NORMAL GENOMIC

• Mendillo ML, Putnam CD, Kolodner RD
• Escherichia coli MutS tetramerization domain structure reveals that stabledimers but not tetramers are essential for DNA mismatch repair in vivo.
• J Biol Chem. 2007; 282: 16345-54
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• The Escherichia coli mispair-binding protein MutS forms dimers andtetramers in vitro, although the functional form in vivo is under debate.Here we demonstrate that the MutS tetramer is extended in solution usingsmall angle x-ray scattering and the crystal structure of the C-terminal34 amino acids of MutS containing the tetramer-forming domain fused tomaltose-binding protein (MBP). Wild-type C-terminal MBP fusions formedtetramers and could bind MutS and MutS-MutL-DNA complexes. In contrast,D835R and R840E mutations predicted to disrupt tetrameric interactionsonly allowed dimerization of MBP. A chromosomal MutS truncation mutationeliminating the dimerization/tetramerization domain eliminated mismatchrepair, whereas the tetramer-disrupting MutS D835R and R840E mutationsonly modestly affected MutS function. These results demonstrate thatdimerization but not tetramerization of the MutS C terminus is essentialfor mismatch repair.

• Cahill D, Carney JP
• Dimerization of the Rad50 protein is independent of the conserved hookdomain.
• Mutagenesis. 2007; 22: 269-74
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• The Mre11 complex (Mre11-Rad50-Nbs1) is involved in a diverse array of DNAmetabolic processes including the response to DNA double-strand breaks(DSBs). The structure of Rad50 plays a key role in the DNA-binding andend-bridging activity of the complex. An interesting feature within thecentral portion of the Rad50 protein is the Rad50 hook region that isdefined by the highly conserved CXXC motif. The structure of thePyrococcus furiosus Rad50 hook region revealed an intermoleculardimerization of Rad50 through the coordination of a zinc ion by the fourcysteines. Biochemical and genetic analysis in Saccharomyces cerevisiaehave shown that mutations in the conserved cysteines impact all functionsof the Mre11 complex including interaction with Mre11, increasedsensitivity to DSB inducing agents, telomere maintenance andintrachromosomal association. Mutations in the yeast hook domain can leadto increased chromosome fragmentation, suggesting that the hook domain ofRad50 is essential for the tethering of chromosome ends. In this study, wehave examined the effects of mutating the key cysteine residues in thehook domain of human Rad50 (hRad50), focusing on the interactions Rad50has with itself, Mre11 and DNA. Our results reveal that mutation of theconserved cysteine residues abrogates dimerization at the hook domain inhRad50; however, disrupting dimerization at this domain does not appear toimpair the interaction of full-length hRad50 with itself and hMre11 oraffect DNA-binding activity of the hMre11-Rad50 complex.

• Matson SW, Robertson AB
• The UvrD helicase and its modulation by the mismatch repair protein MutL.
• Nucleic Acids Res. 2006; 34: 4089-97
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• UvrD is a superfamily I DNA helicase with well documented roles inexcision repair and methyl-directed mismatch repair (MMR) in addition topoorly understood roles in replication and recombination. The MutL proteinis a homodimeric DNA-stimulated ATPase that plays a central role in MMR inEscherichia coli. This protein has been characterized as the masterregulator of mismatch repair since it interacts with and modulates theactivity of several other proteins involved in the mismatch repair pathwayincluding MutS, MutH and UvrD. Here we present a brief summary of recentstudies directed toward arriving at a better understanding of theinteraction between MutL and UvrD, and the impact of this interaction onthe activity of UvrD and its role in mismatch repair.

• Robertson AB, Pattishall SR, Gibbons EA, Matson SW
• MutL-catalyzed ATP hydrolysis is required at a post-UvrD loading step inmethyl-directed mismatch repair.
• J Biol Chem. 2006; 281: 19949-59
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• Methyl-directed mismatch repair is a coordinated process that ensuresreplication fidelity and genome integrity by resolving base pairmismatches and insertion/deletion loops. This post-replicative eventinvolves the activities of several proteins, many of which appear to beregulated by MutL. MutL interacts with and modulates the activities ofMutS, MutH, UvrD, and perhaps other proteins. The purified proteincatalyzes a slow ATP hydrolysis reaction that is essential for its role inmismatch repair. However, the role of the ATP hydrolysis reaction is notunderstood. We have begun to address this issue using two point mutants:MutL-E29A, which binds nucleotide but does not catalyze ATP hydrolysis,and MutL-D58A, which does not bind nucleotide. As expected, both mutantsfailed to complement the loss of MutL in genetic assays. PurifiedMutL-E29A protein interacted with MutS and stimulated the MutH-catalyzednicking reaction in a mismatch-dependent manner. Importantly, MutL-E29Astimulated the loading of UvrD on model substrates. In fact, stimulationof UvrD-catalyzed unwinding was more robust with MutL-E29A than thewild-type protein. MutL-D58A, on the other hand, did not interact withMutS, stimulate MutH-catalyzed nicking, or stimulate the loading of UvrD.We conclude that ATP-bound MutL is required for the incision stepsassociated with mismatch repair and that ATP hydrolysis by MutL isrequired for a step in the mismatch repair pathway subsequent to theloading of UvrD and may serve to regulate helicase loading.

• Ahrends R et al.
• Identifying an interaction site between MutH and the C-terminal domain ofMutL by crosslinking, affinity purification, chemical coding and massspectrometry.
• Nucleic Acids Res. 2006; 34: 3169-80
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• To investigate protein-protein interaction sites in the DNA mismatchrepair system we developed a crosslinking/mass spectrometry techniqueemploying a commercially available trifunctional crosslinker with athiol-specific methanethiosulfonate group, a photoactivatable benzophenonemoiety and a biotin affinity tag. The XACM approach combinesphotocrosslinking (X), in-solution digestion of the crosslinked mixtures,affinity purification via the biotin handle (A), chemical coding of thecrosslinked products (C) followed by MALDI-TOF mass spectrometry (M). Weillustrate the feasibility of the method using a single-cysteine variantof the homodimeric DNA mismatch repair protein MutL. Moreover, wesuccessfully applied this method to identify the photocrosslink formedbetween the single-cysteine MutH variant A223C, labeled with thetrifunctional crosslinker in the C-terminal helix and its activatorprotein MutL. The identified crosslinked MutL-peptide maps to a conservedsurface patch of the MutL C-terminal dimerization domain. Theseobservations are substantiated by additional mutational and chemicalcrosslinking studies. Our results shed light on the potential structuresof the MutL holoenzyme and the MutH-MutL-DNA complex.

• Jacquelin DK, Filiberti A, Argarana CE, Barra JL
• Pseudomonas aeruginosa MutL protein functions in Escherichia coli.
• Biochem J. 2005; 388: 879-87
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• Escherichia coli MutS, MutL and MutH proteins act sequentially in the MMRS(mismatch repair system). MutH directs the repair system to the newlysynthesized strand due to its transient lack of Dam (DNA-adeninemethylase) methylation. Although Pseudomonas aeruginosa does not have thecorresponding E. coli MutH and Dam homologues, and consequently the MMRSseems to work differently, we show that the mutL gene from P. aeruginosais capable of complementing a MutL-deficient strain of E. coli. MutL fromP. aeruginosa has conserved 21 out of the 22 amino acids known to affectfunctioning of E. coli MutL. We showed, using protein affinitychromatography, that the C-terminal regions of P. aeruginosa and E. coliMutL are capable of specifically interacting with E. coli MutH andretaining the E. coli MutH. Although, the amino acid sequences of theC-terminal regions of these two proteins are only 18% identical, they are88% identical in the predicted secondary structure. Finally, by analysing(E. coli-P. aeruginosa) chimaeric MutL proteins, we show that theN-terminal regions of E. coli and P. aeruginosa MutL proteins functionsimilarly, in vivo and in vitro. These new findings support the hypothesisthat a large surface, rather than a single amino acid, constitutes theMutL surface for interaction with MutH, and that the N- and C-terminalregions of MutL are involved in such interactions.

• Mennecier S, Coste G, Servant P, Bailone A, Sommer S
• Mismatch repair ensures fidelity of replication and recombination in theradioresistant organism Deinococcus radiodurans.
• Mol Genet Genomics. 2004; 272: 460-9
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• We have characterized the mismatch repair system (MMR) of the highlyradiation-resistant type strain of Deinococcus radiodurans, ATCC 13939. Weshow that the MMR system is functional in this organism, where itparticipates in ensuring the fidelity of DNA replication andrecombination. The system relies on the activity of two key proteins,MutS1 and MutL, which constitute a conserved core involved in mismatchrecognition. Inactivation of MutS1 or MutL resulted in a seven-foldincrease in the frequency of spontaneous RifR mutagenesis and a ten-foldincrease in the efficiency of integration of a donor point-mutation markerduring bacterial transformation. Inactivation of the mismatchrepair-associated UvrD helicase increased the level of spontaneousmutagenesis, but had no effect on marker integration--suggesting thatbinding of MutS1 and MutL proteins to a mismatched heteroduplex sufficesto inhibit recombination between non identical (homeologous) DNAs. Incontrast, inactivation of MutS2, encoded by the second mutS -related genepresent in D. radiodurans, had no effect on mutagenesis or recombination.Cells devoid of MutS1 or MutL proteins were as resistant to gamma-rays,mitomycin C and UV-irradiation as wild-type bacteria, suggesting that themismatch repair system is not essential for the reconstitution of afunctional genome after DNA damage.

• Giron-Monzon L, Manelyte L, Ahrends R, Kirsch D, Spengler B, Friedhoff P
• Mapping protein-protein interactions between MutL and MutH bycross-linking.
• J Biol Chem. 2004; 279: 49338-45
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• Strand discrimination in Escherichia coli DNA mismatch repair requires theactivation of the endonuclease MutH by MutL. There is evidence that MutHbinds to the N-terminal domain of MutL in an ATP-dependent manner;however, the interaction sites and the molecular mechanism of MutHactivation have not yet been determined. We used a combination ofsite-directed mutagenesis and site-specific cross-linking to identifyprotein interaction sites between the proteins MutH and MutL. Uniquecysteine residues were introduced in cysteine-free variants of MutH andMutL. The introduced cysteines were modified with the cross-linkingreagent 4-maleimidobenzophenone. Photoactivation resulted in cross-linksverified by mass spectrometry of some of the single cysteine variants totheir respective Cys-free partner proteins. Moreover, we mapped the siteof interaction by cross-linking different combinations of single cysteineMutH and MutL variants with thiol-specific homobifunctional cross-linkersof varying length. These results were used to model the MutH.MutL complexand to explain the ATP dependence of this interaction.

• Fukui K, Masui R, Kuramitsu S
• Thermus thermophilus MutS2, a MutS paralogue, possesses an endonucleaseactivity promoted by MutL.
• J Biochem. 2004; 135: 375-84
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• The mismatch repair system (MMR) recognizes and corrects mismatched orunpaired bases caused mainly by DNA polymerase, and contributes to thefidelity of DNA replication in living cells. In Escherichia coli, theMutHLS system is known to function in MMR, and homologues of MutS and MutLare widely conserved in almost all organisms. However, the MutHendonuclease has not been found in the majority of organisms. Suchorganisms, including Thermus thermophilus HB8, often possess the so-calledMutS2 protein, which is highly homologous to MutS but contains an extraC-terminal stretch. To elucidate the function of MutS2, we overexpressedand purified T. thermophilus MutS2 (ttMutS2). ttMutS2 demonstrated theability to bind double-stranded (ds) DNA, but, unlike ttMutS, ttMutS2showed no specificity for mismatched duplexes. ttMutS2 ATPase activity wasalso detected and was stimulated by dsDNA. Our results also showed thatttMutS2 incises dsDNA. ttMutS2 incises not only oligo dsDNA but alsoplasmid DNA, suggesting that ttMutS2 possesses an endonuclease activity.At low concentrations, the incision activity was not retained, but waspromoted by T. thermophilus MutL.

• Lamers MH, Winterwerp HH, Sixma TK
• The alternating ATPase domains of MutS control DNA mismatch repair.
• EMBO J. 2003; 22: 746-56
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• DNA mismatch repair is an essential safeguard of genomic integrity byremoving base mispairings that may arise from DNA polymerase errors orfrom homologous recombination between DNA strands. In Escherichia coli,the MutS enzyme recognizes mismatches and initiates repair. MutS has anintrinsic ATPase activity crucial for its function, but which is poorlyunderstood. We show here that within the MutS homodimer, the twochemically identical ATPase sites have different affinities for ADP, andthe two sites alternate in ATP hydrolysis. A single residue, Arg697,located at the interface of the two ATPase domains, controls theasymmetry. When mutated, the asymmetry is lost and mismatch repair in vivois impaired. We propose that asymmetry of the ATPase domains is anessential feature of mismatch repair that controls the timing of thedifferent steps in the repair cascade.

• Schofield MJ, Hsieh P
• DNA mismatch repair: molecular mechanisms and biological function.
• Annu Rev Microbiol. 2003; 57: 579-608
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• DNA mismatch repair (MMR) guards the integrity of the genome in virtuallyall cells. It contributes about 1000-fold to the overall fidelity ofreplication and targets mispaired bases that arise through replicationerrors, during homologous recombination, and as a result of DNA damage.Cells deficient in MMR have a mutator phenotype in which the rate ofspontaneous mutation is greatly elevated, and they frequently exhibitmicrosatellite instability at mono- and dinucleotide repeats. Theimportance of MMR in mutation avoidance is highlighted by the finding thatdefects in MMR predispose individuals to hereditary nonpolyposiscolorectal cancer. In addition to its role in postreplication repair, theMMR machinery serves to police homologous recombination events and acts asa barrier to genetic exchange between species.

• Li GM
• DNA mismatch repair and cancer.
• Front Biosci. 2003; 8: 9971017-9971017
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• DNA mismatch repair (MMR) is an important genome caretaker system. Itensures genomic stability by correcting mismatches generated during DNAreplication and recombination and by triggering apoptosis of cells withlarge amounts of DNA damage. Protein components responsible for thesereactions are highly conserved through evolution, and homologs ofbacterial MutS and MutL, which are key players in the initiation steps ofboth the strand-specific mismatch correction and MMR-dependent apoptoticsignaling, have been identified in human cells. Inactivation of genesencoding these activities leads to genome-wide instability, particularlyin simple repetitive sequences, and predisposition to certain types ofcancer, including hereditary non-polyposis colorectal cancer.

• Junop MS, Yang W, Funchain P, Clendenin W, Miller JH
• In vitro and in vivo studies of MutS, MutL and MutH mutants: correlationof mismatch repair and DNA recombination.
• DNA Repair (Amst). 2003; 2: 387-405
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• We have used the recently determined crystal structures of Escherichiacoli (E. coli) MutS, MutL and MutH to guide construction of 47 amino-acidsubstitutions in these proteins and analyzed their behavior in mismatchrepair and recombination in vitro and in vivo. We find that the activesite of the MutH endonuclease is composed of regions from two separatestructural domains and that the C-terminal 5 residues of MutH influenceboth DNA binding and cleavage. We also find that the non-specificDNA-binding activity of MutL is required for mismatch repair and probablyfunctions after strand cleavage by MutH. Alteration of residues in eitherthe mismatch recognition domain, the ATPase active site, or the domaininterfaces linking the two activities can diminish the differentialbinding of MutS to homoduplex versus heteroduplex and results in the lossof mismatch-specific MutH activation. Finally, every mutation thatabolishes mismatch repair is deficient in blocking homeologousrecombination, suggesting that mismatch repair and prevention ofhomeologous recombination use the same MutS-MutL complexes for signalingin E. coli.

• Oliver A, Baquero F, Blazquez J
• The mismatch repair system (mutS, mutL and uvrD genes) in Pseudomonasaeruginosa: molecular characterization of naturally occurring mutants.
• Mol Microbiol. 2002; 43: 1641-50
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• We have recently described the presence of a high proportion ofPseudomonas aeruginosa isolates (20%) with an increased mutation frequency(mutators) in the lungs of cystic fibrosis (CF) patients. In four out of11 independent P. aeruginosa strains, the high mutation frequency wasfound to be complemented with the wild-type mutS gene from P. aeruginosaPAO1. Here, we report the cloning and sequencing of two additional P.aeruginosa mismatch repair genes and the characterization, bycomplementation of deficient strains, of these two putative P. aeruginosamismatch repair genes (mutL and uvrD). We also describe the alterations inthe mutS, mutL and uvrD genes responsible for the mutator phenotype ofhypermutable P. aeruginosa strains isolated from CF patients. Seven out ofthe 11 mutator strains were found to be defective in the MMR system (fourmutS, two mutL and one uvrD). In four cases (three mutS and one mutL), thegenes contained frameshift mutations. The fourth mutS strain showed a 3.3kb insertion after the 10th nucleotide of the mutS gene, and a 54nucleotide deletion between two eight nucleotide direct repeats. Thisdeletion, involving domain II of MutS, was found to be the main oneresponsible for mutS inactivation. The second mutL strain presented aK310M mutation, equivalent to K307 in Escherichia coli MutL, a residueknown to be essential for its ATPase activity. Finally, the uvrD strainhad three amino acid substitutions within the conserved ATP binding siteof the deduced UvrD polypeptide, showing defective mismatch repairactivity. Interestingly, cells carrying this mutant allele exhibited afully active UvrABC-mediated excision repair. The results shown hereindicate that the putative P. aeruginosa mutS, mutL and uvrD genes aremutator genes and that their alteration results in a mutator phenotype.

• Biswas I et al.
• Disruption of the helix-u-turn-helix motif of MutS protein: loss ofsubunit dimerization, mismatch binding and ATP hydrolysis.
• J Mol Biol. 2001; 305: 805-16
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• The DNA mismatch repair protein, MutS, is a dimeric protein thatrecognizes mismatched bases and has an intrinsic ATPase activity. Here, aseries of Taq MutS proteins having C-terminal truncations in the vicinityof a highly conserved helix-u-turn-helix (HuH) motif are assessed forsubunit oligomerization, ATPase activity and DNA mismatch binding. Thoseproteins containing an intact HuH region are dimers; those without the HuHregion are predominantly monomers in solution. Steady-state kinetics oftruncated but dimeric MutS proteins reveals only modest decreases in theirATPase activity compared to full-length protein. In contrast, disruptionof the HuH region results in a greatly attenuated ATPase activity. Inaddition, only dimeric MutS proteins are proficient for mismatch binding.Finally, an analysis of the mismatch repair competency of truncatedEscherichia coli MutS proteins in a rifampicin mutator assay confirms thatthe HuH region is critical for in vivo function. These findings indicatethat dimerization is critical for both the ATPase and DNA mismatch bindingactivities of MutS, and corroborate several key features of the MutSstructure recently deduced from X-ray crystallographic studies.

• Nicholson A, Hendrix M, Jinks-Robertson S, Crouse GF
• Regulation of mitotic homeologous recombination in yeast. Functions ofmismatch repair and nucleotide excision repair genes.
• Genetics. 2000; 154: 133-46
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• The Saccharomyces cerevisiae homologs of the bacterial mismatch repairproteins MutS and MutL correct replication errors and preventrecombination between homeologous (nonidentical) sequences. Previously, wedemonstrated that Msh2p, Msh3p, and Pms1p regulate recombination between91% identical inverted repeats, and here use the same substrates to showthat Mlh1p and Msh6p have important antirecombination roles. In addition,substrates containing defined types of mismatches (base-base mismatches;1-, 4-, or 12-nt insertion/deletion loops; or 18-nt palindromes) were usedto examine recognition of these mismatches in mitotic recombinationintermediates. Msh2p was required for recognition of all types ofmismatches, whereas Msh6p recognized only base-base mismatches and 1-ntinsertion/deletion loops. Msh3p was involved in recognition of thepalindrome and all loops, but also had an unexpected antirecombinationrole when the potential heteroduplex contained only base-base mismatches.In contrast to their similar antimutator roles, Pms1p consistentlyinhibited recombination to a lesser degree than did Msh2p. In addition tothe yeast MutS and MutL homologs, the exonuclease Exo1p and the nucleotideexcision repair proteins Rad1p and Rad10p were found to have roles ininhibiting recombination between mismatched substrates.

• Yang W
• Structure and function of mismatch repair proteins.
• Mutat Res. 2000; 460: 245-56
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• DNA mismatch repair is required for maintaining genomic stability and ishighly conserved from prokaryotes to eukaryotes. Errors made during DNAreplication, such as deletions, insertions and mismatched basepairs, aresubstrates for mismatch repair. Mismatch repair is strand-specific andtargets only the newly synthesized daughter strand. To initiate mismatchrepair in Escherichia coli, three proteins are essential, MutS, formismatch recognition, MutH, for introduction of a nick in the targetstrand, and MutL, for mediating the interactions between MutH and MutS.Homologues of MutS and MutL important for mismatch repair have been foundin nearly all organisms. Mutations in MutS and MutL homologues have beenlinked to increased cancer susceptibility in both mice and humans. Here,we review the crystal structures of the MutH endonuclease, a conservedATPase fragment of MutL (LN40), and complexes of LN40 with variousnucleotides. Based on the crystal structure, the active site of MutH hasbeen identified and an evolutionary relationship between MutH and type IIrestriction endonucleases established. Recent crystallographic andbiochemical studies have revealed that MutL operates as a molecular switchwith its interactions with MutH and MutS regulated by ATP binding andhydrolysis. These crystal structures also shed light on the generalmechanism of mismatch repair and the roles of Mut proteins in preventingmutagenesis.

• Westmoreland J, Porter G, Radman M, Resnick MA
• Highly mismatched molecules resembling recombination intermediatesefficiently transform mismatch repair proficient Escherichia coli.
• Genetics. 1997; 145: 29-38
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• The ability of related DNAs to undergo recombination decreases withincreased sequence divergence. Mismatch repair has been proposed to be akey factor in preventing homeologous recombination; however, thecontribution of mismatch repair is not universal. Although mismatch repairhas been proposed to act by preventing strand exchange and/or inactivatingmultiply mismatched heteroduplexes, there has been no systematic study todetermine at what step(s) in recombination mismatch repair acts in vivo.Since heteroduplex is a commonly proposed intermediate in many models ofrecombination, we have investigated the consequences of mismatch repair onplasmids that are multiply mismatched in heteroduplex structures that aresimilar to those that might arise during recombination. Plasmidscontaining multiply mismatched regions were transformed into wild-type andMut+ Escherichia coli mutants. There was only a 30-40% reduction intransformation of Mut+ as compared to mutS and mutL strains for DNAscontaining an 18% mismatched heteroduplex. The products obtained from mutShosts differed from those obtained from Mut+ hosts in that there were manymore colonies containing mixtures of two plasmids, due to survival of bothstrands of the heteroduplex. There were nearly 10 times more recombinantsobtained from the mutS as compared to the wild-type host. Based on theseresults and those from other studies with E. coli and yeast, we proposethat the prevention of recombination between highly diverged DNAs may beat a step earlier than heteroduplex formation.

• Ikejima M, Shimada T
• [Mismatch repair protein]
• Gan To Kagaku Ryoho. 1997; 24: 1392-400
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• A DNA mismatch repair system exists that repairs mispaired bases formedduring DNA replication and genetic recombination. Genetic defects in thismismatch repair system are known to increase the rate of spontaneousmutation in Escherichia coli. Some cases of inherited cancer areassociated with inherited defects of mismatch repair genes, showing theimportance of the mismatch repair system in maintenance of geneticstability and avoidance of cancer susceptibility. This review focused onwhat is known about the mechanisms of mismatch repair in human cells andthe relationship between defects in mismatch repair and carcinogenesis.

• Radman M, Matic I, Halliday JA, Taddei F
• Editing DNA replication and recombination by mismatch repair: frombacterial genetics to mechanisms of predisposition to cancer in humans.
• Philos Trans R Soc Lond B Biol Sci. 1995; 347: 97-103
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• A hereditary form of colon cancer, hereditary non-polyposis colon cancer(HNPCC), is characterized by high instability of short repeated sequencesknown as microsatellites. Because the genes controlling microsatellitestability were known in bacteria and yeast, as was their evolutionaryconservation, the search for human genes responsible for HNPCC became a'targeted' search for known sequences. Mismatch-repair deficiency inbacteria and yeast produces multiple phenotypes as a result of its dualinvolvement in the editing of both replication errors and recombinationintermediates. In addition, mismatch-repair functions are specialized ineukaryotes, characterized by specific mitotic (versus meiotic) functions,and nuclear (versus mitochondrial) localization. Given the number ofphenotypes observed so far, we predict other links between mismatch-repairdeficiency and human genetic disorders. For example, a similar type ofsequence instability has been found in HNPCC tumours and in a number ofneuro-muscular genetic disorders. Several human mitochondrial disordersdisplay genomic instabilities reminiscent of yeast mitochondrialmismatch-repair mutants. In general, the process of mismatch repair isresponsible for the constant maintenance of genome stability and itsfaithful transmission from one generation to the next. However, withoutgenetic alteration, species would not be able to adapt to changingenvironments. It appears that nature has developed both negative andpositive controls for genetic diversity. In bacteria, for example, aninducible system (sos) exists which generates genetic alterations inresponse to environmental stress (e.g. radiation, chemicals, starvation).Hence, the cost of generating diversity to adapt to changing conditionsmight be paid as sporadic gene alterations associated with disease.

• Modrich P
• Methyl-directed DNA mismatch correction.
• J Biol Chem. 1989; 264: 6597-600
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• In 1964 Robin Holliday (1) proposed the correction of DNA base pairmismatches within recombination intermediates as the basis for geneconversion. The existence of the mismatch repair systems implied by thisproposal is now well established. Activities that recognize and processbase pairing errors within the DNA helix have been identified in bacteria,fungi, and mammalian cells. However, the functions and mechanisms of suchsystems are best understood in Escherichia coli, an organism thatpossesses at least three distinct mismatch correction pathways. Thesethree systems are involved not only in the processing of recombinationintermediates but also contribute in a major way to the genetic stabilityof the organism, a function anticipated for mismatch repair by Tiraby andFox and by Wagner and Meselson. The significance of mismatch correction inthe maintenance of low spontaneous mutability becomes apparent when oneconsiders that seven E. coli mutator genes (dam, mutD, mutH, mutL, mutS,mutU, and mutY) have been implicated in mismatch repair. This minireviewwill summarize information on the most extensively studied E. coli systemfor mismatch correction, the methyl-directed pathway for processing of DNAbiosynthetic errors and intermediates in genetic recombination. Adiscussion of other E. coli mismatch correction systems may be found inthe recent literature and in several recent reviews. Mismatch repairpathways in other organisms and descriptions of the structural propertiesof mispaired bases may also be found in several of these reviews.

• Laengle-Rouault F, Maenhaut-Michel G, Radman M
• GATC sequence and mismatch repair in Escherichia coli.
• EMBO J. 1986; 5: 2009-13
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• The Escherichia coli mismatch repair system greatly improves DNAreplication fidelity by repairing single mispaired and unpaired bases innewly synthesized DNA strands. Transient undermethylation of the GATCsequences makes the newly synthesized strands susceptible to mismatchrepair enzymes. The role of unmethylated GATC sequences in mismatch repairwas tested in transfection experiments with heteroduplex DNA of phage phi174 without any GATC sequence or with two GATC sequences, containing inaddition either a G:T mismatch (Eam+/Eam3) or a G:A mismatch (Bam+/Bam16).It appears that only DNA containing GATC sequences is subject to efficientmismatch repair dependent on E. coli mutH, mutL, mutS and mutU genes;however, also in the absence of GATC sequence some mut-dependent mismatchrepair can be observed. These observations suggest that the mismatchrepair enzymes recognize both the mismatch and the unmethylated GATCsequence in DNA over long distances. The presence of GATC sequence(s) inthe substrate appears to be required for full mismatch repair activity andnot only for its strand specificity according to the GATC methylationstate.