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Narayan A, Naganathan AN. Switching Protein Conformational Substates by Protonation and Mutation. J Phys Chem B 2018; 122:11039-11047. [PMID: 30048131 DOI: 10.1021/acs.jpcb.8b05108] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Protein modules that regulate the availability and conformational status of transcription factors determine the rapidity, duration, and magnitude of cellular response to changing conditions. One such system is the single-gene product Cnu, a four-helix bundle transcription co-repressor, which acts as a molecular thermosensor regulating the expression of virulence genes in enterobacteriaceae through modulation of its native conformational ensemble. Cnu and related genes have also been implicated in pH-dependent expression of virulence genes. We hypothesize that protonation of a conserved buried histidine (H45) in Cnu promotes large electrostatic frustration, thus disturbing the H-NS, a transcription factor, binding face. Spectroscopic and calorimetric methods reveal that H45 exhibits a suppressed p Ka of ∼5.1, the protonation of which switches the conformation to an alternate native ensemble in which the fourth helix is disordered. The population redistribution can also be achieved through a mutation H45V, which does not display any switching behavior at pH values greater than 4. The Wako-Saitô-Muñoz-Eaton (WSME) statistical mechanical model predicts specific differences in the conformations and fluctuations of the fourth and first helices of Cnu determining the observed pH response. We validate these predictions through fluorescence lifetime measurements of a sole tryptophan, highlighting the presence of both native and non-native interactions in the regions adjoining the binding face of Cnu. Our combined experimental-computational study thus shows that Cnu acts both as a thermo- and pH-sensor orchestrated via a subtle but quantifiable balance between the weak packing of a structural element and protonation of a buried histidine that promotes electrostatic frustration.
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Affiliation(s)
- Abhishek Narayan
- Department of Biotechnology, Bhupat & Jyoti Mehta School of Biosciences , Indian Institute of Technology Madras , Chennai 600036 , India
| | - Athi N Naganathan
- Department of Biotechnology, Bhupat & Jyoti Mehta School of Biosciences , Indian Institute of Technology Madras , Chennai 600036 , India
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2
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Marimon O, Teixeira JMC, Cordeiro TN, Soo VWC, Wood TL, Mayzel M, Amata I, García J, Morera A, Gay M, Vilaseca M, Orekhov VY, Wood TK, Pons M. An oxygen-sensitive toxin-antitoxin system. Nat Commun 2016; 7:13634. [PMID: 27929062 PMCID: PMC5155140 DOI: 10.1038/ncomms13634] [Citation(s) in RCA: 38] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2016] [Accepted: 10/18/2016] [Indexed: 12/19/2022] Open
Abstract
The Hha and TomB proteins from Escherichia coli form an oxygen-dependent toxin–antitoxin (TA) system. Here we show that YmoB, the Yersinia orthologue of TomB, and its single cysteine variant [C117S]YmoB can replace TomB as antitoxins in E. coli. In contrast to other TA systems, [C117S]YmoB transiently interacts with Hha (rather than forming a stable complex) and enhances the spontaneous oxidation of the Hha conserved cysteine residue to a -SOxH-containing species (sulfenic, sulfinic or sulfonic acid), which destabilizes the toxin. The nuclear magnetic resonance structure of [C117S]YmoB and the homology model of TomB show that the two proteins form a four-helix bundle with a conserved buried cysteine connected to the exterior by a channel with a diameter comparable to that of an oxygen molecule. The Hha interaction site is located on the opposite side of the helix bundle.
Classical toxin–antitoxin systems in bacteria are based on silencing of a toxin by an antitoxin that, when inactivated, releases the toxin, resulting in a change in metabolism. Here, the authors characterize an oxygen-sensitive toxin–antitoxin system and discuss the implications for the role of the Hha antitoxin.
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Affiliation(s)
- Oriol Marimon
- Biomolecular NMR Laboratory, Organic Chemistry Section, Inorganic and Organic Chemistry Department, University of Barcelona, Baldiri Reixac 10-12, Barcelona 08028, Spain
| | - João M C Teixeira
- Biomolecular NMR Laboratory, Organic Chemistry Section, Inorganic and Organic Chemistry Department, University of Barcelona, Baldiri Reixac 10-12, Barcelona 08028, Spain
| | - Tiago N Cordeiro
- Biomolecular NMR Laboratory, Organic Chemistry Section, Inorganic and Organic Chemistry Department, University of Barcelona, Baldiri Reixac 10-12, Barcelona 08028, Spain
| | - Valerie W C Soo
- Department of Chemical Engineering and Department of Biochemistry and Molecular Biology, Pennsylvania State University, University Park, Pennsylvania 16802, USA
| | - Thammajun L Wood
- Department of Chemical Engineering and Department of Biochemistry and Molecular Biology, Pennsylvania State University, University Park, Pennsylvania 16802, USA
| | - Maxim Mayzel
- Swedish NMR Centre, Gothenburg University, PO Box 465, Gothenburg SE-40530, Sweden
| | - Irene Amata
- Biomolecular NMR Laboratory, Organic Chemistry Section, Inorganic and Organic Chemistry Department, University of Barcelona, Baldiri Reixac 10-12, Barcelona 08028, Spain
| | - Jesús García
- Biomolecular NMR Laboratory, Organic Chemistry Section, Inorganic and Organic Chemistry Department, University of Barcelona, Baldiri Reixac 10-12, Barcelona 08028, Spain.,Institute for Research in Biomedicine (IRB-Barcelona), The Barcelona Institute of Science and Technology, Baldiri Reixac 10-12, Barcelona 08028, Spain
| | - Ainara Morera
- Biomolecular NMR Laboratory, Organic Chemistry Section, Inorganic and Organic Chemistry Department, University of Barcelona, Baldiri Reixac 10-12, Barcelona 08028, Spain
| | - Marina Gay
- Institute for Research in Biomedicine (IRB-Barcelona), The Barcelona Institute of Science and Technology, Baldiri Reixac 10-12, Barcelona 08028, Spain
| | - Marta Vilaseca
- Institute for Research in Biomedicine (IRB-Barcelona), The Barcelona Institute of Science and Technology, Baldiri Reixac 10-12, Barcelona 08028, Spain
| | - Vladislav Yu Orekhov
- Swedish NMR Centre, Gothenburg University, PO Box 465, Gothenburg SE-40530, Sweden
| | - Thomas K Wood
- Department of Chemical Engineering and Department of Biochemistry and Molecular Biology, Pennsylvania State University, University Park, Pennsylvania 16802, USA
| | - Miquel Pons
- Biomolecular NMR Laboratory, Organic Chemistry Section, Inorganic and Organic Chemistry Department, University of Barcelona, Baldiri Reixac 10-12, Barcelona 08028, Spain
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Jaiswal S, Paul P, Padhi C, Ray S, Ryan D, Dash S, Suar M. The Hha-TomB Toxin-Antitoxin System Shows Conditional Toxicity and Promotes Persister Cell Formation by Inhibiting Apoptosis-Like Death in S. Typhimurium. Sci Rep 2016; 6:38204. [PMID: 27910884 PMCID: PMC5133643 DOI: 10.1038/srep38204] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2016] [Accepted: 06/03/2016] [Indexed: 11/09/2022] Open
Abstract
Toxin-antitoxin (TA) modules are two component “addictive” genetic elements found on either plasmid or bacterial chromosome, sometimes on both. TA systems perform a wide range of functions like biofilm formation, persistence, programmed cell death, phage abortive infection etc. Salmonella has been reported to contain several such TA systems. However, the hemolysin expression modulating protein (Hha) and its adjacent uncharacterized hypothetical protein TomB (previously known as YbaJ), have not been listed as a TA module in Salmonella. In this study we established that Hha and TomB form a bonafide TA system where Hha serves as a toxin while TomB functions as an antitoxin. Interestingly, the toxicity of Hha was conditional causing cell death under acid stress. The antitoxin attenuated the toxicity of Hha by forming a TA complex through stable interactions. The Hha-TomB TA system was found to increase persistence and inhibit programmed cell death under antibiotic stress where a phenotypically diverse population expressing differential level of TA components was observed. Therefore we propose that Hha and TomB prevent cells from committing suicide thereby promoting persister cell formation.
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Affiliation(s)
- Sangeeta Jaiswal
- School of Biotechnology, KIIT University, Bhubaneswar-751024, Odisha, India
| | - Prajita Paul
- School of Biotechnology, KIIT University, Bhubaneswar-751024, Odisha, India
| | | | - Shilpa Ray
- School of Biotechnology, KIIT University, Bhubaneswar-751024, Odisha, India
| | - Daniel Ryan
- School of Biotechnology, KIIT University, Bhubaneswar-751024, Odisha, India
| | - Shantoshini Dash
- School of Biotechnology, KIIT University, Bhubaneswar-751024, Odisha, India
| | - Mrutyunjay Suar
- School of Biotechnology, KIIT University, Bhubaneswar-751024, Odisha, India
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Abstract
The H-NS family of DNA-binding proteins is the subject of intense study due to its important roles in the regulation of horizontally acquired genes critical for virulence, antibiotic resistance, and metabolism. Xenogeneic silencing proteins, typified by the H-NS protein of Escherichia coli, specifically target and downregulate expression from AT-rich genes by selectively recognizing specific structural features unique to the AT-rich minor groove. In doing so, these proteins facilitate bacterial evolution; enabling these cells to engage in horizontal gene transfer while buffering potential any detrimental fitness consequences that may result from it. Xenogeneic silencing and counter-silencing explain how bacterial cells can evolve effective gene regulatory strategies in the face of rampant gene gain and loss and it has extended our understanding of bacterial gene regulation beyond the classic operon model. Here we review the structures and mechanisms of xenogeneic silencers as well as their impact on bacterial evolution. Several H-NS-like proteins appear to play a role in facilitating gene transfer by other mechanisms including by regulating transposition, conjugation, and participating in the activation of virulence loci like the locus of enterocyte effacement pathogenicity island of pathogenic strains of E. coli. Evidence suggests that the critical determinants that dictate whether an H-NS-like protein will be a silencer or will perform a different function do not lie in the DNA-binding domain but, rather, in the domains that control oligomerization. This suggests that H-NS-like proteins are transcription factors that both recognize and alter the shape of DNA to exert specific effects that include but are not limited to gene silencing.
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Solórzano C, Srikumar S, Canals R, Juárez A, Paytubi S, Madrid C. Hha has a defined regulatory role that is not dependent upon H-NS or StpA. Front Microbiol 2015; 6:773. [PMID: 26284052 PMCID: PMC4519777 DOI: 10.3389/fmicb.2015.00773] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2015] [Accepted: 07/14/2015] [Indexed: 11/13/2022] Open
Abstract
The Hha family of proteins is involved in the regulation of gene expression in enterobacteria by forming complexes with H-NS-like proteins. Whereas several amino acid residues of both proteins participate in the interaction, some of them play a key role. Residue D48 of Hha protein is essential for the interaction with H-NS, thus the D48N substitution in Hha protein abrogates H-NS/Hha interaction. Despite being a paralog of H-NS protein, StpA interacts with HhaD48N with higher affinity than with the wild type Hha protein. To analyze whether Hha is capable of acting independently of H-NS and StpA, we conducted transcriptomic analysis on the hha and stpA deletion strains and the hhaD48N substitution strain of Salmonella Typhimurium using a custom microarray. The results obtained allowed the identification of 120 genes regulated by Hha in an H-NS/StpA-independent manner, 38% of which are horizontally acquired genes. A significant number of the identified genes are involved in functions related to cell motility, iron uptake, and pathogenicity. Thus, motility assays, siderophore detection and intra-macrophage replication assays were performed to confirm the transcriptomic data. Our findings point out the importance of Hha protein as an independent regulator in S. Typhimurium, highlighting a regulatory role on virulence.
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Affiliation(s)
- Carla Solórzano
- Departament de Microbiologia, Universitat de Barcelona Barcelona, Spain
| | | | - Rocío Canals
- Institute of Integrative Biology, University of Liverpool Liverpool, UK
| | - Antonio Juárez
- Departament de Microbiologia, Universitat de Barcelona Barcelona, Spain ; Institut de Bioenginyeria de Catalunya, Parc Científic de Barcelona Barcelona, Spain
| | - Sonia Paytubi
- Departament de Microbiologia, Universitat de Barcelona Barcelona, Spain
| | - Cristina Madrid
- Departament de Microbiologia, Universitat de Barcelona Barcelona, Spain
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Cordeiro TN, García J, Bernadó P, Millet O, Pons M. A Three-protein Charge Zipper Stabilizes a Complex Modulating Bacterial Gene Silencing. J Biol Chem 2015; 290:21200-12. [PMID: 26085102 DOI: 10.1074/jbc.m114.630400] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2014] [Indexed: 12/31/2022] Open
Abstract
The Hha/YmoA nucleoid-associated proteins help selectively silence horizontally acquired genetic material, including pathogenicity and antibiotic resistance genes and their maintenance in the absence of selective pressure. Members of the Hha family contribute to gene silencing by binding to the N-terminal dimerization domain of H-NS and modifying its selectivity. Hha-like proteins and the H-NS N-terminal domain are unusually rich in charged residues, and their interaction is mostly electrostatic-driven but, nonetheless, highly selective. The NMR-based structural model of the complex between Hha/YmoA and the H-NS N-terminal dimerization domain reveals that the origin of the selectivity is the formation of a three-protein charge zipper with interdigitated complementary charged residues from Hha and the two units of the H-NS dimer. The free form of YmoA shows collective microsecond-millisecond dynamics that can by measured by NMR relaxation dispersion experiments and shows a linear dependence with the salt concentration. The number of residues sensing the collective dynamics and the population of the minor form increased in the presence of H-NS. Additionally, a single residue mutation in YmoA (D43N) abolished H-NS binding and the dynamics of the apo-form, suggesting the dynamics and binding are functionally related.
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Affiliation(s)
- Tiago N Cordeiro
- From the Biomolecular NMR Laboratory, Department of Organic Chemistry, University of Barcelona, 08028 Barcelona, Spain, Centre de Biochimie Structurale, INSERM U1054, CNRS UMR 5048, Université Montpellier 1 and 2, 34092 Montpellier, France
| | - Jesús García
- Institute for Research in Biomedicine (IRB-Barcelona), 08028 Barcelona, Spain, and
| | - Pau Bernadó
- Centre de Biochimie Structurale, INSERM U1054, CNRS UMR 5048, Université Montpellier 1 and 2, 34092 Montpellier, France
| | - Oscar Millet
- the Structural Biology Unit, Center for Cooperative Research in Biosciences (CIC-bioGUNE), 48160 Elexalde, Derio, Spain
| | - Miquel Pons
- From the Biomolecular NMR Laboratory, Department of Organic Chemistry, University of Barcelona, 08028 Barcelona, Spain,
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Wang H, Yehoshua S, Ali SS, Navarre WW, Milstein JN. A biomechanical mechanism for initiating DNA packaging. Nucleic Acids Res 2014; 42:11921-7. [PMID: 25274732 PMCID: PMC4231757 DOI: 10.1093/nar/gku896] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/03/2022] Open
Abstract
The bacterial chromosome is under varying levels of mechanical stress due to a high degree of crowding and dynamic protein–DNA interactions experienced within the nucleoid. DNA tension is difficult to measure in cells and its functional significance remains unclear although in vitro experiments have implicated a range of biomechanical phenomena. Using single-molecule tools, we have uncovered a novel protein–DNA interaction that responds to fluctuations in mechanical tension by condensing DNA. We combined tethered particle motion (TPM) and optical tweezers experiments to probe the effects of tension on DNA in the presence of the Hha/H-NS complex. The nucleoid structuring protein H-NS is a key regulator of DNA condensation and gene expression in enterobacteria and its activity in vivo is affected by the accessory factor Hha. We find that tension, induced by optical tweezers, causes the rapid compaction of DNA in the presence of the Hha/H-NS complex, but not in the presence of H-NS alone. Our results imply that H-NS requires Hha to condense bacterial DNA and that this condensation could be triggered by the level of mechanical tension experienced along different regions of the chromosome.
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Affiliation(s)
- Haowei Wang
- Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada
| | - Samuel Yehoshua
- Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada Department of Physics, University of Toronto, Toronto, ON M5S 1A7, Canada
| | - Sabrina S Ali
- Department of Molecular Genetics, University of Toronto, Toronto, ON M5S 1A8, Canada
| | - William Wiley Navarre
- Department of Molecular Genetics, University of Toronto, Toronto, ON M5S 1A8, Canada
| | - Joshua N Milstein
- Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada Department of Physics, University of Toronto, Toronto, ON M5S 1A7, Canada
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Ali SS, Whitney JC, Stevenson J, Robinson H, Howell PL, Navarre WW. Structural insights into the regulation of foreign genes in Salmonella by the Hha/H-NS complex. J Biol Chem 2013; 288:13356-69. [PMID: 23515315 DOI: 10.1074/jbc.m113.455378] [Citation(s) in RCA: 51] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
BACKGROUND Hha facilitates H-NS-mediated silencing of foreign genes in bacteria. RESULTS Two Hha monomers bind opposing faces of the H-NS N-terminal dimerization domain. CONCLUSION Hha binds the dimerization domain of H-NS and may contact DNA via positively charged surface residues. SIGNIFICANCE The structure of Hha and H-NS in complex provides a mechanistic model of how Hha may affect gene regulation. The bacterial nucleoid-associated proteins Hha and H-NS jointly repress horizontally acquired genes in Salmonella, including essential virulence loci encoded within Salmonella pathogenicity islands. Hha is known to interact with the N-terminal dimerization domain of H-NS; however, the manner in which this interaction enhances transcriptional silencing is not understood. To further understand this process, we solved the x-ray crystal structure of Hha in complex with the N-terminal dimerization domain of H-NS (H-NS(1-46)) to 3.2 Å resolution. Two monomers of Hha bind to symmetrical sites on either side of the H-NS(1-46) dimer. Disruption of the Hha/H-NS interaction by the H-NS site-specific mutation I11A results in increased expression of the Hha/H-NS co-regulated gene hilA without affecting the expression levels of proV, a target gene repressed by H-NS in an Hha-independent fashion. Examination of the structure revealed a cluster of conserved basic amino acids that protrude from the surface of Hha on the opposite side of the Hha/H-NS(1-46) interface. Hha mutants with a diminished positively charged surface maintain the ability to interact with H-NS but can no longer regulate hilA. Increased expression of the hilA locus did not correspond to significant depletion of H-NS at the promoter region in chromatin immunoprecipitation assays. However, in vitro, we find Hha improves H-NS binding to target DNA fragments. Taken together, our results show for the first time how Hha and H-NS interact to direct transcriptional repression and reveal that a positively charged surface of Hha enhances the silencing activity of H-NS nucleoprotein filaments.
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Affiliation(s)
- Sabrina S Ali
- Departments of Molecular Genetics, University of Toronto, Toronto, Ontario M5S 1A8, Canada
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Cytosporone B, an inhibitor of the type III secretion system of Salmonella enterica serovar Typhimurium. Antimicrob Agents Chemother 2013; 57:2191-8. [PMID: 23459474 DOI: 10.1128/aac.02421-12] [Citation(s) in RCA: 60] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Bacterial virulence factors have been increasingly regarded as attractive targets for development of novel antibacterial agents. Virulence inhibitors are less likely to generate bacterial resistance, which makes them superior to traditional antibiotics that target bacterial viability. Salmonella enterica serovar Typhimurium, an important food-borne human pathogen, has type III secretion system (T3SS) as its major virulence factor. T3SS secretes effector proteins to facilitate invasion into host cells. In this study, we identified several analogs of cytosporone B (Csn-B) that strongly block the secretion of Salmonella pathogenicity island 1 (SPI-1)-associated effector proteins, without affecting the secretion of flagellar protein FliC in vitro. Csn-B and two other derivatives exhibited a strong inhibitory effect on SPI-1-mediated invasion to HeLa cells, while no significant toxicity to bacteria was observed. Nucleoid proteins Hha and H-NS bind to the promoters of SPI-1 regulator genes hilD, hilC, and rtsA to repress their expression and consequently regulate the expression of SPI-1 apparatus and effector genes. We found that Csn-B upregulated the transcription of hha and hns, implying that Csn-B probably affected the secretion of effectors through the Hha-H-NS regulatory pathway. In summary, this study presented an effective SPI-1 inhibitor, Csn-B, which may have potential in drug development against antibiotic-resistant Salmonella.
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Yun SH, Ji SC, Jeon HJ, Wang X, Lee Y, Choi BS, Lim HM. A mutational study of Cnu reveals attractive forces between Cnu and H-NS. Mol Cells 2012; 33:211-6. [PMID: 22358512 PMCID: PMC3887714 DOI: 10.1007/s10059-012-0006-5] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2012] [Accepted: 01/09/2012] [Indexed: 10/28/2022] Open
Abstract
Cnu is a small 71-amino acid protein that complexes with H-NS and binds to a specific sequence in the replication origin of the E. coli chromosome. To understand the mechanism of interaction between Cnu and H-NS, we used bacterial genetics to select and analyze Cnu variants that cannot complex with H-NS. Out of 2,000 colonies, 40 Cnu variants were identified. Most variants (82.5%) had a single mutation, but a few variants (17.5%) had double amino acid changes. An in vitro assay was used to identify Cnu variants that were truly defective in H-NS binding. The changes in these defective variants occurred exclusively at charged amino acids (Asp, Glu, or Lys) on the surface of the protein. We propose that the attractive force that governs the Cnu-H-NS interaction is an ionic bond, unlike the hydrophobic interaction that is the major attractive force in most proteins.
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Affiliation(s)
- Sang Hoon Yun
- Department of Biology, College of Biological Sciences and Biotechnology, Chungnam National University, Daejeon 305-764,
Korea
| | - Sang Chun Ji
- Department of Biology, College of Biological Sciences and Biotechnology, Chungnam National University, Daejeon 305-764,
Korea
- Present address: Department of Pharmacology and Clinical Pharmacology, Seoul National University College of Medicine and Hospital, Seoul 110-799,
Korea
| | - Heung Jin Jeon
- Department of Biology, College of Biological Sciences and Biotechnology, Chungnam National University, Daejeon 305-764,
Korea
| | - Xun Wang
- Department of Biology, College of Biological Sciences and Biotechnology, Chungnam National University, Daejeon 305-764,
Korea
| | - Younghoon Lee
- Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 305-701,
Korea
| | - Byong-Seok Choi
- Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 305-701,
Korea
| | - Heon M. Lim
- Department of Biology, College of Biological Sciences and Biotechnology, Chungnam National University, Daejeon 305-764,
Korea
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