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M Radman

Publications and source records attributed to M Radman.

At least 55 records · Page 3Linked to original sources
PubMed1995

Interspecies gene exchange in bacteria: the role of SOS and mismatch repair systems in evolution of species.

Analysis of interspecies matings between S. typhimurium and E. coli indicates that the genetic barrier that separates these (and perhaps many other) related species is primarily recombinational. The structural component of this barrier is genomic sequence divergence. The mismatch repair enzymes act as potent inhibitors of interspecies recombination, whereas the SOS system acts as an inducible positive regulator. Interspecies mating triggers a RecBC-dependent SOS response in female bacteria that increases recombination mainly through overproduction of the RecA protein. Mismatch repair acts to reduce the mutation rate and recombination between similar sequences, whereas SOS acts to increase both. These opposing activities allow mismatch repair and SOS systems to determine both the rate of accumulation of sequence divergence and the extent of genetic isolation, which are the key components of the speciation process.

Biological Evolution↗
PubMed1995

Editing DNA replication and recombination by mismatch repair: from bacterial genetics to mechanisms of predisposition to cancer in humans.

A hereditary form of colon cancer, hereditary non-polyposis colon cancer (HNPCC), is characterized by high instability of short repeated sequences known as microsatellites. Because the genes controlling microsatellite stability were known in bacteria and yeast, as was their evolutionary conservation, the search for human genes responsible for HNPCC became a 'targeted' search for known sequences. Mismatch-repair deficiency in bacteria and yeast produces multiple phenotypes as a result of its dual involvement in the editing of both replication errors and recombination intermediates. In addition, mismatch-repair functions are specialized in eukaryotes, characterized by specific mitotic (versus meiotic) functions, and nuclear (versus mitochondrial) localization. Given the number of phenotypes observed so far, we predict other links between mismatch-repair deficiency and human genetic disorders. For example, a similar type of sequence instability has been found in HNPCC tumours and in a number of neuro-muscular genetic disorders. Several human mitochondrial disorders display genomic instabilities reminiscent of yeast mitochondrial mismatch-repair mutants. In general, the process of mismatch repair is responsible for the constant maintenance of genome stability and its faithful transmission from one generation to the next. However, without genetic alteration, species would not be able to adapt to changing environments. It appears that nature has developed both negative and positive controls for genetic diversity. In bacteria, for example, an inducible system (sos) exists which generates genetic alterations in response to environmental stress (e.g. radiation, chemicals, starvation). Hence, the cost of generating diversity to adapt to changing conditions might be paid as sporadic gene alterations associated with disease.

Amino Acid Sequence↗
PubMed1994

Cloning and expression of the Xenopus and mouse Msh2 DNA mismatch repair genes.

Bacterial MutS protein and its yeast and human homologs MSH2 trigger the mismatch repair process by their initial binding to mispaired and unpaired bases in DNA. We describe the cloning and sequencing of genes from Xenopus laevis and Mus musculus encoding the homolog of the Saccharomyces cerevisiae MSH2 (the major DNA mismatch binding protein). Mutations in the human homolog of this gene have recently been implicated in microsatellite instability and DNA mismatch repair deficiency in tumour cells from patients with the most common hereditary predisposition to cancer (Lynch syndrome, or hereditary non-polyposis colorectal cancer, HNPCC), as well as in a significant percentage of sporadic tumours. Expression of the amphibian and murine Msh2 gene in different tissues appears to be ubiquitous. The Xenopus gene is highly expressed in eggs, a model system for the biochemistry of DNA mismatch repair. Expression of the murine gene is low in all tissues examined, and is relatively high in a rapidly dividing cell line. These data are suggestive of a role for MSH2 during DNA replication.

Animals↗
PubMed1994

Mismatch repair proteins MutS and MutL inhibit RecA-catalyzed strand transfer between diverged DNAs.

Bacterial mutS and mutL mutations confer large increases in recombination between sequences that are divergent by several percent at the nucleotide level, an effect attributed to a role for products of these genes in control of recombination fidelity. Since MutS and MutL are proteins involved in the earliest steps of mismatch repair, including mismatch recognition by MutS, we have tested the possibility that they may affect strand exchange in response to occurrence of mispairs within the recombination heteroduplex. We show that MutS abolishes RecA-catalyzed strand transfer between fd and M13 bacteriophage DNAs, which vary by 3% at the nucleotide level, but is without effect on M13-M13 or fd-fd exchange. Although MutL alone has no effect on M13-fd heteroduplex formation, the protein dramatically enhances the inhibition of strand transfer mediated by MutS. Analysis of strand-transfer intermediates that accumulate in the presence of MutS and MutL indicates that the proteins block branch migration, presumably in response to occurrence of mispairs within newly formed heteroduplex.

Adenosine Triphosphatases↗
PubMed1994

Structure of recombinants from conjugational crosses between Escherichia coli donor and mismatch-repair deficient Salmonella typhimurium recipients.

To get more insight into the control of homologous recombination between diverged DNA by the Mut proteins of the long-patch mismatch repair system, we have studied interspecies Escherichia coli/Salmonella typhimurium recombination. Knowing that the same recombination pathway (RecABCD) is responsible for intraspecies and interspecies recombination, we have now studied the structure (replacement vs. addition-type or other rearrangement-type recombinants) of 81 interspecies recombinants obtained in conjugational crosses between E. coli donor and mutL, mutS, mutH, mutU or mut+ S. typhimurium recipients. Taking advantage of high interspecies sequence divergence, a physical analysis was performed on one third of the E. coli Hfr genome, which was expected to be transferred to S. typhimurium F- recipients during 40 min before interruption of the mating. Probes specific for each species were hybridized on dot blots of genomic DNA, or on colonies, and the composition of the rrn operons was determined from purified genomic DNA. With very few exceptions, the structure of these interspecies recombinants corresponds to replacements of one continuous block of the recipient genome by the corresponding region of the donor genome.

Base Sequence↗
PubMed1993

Mismatch recognition in chromosomal interactions and speciation.

Homologous chromosomes interact during meiosis by means of proteins involved in recombination and in the recognition and repair of mismatched base pairs. Recombination proteins bring homologous chromosomes or chromosomal regions together by facilitating the search for DNA homology and by catalyzing strand exchange between homologous molecules or regions. Mismatch recognition and repair proteins act as editors of recombination and appear to disrupt those DNA associations that contain mismatched base pairs. Thus, it may be that, as chromosomes diverge in their primary sequence and become increasingly polymorphic, recombinational interactions leading to chromosome pairing and recombination tend to be inhibited. Decreasing homologous interactions within and between chromosomes will clearly contribute to maintaining the integrity of individual chromosomes and may ultimately lead, as a result of sterile meioses, to the reproductive isolation of closely related species.

Biological Evolution↗
PubMed1992

Duplication-targeted DNA methylation and mutagenesis in the evolution of eukaryotic chromosomes.

Mammalian genomes are threatened with gene inactivation and chromosomal scrambling by recombination between repeated sequences such as mobile genetic elements and pseudogenes. We present and test a model for a defensive strategy based on the methylation and subsequent mutation of CpG dinucleotides in those DNA duplications that create uninterrupted homologous sequences longer than about 0.3 kilobases. The model helps to explain both the diversity of CpG frequencies in different genes and the persistence of gene fragmentation into exons and introns.

Animals↗
PubMed1991

Mismatch-stimulated plasmid integration in yeast.

A single base pair mismatch (G:T or A:C) in the CYC1 gene of the integrative plasmid pAB218 stimulates up to a five-fold integration into the yeast chromosome. Analysis of chromosomal sites of plasmid integration suggests that the mismatch-stimulated integration is not targeted as would be expected if crossovers, localised in the region of the mismatch, were a necessary step in mismatch repair. Instead, the observed mismatch-stimulated plasmid integration could be due to potentially recombinogenic structures formed during mismatch repair, such as single-stranded gaps or denatured DNA regions extending around the plasmid molecule.

Base Composition↗
PubMed1991

Counterselection of GATC sequences in enterobacteriophages by the components of the methyl-directed mismatch repair system.

Weak to severe deficit of GATC sequences in the DNA of enterobacteriophages appears to be correlated with their undermethylation during growth in dam+ (GATC ade-methylase) bacteria. This observation is corroborated by the sequence analysis showing no evidence for site-specific mutagenicity of 6meAde. The MutH protein of the methyl-directed mismatch repair system recognizes and cleaves the undermethylated GATC sequences in the course of mismatch repair. To enquire whether the MutH function of the methyl-directed mismatch repair system participates in counterselection of GATC sequences in enterobacteriophages, we have studied the yield of bacteriophage phi X174 containing either 0, 1, or 2 GATC sequences, in wild type, dam, and mut (H, L, S, U) Escherichia coli. Following transfection with unmethylated DNA containing two GATC sequences, a net decrease in the yield of infective particles was observed in all bacterial mutH+ dam- strains, whereas no detectable decrease was observed in bacteria infected by DNA without GATC sequence. This effect of the MutH function is maximum in wild type and mutL and mutS bacteria whereas the effect is not significant in mutU bacteria, suggesting an interaction of the helicase II with the MutH protein. However, in dam+ bacteria, the presence of GATC sequences leads to an increased yield of infective particles. The effect of GATC sequence and its Dam methylation system on phage yield in mutH- bacteria reveals that methylated GATC sequences are advantageous to the phage.(ABSTRACT TRUNCATED AT 250 WORDS)

Base Sequence↗
PubMed1991

Avoidance of inter-repeat recombination by sequence divergence and a mechanism of neutral evolution.

Eucaryotic genomes are loaded with diverse repeated sequences and are therefore threatened by rearrangements via inter-repeat crossovers and by gene-inactivating conversions between genes and their inactive pseudogenes. Such repeated DNA sequences are usually diverged and polymorphic. Sequence divergence by well-spread point mutations is a potent inhibitor of homologous recombination due to the loss of recombination initiation sites and to the editing of recombinational intermediates by the mismatch repair system. Evidence is reviewed suggesting that a germ line process can identify duplicated sequences by homologous pairing, modify them by methylation and mutate by C----T transitions. Since this process requires a minimum contiguous homology that is larger than the average exon size, it is proposed that fragmentation by intron inserts protects the coding sequences from inactivation by homologous interactions with their pseudogene sequences.

Animals↗
PubMed1991

Interspecific recombination between Escherichia coli and Salmonella typhimurium occurs by the RecABCD pathway.

Interspecific recombination in conjugation between Escherichia coli and Salmonella typhimurium is several orders of magnitude lower than intraspecies recombination and is dependent on the RecA function. This low efficiency is due to a 20% divergence in the DNA sequence. The methyl-directed (mut H,L,S dependent) mismatch repair system appears to control the fidelity of homologous recombination; inactivating one of the Mut functions increases the interspecies recombination at least by 10(3)-fold. The interspecific recombination in mutS or mutL mutants is only approximately 10-fold lower than recombination in homospecific crosses as found after correction for the efficiency of mating and DNA transfer by zygotic induction experiments. The interspecific recombination is dependent on the RecABCD pathway: it was abolished in a recA mutant and decreased approximately 10(3)-fold in a recC mutant.

Conjugation, Genetic↗
PubMed1991

Control of large chromosomal duplications in Escherichia coli by the mismatch repair system.

Excessive recombination between repeated, interspersed, and diverged DNA sequences is a potential source of genomic instability. We have investigated the possibility that a mechanism exists to suppress genetic exchange between these quasi-homologous (homeologous) sequences. We examined the role of the general mismatch repair system of Escherichia coli because previous work has shown that the mismatch repair pathway functions as a barrier to interspecies recombination between E. coli and Salmonella typhimurium. The formation of large duplications by homeologous recombination in E. coli was increased some tenfold by mutations in the mutL and mutS genes that encode the mismatch recognition proteins. These findings indicate that the mismatch recognition proteins act to prevent excessive intrachromosomal exchanges. We conclude that mismatch repair proteins serve as general controllers of the fidelity of genetic inheritance, acting to suppress chromosomal rearrangements as well as point mutations.

Chromosomes, Bacterial↗
PubMed1990

Some restriction endonucleases tolerate single mismatches of the pyrimidine.purine type.

DNAs from phage mutants M13mp18 and M13mp18/MP-1 were used to construct two closed circular heteroduplexes. One of them carried the sequence 5'-CCTGGG-3' 3'-GGGCCC-5' with a T.G mismatch at the position 6248. The other carried the sequence 5'-CCCGGG-3' 3'-GGACCC-5' with a C.A mismatch at the same position. Heteroduplexes were exposed to 7 restriction endonucleases having recognition sites within the sequence 5'-CCCGGG-3' 3'-GGGCCC-5' and to 1 restriction endonuclease having a recognition site within the sequence 5'-CCTGGG-3' 3'-GGACCC-5'. All tested enzymes cleaved at least one mismatch-containing sequence although with reduced efficiency. Smal and Xmal tolerated both mismatch-containing sequences. Aval, Hpall, Mspl, Ncil and Nsplll were able to tolerate only the T.G containing sequence, while BstNl was able to tolerate only the C.A containing sequence. It is inferred that the tolerance displayed by Smal and Xmal depends on the presence of either the original purines or the original pyrimidines in mismatches of both the T.G and C.A type and that all other tested enzymes require the presence of the original purines in mismaches of both types.

Bacteriophages↗
PubMed1990

Genotoxic potency of monofunctional alkylating agents in E. coli: comparison with carcinogenic potency in rodents.

A quantitative correlation between carcinogenicity and genotoxicity was investigated by a comparison between the carcinogenic potency in rodents and the mutagenic (M), recombinogenic (R) and SOS-inducing (I) potencies in a bacterial test (E. coli multitest) for 9 monofunctional alkylating agents: N-nitroso-N-methylurethane, N-nitroso-N-ethylurea, epichlorohydrin, N-nitroso-N-methylurea, N-nitroso-N-methyl-N'-nitroguanidine, methyl methanesulfonate, diethylsulfate, dimethylsulfate, ethyl methanesulfonate. A significant positive correlation between the carcinogenic potency and the product of the mutagenic and recombinogenic potencies was found for all tested compounds. Thus, the E. coli multitest may be used as a simple test to search for correlations between carcinogenicity and genotoxicity of DNA-damaging agents.

Alkylating Agents↗
PubMed1990

DNA mismatch repair in Xenopus egg extracts: repair efficiency and DNA repair synthesis for all single base-pair mismatches.

Repair of all 12 single base-pair mismatches by Xenopus egg extracts was measured by a physical assay with a sequence containing four overlapping restriction sites. The heteroduplex substrates, derivatives of M13 phage DNA, differed in sequence at the mismatch position only and permitted measurement of repair to both strands. The efficiency of repair varied about 4-fold between the most and least effectively repaired mismatches. Repair was most active with C/A and T/C mismatches but the efficiency varied depending on the orientation of the mismatch. Mismatch-specific DNA repair synthesis was also observed but the extent of repair was not always predictive of the extent of synthesis, suggesting the presence of different repair systems or different modes of mismatch recognition.

Animals↗
PubMed1989

The barrier to recombination between Escherichia coli and Salmonella typhimurium is disrupted in mismatch-repair mutants.

The requirement for DNA sequence homology in generalized genetic recombination is greatly relaxed in bacterial mutL, mutS and mutH mutants deficient in mismatch repair. In such mutants, intergeneric recombination occurs efficiently between Escherichia coli and Salmonella typhimurium, which are approximately 20% divergent in DNA sequence. This finding has implications for speciation, for regulating recombination between diverged repeated sequences, and for hitherto difficult interspecies hybridizations.

Alleles↗
PubMed1989

The extreme mutator effect of Escherichia coli mutD5 results from saturation of mismatch repair by excessive DNA replication errors.

Escherichia coli mutator mutD5 is the most potent mutator known. The mutD5 mutation resides in the dnaQ gene encoding the proofreading exonuclease of DNA polymerase III holoenzyme. It has recently been shown that the extreme mutability of this strain results, in addition to a proofreading defect, from a defect in mutH, L, S-encoded postreplicational DNA mismatch repair. The following measurements of the mismatch-repair capacity of mutD5 cells demonstrate that this mismatch-repair defect is not structural, but transient. mutD5 cells in early log phase are as deficient in mismatch repair as mutL cells, but they become as proficient as wild-type cells in late log phase. Second, arrest of chromosomal replication in a mutD5-dnaA(Ts) strain at a nonpermissive temperature restores mismatch repair, even from the early log phase of growth. Third, transformation of mutD5 strains with multicopy plasmids expressing the mutH or mutL gene restores mismatch repair, even in rapidly growing cells. These observations suggest that the mismatch-repair deficiency of mutD strains results from a saturation of the mutHLS-mismatch-repair system by an excess of primary DNA replication errors due to the proofreading defect.

Base Sequence↗