Graham Walker
Massachusetts Institute of Technology (MIT), Biology, Faculty Member
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Research Interests: Molecular Biology, Bacteriology, Gene regulation, Biology, DNA repair, and 15 moreMedicine, Biological Sciences, Electromagnetic Radiation, Mutation, Escherichia coli, Temperature, Gene, Plasmids, Genotype, Heat Shock, Production Function, Heat Shock Proteins, Organic Compound, Medical and Health Sciences, and GroEl
Significance Mutagenic translesion synthesis (TLS) allows cells to increase their survival after DNA damage by bypassing lesions that normally block DNA replication but at the cost of introducing mutations. Interfering with this TLS... more
Significance Mutagenic translesion synthesis (TLS) allows cells to increase their survival after DNA damage by bypassing lesions that normally block DNA replication but at the cost of introducing mutations. Interfering with this TLS pathway genetically or with the small molecule inhibitor JH-RE-06 has been shown to improve cisplatin chemotherapy by suppressing tumor growth and enhancing survival in mouse xenograft tumor models. Deleting Rev7, which is a component of both the TLS machinery and a complex that regulates the choice of double-strand break repair pathways, strikingly potentiates cisplatin chemotherapy in a lung cancer model. Moreover, while cisplatin monotherapy resulted in tumor cell apoptosis, Rev7 deletion promoted a cisplatin-induced senescence phenotype, suggesting that targeting Rev7 is an attractive strategy to improve chemotherapy.
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Rhizobium meliloti strains mutant in the exoR gene overproduce an exopolysaccharide called succinoglycan or EPS I. Protein fusions to several different exo genes required for EPS I biosynthesis are expressed at a higher level in an exoR... more
Rhizobium meliloti strains mutant in the exoR gene overproduce an exopolysaccharide called succinoglycan or EPS I. Protein fusions to several different exo genes required for EPS I biosynthesis are expressed at a higher level in an exoR strain than in a wild-type strain, showing that the overproduction of EPS I in exoR strains results at least in part from increased gene expression. This regulation is important to nodulation, since exoR mutants fail to invade alfalfa nodules unless secondary suppressor mutations that cause a decrease in EPS I production occur. Here, we show that an exoR strain contains higher levels of mRNA for other exo genes than does the wild-type parental strain. ExoR therefore most probably exerts its regulatory effect at the level of transcription. In addition, we have localized, subcloned, and sequenced the exoR gene. A newly constructed insertion allele of exoR has the same phenotype as the original mutant. The deduced sequence of ExoR is 268 amino acids lon...
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Research Interests: Molecular Biology, Biology, RNA polymerase, Medicine, Gene expression, and 13 moreCell Division, Biological Sciences, Mutation, Escherichia coli, Bacteria, Temperature, Molecular cloning, Heat Shock, Heat Shock Proteins, Heat Shock Protein, Subunit, Nucleotide sequence, and Medical and Health Sciences
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Antimicrobial peptide transporter SbmA is an evolutionary link between ABC and proton-driven transporters.
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Significance Mutagenic translesion synthesis (TLS) increases cell survival after DNA damage by bypassing lesions that normally block DNA replication but introduces mutations. In cancer cells, REV1/POLζ-dependent mutagenic TLS can... more
Significance Mutagenic translesion synthesis (TLS) increases cell survival after DNA damage by bypassing lesions that normally block DNA replication but introduces mutations. In cancer cells, REV1/POLζ-dependent mutagenic TLS can contribute to intrinsic chemoresistance, while the mutations it introduces can underlie acquired chemoresistance. Interfering with this TLS pathway genetically or with the small molecule inhibitor JH-RE-06 has been shown to improve cisplatin chemotherapy by suppressing tumor growth and enhancing survival in mouse xenograft tumor models. Cisplatin chemotherapy commonly exerts its antitumor effects via DNA damage-mediated apoptosis. However, in two mouse xenograft models and four mammalian cell lines, the JH-RE-06 unexpectedly profoundly alters the biological response to cisplatin. Apoptosis is suppressed and surprisingly numerous hallmarks of senescence are induced prior to cell death.
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Fusions of the lac genes to the promoters of four structural genes in the methionine biosynthetic pathway, metA, metB, metE, and metF, were obtained by the use of the Mu d(Ap lac) bacteriophage. The levels of beta-galactosidase in these... more
Fusions of the lac genes to the promoters of four structural genes in the methionine biosynthetic pathway, metA, metB, metE, and metF, were obtained by the use of the Mu d(Ap lac) bacteriophage. The levels of beta-galactosidase in these strains could be derepressed by growth under methionine-limiting conditions. Furthermore, growth in the presence of vitamin B12 repressed the synthesis of beta-galactosidase in strains containing a fusion of lacZ to the metE promoter, phi(metE'-lacZ+). Mutations affecting the regulation of met-lac fusions were generated by the insertion of Tn5. Tn5 insertions were obtained at the known regulatory loci metJ and metK. Interestingly, a significant amount of methionine adenosyltransferase activity remained in the metK mutant despite the fact that the mutation was generated by an insertion. Several Tn5-induced regulatory mutations were isolated by screening for high-level beta-galactosidase expression in a phi(metE'-lacZ+) strain in the presence o...
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Spontaneous mutators of Salmonella typhimurium LT2 were generated by inserting the transposable element Tn5 or Tn10 into the bacterial chromosome. Two mutators mapped at the position of the mutH and mutL loci of S. typhimurium, and two... more
Spontaneous mutators of Salmonella typhimurium LT2 were generated by inserting the transposable element Tn5 or Tn10 into the bacterial chromosome. Two mutators mapped at the position of the mutH and mutL loci of S. typhimurium, and two other mutators mapped at positions corresponding to the mutS and uvrD loci of Escherichia coli. A fifth mutator, mutB, did not map at a position corresponding to any of the known mutators of S. typhimurium or E. coli. The mutH,L,S and uvrD alleles increased the frequency of both spontaneous base substitution and frameshift mutations, whereas the mutB allele increased the frequency only of spontaneous base substitution mutations. The increased frequency of base substitution mutations was recA+ independent in the mutH, mutL, and uvrD strains and partially recA+ independent in the mutS strain. The uvrD mutation decreased the resistance of the cells to killing by ultraviolet irradiation. The mutH,L,S and uvrD strains showed an increased sensitivity to mut...
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YbeY is a highly conserved, multifunctional endoribonuclease that plays a significant role in ribosome biogenesis and has several additional roles. Here we show that overexpression of the conserved GTPase Era in partially suppresses the... more
YbeY is a highly conserved, multifunctional endoribonuclease that plays a significant role in ribosome biogenesis and has several additional roles. Here we show that overexpression of the conserved GTPase Era in partially suppresses the growth defect of a Δ strain while improving 16S rRNA processing and 70S ribosome assembly. This suppression requires both the ability of Era to hydrolyze GTP and the function of three exoribonucleases, RNase II, RNase R, and RNase PH, suggesting a model for the action of Era. Overexpression of Era similarly partially suppresses the defects of an Δ strain, indicating that this property of Era is conserved in bacteria other than This work provides insight into the critical, but still incompletely understood, mechanism of processing of the 16S rRNA 3' terminus. The highly conserved GTPase Era is known to bind to the precursor of the 16S rRNA near its 3' end. Both the endoribonuclease YbeY, which binds to Era, and four exoribonucleases have been ...
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We constructed an ada deletion by gene replacement in a recD1014 strain of Escherichia coli. Characterization of a delta ada-25 recD+ strain revealed the presence of a second DNA methyltransferase activity in E. coli K-12 which transfers... more
We constructed an ada deletion by gene replacement in a recD1014 strain of Escherichia coli. Characterization of a delta ada-25 recD+ strain revealed the presence of a second DNA methyltransferase activity in E. coli K-12 which transfers a methyl group from methylated DNA to a protein with a molecular weight of 18,000 to 20,000.
Research Interests: Bacteriology, Biology, DNA repair, Medicine, Biological Sciences, and 15 moreDNA, Mutation, Escherichia coli, Methylation, Gene, Enzyme, Chemical Reaction, Enzyme activity, Alkylation, Molecular weight, Organic Compound, Methyltransferase, Nucleic Acid, Chromosome deletion, and Medical and Health Sciences
The GroE proteins of Escherichia coli are heat shock proteins which have also been shown to be molecular chaperone proteins. Our previous work has shown that the GroE proteins of E. coli are required for UV mutagenesis. This process... more
The GroE proteins of Escherichia coli are heat shock proteins which have also been shown to be molecular chaperone proteins. Our previous work has shown that the GroE proteins of E. coli are required for UV mutagenesis. This process requires the umuDC genes which are regulated by the SOS regulon. As part of the UV mutagenesis pathway, the product of the umuD gene, UmuD, is posttranslationally cleaved to yield the active form, UmuD'. In order to investigate what role the groE gene products play in UV mutagenesis, we measured UV mutagenesis in groE+ and groE strains which were expressing either the umuDC or umuD'C genes. We found that expression of umuD' instead of umuD will suppress the nonmutability conferred by the groE mutations. However, cleavage of UmuD to UmuD' is unaffected by mutations at the groE locus. Instead we found that the presence of UmuD' increased the stability of UmuC in groE strains. In addition, we obtained evidence which indicates that GroEL ...
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During the symbiotic interaction between alfalfa and the nitrogen-fixing bacterium Rhizobium meliloti, the bacterium induces the formation of nodules on the plant roots and then invades these nodules. Among the bacterial genes required... more
During the symbiotic interaction between alfalfa and the nitrogen-fixing bacterium Rhizobium meliloti, the bacterium induces the formation of nodules on the plant roots and then invades these nodules. Among the bacterial genes required for nodule invasion are the exo genes, involved in production of an extracellular polysaccharide, and the ndv genes, needed for production of a periplasmic cyclic glucan. Mutations in the exoD gene result in altered exopolysaccharide production and in a nodule invasion defect. In this work we show that the stage of symbiotic arrest of exoD mutants is similar to that of other exo and ndv mutants. However, the effects of exoD mutations on exopolysaccharide production and growth on various media are different from the effects of other exo and ndv mutations. Finally, exoD mutations behave differently from other exo mutations in their ability to be suppressed or complemented extracellularly. The results suggest that exoD represents a new class of Rhizobium...
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On the basis of characterizations of a set of UmuD monocysteine derivatives, we had suggested that positions 24, 34, and 44 are closer to the intact UmuD homodimer interface than other positions tested (M. H. Lee, T. Ohta, and G. C.... more
On the basis of characterizations of a set of UmuD monocysteine derivatives, we had suggested that positions 24, 34, and 44 are closer to the intact UmuD homodimer interface than other positions tested (M. H. Lee, T. Ohta, and G. C. Walker, J. Bacteriol. 176:4825-4837, 1994). Because this region of UmuD also appeared to be important for interactions with RecA, we followed up on our previous study by constructing a second set of monocysteine UmuD derivatives with single cysteine substitutions at positions 30 to 42. We found that like the VC34 mutant, UmuD derivatives with monocysteine substitutions at positions 32 and 35 showed deficiencies in in vivo and in vitro RecA-mediated cleavage as well as in UV mutagenesis, suggesting that the position 32 to 35 region may be important for RecA-mediated cleavage of UmuD. Interestingly, UmuD with monocysteine substitutions at residues 33 and 40 showed a reduction in UV mutagenesis while retaining the ability to be cleaved by RecA in vivo, sugg...
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The detailed structure of the symbiotically important exopolysaccharide succinoglycan from Rhizobium meliloti Rm1021 was determined by mass spectrometry with electrospray ionization and collision-induced dissociation of the octameric... more
The detailed structure of the symbiotically important exopolysaccharide succinoglycan from Rhizobium meliloti Rm1021 was determined by mass spectrometry with electrospray ionization and collision-induced dissociation of the octameric oligosaccharide repeating unit. Previously undetermined locations of the succinyl and acetyl modifications were determined, in respect to both residue locations within the octamer and the carbon positions within the pyranose ring. Glycosidic linkages determined previously by methylation analysis were also verified.
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To isolate strains with new recA mutations that differentially affect RecA protein functions, we mutagenized in vitro the recA gene carried by plasmid mini-F and then introduced the mini-F-recA plasmid into a delta recA host that was... more
To isolate strains with new recA mutations that differentially affect RecA protein functions, we mutagenized in vitro the recA gene carried by plasmid mini-F and then introduced the mini-F-recA plasmid into a delta recA host that was lysogenic for prophage phi 80 and carried a lac duplication. By scoring prophage induction and recombination of the lac duplication, we isolated new recA mutations. A strain carrying mutation recA1734 (Arg-243 changed to Leu) was found to be deficient in phi 80 induction but proficient in recombination. The mutation rendered the host not mutable by UV, even in a lexA(Def) background. Yet, the recA1734 host became mutable upon introduction of a plasmid encoding UmuD*, the active carboxyl-terminal fragment of UmuD. Although the recA1734 mutation permits cleavage of lambda and LexA repressors, it renders the host deficient in the cleavage of phi 80 repressor and UmuD protein. Another strain carrying mutation recA1730 (Ser-117 changed to Phe) was found to b...
Research Interests: Genetics, Molecular Biology, Bacteriology, Biology, DNA repair, and 15 moreMedicine, Biological Sciences, Crystal structure, Mutation, Escherichia coli, Protein Function, Molecular cloning, Radiation Effect, Genetic Recombination, Plasmid, Prophage, Lysogeny, DNA mutational analysis, Medical and Health Sciences, and biological effect
Summary Genetic experiments have indicated that succinoglycan (EPS I), the acidic Calcofluor-binding exopolysaccharide, of the nitrogen-fixing bacterium Rhizobium meliloti strain Rm1021 is required for nodule invasion and possibly for... more
Summary Genetic experiments have indicated that succinoglycan (EPS I), the acidic Calcofluor-binding exopolysaccharide, of the nitrogen-fixing bacterium Rhizobium meliloti strain Rm1021 is required for nodule invasion and possibly for later events in nodule development on alfalfa and other hosts. Fourteen exo loci on the second megaplasmid have been identified that are required for, or affect, the synthesis of EPS I. Mutations in certain of these loci completely abolish the production of EPS I and result in mutants that form empty Fix- nodules. We have identified two loci, exoR and exoS, that are involved in the regulation of EPS I synthesis in the free-living state. Certain exo mutations which completely abolish EPS I production are lethal in an exoR95 or exoS96 background. Histochemical analyses of the expression of exo genes during nodulation using exo :: TnphoA fusions have indicated that the exo genes are expressed most strongly in the invasion zone. In addition, we have discov...
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SummarySinorhizobium meliloti is a soil bacterium that invades the root nodules it induces on Medicago sativa, whereupon it undergoes an alteration of its cell cycle and differentiates into nitrogen‐fixing, elongated and polyploid... more
SummarySinorhizobium meliloti is a soil bacterium that invades the root nodules it induces on Medicago sativa, whereupon it undergoes an alteration of its cell cycle and differentiates into nitrogen‐fixing, elongated and polyploid bacteroid with higher membrane permeability. In Caulobacter crescentus, a related alphaproteobacterium, the principal cell cycle regulator, CtrA, is inhibited by the phosphorylated response regulator DivK. The phosphorylation of DivK depends on the histidine kinase DivJ, while PleC is the principal phosphatase for DivK. Despite the importance of the DivJ in C. crescentus, the mechanistic role of this kinase has never been elucidated in other Alphaproteobacteria. We show here that the histidine kinases DivJ together with CbrA and PleC participate in a complex phosphorylation system of the essential response regulator DivK in S. meliloti. In particular, DivJ and CbrA are involved in DivK phosphorylation and in turn CtrA inactivation, thereby controlling corr...
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Cells are constantly under threat from the cytotoxic and mutagenic effects of DNA damaging agents. These agents can either be exogenous or formed within cells. Environmental DNA-damaging agents include UV light and ionizing radiation, as... more
Cells are constantly under threat from the cytotoxic and mutagenic effects of DNA damaging agents. These agents can either be exogenous or formed within cells. Environmental DNA-damaging agents include UV light and ionizing radiation, as well as a variety of chemicals encountered in foodstuffs, or as air- and water-borne agents. Endogenous damaging agents include methylating species and the reactive oxygen species that arise during respiration. Although diverse responses are elicited in cells following DNA damage, this review focuses on three aspects: DNA repair mechanisms, cell cycle checkpoints, and apoptosis. Because the areas of nucleotide excision repair and mismatch repair have been covered extensively in recent reviews ( 1 , 2 , 3 , 4 , 5 , 6 ), we restrict our coverage of the DNA repair field to base excision repair and DNA double-strand break repair.
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The most abundant carbon source transported into legume root nodules is photosynthetically produced sucrose, yet the importance of its metabolism by rhizobia in planta is not yet known. To identify genes involved in sucrose uptake and... more
The most abundant carbon source transported into legume root nodules is photosynthetically produced sucrose, yet the importance of its metabolism by rhizobia in planta is not yet known. To identify genes involved in sucrose uptake and hydrolysis, we screened a Sinorhizobium meliloti genomic library and discovered a segment of S. meliloti DNA which allows Ralstonia eutropha to grow on the α-glucosides sucrose, maltose, and trehalose. Tn 5 mutagenesis localized the required genes to a 6.8-kb region containing five open reading frames which were named agl , for α-glucoside utilization. Four of these ( aglE , aglF , aglG , and aglK ) appear to encode a periplasmic-binding-protein-dependent sugar transport system, and one ( aglA ) appears to encode an α-glucosidase with homology to family 13 of glycosyl hydrolases. Cosmid-borne agl genes permit uptake of radiolabeled sucrose into R. eutropha cells. Analysis of the properties of agl mutants suggests that S. meliloti possesses at least one...
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Effective invasion of alfalfa by Rhizobium meliloti Rm1021 normally requires the presence of succinoglycan, an exopolysaccharide (EPS) produced by the bacterium. However, Rm1021 has the ability to produce a second EPS (EPS II) that can... more
Effective invasion of alfalfa by Rhizobium meliloti Rm1021 normally requires the presence of succinoglycan, an exopolysaccharide (EPS) produced by the bacterium. However, Rm1021 has the ability to produce a second EPS (EPS II) that can suppress the symbiotic defects of succinoglycan-deficient strains. EPS II is a polymer of modified glucose-(beta-1,3)-galactose subunits and is produced by Rm1021 derivatives carrying either an expR101 or mucR mutation. If the ability to synthesize succinoglycan is blocked genetically, expR101 derivatives of Rm1021 are nodulation-proficient, whereas mucR derivatives of Rm1021 are not. The difference in nodulation proficiency between these two classes of EPS II-producing strains is due to the specific production of a low molecular weight form of EPS II by expR101 strains. A low molecular weight EPS II fraction consisting of 15-20 EPS II disaccharide subunits efficiently allows nodule invasion by noninfective strains when present in amounts as low as 7 ...
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In this article, we describe an exploratory study of a small-scale, concept-driven, voluntary
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We have identified a DNA methyltransferase activity of the nitrogen-fixing bacterium, Rhizobium meliloti, that repairs O6-methylguanine lesions. Repair of the O6-methylguanine residue results in transfer of the methyl group to a cysteine... more
We have identified a DNA methyltransferase activity of the nitrogen-fixing bacterium, Rhizobium meliloti, that repairs O6-methylguanine lesions. Repair of the O6-methylguanine residue results in transfer of the methyl group to a cysteine residue of a 28,000-dalton protein. The O6-methyltransferase activity is expressed constitutively and R. meliloti does not exhibit an adaptive response to alkylating agents.
Research Interests: Biology, DNA repair, Medicine, DNA, Mutation, and 4 moreEscherichia coli, Methylation, Rhizobium, and Methyltransferase
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Succinoglycan, a symbiotically important exopolysaccharide of Rhizobium meliloti , is composed of polymerized octasaccharide subunits, each of which consists of one galactose and seven glucoses with succinyl, acetyl, and pyruvyl... more
Succinoglycan, a symbiotically important exopolysaccharide of Rhizobium meliloti , is composed of polymerized octasaccharide subunits, each of which consists of one galactose and seven glucoses with succinyl, acetyl, and pyruvyl modifications. Production of specific low molecular weight forms of R. meliloti exported and surface polysaccharides, including succinoglycan, appears to be important for nodule invasion. In a previous study of the roles of the various exo gene products in succinoglycan biosynthesis, exoP, exoQ, and exoT mutants were found to synthesize undecaprenol-linked fully modified succinoglycan octasaccharide subunits, suggesting possible roles for their gene products in polymerization or transport. Using improved techniques for analyzing succinoglycan biosynthesis by these mutants, we have obtained evidence indicating that R. meliloti has genetically separable systems for the synthesis of high molecular weight succinoglycan and the synthesis of a specific class of lo...
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<p><b>A</b>) Total RNA was isolated from C6706 Wt pY and the Δ<i>ybeY</i> pY strain grown in the presence of arabinose or glucose as specified and analyzed by agarose gel electrophoresis. The positions of 23... more
<p><b>A</b>) Total RNA was isolated from C6706 Wt pY and the Δ<i>ybeY</i> pY strain grown in the presence of arabinose or glucose as specified and analyzed by agarose gel electrophoresis. The positions of 23 S, 17 S, and 16 S rRNAs are indicated based on their mobility. <b>B</b>) The 5′ and 3′ termini of 16 S rRNA from C6706 Wt pY and the Δ<i>ybeY</i> pY strain were mapped as described in detail in <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004175#s4" target="_blank">Materials and Methods</a>. “P” and “M” specify the positions of bands derived from the precursor and mature forms of 16 S rRNA. <b>C</b>) Ribosome profiles for C6706 Wt pY and the Δ<i>ybeY</i> pY strain (top); quantitation of polysomes, 70 S, 50 S and 30 S ribosomes (bottom pie charts). “pY” indicates that <i>ybeY</i> is expressed from a plasmid. Ara+, cells were grown in LB in the presence of arabinose. Gluc+, cells were grown in LB in the presence of glucose. Ara+/Gluc+, intermediate YbeY depletion by switching the carbon source of the Δ<i>ybeY</i> pY strain in early exponential phase from arabinose to glucose (for details see <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004175#s4" target="_blank">Materials and Methods</a>).</p
Research Interests: Biochemistry, Genetics, Microbiology, Biofilms, Enzymology, and 15 moreBacteriology, Microbial Ecology, Medical Microbiology, Biology, Ecology, Gene expression, Microbial Physiology, Biological Sciences, Enzymes, Bacterial Physiology, Bacterial Biofilms, Gene Function, Host Pathogen Interactions, Bacterial Pathogens, and Microbial Pathogens
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closely linked to degP. degP: two loci required for symbiosis are bacA-phoA fusion results in identification of Genetic analysis of Rhizobium meliloti