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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    FEMS microbiology reviews 19 (1996), S. 0 
    ISSN: 1574-6976
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    FEMS microbiology reviews 19 (1996), S. 0 
    ISSN: 1574-6976
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract: The growth of the bacterial cell involves the co-ordination of the fluxes of carbon into a considerable diversity of products that are the components of the cell. Fortunately the monomers from which the cell's polymers are made are themselves synthesised from a relatively small group of precursors that are the products of the central metabolic pathways. This simplification renders cell metabolism accessible to flux analysis, a method for handling experimental data to derive metabolic fluxes. Through such analysis of the growth of Escherichia coli ML308 on 11 single carbon sources in batch, turbidostat or chemostat culture general patterns are discernible. Most significant among these are that growth on different carbon sources is achieved without any obvious enzyme acting as a regulator of metabolic flux, except when acetate is the sole source of carbon. In this case a junction is created at which isocitrate dehydrogenase (ICDH) and isocitrate lyase (ICL) compete for their common substrate and this competition is resolved by partial inactivation of ICDH to match flux through ICL and this balance limits growth rate. In this sense, flux through ICDH aand ICL is ‘rate-limiting’. Uptake of six of the remaining carbon inputs exceeds the capacity of the central metabolic pathways (CMPs) to sustain flux to the precursors required for growth and the CMPs are balanced by excretion of acetate. Restriction of carbon uptake by chemostat progressively diminishes growth rate and acetate excretion until acetate excretion is prevented. For the four remaining carbon sources, uptake is apparently restricted and the products are biomass, carbon dioxide and water. Carbon sources feeding the phosphorylated parts of the CMPs flux relatively more carbon to precursors (Pre-C) than CO2 when compared with carbon sources which feed into the non-phosphorylated pathways. Pre-C/CO2 ratios for the former are 1.73–3.91 and for the latter are 0.46–0.78. Flux analysis of all 11 carbon sources shows that there is an overabundant supply of ‘energy’ (ATP+[2H]), generated by the CMPs, in all phenotypes and conditions down to a glucose chemostat at μ of 0.72. This excess energy is a thermodynamic inefficiency which must be dissipated as heat. E. coli ML308 probably evolved in circumstances of ‘feast’ and ‘famine’. The two strategies selected (excretion of surplus carbon and restriction of μ) would appear to be defences against ‘feast’. Presumably there are defences against ‘famine’. These are not made obvious by flux analysis but allosteric control of irreversible enzymes would protect pools of essential nutrients from rapid depletion on the sudden onset of ‘famine’.
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    FEMS microbiology reviews 18 (1996), S. 0 
    ISSN: 1574-6976
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract: The blood-brain barrier (BBB) is formed by the tight junctions of the cerebral capillary endothelium and the choroid plexus epithelium. The molecular anatomy of the tight junction resembles that of a polarized, transporting epithelium, suggesting some model cell culture systems can provide insight into traffic into the central nervous system. Pathogens target both the endothelium, causing encephalitis, and the choroid plexus, leading to meningitis. Routes of entry are diverse including paracellular and transcellular penetration. In addition, circulating microbial products can induce loss of BBB function. Understanding the heterogeneous molecular interactions between pathogens and the BBB may provide avenues to interrupt the devastating neurological sequelae that accompany central nervous system infections.
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    FEMS microbiology reviews 18 (1996), S. 0 
    ISSN: 1574-6976
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    FEMS microbiology reviews 18 (1996), S. 0 
    ISSN: 1574-6976
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1574-6976
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract: Free-living bacteria are expert in adapting to variations in nutrient availability, often using an array of transport systems of different affinities to scavenge for particular substrates (multiport). This review concentrates on the regulation of expression of different transporters contributing to multiport in response to varying nutrient levels. A novel mechanism of controlling bacterial transport affinity under sugar limitation is described. In particular, switching from glucose-rich to glucose-limited conditions results in Escherichia coli orchestrating outer membrane changes as well as the induction of a periplasmic binding protein-dependent (ABC-type) transport system. The changes leading to the high affinity transport pathway are directed towards uptake of rapidly utilisable concentrations and are optimal close to 10−6 M medium glucose. High affinity transport is absent under both glucose-rich ‘feast’ and glucose-starved ‘famine’ conditions hence high affinity transporters are not simply repressed by excess nutrient. Rather, the improvement in glucose scavenging involves induction of genes in 2 distinct regulons (mgl/gal and mal/lamB) through synthesis of 2 different endogenous inducer molecules (galactose, maltotriose). Endoinducer levels are tightly controlled by extracellular glucose concentration at different glucose-limited growth rates. Aside from endoinducers, the elevated intracellular level of cAMP plays a role in induction of the high-affinity pathway but CAMP-mediated relief from catabolite repression is not itself sufficient for high affinity transport. In contrast to the repressive role of glucose when present at millimolar concentrations, micromolar glucose also leads to the induction of transport systems for other sugars, further broadening the scavenging potential of nutrient-limited bacteria for other substrates.
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    FEMS microbiology reviews 18 (1996), S. 0 
    ISSN: 1574-6976
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract: Tungsten (atomic number 74) and the chemically analogous and very similar metal molybdenum (atomic number 42) are minor yet equally abundant elements on this planet. The essential role of molybdenum in biology has been known for decades and molybdoenzymes are ubiquitous. Yet, it is only recently that a biological role for tungsten has been established in prokaryotes, although not as yet in eukaryotes. The best characterized organisms with regard to their metabolism of tungsten are certain species of hyperthermophilic archaea (Pyrococcus furiosus and Thermococcus litoralis), methanogens (Methanobacterium thermoautotrophicum and Mb. wolfei), Gram-positive bacteria (Clostridium thermoaceticum, C. formicoaceticum and Eubacterium acidaminophilum), Gram-negative anaerobes (Desulfovibrio gigas and Pelobacter acetylenicus) and Gram-negative aerobes (Methylobacterium sp. RXM). Of these, only the hyperthermophilic archaea appear to be obligately tungsten-dependent. Four different types of tungstoenzyme have been purified: formate dehydrogenase, formyl methanofuran dehydrogenase, acetylene hydratase, and a class of phylogenetically related oxidoreductases that catalyze the reversible oxidation of aldehydes. These are carboxylic reductase, and three ferredoxin-dependent oxidoreductases which oxidize various aldehydes, formaldehyde and glyceraldehyde 3-phosphate. All tungstoenzymes catalyze redox reactions of very low potential (≤−420 mV) except one (acetylene hydratase) which catalyzes a hydration reaction. The tungsten in these enzymes is bound by a pterin moiety similar to that found in molybdoenzymes. The first crystal structure of a tungsten- or pterin-containing enzyme, that of aldehyde ferredoxin oxidoreductase from P. furiosus, has revealed a catalytic site with one W atom coordinated to two pterin molecules which are themselves bridged by a magnesium ion. The geochemical, ecological, biochemical and phylogenetic basis for W- vs. Mo-dependent organisms is discussed.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    FEMS microbiology reviews 18 (1996), S. 0 
    ISSN: 1574-6976
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract: In order to achieve genetic rearrangement in a sexual cycle, eukaryotes go through the processes of meiosis and mating. Different mating types assure that mating is only possible between two genetically diverse individuals. Basidiomycetous fungi display thousands of different mating types that are determined by two genetically unlinked loci. One locus is multiallelic and contains genes for homeodomain transcription factors which are able to form heterodimers. The activation of target genes is dependent on heterodimers formed from the monomeric transcription factor proteins originating from different alleles of this genetic locus. The interactions between the two monomeric transcription factors and the activation of target genes by the heterodimeric proteins make this regulatory system both complex and interesting. The second locus contains a pheromone receptor system: the pheromone receptor is similar to the G protein-linked serpentine receptors in Saccharomyces cerevisiae that activate the pheromone response via a phosphorylation signal transduction cascade in S. cerevisiae. This pheromone perception is a trigger of sexual development and not, as with yeast, itself under control of mating type genes. Rather it directly senses diversity at the mating type loci. Whereas heterobasidiomycetes display a bi-allelic structure for this locus with recognition between one receptor and the opposite pheromone, homobasidiomycetes contain multiple specificities for pheromone receptors and pheromones.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    FEMS microbiology reviews 17 (1995), S. 0 
    ISSN: 1574-6976
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1574-6976
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract: Most of the presently studied acetone-butanol (solvent)-producing bacteria are labelled as Clostridium acetobutylicum. This situation contrasts what was experienced by investigators of the 1940s who faced a plurality of names for solvent-producing bacteria. Significant phenotypic differences, however, exist among the presently studied strains of C. acetobutylicum, which raised the question of whether or not these organisms can truly be considered as members of one species. Furthermore, two cultures (ATCC 824 and NCIMB 8052) that are thought to be equivalent in serving as the type strain of C. acetobutylicum have significantly different properties. To assess the relatedness of these bacteria as members of a species, a comparison of similarity of their genomic DNA is most effective. DNAs from cultures of clostridia labelled as C. acetobutylicum, ‘C. butylicum’, and C. saccharoperbutylacetonicum from several collections have been compared with DNAs from reference strains, including the type strain of C. acetobutylicum and C. beijerinckii. Based on DNA reassociation, which measures sequence similarities, four distinct groups or species (with inter-group similarities below 30%) were identified: (i) those having 〉 80% DNA sequence similarity with the type strain of C. acetobutylicum; (ii) those, including NCIMB 8052, having 〉 70% DNA sequence similarity with the type strain of C. beijerinckii; (iii) two cultures (NRRL B643 and NCP 262) having 94% similarity between them; and (iv) C. saccharoperbutylacetonicum. Identification of four species from these solvent-producing clostridia explains the discrepancies reported by different laboratories, and classification of these bacteria on the basis of their genomic relatedness should facilitate future genetic experiments. It is noteworthy that after the carbon source was switched from starch (corn mash) to sugars (molasses), the industrial solvent fermentation indeed utilized organisms (represented by groups 2, 3, and 4) genetically distinct from C. acetobutylicum.
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