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We report on the observation of nanoscale conduction at ferroelectric domain walls in hexagonal HoMnO(3) protected by the topology of multiferroic vortices using in situ conductive atomic force microscopy, piezoresponse force microscopy, and Kelvin-probe force microscopy at low temperatures. In addition to previously observed Schottky-like rectification at low bias [Phys. Rev. Lett. 104, 217601 (2010)], conductance spectra reveal that negatively charged tail-to-tail walls exhibit enhanced conduction at high forward bias, while positively charged head-to-head walls exhibit suppressed conduction at high reverse bias. Our results pave the way for understanding the semiconducting properties of the domains and domain walls in small-gap ferroelectrics.
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By correlating room temperature conductive atomic force microscopy with low temperature electrostatic force microscopy images of the same sample region, we demonstrate that nanoscale electric conduction between a sharp tip and the surface of ferroelectric HoMnO3 is intrinsically modulated by the polarization of ferroelectric domains. Conductance spectra reveal that the electric conduction is described by polarization-induced Schottky-like rectification at low bias, but dominated by a space-charge limited conduction mechanism at high bias. Our observation demonstrates visualization of ferroelectric domain structure by electric conduction, which may be used for nondestructive readout of nanoscale ferroelectric memories and/or ferroelectric sensors.
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We report on variable-temperature STM investigations of the spontaneous long-range magnetic order of Mn monolayer nanostructures epitaxially grown on stepped W(110). The measurements reveal that the onset of the antiferromagnetic order is closely related to the Mn nanostructure width along the [001] direction, with a decreasing Néel temperature as we move from a 2D toward a quasi-1D system. In contrast, lateral confinement along the [110] direction seems to play a less important role. The results are discussed in terms of anisotropic exchange coupling and of boundary effects, both potentially stabilizing long-range magnetic order in nanostructures confined in the [110] direction.
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We combined coherent nonlinear optical spectroscopy with nano-electron volt energy resolution and low-temperature near-field microscopy with subwavelength resolution (
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Escherichia coli can grow under either aerobic or anaerobic conditions, deriving energy from a variety of respiratory of fermentative processes. The switch between different metabolic modes depends on the availability of oxygen or alternative electron acceptors, and it is controlled by regulatory mechanisms which ensure that the most energetically favourable metabolic mode is adopted in a specific environment. This article reviews the properties of two transcriptional regulators, ArcA and FNR, which control the expression of networks of genes in response to oxygen limitation.
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Proteínas de Escherichia coli , Escherichia coli/metabolismo , Proteínas Ferro-Enxofre , Proteínas Repressoras , Aerobiose , Anaerobiose , Proteínas da Membrana Bacteriana Externa/genética , Proteínas de Bactérias/genética , Sequência de Bases , DNA Bacteriano/genética , Escherichia coli/genética , Genes BacterianosRESUMO
The aconitase family contains a diverse group of iron-sulphur (Fe-S) isomerases and two types of iron regulatory protein (IRP). Structural comparisons have revealed three architecturally distinct variants in which one of the four structural domains is covalently linked at either the amino- or carboxy-terminal end of a single polypeptide or else this domain exists as an independent subunit.
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Aconitato Hidratase/genética , Família Multigênica , Estrutura Terciária de Proteína , Animais , Evolução Biológica , Humanos , FilogeniaRESUMO
Bacteria which can grow in different environments have developed regulatory systems which allow them to exploit specific habitats to their best advantage. In the facultative anaerobe Escherichia coli two transcriptional regulators controlling independent networks of oxygen-regulated gene expression have been identified. One is a two-component sensor-regulator system (ArcB-A), which represses a wide variety of aerobic enzymes under anaerobic conditions. The other is FNR, the transcriptional regulator which is essential for expressing anaerobic respiratory processes. The purpose of this review is to summarize what is known about FNR. The fnr gene was initially defined by the isolation of some pleiotropic mutants which characteristically lacked the ability to use fumarate and nitrate as reducible substrates for supporting anaerobic growth and several other anaerobic respiratory functions. Its role as a transcriptional regulator emerged from genetic and molecular studies in which its homology with CRP (the cyclic AMP receptor protein which mediates catabolite repression) was established and has since been particularly important in identifying the structural basis of its regulatory specificities. FNR is a member of a growing family of CRP-related regulatory proteins which have a DNA-binding domain based on the helix-turn-helix structural motif, and a characteristic beta-roll that is involved in nucleotide-binding in CRP. The FNR protein has been isolated in a monomeric form (Mr 30,000) which exhibits a high but as yet non-specific affinity for DNA. Nevertheless, the DNA-recognition site and important residues conferring the functional specificity of FNR have been defined by site-directed mutagenesis. A consensus for the sequences that are recognized by FNR in the promoter regions of FNR-regulated genes, has likewise been identified. The basic features of the genes and operons regulated by FNR are reviewed, and examples in which FNR functions negatively as an anaerobic repressor as well as positively as an anaerobic activator, are included. Less is known about the way in which FNR senses anoxia and is thereby transformed into its 'active' form, but it seems likely that cysteine residues and possibly a metal ion are involved. Four of the five cysteine residues of FNR are clustered in an essential N-terminal 'domain' which is conserved in FNR and the HlyX protein of Actinobacillus pleuropneumoniae, but not in CRP or the FixK protein of Rhizobium meliloti. The relationships between FNR and other oxygen-related systems in E. coli are discussed, as well as parallel systems in other organisms.(ABSTRACT TRUNCATED AT 400 WORDS)
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Proteínas de Bactérias/metabolismo , Proteínas de Escherichia coli , Escherichia coli/fisiologia , Regulação Bacteriana da Expressão Gênica , Proteínas Ferro-Enxofre , Oxigênio/fisiologia , Fatores de Transcrição/metabolismo , Aerobiose , Sequência de Aminoácidos , Bactérias/genética , Bactérias/metabolismo , Proteínas de Bactérias/genética , Sequência de Bases , Escherichia coli/efeitos dos fármacos , Escherichia coli/genética , Regulação Bacteriana da Expressão Gênica/efeitos dos fármacos , Genes Bacterianos , Modelos Moleculares , Dados de Sequência Molecular , Conformação Proteica , Salmonella typhimurium/genética , Salmonella typhimurium/metabolismo , Homologia de Sequência do Ácido Nucleico , Fatores de Transcrição/genética , Transcrição GênicaRESUMO
Coral cover on reefs is declining globally due to coastal development, overfishing and climate change. Reefs isolated from direct human influence can recover from natural acute disturbances, but little is known about long term recovery of reefs experiencing chronic human disturbances. Here we investigate responses to acute bleaching disturbances on turbid reefs off Singapore, at two depths over a period of 27 years. Coral cover declined and there were marked changes in coral and benthic community structure during the first decade of monitoring at both depths. At shallower reef crest sites (3-4 m), benthic community structure recovered towards pre-disturbance states within a decade. In contrast, there was a net decline in coral cover and continuing shifts in community structure at deeper reef slope sites (6-7 m). There was no evidence of phase shifts to macroalgal dominance but coral habitats at deeper sites were replaced by unstable substrata such as fine sediments and rubble. The persistence of coral dominance at chronically disturbed shallow sites is likely due to an abundance of coral taxa which are tolerant to environmental stress. In addition, high turbidity may interact antagonistically with other disturbances to reduce the impact of thermal stress and limit macroalgal growth rates.
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Antozoários/fisiologia , Monitoramento Ambiental/métodos , Animais , Antozoários/microbiologia , Mudança Climática , Recifes de Corais , Resistência à Doença , Dinâmica Populacional , SingapuraRESUMO
Autism spectrum disorders (ASD) are highly heterogeneous developmental conditions characterized by deficits in social interaction, verbal and nonverbal communication, and obsessive/stereotyped patterns of behavior and repetitive movements. Social interaction impairments are the most characteristic deficits in ASD. There is also evidence of impoverished language and empathy, a profound inability to use standard nonverbal behaviors (eye contact, affective expression) to regulate social interactions with others, difficulties in showing empathy, failure to share enjoyment, interests and achievements with others, and a lack of social and emotional reciprocity. In developed countries, it is now reported that 1%-1.5% of children have ASD, and in the US 2015 CDC reports that approximately one in 45 children suffer from ASD. Despite the intense research focus on ASD in the last decade, the underlying etiology remains unknown. Genetic research involving twins and family studies strongly supports a significant contribution of environmental factors in addition to genetic factors in ASD etiology. A comprehensive literature search has implicated several environmental factors associated with the development of ASD. These include pesticides, phthalates, polychlorinated biphenyls, solvents, air pollutants, fragrances, glyphosate and heavy metals, especially aluminum used in vaccines as adjuvant. Importantly, the majority of these toxicants are some of the most common ingredients in cosmetics and herbicides to which almost all of us are regularly exposed to in the form of fragrances, face makeup, cologne, air fresheners, food flavors, detergents, insecticides and herbicides. In this review we describe various scientific data to show the role of environmental factors in ASD.
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Transtorno do Espectro Autista/etiologia , Exposição Ambiental , Poluentes Ambientais/efeitos adversos , Substâncias Perigosas/efeitos adversos , Adolescente , Adulto , Transtorno do Espectro Autista/induzido quimicamente , Criança , Pré-Escolar , Feminino , Feto/efeitos dos fármacos , Humanos , Lactente , Recém-Nascido , Masculino , Pessoa de Meia-IdadeRESUMO
While many studies of coral bleaching report on broad, regional scale responses, fewer examine variation in susceptibility among coral taxa and changes in community structure, before, during and after bleaching on individual reefs. Here we report in detail on the response to bleaching by a coral community on a highly disturbed reef site south of mainland Singapore before, during and after a major thermal anomaly in 2010. To estimate the capacity for resistance to thermal stress, we report on: a) overall bleaching severity during and after the event, b) differences in bleaching susceptibility among taxa during the event, and c) changes in coral community structure one year before and after bleaching. Approximately two thirds of colonies bleached, however, post-bleaching recovery was quite rapid and, importantly, coral taxa that are usually highly susceptible were relatively unaffected. Although total coral cover declined, there was no significant change in coral taxonomic community structure before and after bleaching. Several factors may have contributed to the overall high resistance of corals at this site including Symbiodinium affiliation, turbidity and heterotrophy. Our results suggest that, despite experiencing chronic anthropogenic disturbances, turbid shallow reef communities may be remarkably resilient to acute thermal stress.
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Antozoários/fisiologia , Resistência à Doença/fisiologia , Animais , Recifes de Corais , Ecossistema , Temperatura Alta , Oceano ÍndicoRESUMO
A composite protein sequence database was searched for amino acid sequences similar to the C-terminal domain of the dihydrolipoamide acetyltransferase subunit (E2p) of the pyruvate dehydrogenase complex of Escherichia coli. Nine sequences with extensive similarity were found, of which eight were E2 subunits. The other was for a putative mitochondrial ribosomal protein, MRP3, from Neurospora crassa. Alignment of the MRP3 and E2 sequences showed that the similarity extends through the entire MRP3 sequence and that MRP3 is most closely related to the E2p subunit of the pyruvate dehydrogenase complex from Saccharomyces cerevisiae, with 54% identical residues and a further 36% that are conservatively substituted. Other features of the MRP3 gene and protein are also consistent with it being the acyltransferase subunit of a 2-oxo acid dehydrogenase complex. A multiple alignment of 13 E2 sequences indicated that 120 (34%) of 353 equivalenced residues are identical or show some degree of conservation. It also identified residues that are potentially important for the structure, catalytic activity and substrate-specificity of the acyltransferases.
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Acetiltransferases/genética , Escherichia coli/genética , Neurospora crassa/genética , Saccharomyces cerevisiae/genética , Sequência de Aminoácidos , Di-Hidrolipoil-Lisina-Resíduo Acetiltransferase , Escherichia coli/enzimologia , Proteínas Fúngicas/genética , Genes Fúngicos , Dados de Sequência Molecular , Neurospora crassa/enzimologia , Complexo Piruvato Desidrogenase/genética , Proteínas Ribossômicas/genética , Saccharomyces cerevisiae/enzimologia , Proteínas de Saccharomyces cerevisiae , Homologia de Sequência do Ácido NucleicoRESUMO
Succinyl-CoA synthetase (EC 6.2.1.5, succinate:CoA ligase (ADP-forming] of Escherichia coli is an alpha 2 beta 2 tetramer, with the active site believed to be located at the point of contact between the two subunit types. It has been previously established that the reaction involves the intermediate participation of a phosphorylated enzyme form in the process of catalysis. The site of phosphorylation (His-246) and the binding sites for the substrates ADP and ATP are located in the alpha subunit, and the succinate and CoA binding sites are in beta. A mutant form of this enzyme, with the active site histidine residue replaced by aspartate, has been produced in large quantities and purified to homogeneity. This form appears to be indistinguishable from the native enzyme with respect to its subunit assembly, but has no ability to catalyze the overall reaction. As expected, the His-246 alpha----Asp mutant is incapable of undergoing phosphorylation. We have developed an assay based upon the arsenolysis of succinyl-CoA that effectively isolates the partial reaction that occurs in the portion of the active site contributed by the beta subunit; this reaction does not involve covalent participation of His-246 alpha. We have found that the His-246 alpha----Asp mutant is also devoid of activity in this arsenolysis reaction, indicating that an intact His-246 alpha is required for the establishment of the microenvironment in this portion of the active site that is required for the corresponding step of the overall reaction.
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Escherichia coli/enzimologia , Succinato-CoA Ligases/metabolismo , Difosfato de Adenosina/metabolismo , Trifosfato de Adenosina/metabolismo , Sítios de Ligação , Eletroforese em Gel de Poliacrilamida , Escherichia coli/genética , Expressão Gênica , Histidina/metabolismo , Cinética , Mutação , Fosforilação , Succinato-CoA Ligases/química , Succinato-CoA Ligases/genéticaRESUMO
The pyruvate dehydrogenase multienzyme complex of Pseudomonas aeruginosa was subjected to a steady-state kinetic analysis using the exponential model for a regulatory enzyme and a sensitive statistical fitting procedure. This showed that all the substrates, pyruvate, CoA and NAD+, exhibit cooperative kinetics towards the native multienzyme complex.
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Pseudomonas aeruginosa/enzimologia , Complexo Piruvato Desidrogenase/metabolismo , Coenzima A , Cinética , NAD/farmacologia , Complexo Piruvato Desidrogenase/antagonistas & inibidores , Piruvatos , Estatística como AssuntoRESUMO
Biochemical studies with strains of Escherichia coli that are amplified for the products of the three fumarase genes, fumA (FUMA), fumB (FUMB) and fumC (FUMC), have shown that there are two distinct classes of fumarase. The Class I enzymes include FUMA, FUMB, and the immunologically related fumarase of Euglena gracilis. These are characteristically thermolabile dimeric enzymes containing identical subunits of Mr 60,000. FUMA and FUMB are differentially regulated enzymes that function in the citric acid cycle (FUMA) or to provide fumarate as an anaerobic electron acceptor (FUMB), and their affinities for fumarate and L-malate are consistent with these roles. The Class II enzymes include FUMC, and the fumarases of Bacillus subtilis, Saccharomyces cerevisiae and mammalian sources. They are thermostable tetrameric enzymes containing identical subunits Mr 48,000-50,000. The Class II fumarases share a high degree of sequence identity with each other (approx. 60%) and with aspartase (approx. 38%) and argininosuccinase (approx. 15%), and it would appear that these are all members of a family of structurally related enzymes. It is also suggested that the Class I enzymes may belong to a wider family of iron-dependent carboxylic acid hydro-lyases that includes maleate dehydratase and aconitase. Apart from one region containing a Gly-Ser-X-X-Met-X-X-Lys-X-Asn consensus sequence, no significant homology was detected between the Class I and Class II fumarases.
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Proteínas de Bactérias/classificação , Escherichia coli/enzimologia , Fumarato Hidratase/classificação , Sequência de Aminoácidos , Proteínas de Bactérias/genética , Proteínas de Bactérias/imunologia , Proteínas de Bactérias/fisiologia , Escherichia coli/genética , Fumarato Hidratase/genética , Fumarato Hidratase/imunologia , Fumarato Hidratase/fisiologia , Genes Bacterianos , Temperatura Alta , Dados de Sequência Molecular , Plasmídeos , Proteínas Recombinantes , Homologia de Sequência do Ácido Nucleico , Especificidade por SubstratoRESUMO
The acetyltransferase chains of the pyruvate dehydrogenase complex of Escherichia coli contain conformationally mobile (alanine + proline)-rich segments that link the lipoyl domains to each other and to the subunit-binding and catalytic domain, and facilitate the intramolecular coupling of active sites in the complex. A deletion of 12 of the 32 residues of the (Ala + Pro)-rich segment of an acetyltransferase containing only one lipoyl domain was made by deleting the corresponding segment of the aceF gene. A pyruvate dehydrogenase complex was still produced and the catalytic activity of the restructured complex, including active-site coupling, was not detectably impaired.
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Proteínas de Bactérias/genética , Escherichia coli/genética , Complexo Piruvato Desidrogenase/genética , Acetiltransferases/genética , Acetiltransferases/metabolismo , Alanina , Sequência de Aminoácidos , Proteínas de Bactérias/metabolismo , Sítios de Ligação , Catálise , Genes , Genes Bacterianos , Prolina , Conformação Proteica , Complexo Piruvato Desidrogenase/metabolismoRESUMO
The 70-amino-acid-residue N-terminal sequence of the bacterioferritin (BFR) of Azotobacter vinelandii was determined and shown to be highly similar to the N-terminal sequences of the Escherichia coli and Nitrobacter winogradskyi bacterioferritins. Electrophoretic and immunological analyses further indicate that the bacterioferritins of E. coli, A. vinelandii and Pseudomonas aeruginosa are closely related. A novel, two-subunit assembly state that predominates over the 24-subunit form of BFR at low pH was demonstrated. The results indicate that the bacterioferritins form a family of proteins that are distinct from the ferritins of plants and animals.
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Azotobacter/análise , Proteínas de Bactérias/química , Grupo dos Citocromos b/química , Escherichia coli/análise , Ferritinas/química , Nitrobacter/análise , Pseudomonas aeruginosa/análise , Sequência de Aminoácidos , Proteínas de Bactérias/imunologia , Fenômenos Químicos , Físico-Química , Reações Cruzadas , Grupo dos Citocromos b/imunologia , Eletroforese em Gel de Poliacrilamida , Ferritinas/imunologia , Imunodifusão/métodos , Focalização Isoelétrica , Dados de Sequência Molecular , Especificidade da EspécieRESUMO
The regulatory mechanism of substrate activation in yeast pyruvate decarboxylase is triggered by the interaction of pyruvic acid with C221 located on the beta domain at >20 A from the thiamin diphosphate (ThDP). To trace the putative information transfer pathway, substitutions were made at H92 on the alpha domain, across the domain divide from C221, at E91, next to H92 and hydrogen bonded to W412, the latter being intimately involved in the coenzyme binding locus. Additional substitutions were made at D28, E51, H114, H115, I415 and E477, all near the active center. The pH-dependent steady-state kinetic parameters, including the Hill coefficient, provide useful insight to this effort. In addition to C221, the residues H92, E91, E51 and H114 and H115 together appear to have a critical impact on the Hill coefficient, providing a pathway for information transfer. To study the activation by ThDP.Mg(II), variants at G231 (of the conserved GDG triplet) and at N258 and C259 (all three being part of the putative ThDP fold) of the E1 component of the Escherichia coli pyruvate dehydrogenase multienzyme complex were studied. Kinetic and spectroscopic evidence suggests that the Mg(II) ligands are very important to activation of the enzymes by cofactors.
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Magnésio/metabolismo , Conformação Proteica , Estrutura Terciária de Proteína , Piruvato Descarboxilase/metabolismo , Complexo Piruvato Desidrogenase/metabolismo , Tiamina Pirofosfato/metabolismo , Sítios de Ligação , Cristalografia por Raios X , Ativação Enzimática , Modelos Moleculares , Mutagênese Sítio-Dirigida , Piruvato Descarboxilase/química , Piruvato Descarboxilase/genética , Ácido Pirúvico/metabolismoRESUMO
The fnr gene is essential for the expression of anaerobic respiratory metabolism in Escherichia coli. Genetic and biochemical studies support the view that its product. Fnr, is a transcriptional regulatory protein specific for genes encoding anaerobic respiratory functions (fumarate, nitrate and nitrite reductases, hydrogenase, etc.). In this respect Fnr may be considered analogous to the well-characterized catabolite gene activator protein (CAP), which mediates the control of catabolite-sensitive gene transcription. With a view to identifying its function, the fnr gene has recently been cloned and the primary structure of the Fnr protein deduced from the nucleotide sequence. This has revealed the presence of three regions of sequence homology with CAP. One corresponds to the DNA-binding site, a region of about 20 highly conserved amino acids that is believed to form a characteristic three-dimensional structure in several transcriptional regulators. The other regions of homology are in the nucleotide binding domain of CAP but the residues that interact with cAMP are not identical in Fnr. These homologies suggest that Fnr and CAP may have similar three-dimensional structures and that the regulation of anaerobic energy metabolism may involve interaction between Fnr and an unidentified effector molecule.
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Anaerobiose , Escherichia coli/genética , Metabolismo , Aminoácidos/análise , Sequência de Bases , DNA Bacteriano , Escherichia coli/metabolismo , Genes Reguladores , Biossíntese de Proteínas , Receptores de AMP Cíclico/análise , Receptores de AMP Cíclico/biossíntese , Transcrição GênicaRESUMO
The elucidation of the primary structure of the Escherichia coli lipoamide dehydrogenase (EC 1.8.1.4) by sequencing the corresponding structural gene (lpd) has enabled a detailed structural comparison between lipoamide dehydrogenase and the related disulphide oxido-reductase, human erythrocyte glutathione reductase (EC 1.6.4.2). Some 28% of the amino acid residues were found to be identical and a striking degree of homology was apparent throughout the polypeptide chains. It was concluded that the two enzymes possess very similar three-dimensional structures with particularly strong conservation of residues around the FAD and NAD(P) binding sites and at the redox centres of the molecules. Significant amino acid substitutions occur in the substrate binding pocket and these include an extra 18 amino acid residues at the C terminus of lipoamide dehydrogenase. Under physiological conditions, lipoamide dehydrogenase and glutathione reductase act in opposite directions, passing reducing equivalents to NAD+ or from NADPH (respectively), and two key substitutions near the redox centre could be associated with this difference in function. This study represents the first direct structural comparison between two related enzymes that are NADP+-linked (glutathione reductase) and NAD+-linked (lipoamide dehydrogenase). The differential recognition of these two cofactors could be explained in terms of amino acid substitutions. A divergent evolutionary relationship between the two enzymes including their NAD and NADP binding domains is fully supported by this analysis.
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Di-Hidrolipoamida Desidrogenase , Glutationa Redutase , Sequência de Aminoácidos , Sítios de Ligação , Catálise , Flavina-Adenina Dinucleotídeo , Modelos Moleculares , NAD , NADP , Ligação Proteica , Conformação ProteicaRESUMO
Site-directed mutagenesis of the aceF gene of Escherichia coli was used to generate a nested set of deletions in the long (alanine + proline)-rich sequence that separates the lipoyl domain from the dihydrolipoamide dehydrogenase-binding domain in the "one-lipoyl domain" dihydrolipoamide acetyltransferase polypeptide chains of a pyruvate dehydrogenase multienzyme complex. The deletions reduced the number of residues in this sequence successively from 32 to 20, 13, 7 and just 1 residue. In all instances, pyruvate dehydrogenase complexes were still assembled in vivo around cores containing the deleted chains, and those with the two shortest deletions were essentially fully active. However, the two most severe deletions caused falls of 50% or more in specific catalytic activity. Similarly, although shortening the interdomain sequence to 20 residues left the system of active-site coupling unimpaired, cutting it to 13 residues or less caused substantial falls in the reductive acetylation of the lipoyl domains and corresponding losses of active-site coupling. The changes in specific catalytic activity and active-site coupling that accompanied the shortening of the (alanine + proline)-rich segment were reflected in the poorer growth rates of the relevant strains of E. coli on stringent substrates. All these results are consistent with this (alanine + proline)-rich sequence acting as a linker region that facilitates the movements of the lipoyl domains required for full catalytic activity and active-site coupling in the complex. The other two such sequences that separate the additional lipoyl domains in the N-terminal half of the wild-type "three-lipoyl domain" dihydrolipoamide acetyltransferase chain are presumed to function similarly. This role is consistent with the conformational flexibility assigned to these segments from previous studies based on 1H nuclear magnetic resonance spectroscopy and protein engineering.