of spontaneous base substitution sites (Figure 1). There was an 18:1 bias in favor of the G:C base pair as the site of base substitutions, while transversions were predominant over transitions. The strong preference for CoCl2-induced base substitutions at G:C sites suggests that the N7G–Co(II) adduct is the lesions responsible for inducing these mutations.
|
In contrast, Tkeshelashvili et al. (1991) demonstrated that Cu(I) and/or Cu(II) ions induced C
T transitions caused by oxygen free radicals and GT transversions mediated by binding of copper ions to the N7G in 
X174 am3 DNA. A different result was observed with FeSO4-induced base substitutions in the same supF gene of E.coli (Akasaka and Yamamoto, 1995). The mutation spectra induced by Fe(II) ion-generated oxygen free radicals suggested that 8-hydroxydeoxyguanosine (8-OHdG) is responsible for G:CT:A transversions, whereas neither 8-OHdG nor 2,6-diamino-4-hydroxy-5-formamidopyrimidine can be the lesions responsible for G:CC:G transversions. However, as the premutagenic lesions are not clearly documented, further research and development in this area is required. Finally, one site of base substitutions, 133, was found to be a warm-spot which was a part of the anticodon in the supF tRNA gene, as four mutations were recovered. In addition, the observed mutational events were single base changes with one exception, which was a tandem double base substitution found at site 108–113 (CC19, Table III).
Frameshifts.
The sites and sequence contexts of frameshift mutations are presented in Table IV. Sequences with a run of a few identical residues were found at four of five sites (70–72, 102–105, 115–116 and 172–176) and mutations at these sites involved deletion or addition of 1 bp. One of these sites, 70–72, could be a warm-spot, as four mutations were recovered. Since the site was not a hot-spot for spontaneous mutations (Akasaka et al., 1992), it probably reflected specific interaction of CoCl2 with the site. On estimating slippage events which were triggered by damaged bases using each of those sequence contexts, eight of 10 (80%) frameshifts occurred at G:C sites, while the remainder were found at A:T sites. These results indicate that the N7G–Co(II) adduct is the lesion responsible for inducing these mutations. Furthermore, this phenomenon also seems to support our previous study (Ogawa et al., 1986, 1988), where CoCl2 induced specific frameshift mutations in the Ames Salmonella tester strains TA1537 and TA2637, but not in strains TA98 and TA100. Specifically the hisC3706 frameshift mutant had an additional G base in the -GGG- site of the hisC gene, where reverse mutations were detectable depending on the deletion of one G (Ames et al., 1973, 1975).
|
Insertion sequences. Five mutants, CC31, CC56, CC57, CC59 and CC64, were found to contain the sequences of IS insertion elements; the frequency was 2.5x10–6x8% = 2.0x10–7. They may be of spontaneous origin, since IS elements are very common (25%) among spontaneous mutations (the frequency is 3.1x10–7x25% = 7.75x10–8; Table I
). Four of five were IS50R insertions and one was an IS186 insertion. IS50R and IS186 were inserted into sites 124–125 and 114–115, respectively. IS50R was probably derived from the Tn5 in strain DL16 (Akasaka et al., 1992; Berg and Howe, 1989). We conclude that exposure of plasmid pUB3 DNA to CoCl2 primarily induced deletions and frameshifts followed by base substitutions, although very weakly, in the supF gene of E.coli. Eight of 10 frameshifts and 18 of 19 base substitutions occurred at G:C sites. Thus, N7G–Co(II) adducts contributed to 40% (26 of 65) of CoCl2 mutagenesis. Slippage–misalignment was the suggested mechanism for generation of deletions, including CoCl2-induced frameshifts. Furthermore, use of the SY1032/pKY241 system may provide an easy approach to DNA sequence analysis of metal ion mutagenesis in E.coli.
|
Acknowledgments |
|---|
This work was supported by a Grant-in-Aid from the Ministry of Education, Science and Culture, Japan (no. 07558077) and in part by Special Coordination Funds for Promoting Science and Technology from the Science and Technology Agency of the Japanese Government.
|
Notes |
|---|
4 To whom correspondence should be addressed. Tel: +81 93 884 3315; Fax: +81 93 884 3300; Email: ogawahi{at}che.kyutech.ac.jp
|
References |
|---|
|
TopAbstractIntroductionMaterials and methodsResults and discussionReferences |
|---|
-
Akagi,T., Morota,K., Ogawa,H,-I., Kimura,H. and Kato,T. (1990) Mutational specificity of the carcinogen 3-amino-1-methyl-5H-pyrido[4,3-b]-indole in mammalian cells. Carcinogenesis, 11, 841–846.
Akasaka,S. and Yamamoto,K. (1995) Mutational specificity of the ferrous ion in a supF gene of Escherichia coli. Biochem. Biophys. Res. Commun., 213, 74–80.[Web of Science][Medline]
Akasaka,S., Takimoto,K. and Yamamoto,K. (1992) G:CT:A and G:CC:G transversions are the predominant spontaneous mutations in the Escherichia coli supF gene: an improved lacZ(am) E.coli host designed for assaying pZ189 supF mutational specificity. Mol. Gen. Genet., 235, 173–178.[Web of Science][Medline]
Ames,B.N., Lee,F.D. and Durston,W.E. (1973) An improved bacterial test system for the detection and classification of mutagens and carcinogens. Proc. Natl Acad. Sci. USA, 70, 782–786.
Ames,B.N., McCann,J. and Yamasaki,E. (1975) Methods for detecting carcinogens and mutagens with Salmonella/mammalian-microsome mutagenicity test. Mutat. Res., 31, 347–364.[Web of Science][Medline]
Berg,D.E. and Howe,M.M. (1989) Mobile DNA. American Society for Microbiology, Washington, DC, pp. 163–184.
Birnboim,H.C. and Doly,J. (1979) A rapid alkaline extraction procedure for screening recombinant plasmid DNA. Nucleic Acids Res., 7, 1513–1523.
Eichhorn,G.L. and Shin,Y.A. (1968) Interaction of metal ions with polynucleotides and related compounds. XII. The relative effect of various metal ions on DNA helicity. J. Am. Chem. Soc., 90, 7323–7328.[Web of Science][Medline]
Hamilton-Koch,W., Snyder,R.D. and La Velle,J.M. (1986) Metal-induced DNA damage and repair in human diploid fibroblasts and Chinese hamster ovary cells. Chem. Biol. Interact., 59, 17–28.[Web of Science][Medline]
Hartwig,A., Kasten,U., Boakye-Dankwa,K. and Beyersmann,D. (1990) Uptake and genotoxicity of micromolar concentration of cobalt chloride in mammalian cells. Toxicol. Environ. Chem., 28, 205–215.
Hauser,J., Seidman,M.M., Sidur,K. and Dixon,K. (1986) Sequence specificity of point mutations induced during passage of a UV-irradiated shuttle vector plasmid in monkey cells. Mol. Cell. Biol., 6, 277–285.
Heath,J.C. (1956) The production of malignant tumours by cobalt in the rat. Br. J. Cancer, 10, 668–673.[Web of Science][Medline]
Heath,J.C. and Webb,M. (1967) Content and intracellular distribution of the inducing metal in the primary rhabdomyosarcomata induced in the rat by cobalt, nickel and cadmium. Br. J. Cancer, 21, 768–779.[Web of Science][Medline]
Ikehata,H., Akagi,T., Kimura,H., Akasaka,S. and Kato,T. (1989) Spectrum of spontaneous mutations in a cDNA of the human hprt gene integrated in chromosomal DNA. Mol. Gen. Genet., 219, 349–358.[Web of Science][Medline]
Kadiiska,M.B., Maples,K.R. and Mason,R.P. (1989) A comparison of cobalt(II) and iron(II) hydroxyl and superoxide free radical formation. Arch. Biochem. Biophys., 275, 98–111.[Web of Science][Medline]
Kimura,H., Higuchi,H., Ogawa,H.-I. and Kato,T. (1993) Sequence analysis of X-ray-induced mutations occurring in a cDNA of the human hprt gene integrated into mammalian chromosomal DNA. Radiat. Res., 134, 202–208.[Web of Science][Medline]
Lutze,L.H. and Winegar,R.A. (1990) A quick and efficient method for the recovery of plasmid or viral DNA from mammalian cells. Nucleic Acids Res., 18, 6150.
Maniatis,T., Fritsch,E.F. and Sambrook,J. (1982) Molecular Cloning: A Laboratory Manual. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, pp. 252–253.
Moorhouse,C.P., Halliwell,B., Grootveld,M. and Gutteridge,J.M.C. (1985) Cobalt(II) ion as a promoter of hydroxyl radical and possible `crypto-hydroxyl' radical formation under physiological conditions. Differential effects of hydroxyl radical scavengers. Biochim. Biophys. Acta, 843, 261–268.[Medline]
Morita,H., Umeda,M. and Ogawa,H.I. (1991) Mutagenicity of various chemicals including nickel and cobalt compounds in cultured mouse FM3A cells. Mutat. Res., 261, 131–137.[Web of Science][Medline]
Ogawa,H.I., Sakata,K., Inouye,T., Jyosui,S., Niyitani, Y., Kakimoto,K., Morishita,M., Tsuruta,S. and Kato,Y. (1986) Combined mutagenicity of cobalt(II) salt and heteroaromatic compounds in Salmonella typhimurium. Mutat. Res., 172, 97–104.[Web of Science][Medline]
Ogawa,H.I., Tsuruta,S., Niyitani,Y., Mino,H., Sakata,K. and Kato,Y. (1987a) Mutagenicity of metal salts in combination with 9-aminoacridine in Salmonella typhimurium. Jpn. J. Genet., 62, 159–162.
Ogawa,H.I., Sakata,K., Liu,S.-Y., Mino,H., Tsuruta,S. and Kato,Y. (1987b) Cobalt(II) salt–quinoline compound interaction: combined mutagenic activity in Salmonella typhimurium and strength of coordinate bond in the mixtures. Jpn. J. Genet., 62, 485–491.
Ogawa,H.I., Liu,S.-Y., Sakata,K., Niyitani,Y., Tsuruta,S. and Kato,Y. (1988) Inverse correlation between combined mutagenicity in Salmonella typhimurium and strength of coordinate bond in mixtures of cobalt(II) chloride and 4-substituted pyridines. Mutat. Res., 204, 117–121.[Web of Science][Medline]
Ogawa,H.I., Kimura,H., Koya,M., Higuchi,H. and Kato,T. (1993) N-Acetoxy-N-acetyl-2-aminofluorene-induced mutation spectrum in a human hprt cDNA shuttle vector integrated into mammalian cells. Carcinogenesis, 14, 2245–2250.
Ogawa,H.I., Shibahara,T., Iwata,H., Okada,T., Tsuruta,S., Kakimoto,K., Sakata,K., Kato,Y., Ryo,H., Itoh,T. and Fujikawa,K. (1994) Genotoxic activities in vivo of cobaltous chloride and other metal chlorides as assayed in the Drosophila wing spot test. Mutat. Res., 320, 133–140.[Web of Science][Medline]
Otsuji,N., Iyehara,H. and Hideshima,Y. (1974) Isolation and characterization of an Escherichia coli ruv mutant which forms nonseptate filaments after low doses of ultraviolet light irradiation. J. Bacteriol., 117, 337–344.
Robison,S.H., Cantoni,O. and Costa,M. (1982) Strand breakage and decreased molecular weight of DNA induced by specific metal compounds. Carcinogenesis, 3, 657–662.
Rodriguez,H., Snow,E.T., Bhat,U. and Loechler,E.L. (1992) An Escherichia coli plasmid-based, mutational system in which supF mutants are selectable: insertion elements dominate the spontaneous spectra. Mutat. Res., 270, 219–231.[Web of Science][Medline]
Sanger,F., Nicklen,S. and Coulson,A.R. (1977) DNA sequencing with chain-terminating inhibitors. Proc. Natl Acad. Sci. USA, 74, 5463–5467.
Sirover,M.A. and Loeb,L.A. (1976) Infidelity of DNA synthesis in vitro: screening for potential metal mutagens or carcinogens. Science, 194, 1434–1436.
Tkeshelashvili,L.K., McBride,T., Spence,K. and Loeb,L.A. (1991) Mutation spectrum of copper-induced DNA damage. J. Biol. Chem., 266, 6401–6406.
Uematsu,N., Eda,S. and Yamamoto,K. (1997) An Escherichia coli topB mutant increases deletion and frameshift mutations in the supF target gene. Mutat. Res., 383, 223–230.[Web of Science][Medline]
Yamamoto,K., Inoue,S., Yamazaki,A., Yoshinaga,T. and Kawanishi,S. (1989) Site-specific DNA damage induced by cobalt(II) ion and hydrogen peroxide: role of singlet oxygen. Chem. Res. Toxicol., 2, 234–239.[Web of Science][Medline]
Zimmer,Ch., Luck,G. and Triebel,H. (1974) Conformation and reactivity of DNA. IV. Base binding ability of transition metal ions to native DNA and effect on helix conformation with special reference to DNA–Zn(II) complex. Biopolymers, 13, 425–453.[Web of Science][Medline]
Received on October 9, 1998; accepted on December 8, 1998.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  H. Sone, Y. J. Li, M. Ishizuka, Y. Aoki, and M. Nagao Increased Mutant Frequency and Altered Mutation Spectrum of the lacI Transgene in Wilson Disease Rats with Hepatitis Cancer Res., September 1, 2000; 60(18): 5080 - 5086. [Abstract] [Full Text]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||