Search PubMed⌕ Search

Biomedical subjects

M Radman

Publications and source records attributed to M Radman.

At least 91 records · Page 5Linked to original sources
PubMed1986

Mismatch-stimulated killing.

DNA duplexes with or without mismatches and with or without adenine-methylated GATC sequences were prepared from separated strands of bacteriophage lambda DNA and used to transfect Escherichia coli. Unmethylated heteroduplexes containing one or more repairable mismatches transfect cells with a functioning mismatch repair system less efficiently than they transfect cells deficient in mismatch repair. No difference is observed when the duplexes contain no mismatch or a poorly repaired mismatch or when the heteroduplexes are fully or hemimethylated. These results and the phenotypes of E. coli dam mutants suggest that the E. coli mismatch repair system may introduce double-strand breaks in unmethylated DNA at or near repairable mismatches.

Adenine↗
PubMed1985

A system for detection of genetic and epigenetic alterations in Escherichia coli induced by DNA-damaging agents.

In order to compare the genetic and epigenetic effects of genotoxic agents, we have constructed Escherichia coli K12 strains that allow the detection of mutagenesis, SOS induction (epigenetic effect) and genetic recombination in the same genetic background. The epigenetic effect was detected in a similar way to any genetic alteration, i.e. by counting altered clones (colonies), using a gene fusion system that responds to a temporary epigenetic effect by a stable, heritable switch. The gene fusion consists of the E. coli gal operon and a partially deleted prophage lambda, resulting in the gal operon coming under the control of the cI and cro genes. It allows the detection of SOS induction and forward mutagenesis in the cI gene. Even a temporary inactivation of the CI repressor in this particular system leads to a stable epigenetic switch transmitted to the cellular progeny, which can be detected as Gal+ (red) colonies. The genetic (mutational inactivation of gene cI) and epigenetic (proteolytic inactivation of the product of gene cI) mechanisms leading to gal expression can be distinguished. Genetic recombination between two heteroallelic lacZ genes, one located in the bacterial chromosome, the other on an F'lac plasmid, can be detected as Lac+ colonies. Radiation and several chemical mutagens show very different capacities in generating mutants, inductants and recombinants; therefore, a dose range of any physical or chemical agent generates a set of relative values for the generation of mutants, inductants and recombinants that are characteristic of the agent.

Aflatoxin B1↗
PubMed1985

Involvement of DNA polymerase III in UV-induced mutagenesis of bacteriophage lambda.

It has been proposed that the mutation fixation processes stimulated by SOS induction result from an induced infidelity of DNA replication (Radman 1974). The aim of this study was to determine if mutator mutations in the E. coli DNA polymerase III might affect UV-induced mutagenesis. Using a phage lambda mutation assay which can discriminate between targeted and untargeted mutations, we show that the polC74 mutator mutation (Sevastopoulos and Glaser 1977) primarily affects untargeted mutagenesis, which occurs in a recA1 genetic background and is amplified in the recA+ genetic background. The polC74 mutation also increases the UV-induced mutagenesis of the bacterial chromosome. These results suggest that DNA polymerase III is involved in the process of UV-induced mutagenesis in E. coli.

Bacteriophage lambda↗
PubMed1984

SOS mutator effect in E. coli mutants deficient in mismatch correction.

We have used bacteriophage lambda to characterize the mutator effect of the SOS response induced by u.v. irradiation of Escherichia coli. Mutagenesis of unirradiated phages grown in irradiated or unirradiated bacteria was detected by measuring forward mutagenesis in the immunity genes or reversion mutagenesis of an amber codon in the R gene. Relative to the wild-type, the SOS mutator effect was higher in E. coli mismatch correction-deficient mutants (mutH, mutL and mutS) and lower in an adenine methylation-deficient mutant ( dam3 ). We conclude that a large proportion of SOS-induced 'untargeted' mutations are removed by the methyl-directed mismatch correction system, which acts on newly synthesized DNA strands. The lower SOS mutator effect observed in E. coli dam mutants may be due to a selective killing of mismatch-bearing chromosomes resulting from undirected mismatch repair. The SOS mutator effect on undamaged lambda DNA, induced by u.v. irradiation of the host, appears to result from decreased fidelity of DNA synthesis.

Adenine↗
PubMed1982

Chromosomal rearrangement and carcinogenesis.

All carcinogens that have been thoroughly tested have been found to induce some kind of chromosomal rearrangement. Chromosomal rearrangements are associated with a variety of human and rodent cancers and are associated, with in vitro cell transformation. The DNA from non-malignant cells can transform other non-malignant cells under conditions that may involve chromosomal rearrangement. These findings support the view that chromosomal rearrangement can be a step in carcinogenesis. Available evidence indicates that carcinogens can act to induce chromosomal rearrangement by creating or revealing sites on DNA for recombination, or by inducing or activating cellular systems resulting in a stimulation of recombination. Chromosomal rearrangement may affect carcinogenesis by altering gene expression. Perhaps by allowing the activation of cellular cancer genes.

Carcinogens↗
PubMed1981

Studies on the miscoding properties of 1,N6-ethenoadenine and 3,N4-ethenocytosine, DNA reaction products of vinyl chloride metabolites, during in vitro DNA synthesis.

1,N6-Ethenoadenine (epsilon A) and 3,N4-ethenocytosine (epsilon C) are formed when electrophilic vinyl chloride (VC) metabolites, chloroethylene oxide (CEO) or chloroacetaldehyde (CAA) react with adenine and cytosine residues in DNA. They were assayed for their miscoding properties in an in vitro system using Escherichia coli DNA polymerase I and synthetic templates prepared by reaction of poly(dA) and poly(dC) with increasing concentrations of CEO or CAA. Following the introduction of etheno groups, an increasing inhibition of DNA synthesis was observed. dGMP was misincorporated on CAA- or CEO-treated poly(dA) templates and dTMP was misincorporated on CAA- or CEO-treated poly(dC) templates, suggesting that epsilon A and epsilon C may miscode. The error rates augmented with the extent of reaction of CEO or CAA with the templates. Base-pairing models are proposed for the epsilon A.G. and epsilon C.T pairs. The potentially miscoding properties of epsilon A and epsilon C may explain why metabolically-activated VC and its reactive metabolites specifically induce base-pair substitution mutations in Salmonella typhimurium. Promutagenic lesions may represent one of the initial steps in VC- or CEO-induced carcinogenesis.

Acetaldehyde↗
PubMed1980

Escherichia coli mutator mutants deficient in methylation-instructed DNA mismatch correction.

Our approach to the isolation of DNA mismatch-correction-deficient mutants was based upon the isolation of 2-aminopurine-resistant second-site revertants of Escherichia coli dam- mutants. We isolated such second-site revertants which, when separated from the dam- mutation, have a mutator character of their own. These new mutators all mapped at three known mutator loci, mutH, mutL, and mutS, which exhibit the same mutagenic spectrum as the dam- mutator: increased levels of base substitution and frameshift mutations. The mutator potencies of double and triple mut- mutants suggest that these mutators are involved in the same general mismatch-repair pathway. All these mutations result in a hyper-recombination phenotype, but in four-factor crosses among lambda phages, a specific loss of intragenic recombination (Pam3 X Pam80) was found in mutL and mutS mutants, as would be predicted from the postulated role of mismatch correction in gene conversion and high negative interference phenomena.

2-Aminopurine↗
PubMed1980

Inhibition of carcinogen-induced chromosomal aberrations by an anticarcinogenic protease inhibitor.

It was hypothesized that chemicals- and radiation-induced carcinogenesis might require at least two specific chromosomal events that must coincide within a single target cell: (i) induction of chromosomal changes, possibly mutations, that are recessive and therefore latent in diploid somatic cells and (ii) aberrant mitotic segregation events that will convert the heterozygous cell, created by the first process, into a homozygous or hemizygous cell through chromosomal rearrangements. Hence, we tested the prediction that an inhibitor of induced carcinogenesis may inhibit one or both of these chromosomal events by studying the effects of antipain, a protease inhibitor and known inhibitor of carcinogenesis, on N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced mutagenesis, chromosomal aberrations, sister chromatid exchanges, and cell killing in V79 Chinese hamster cells. We show that antipain inhibited MNNG-induced chromosomal exchanges and all other chromosomal aberrations exclusively. This results leads us to postulate that MNNG-induced DNA lesions cause chromosomal aberrations which arise through an antipain-sensitive cellular process, that some chromosomal rearrangement is a rate-limiting step in carcinogenesis, and that mutagenesis alone, if required, is not sufficient to accomplish carcinogenesis.

Animals↗
PubMed1980

Induction of a stable, heritable epigenetic change by mutagenic carcinogens: a new test system.

The expression of the E. coli gal operon was set under the direct negative control of phage lambda repressor by a fusion between the gal operon an the active cI cro segment of phage lambda genoma. This system can exist in two stable but reversible epigenetic states: (A) the immune cI+ cro- gal- state (white colonies on McConkey gal plates) and (B) the nonimmune cI- cro+ gal+ state (red colonies). Transcription of this gal operon depends upon activation of the PR promoter; hence requiring the removal of lambda immunity repressor (cI) from the oR operator. Short exposure of this E. coli strain to radiation or chemical mutagens/carcinogens leads to a stable, inherited loss of the cI repressor (due to the take-over by the cro repressor, repressing the cI gene) and subsequent constitutive expression of the gal operon. This switch, from the (A) state to the (B) state, depends upon E. coli recA+ and lambda cro+ genes and is reversible upon supply of lambda cI repressor; this is consistent with the fact that cI and cro proteins are mutual repressors, i.e., only one of them can be expressed at a given time. This E. coli strain provides the easiest, most accurate and sensitive assay for all mutagenic and SOS-inducing agents.

Bacteriophage lambda↗
PubMed1980

Chromosomal events in carcinogenic initiation and promotion: implications for carcinogenicity testing and cancer prevention strategies.

We have provided experimental evidence in favour of the hypothesis that carcinogenesis is triggered by at least two chromosomal events, which must occur in a single diploid somatic cell in a specific time sequence: (i) specific recessive mutational or epigenetic chromosomal change(s) resulting in a heterozygous (m/+), latently premalignant state (initiation); this must be followed by (ii) a chromosomal rearrangement involving the affected locus, and leading to homozygosity (m/m) or hemizygosity (m/o), and subsequent expression of the recessive malignant character (promotion). The complete carcinogen, MNNG, induced mutations (6-thioguanine-resistance), chromosomal rearrangements and SCEs in V79 Chinese hamster cells. TPA, a potent tumour promoter, induced only SCEs and specific chromosomal effects. Antipain, a protease inhibitor and a known inhibitor of both carcinogenesis and tumour promotion, inhibited only the MMNG-induced chromosomal rearrangements (but not mutagenesis and SCEs) and the TPA-induced chromosomal events. These results suggest that (1) both TPA-induced and MNNG-induced chromosomal rearrangements are caused by the activation or induction of mitotic recombination and hence appear to be preventable; (2) chromosomal rearrangement is a rate-limiting step in carcinogenesis; and (3) if mutagenesis is involved in carcinogenesis, it is probably not sufficient. The existence of the human cancer-prone syndromes, Bloom's, Fanconi's anaemia and ataxia telangiectasia, which involve spontaneous chromosomal rearrangements analogous to those induced by carcinogens in normal cells, strongly supports our hypothesis that carcinogenesis involves two chromosomal events. We discuss the implications of this work to carcinogenicity testing and cancer prevention strategies.

Animals↗