DNA Repair Enzyme
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17 results
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Direct DNA Lesion Reversal and Excision Repair in Escherichia coli
- Authors: Sophie Couvé, Alexander A. Ishchenko, Olga S. Fedorova, Erlan M. Ramanculov, Jacques Laval, and Murat Saparbaev
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Citation: Couvé S, Ishchenko A, Fedorova O, Ramanculov E, Laval J, Saparbaev M. 2013. Direct DNA Lesion Reversal and Excision Repair in Escherichia coli, EcoSal Plus 2013; doi:10.1128/ecosalplus.7.2.4
- DOI 10.1128/ecosalplus.7.2.4
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Abstract:
Cellular DNA is constantly challenged by various endogenous and exogenous genotoxic factors that inevitably lead to DNA damage: structural and chemical modifications of primary DNA sequence. These DNA lesions are either cytotoxic, because they block DNA replication and transcription, or mutagenic due to the miscoding nature of the DNA modifications, or both, and are believed to contribute to cell lethality and mutagenesis. Studies on DNA repair in Escherichia coli spearheaded formulation of principal strategies to counteract DNA damage and mutagenesis, such as: direct lesion reversal, DNA excision repair, mismatch and recombinational repair and genotoxic stress signalling pathways. These DNA repair pathways are universal among cellular organisms. Mechanistic principles used for each repair strategies are fundamentally different. Direct lesion reversal removes DNA damage without need for excision and de novo DNA synthesis, whereas DNA excision repair that includes pathways such as base excision, nucleotide excision, alternative excision and mismatch repair, proceeds through phosphodiester bond breakage, de novo DNA synthesis and ligation. Cell signalling systems, such as adaptive and oxidative stress responses, although not DNA repair pathways per se, are nevertheless essential to counteract DNA damage and mutagenesis. The present review focuses on the nature of DNA damage, direct lesion reversal, DNA excision repair pathways and adaptive and oxidative stress responses in E. coli.
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Translesion DNA Synthesis
- Authors: Alexandra Vaisman, John P. McDonald, and Roger Woodgate
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Citation: Vaisman A, McDonald J, Woodgate R. 2012. Translesion DNA Synthesis, EcoSal Plus 2012; doi:10.1128/ecosalplus.7.2.2
- DOI 10.1128/ecosalplus.7.2.2
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Abstract:
All living organisms are continually exposed to agents that damage their DNA, which threatens the integrity of their genome. As a consequence, cells are equipped with a plethora of DNA repair enzymes to remove the damaged DNA. Unfortunately, situations nevertheless arise where lesions persist, and these lesions block the progression of the cell's replicase. In these situations, cells are forced to choose between recombination-mediated "damage avoidance" pathways or a specialized DNA polymerase (pol) to traverse the blocking lesion. The latter process is referred to as Translesion DNA Synthesis (TLS). As inferred by its name, TLS not only results in bases being (mis)incorporated opposite DNA lesions but also bases being (mis)incorporated downstream of the replicase-blocking lesion, so as to ensure continued genome duplication and cell survival. Escherichia coli and Salmonella typhimurium possess five DNA polymerases, and while all have been shown to facilitate TLS under certain experimental conditions, it is clear that the LexA-regulated and damage-inducible pols II, IV, and V perform the vast majority of TLS under physiological conditions. Pol V can traverse a wide range of DNA lesions and performs the bulk of mutagenic TLS, whereas pol II and pol IV appear to be more specialized TLS polymerases.
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The DNA Exonucleases of Escherichia coli
- Author: Susan T. Lovett
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Citation: Lovett S. 2011. The DNA Exonucleases of Escherichia coli, EcoSal Plus 2011; doi:10.1128/ecosalplus.4.4.7
- DOI 10.1128/ecosalplus.4.4.7
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Abstract:
DNA exonucleases, enzymes that hydrolyze phosphodiester bonds in DNA from a free end, play important cellular roles in DNA repair, genetic recombination and mutation avoidance in all organisms. This article reviews the structure, biochemistry, and biological functions of the 17 exonucleases currently identified in the bacterium Escherichia coli. These include the exonucleases associated with DNA polymerases I (polA), II (polB), and III (dnaQ/mutD); Exonucleases I (xonA/sbcB), III (xthA), IV, VII (xseAB), IX (xni/xgdG), and X (exoX); the RecBCD, RecJ, and RecE exonucleases; SbcCD endo/exonucleases; the DNA exonuclease activities of RNase T (rnt) and Endonuclease IV (nfo); and TatD. These enzymes are diverse in terms of substrate specificity and biochemical properties and have specialized biological roles. Most of these enzymes fall into structural families with characteristic sequence motifs, and members of many of these families can be found in all domains of life.
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Stationary-Phase Gene Regulation in Escherichia coli §
- Author: Regine Hengge
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Citation: Hengge R. 2011. Stationary-Phase Gene Regulation in Escherichia coli §, EcoSal Plus 2011; doi:10.1128/ecosalplus.5.6.3
- DOI 10.1128/ecosalplus.5.6.3
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Abstract:
In their stressful natural environments, bacteria often are in stationary phase and use their limited resources for maintenance and stress survival. Underlying this activity is the general stress response, which in Escherichia coli depends on the σS (RpoS) subunit of RNA polymerase. σS is closely related to the vegetative sigma factor σ70 (RpoD), and these two sigmas recognize similar but not identical promoter sequences. During the postexponential phase and entry into stationary phase, σS is induced by a fine-tuned combination of transcriptional, translational, and proteolytic control. In addition, regulatory "short-cuts" to high cellular σS levels, which mainly rely on the rapid inhibition of σS proteolysis, are triggered by sudden starvation for various nutrients and other stressful shift conditons. σS directly or indirectly activates more than 500 genes. Additional signal input is integrated by σS cooperating with various transcription factors in complex cascades and feedforward loops. Target gene products have stress-protective functions, redirect metabolism, affect cell envelope and cell shape, are involved in biofilm formation or pathogenesis, or can increased stationary phase and stress-induced mutagenesis. This review summarizes these diverse functions and the amazingly complex regulation of σS. At the molecular level, these processes are integrated with the partitioning of global transcription space by sigma factor competition for RNA polymerase core enzyme and signaling by nucleotide second messengers that include cAMP, (p)ppGpp, and c-di-GMP. Physiologically, σS is the key player in choosing between a lifestyle associated with postexponential growth based on nutrient scavenging and motility and a lifestyle focused on maintenance, strong stress resistance, and increased adhesiveness. Finally, research with other proteobacteria is beginning to reveal how evolution has further adapted function and regulation of σS to specific environmental niches.
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Sensing and Responding to Reactive Oxygen and Nitrogen Species
- Authors: Gisela Storz, Stephen Spiro
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Source: Bacterial Stress Responses, Second Edition , pp 157-173
Publication Date :
January 2011
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This chapter provides an overview of what is known about the regulation and defenses against reactive oxygen species and nitrogen species. In Escherichia coli and a subset of enteric bacteria, SoxR only controls the expression of SoxS, an AraC-type transcription regulator. Structural and spectral studies of oxidized SoxR bound to DNA show that the activated protein introduces a significant distortion into the DNA in order to activate transcription, as has been observed for other MerR family members. In Salmonella enterica and E. coli, OxyR regulates numerous genes whose expression is induced by exposure to H2O2 and that make sense in terms of a defense against peroxides, including genes encoding catalase and the AhpC peroxiredoxin together with the AhpF reductase, the iron-sequestering protein Dps, and the disulfide bond reducing enzymes thioredoxin and glutaredoxin. In the denitrifying organism Ralstonia eutropha, the single subunit quinol-oxidizing respiratory free radical nitric oxide (NO) reductase is encoded by the norB gene, which is co-transcribed with norA under the control of the transcription activator encoded by the divergently transcribed norR gene. In higher eukaryotes, the major receptor for NO is the soluble guanylate cyclase (sGC). DosS (DevS) and DosT are paralogous histidine kinases from Mycobacterium tuberculosis that each contain a heme cofactor bound to an N-terminal GAF domain. In all cases, the redox-active centers contain reactive metals or cysteine or histidine residues, although the types of modifications that occur are surprisingly varied.
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The General Stress Response in Alphaproteobacteria
- Authors: Anne Francez-Charlot, Julia Frunzke, Julia A. Vorholt
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Source: Bacterial Stress Responses, Second Edition , pp 291-300
Publication Date :
January 2011
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Abstract:
Extensive studies with the model organisms Escherichia coli and Bacillus subtilis have led to the identification of two regulatory networks controlling general stress response. Genes encoding histidine kinases are found in the vicinity of phyR in several Alphaproteobacteria. However, the involvement of one of these histidine kinases in the PhyR cascade has not been demonstrated so far. Francez-Charlot et al. proposed that σEcfG or several members of the σEcfG family are responsible for transcription of stress-related genes. In several Alphaproteobacteria, a gene encoding a histidine kinase is found at the phyR locus. Signal perception in the σS and σB regulatory cascades has been described as highly complex, as a result of the necessity to integrate multiple signals; such complexity can be expected in Alphaproteobacteria too and may reflect an adaptation to the various environments in which the organisms live. In Rhizobium etli, the RpoE2/ σT/σEcfG homolog, RpoE4, was shown to be involved in oxidative and osmotic stresses. Genes encoding catalase, the DNA protection protein Dps or DNA repair enzymes, known to be crucial for σS-dependent resistance to oxidative stress, are controlled by the cascade in several species. All regulons contain several regulators whose functions are mainly unknown, such as kinases and response regulators of His-Asp phosphorelays, onecomponent systems, and sigma factors, such as RpoH.
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Sensing Extracellular Signals in Cryptococcus neoformans
- Authors: Alexander Idnurm, Yong-Sun Bahn, Wei-Chiang Shen, Julian C. Rutherford, Fritz A. Mühlschlegel
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Source: Cryptococcus , pp 175-187
Publication Date :
January 2011
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Abstract:
The chapter describes the abilities of Cryptococcus neoformans to sense and adapt to a subset of environmental conditions it encounters. The mechanisms for sensing extracellular signals such as stress, light, nitrogen, carbon dioxide, and oxygen are emerging. There are several future paths toward understanding the light responses of C. neoformans. First, the functions of the opsin and phytochrome in light sensing, if any, are currently unknown. Second, the conformational effects of light on the white collar complex are also unknown in C. neoformans or in detail from any fungal system since the proteins are notoriously difficult to purify in abundance for structural and other studies. Third, the genes that are regulated by light remain to be fully elucidated, although this is being addressed through transcript profiling microarray and genetic screens, particularly since these genes should control C. neoformans ability to proliferate in the wild and cause disease in humans. The current model of the CO2-sensing system in C. neoformans suggests that under limiting concentrations this molecule diffuses into the cell and is subsequently hydrated to HCO3 - and fixed inside the cell by the CA Can2p. Analysis of deletion mutants identified two genes in addition to SRE1 and SCP1 that were required for full growth under hypoxic conditions. It is of interest to note that mutation of SRE1 and SCP1 leads to an increased sensitivity to ROS, which led Ingavale et al. to propose that there is a link in oxygen sensing between hypoxia, mitochondrial function, and ROS generation.
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Oxidative Stress
- Author: James A. Imlay
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Citation: Imlay J. 2009. Oxidative Stress, EcoSal Plus 2009; doi:10.1128/ecosalplus.5.4.4
- DOI 10.1128/ecosalplus.5.4.4
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The ancestors of Escherichia coli and Salmonella ultimately evolved to thrive in air-saturated liquids, in which oxygen levels reach 210 μM at 37°C. However, in 1976 Brown and colleagues reported that some sensitivity persists: growth defects still become apparent when hyperoxia is imposed on cultures of E. coli. This residual vulnerability was important in that it raised the prospect that normal levels of oxygen might also injure bacteria, albeit at reduced rates that are not overtly toxic. The intent of this article is both to describe the threat that molecular oxygen poses for bacteria and to detail what we currently understand about the strategies by which E. coli and Salmonella defend themselves against it. E. coli mutants that lack either superoxide dismutases or catalases and peroxidases exhibit a variety of growth defects. These phenotypes constitute the best evidence that aerobic cells continually generate intracellular superoxide and hydrogen peroxide at potentially lethal doses. Superoxide has reduction potentials that allow it to serve in vitro as either a weak univalent reductant or a stronger univalent oxidant. The addition of micromolar hydrogen peroxide to lab media will immediately block the growth of most cells, and protracted exposure will result in the loss of viability. The need for inducible antioxidant systems seems especially obvious for enteric bacteria, which move quickly from the anaerobic gut to fully aerobic surface waters or even to ROS-perfused phagolysosomes. E. coli and Salmonella have provided two paradigmatic models of oxidative-stress responses: the SoxRS and OxyR systems.
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Respiration of Nitrate and Nitrite
- Authors: Jeffrey A. Cole, and David J. Richardson
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Citation: Cole J, Richardson D. 2008. Respiration of Nitrate and Nitrite, EcoSal Plus 2008; doi:10.1128/ecosal.3.2.5
- DOI 10.1128/ecosal.3.2.5
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Abstract:
Nitrate reduction to ammonia via nitrite occurs widely as an anabolic process through which bacteria, archaea, and plants can assimilate nitrate into cellular biomass. Escherichia coli and related enteric bacteria can couple the eight-electron reduction of nitrate to ammonium to growth by coupling the nitrate and nitrite reductases involved to energy-conserving respiratory electron transport systems. In global terms, the respiratory reduction of nitrate to ammonium dominates nitrate and nitrite reduction in many electron-rich environments such as anoxic marine sediments and sulfide-rich thermal vents, the human gastrointestinal tract, and the bodies of warm-blooded animals. This review reviews the regulation and enzymology of this process in E. coli and, where relevant detail is available, also in Salmonella and draws comparisons with and implications for the process in other bacteria where it is pertinent to do so. Fatty acids may be present in high levels in many of the natural environments of E. coli and Salmonella in which oxygen is limited but nitrate is available to support respiration. In E. coli, nitrate reduction in the periplasm involves the products of two seven-gene operons, napFDAGHBC, encoding the periplasmic nitrate reductase, and nrfABCDEFG, encoding the periplasmic nitrite reductase. No bacterium has yet been shown to couple a periplasmic nitrate reductase solely to the cytoplasmic nitrite reductase NirB. The cytoplasmic pathway for nitrate reduction to ammonia is restricted almost exclusively to a few groups of facultative anaerobic bacteria that encounter high concentrations of environmental nitrate.
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Reversal of Base Damage Caused by UV Radiation
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Source: DNA Repair and Mutagenesis, Second Edition , pp 109-138
Publication Date :
January 2006
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This chapter considers several examples of the direct reversal of DNA damage, all of which are catalyzed by single polypeptide enzymes. Both cyclobutane pyrimidine dimers (CPD) and (6-4) pyrimidine-pyrimidone photoproducts [(6-4)PP] constitute quantitatively and qualitatively important sources of base damage following the exposure of cells to UV radiation at wavelengths near the absorption maximum of DNA. The chapter discusses a specific DNA repair mode called enzymatic photoreactivation (EPR), or simply photoreactivation. Enzymes that catalyze EPR of CPD in DNA are referred to as pyrimidine dimer-DNA photolyases (PD-DNA photolyase), pyrimidine dimerdeoxyribodipyrimidine photolyases, or pyrimidine dimerphotoreactivating enzymes. The prefix “pyrimidine dimer” is added to distinguish these enzymes from those that catalyze the repair of (6-4)PP by an essentially identical mechanism. These are called (6-4) photoproduct-DNA photolyases ((6-4)PP-DNA photolyase). The chapter describes distribution, properties, structural studies and mechanism of action of PD-DNA photolyase. It also describes PD-DNA photolyase from other organisms and its therapeutic use for protection against sunlight. Next, it explains mechanism of action and C-terminal region of (6-4)PP-DNA photolyase. In addition, the phylogenetic relationships between PD-DNA photolyases, (6-4)PP-DNA photolyases, and blue-light receptor proteins have been suggested, and the following primary subfamilies of proteins have been defined: (i) PD-DNA photolyase proteins; (ii) plant CRY and plant and animal (6-4)PP-DNA photolyase proteins; and (iii) animal CRY proteins. Other covered topics are repair of thymine dimers by a deoxyribozyme, photoreactivation of RNA, and reversal of spore photoproduct in DNA.
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Repair of Mitochondrial DNA Damage
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Source: DNA Repair and Mutagenesis, Second Edition , pp 449-459
Publication Date :
January 2006
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The cells of higher eukaryotes typically contain several thousand copies of mitochondrial DNA (mtDNA). This chapter first talks about mitochondrial mutagenesis and DNA damage in the mitochondrial genome. Several factors can facilitate preferential mtDNA damage. While a number of early studies provided suggestive hints about mtDNA repair, the first definitive study demonstrating active base excision repair (BER) in mitochondria in mammalian cells documented the formation and repair of N-methylpurines in an insulinoma cell line exposed to the naturally occurring nitrosamine streptozotocin. The chapter discusses loss of specific types of base damage from mtDNA and repair of oxidative damage. Other covered topics are enzymes for BER repair in mitochondrial extracts, short-patch BER of mitochondrial DNA, age-related studies of mitochondrial DNA repair, alternative excision repair pathway, and recombinational repair in mtDNA. The determination of the number and type of distinct DNA repair pathways that operate in mtDNA in mammalian cells remains an important challenge and that results obtained with lower eukaryotes cannot be extrapolated to higher organisms.
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Dr. Jekyll and Mr. Hyde: How the MutSLH Repair System Kills the Cell
- Author: M. G. Marinus
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Source: The Bacterial Chromosome , pp 413-430
Publication Date :
January 2005
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Abstract:
This chapter concentrates on the repair mechanism in Escherichia coli, but the lessons learned in this organism should also apply to analogous systems in other organisms. Although there are several distinct DNA mismatch repair systems, in this chapter the term is used to denote the MutSLH system. DNA polymerase III, the replicative enzyme, catalyzes resynthesis of nucleotides and ligation followed by Dam methylation to complete the process. An alternative to the futile cycling model based on double-strand DNA breaks (DSBs) recombinational repair is described in the chapter to explain how mismatch repair sensitizes E. coli dam mutants (and human cells) to methylating agents and cisplatin. In dam mutants there is constant repair of DSBs, and the recombinational capacity of the cell is probably near its maximum. This conclusion is based on the higher basal level of transcription of certain SOS genes in dam cells, suggesting that one or more of the RecA or RuvA or RuvB proteins is limiting. The hypothesis that dam bacteria are sensitive to these agents because of inability to repair all DSBs is quite plausible. An important common theme is the requirement for replication forks to stall or collapse at lesions. The hyperrecombination phenotype is explained by the increased number of DSBs leading to increased initiation of recombination. Together with the roles for Dam methylation in controlling transcription initiation and its role in regulating initiation of chromosome replication and its synchronization, almost all the phenotypic properties can now be explained.
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Reactive Oxygen and Reactive Nitrogen Metabolites as Effector Molecules against Infectious Pathogens
- Author: Christian Bogdan
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Source: The Innate Immune Response to Infection , pp 357-396
Publication Date :
January 2004
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This chapter focuses on the sources, the regulation, the spectrum of activities, and the viral and microbial targets of reactive oxygen intermediates (ROIs) and reactive nitrogen intermediates (RNIs) generated by mammalian host cells. With respect to the control of infectious agents, the two most important oxygen-dependent pathways for the generation of antiviral or antimicrobial effector molecules are the phagocyte NADPH oxidase (Phox) and the inducible nitric oxide synthase (iNOS) pathways. One of the most intriguing discoveries in the field of ROIs in recent years was the observation by Wentworth and colleagues that antibodies, independent of their source or antigen specificity, can catalyze the generation of ROIs. Members of all groups of infectious pathogens (viruses, bacteria, protozoa, helminths, and fungi) were found to be controlled by RNIs. The significant improvement of certain infectious diseases after inhibition or genetic deletion of iNOS, which was without negative effects on the pathogen clearance, was unexpected. It can be explained by the inhibition of T-cell proliferation or induction of T-cell apoptosis via iNOS-positive suppressor cells (macrophages and dendritic cells) or by the tissue-damaging properties of RNIs. Transgenic mouse models have been extremely helpful to elucidate the relative contributions of ROIs and RNIs for the control of infectious pathogens. Viruses, bacteria, parasites, and fungi have developed multiple strategies to evade killing by oxygen-dependent effector mechanisms. Current research projects aim at the development of ROI or RNI precursors that enter only certain types of host cells and are activated by the infectious pathogens themselves.
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Antibiotics That Block DNA Replication and Repair: the Quinolones
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Source: Antibiotics , pp 70-77
Publication Date :
January 2003
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Inhibition of DNA replication and repair enzymes would seem a logical target for antibacterial action by natural products elaborated by microbes to kill their neighbors. One such class of molecules, the coumarins, represented by such streptomycete metabolites as novobiocin and coumermycin, has been studied for many years and served to pinpoint enzymes called DNA type II topoisomerases, specifically DNA gyrase, as the killing target. The newest generation of quinolones in particular, such as gatifloxacin, have increased potency against gram-positive pathogens. Thousands of fluoroquinolones have been synthesized around the core planar heterocyclic nucleus that gives the family its name. Extensive analysis has indicated that quinolones affect the double-strand cleavage/double-strand religation equilibrium in gyrase and topo IV catalytic cycles, such that the cleaved complex accumulates. There has been speculation about whether quinolones speed up the double-cleavage step of bound DNA or selectively slow the double-religation step, without definitive evidence for either interpretation. The mechanism by which quinolones induce the accumulation of the doubly cut covalent DNA-enzyme intermediate is likewise still mysterious. As the quinolone-covalent gyrase-doubly cut DNA intermediate accumulates, the killing action is thought to be from the downstream effect this block has on the progression of DNA replication forks which are halted by this. It may be that DNA repair machinery is recruited, attempts to come to the rescue, and fails as the recalcitrant quinolone-stabilized gyrase-DNA intermediate persists. This may be the signal that turns on the signaling processes that lead to the rapid killing of bacteria induced by the quinolones.
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Retroviral DNA Integration
- Author: Robert Craigie
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Source: Mobile DNA II , pp 613-630
Publication Date :
January 2002
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This chapter provides an overview of the current understanding of the molecular mechanism of retroviral DNA integration and points to some of the issues that are not yet well understood. The mechanism of retroviral DNA integration is closely related to the mechanism by which many transposons and retrotransposons move from one location to another in the genome of the host cell. Retroviral integrase is encoded by the 3' part of the pol gene and is assembled into virus particles as the Gag-Pol polyprotein precursor. Retroviral integrases share a common domain structure, and the biochemical activities of integrase proteins from different retroviruses are fundamentally similar. Integrase has a nonspecific nuclease activity that is most easily monitored by observing nicking of closed circular DNA. Indeed, this nonspecific nuclease activity was the first biochemical activity detected for a retroviral integrase. This activity may reflect an inefficient 3' processing reaction acting on an aberrant DNA substrate. Complementation experiments demonstrate that HIV-1 integrase functions as a multimer. Individual proteins lacking either the N-terminal or the C-terminal domain are inactive both for 3' end processing and DNA strand transfer. Mu-mediated PCR footprinting has been used to probe the nucleoprotein organization within preintegration complexes (PICs). Several cellular proteins have been implicated to play a role in retroviral DNA integration. Further studies of retroviral DNA integration will be needed to address these and other questions for which there are only partial answers at this time.
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Big Questions and Future Prospects
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Source: Biocatalysis and Biodegration , pp 213-216
Publication Date :
January 2001
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This chapter discusses questions related to microbial biocatalysis and biodegradation. These questions were selected for their important impact on applications in biocatalysis and biodegradation and on one's basic understanding of microbial physiology, genomics, and ecology. The preponderance of Pseudomonas and related species in the biodegradation literature is due, at least in part, to the conditions used for their enrichment and cultivation. Comparative genomics is beginning to show, not unexpectedly, that the amount of catabolic metabolism encoded in the genome varies considerably, depending on whether the bacterium is an obligate parasitic pathogen or a soil organism known to have extensive biodegradative capabilities, such as Pseudomonas spp. and Sphingomonas spp. It is unclear how rapidly new metabolism can evolve, practically and theoretically. One approach that promises to shed light on this is the use of directed evolution in the laboratory. This can reveal the plasticity of microbial enzymes and pathways and point the way toward a better understanding of what happens in the soils of the world. It seems very likely that even more challenges will be posed for microbial metabolism, and one will need to continually discover new microbial metabolism to match the discovery of new organic compounds.
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DNA Repair Systems
- Authors: Ronald E. Yasbin, David Cheo, David Bol
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Source: Bacillus subtilis and Other Gram-Positive Bacteria , pp 529-537
Publication Date :
January 1993
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Escherichia coli has served as the principal model for investigations into DNA repair mechanisms. The DNA repair systems identified in this paradigm have also been discovered in most other organisms studied. This chapter attempts to look at these repair systems with respect to differentiation processes and developmental biology as studied in the gram-positive bacterium Bacillus subtilis. Originally, DNA repair systems were considered integral parts of an organism's ability to survive the effects of environmental insults and metabolic processes. However, as the molecular characterization of these repair systems proceeded it became obvious that in addition to determining mutation frequency and cell survival, DNA repair systems also play important roles in viral activation, DNA replication, genetic recombination, metabolism, and cancer. In order to investigate systematically the interrelationship(s) between DNA repair systems and these other phenomena, an appropriate model system must be identified. These systems are generally linked in their expression and activity with one or more of the developmental states that have been identified for the bacterium. It is in the elucidation of this linkage that B. subtilis becomes a critical model for the understanding of how organisms respond at the molecular level to stressful situations. Similar characterizations of these repair systems in other gram-positive bacteria, especially among the extremely resistant bacteria, will determine whether or not B. subtilis represents a paradigm for gram-positive bacteria or for bacteria that have distinct developmental cycles.