Showing papers by "Bielefeld University published in 1993"

Common randomness in information theory and cryptography. I. Secret sharing

Abstract: As the first part of a study of problems involving common randomness at distance locations, information-theoretic models of secret sharing (generating a common random key at two terminals, without letting an eavesdropper obtain information about this key) are considered. The concept of key-capacity is defined. Single-letter formulas of key-capacity are obtained for several models, and bounds to key-capacity are derived for other models. >

On sparse spanners of weighted graphs

Abstract: Given a graphG, a subgraphG' is at-spanner ofG if, for everyu,v ?V, the distance fromu tov inG' is at mostt times longer than the distance inG. In this paper we give a simple algorithm for constructing sparse spanners for arbitrary weighted graphs. We then apply this algorithm to obtain specific results for planar graphs and Euclidean graphs. We discuss the optimality of our results and present several nearly matching lower bounds.

The transition state of the f + h2 reaction.

Abstract: The transition state region of the F + H(2) reaction has been studied by photoelectron spectroscopy of FH(2)(-). New para and normal FH(2)(-)photoelectron spectra have been measured in refined experiments and are compared here with exact three-dimensional quantum reactive scattering simulations that use an accurate new ab initio potential energy surface for F + H(2). The detailed agreement that is obtained between this fully ab initio theory and experiment is unprecedented for the F + H(2) reaction and suggests that the transition state region of the F + H(2) potential energy surface has finally been understood quantitatively.

Identification of a new class of nitrogen fixation genes in Rhodobacter capsulatus: a putative membrane complex involved in electron transport to nitrogenase

Abstract: DNA sequence analysis of a 12236 bp fragment, which is located upstream of nifE in Rhodobacter capsulatus nif region A, revealed the presence of ten open reading frames With the exception of fdxC and fdxN, which encode a plant-type and a bacterial-type ferredoxin, the deduced products of these coding regions exhibited no significant homology to known proteins Analysis of defined insertion and deletion mutants demonstrated that six of these genes were required for nitrogen fixation Therefore, we propose to call these genes rnfA, rnfB, rnfC, rnfD, rnfE and rnfF (for Rhodobacter nitrogen fixation) Secondary structure predictions suggested that the rnf genes encode four potential membrane proteins and two putative iron-sulphur proteins, which contain cysteine motifs (C-X2-C-X2-C-X3-C-P) typical for [4Fe--4S] proteins Comparison of the in vivo and in vitro nitrogenase activities of fdxN and rnf mutants suggested that the products encoded by these genes are involved in electron transport to nitrogenase In addition, these mutants were shown to contain significantly reduced amounts of nitrogenase The hypothesis that this new class of nitrogen fixation genes encodes components of an electron transfer system to nitrogenase was corroborated by analysing the effect of metronidazole Both the fdxN and rnf mutants had higher growth yields in the presence of metronidazole than the wild type, suggesting that these mutants contained lower amounts of reduced ferredoxins

Blue shift of the Mie plasma frequency in Ag clusters and particles

Abstract: Photodepletion spectroscopy is used to measure the optical activity of sputtered Ag ionic clusters with up to 70 atoms. With decreasing cluster size the giant resonance caused by collective excitation of the valence electrons shifts to higher frequencies. This blue shift is qualitatively explained in terms of a reduced s-d screening interaction in the surface region of the particles.

Implementation of self-organizing neural networks for visuo-motor control of an industrial robot

Abstract: The implementation of two neural network algorithms for visuo-motor control of an industrial robot (Puma 562) is reported. The first algorithm uses a vector quantization technique, the 'neural-gas' network, together with an error correction scheme based on a Widrow-Hoff-type learning rule. The second algorithm employs an extended self-organizing feature map algorithm. Based on visual information provided by two cameras, the robot learns to position its end effector without an external teacher. Within only 3000 training steps, the robot-camera system is capable of reducing the positioning error of the robot's end effector to approximately 0.1% of the linear dimension of the work space. By employing adaptive feedback the robot succeeds in compensating not only slow calibration drifts, but also sudden changes in its geometry. Hardware aspects of the robot-camera system are discussed. >

Plant defense and delayed infection of alfalfa pseudonodules induced by an exopolysaccharide (EPS I)-deficient Rhizobium meliloti mutant

Abstract: Mutants of the symbiotic soil bacterium Rhizobium meliloti that fail to synthesize the acidic exopolysaccharide EPS I were unable to induce infected root nodules on Medicago sativa L. (alfalfa). These strains, however, elicited pseudonodules that contained no infection threads or bacteroids. The cortical cell walls of the pseudonodules were abnormally thick and incrusted with an autofluorescent material. Parts of these cell walls and wall appositions contained callose. Biochemical analysis of nodules induced by the EPS I-deficient R. meliloti mutant revealed an increase of phenolic compounds bound to the nodule cell walls when compared with the wild-type strain. These microscopic and biochemical data indicated that a general plant defence response against the EPS I-deficient mutant of R. meliloti was induced in alfalfa pseudonodules. Following prolonged incubation with the EPS I-deficient R. meliloti mutant, the defence system of the alfalfa plant could be overcome by the rhizobium mutant. In the case of the delayed infections, the mutants colonized lobes of the pseudonodules, but the infection threads in these nodules had an abnormal morphology. They were greatly enlarged and did not contain the typical gum-like matrix inside. The bacteria were tightly packed. Based on the mechanism of phytopathogenic interactions, we propose that EPS I or a related compound may act as a suppressor of the alfalfa plant defence system, enabling R. meliloti to infect the plant.

Two loop two point functions with masses: Asymptotic expansions and Taylor series, in any dimension

Abstract: In all mass cases needed for quark and gluon self-energies, the two-loop master diagram is expanded at large and smallq2, ind dimensions, using identities derived from integration by parts Expansions are given, in terms of hypergeometric series, for all gluon diagrams and for all but one of the quark diagrams; expansions of the latter are obtained from differential equations Pade approximants to truncations of the expansions are shown to be of great utility As an application, we obtain the two-loop photon self-energy, for alld, and achieve highly accelerated convergence of its expansions in powers ofq2/m2 orm2/q2, ford=4

Thermal hadron production in high energy heavy ion collisions

Abstract: We provide a method to test if hadrons produced in high energy heavy ion collisions were emitted at freeze-out from an equilibrium hadron gas. Our considerations are based on an ideal gas at fixed temperatureTf, baryon number densitynB, and vanishing total strangeness. The constituents of this gas are all hadron resonances up to a mass of 2 GeV; they are taken to decay according to the experimentally observed branching ratios. The ratios of the various resulting hadron production rates are tabulated as functions ofTf andnB. These tables can be used for the equilibration analysis of any heavy ion data; we illustrate this for some specific cases.

Fibroblast and neutrophil collagenases cleave at two sites in the cartilage aggrecan interglobular domain.

Abstract: The actions of recombinant human fibroblast collagenase (MMP1), purified polymorphonuclear leucocyte collagenase (MMP8) and their N-terminal catalytic domain fragments against cartilage aggrecan and an aggrecan G1-G2 fragment have been investigated in vitro. After activation with recombinant human stromelysin and typsin, both collagenases were able to degrade human and porcine aggrecans to a similar extent. An N-terminal G1-G2 fragment (150 kDa) was used to identify specific cleavage sites occurring within the proteinase-sensitive interglobular domain between G1 and G2. Two specific sites were found; one at an Asn341-Phe342 bond and another at Asp441-Leu442 (human sequence). This specificity of the collagenases for aggrecan G1-G2 was identical with that of the truncated metalloproteinase matrilysin (MMP7), but different from those of stromelysin (MMP3) and the gelatinases (MMP2 or gelatinase A; MMP9 or gelatinase B) which cleave at the Asn-Phe site, but not the Asp-Leu site. In addition, collagenase catalytic fragments lacking C-terminal hemopexin-like domains were tested and shown to exhibit the same specificities for the G1-G2 fragment as the full-length enzymes. Thus the specificity of the collagenases for cartilage aggrecan was not influenced by the presence or absence of the C-terminal domain. Together with our previous findings, the results show that stromelysin-1, matrilysin, gelatinases A and B and fibroblast and neutrophil collagenases cleave at a common, preferred site in the aggrecan interglobular domain, and additionally that both fibroblast and neutrophil collagenases cleave at a second site in the interglobular domain that is not available to stromelysin or gelatinases.

Evidence for plasmid-encoded virulence factors in the phytopathogenic bacterium Clavibacter michiganensis subsp. michiganensis NCPPB382.

Abstract: The tomato pathogen Clavibacter michiganensis subsp. michiganensis NCPPB382, which causes bacterial wilt, harbors two plasmids pCM1 (27.5 kb) and pCM2 (72 kb). After curing of the plasmids, bacterial derivatives were still proficient in the ability to colonize the host plant and in the production of exopolysaccharides but exhibited a reduced virulence. When one of the two plasmids is lost, there is a significant delay in the development of wilting symptoms after infection and a plasmid-free derivative is not able to induce disease symptoms. By cloning of restriction fragments of both plasmids in the plasmid-free strain CMM100, two DNA fragments which restored the virulent phenotype were identified. Further analysis suggested that a fragment of plasmid pCM1 encodes an endocellulase which is involved in the expression of the pathogenic phenotype.

Characterization of Rhodobacter capsulatus genes encoding a molybdenum transport system and putative molybdenum-pterin-binding proteins.

Abstract: The alternative, heterometal-free nitrogenase of Rhodobacter capsulatus is repressed by traces of molybdenum in the medium. Strains carrying mutations located downstream of nifB copy II were able to express the alternative nitrogenase even in the presence of high molybdate concentrations. DNA sequence analysis of a 5.5-kb fragment of this region revealed six open reading frames, designated modABCD, mopA, and mopB. The gene products of modB and modC are homologous to ChlJ and ChlD of Escherichia coli and represent an integral membrane protein and an ATP-binding protein typical of high-affinity transport systems, respectively. ModA and ModD exhibited no homology to known proteins, but a leader peptide characteristic of proteins cleaved during export to the periplasm is present in ModA, indicating that ModA might be a periplasmic molybdate-binding protein. The MopA and MopB proteins showed a high degree of amino acid sequence homology to each other. Both proteins contained a tandem repeat of a domain encompassing 70 amino acid residues, which had significant sequence similarity to low-molecular-weight molybdenum-pterin-binding proteins from Clostridium pasteurianum. Compared with that for the parental nifHDK deletion strain, the molybdenum concentrations necessary to repress the alternative nitrogenase were increased 4-fold in a modD mutant and 500-fold in modA, modB, and modC mutants. No significant inhibition of the heterometal-free nitrogenase by molybdate was observed for mopA mopB double mutants. The uptake of molybdenum by mod and mop mutants was estimated by measuring the activity of the conventional molybdenum-containing nitrogenase. Molybdenum transport was not affected in a mopA mopB double mutant, whereas strains carrying lesions in the binding-protein-dependent transport system were impaired in molybdenum uptake.

Identification and analysis of the Rhizobium meliloti exoAMONP genes involved in exopolysaccharide biosynthesis and mapping of promoters located on the exoHKLAMONP fragment.

Abstract: Sequence analysis of a 7494 kb DNA fragment from megaplasmid 2 of Rhizobium meliloti 2011 involved in exopolysaccharide I (EPS 1) biosynthesis revealed the presence of five exo genes designated exoA, exoM, exoN, exoO, and exoP ExoN was found to show strong homology to a UDP-glucose pyrophosphorylase from Acetobacter xylinum, whereas ExoO displayed weak homologies to the NodC proteins from R meliloti and R loti Supprisingly, different mutations in exoP resulted in divergent phenotypes One exoP mutant was able to establish an effective symbiosis with alfalfa, although no EPS I polymer could be detected In contrast, other exoP mutations prevented the formation of an effective symbiosis The transcriptional organization of the exoA-exoP gene region has been analysed in conjunction with the exoH, exoK and exoL genes Using exo-lacZ transcription fusions in association with plasmid integration mutagenesis a strong promoter was identified upstream of exoH, which is able to direct transcription of the whole exoHKLAMONP gene cluster A much weaker promoter upstream of exoL was found to be involved in the transcription of the exoLAMONP genes In addition, weak promoters were identified upstream of exoK, exoA, exoN and exoP

Genetic analysis of the Rhizobium meliloti exoYFQ operon: ExoY is homologous to sugar transferases and ExoQ represents a transmembrane protein.

Abstract: The nucleotide sequence of a 4.8-kb ClaI-EcoRI DNA fragment of megaplasmid 2 of Rhizobium meliloti Rm2011 involved in succinoglucan (EPS I) synthesis and nodule infection was determined. Four open reading frames (ORFs) were identified on this fragment. A mutational analysis revealed that these ORFs represent genes that were termed exoX, exoY, exoF, and exoQ. The locations of transposon insertions in these exo genes were determined at the nucleotide level. Plasmid integration mutagenesis revealed that the genes exoY, exoF, and exoQ are organized in an operon. The exoX gene running in opposite direction forms a monocistronic transcriptional unit. The exoX gene was shown to negatively influence the amount of EPS I synthesized. The exoY gene is coding for a membrane associated protein homologous to the C-terminal part of the Xanthomonas campestris glucosyltransferase GumD and the Salmonella typhimurium galactose transferase RfbP. ExoF, a probable periplasmatic protein, is nearly identical to the protein encoded by ORF1 of Rhizobium sp. strain NGR234. ExoQ is most probably a membrane associated protein as deduced by its hydrophobic structural features. All three genes of the exoYFQ operon were shown to be essential for succinoglucan synthesis and nodule infection.

Non-Abelian Debye mass at next-to-leading order.

Abstract: It is shown that, after a resummation of leading high-temperature contributions, a complete and gauge-independent result for the non-Abelian Debye screening mass at next-to-leading order can be extracted from the static gluon propagator. In contrast with previous, incomplete results, the correction to the Debye mass is found to be logarithmically sensitive to the nonperturbative magnetic mass and positive, in accordance with recent high-statistics results from lattice calculations.

nif gene expression studies in Rhodobacter capsulatus: ntrC-independent repression by high ammonium concentrations.

Abstract: The expression of nif genes in Rhodobacter capsulatus depends on the two regulatory genes, rpoN and nifA, encoding a nif-specific alternative sigma factor of RNA polymerase and a nif-specific transcriptional activator, respectively. The expression of the rpoN gene itself is also RPON/NIFA dependent. In order to better characterize the regulation of nif gene induction, chromosomal nifH-, rpoN-, nifA1- and nifA2- lacZ fusions were constructed and the expression of these different nif-lacZ fusions was determined under photoheterotrophic conditions at different starting ammonium concentrations. The two nifA genes were found to be induced first, followed by nifH and finally by rpoN upon weak, medium and strong nitrogen starvation, respectively. This induction profile and the correlation between the expression of the different nif genes suggested that nifA1 expression is the limiting factor for nif gene induction. This hypothesis was tested by construction of different nifA1 overexpressing mutants. Contrary to the current model of nif gene expression in R. capsulatus, which predicted constitutive nif gene expression in such mutants, a strong repression of nifH and rpoN was found at high ammonium concentration. The low nifH expression under these conditions is unaffected by nifA2 and is not increased in a ntrC mutant, ruling out any role of NTRC as a mediator of this repression. This finding implies an additional, so far unidentified, regulation by fixed nitrogen in R. capsulatus. Changing the expression level of rpoN indicated that low levels of RPON are already sufficient for full nifH induction. The nifA1 and rpoN expression mutants were also tested for diazotrophic growth. Similar generation times were determined for the mutants and for the wild type, but diazotrophic growth of the nifA1 over-expressing ntrC mutant RCM14 did not start until after a prolonged lag phase of two to three days.

The Composition Algebra of a Cyclic Quiver

Abstract: Note that A is also called the quiver of type An_x with cyclic orientation. We denote by Ao the set of vertices of A (and often we will identify Ao with Z/nZ, or also with the set {xx, x2, •-, xn) or just with {1, 2, ..., n}, with arrows xt—>xi+l or i->i + l). There are n one-dimensional representations, corresponding to the vertices of A; these are (up to isomorphism) all the simple objects of T. The simple representation corresponding to the vertex a of A will be denoted by 5(a), and if S' is isomorphic to S(a), we will write [5'] = a. Given a simple representation 5, and I eNu there is (up to isomorphism) a unique indecomposable representation S[l] of length / with top 5, and we obtain in this way all indecomposable representations (again up to isomorphism). It follows that we can index the isomorphism classes in T by the set IT of n-tuples of partitions; the representation of A corresponding to n e U. will be denoted by M{JZ); see § 3.3.

Simulation of an ensemble with varying magnetic field: A numerical determination of the order-order interface tension in the D=2 Ising model.

Abstract: In analogy with a recently proposed multicanonical ensemble we introduce an ensemble where the partition function is simulated with a term in the action containing a varying magnetic field. Using this ensemble we demonstrate on lattices with periodic boundary conditions that it is possible to enhance the appearance of order-order interfaces by many orders of magnitude. To perform a stringent test of the method we consider the D=2 Ising model at \ensuremath{\beta}=0.5 and simulate square lattices up to size 100\ifmmode\times\else\texttimes\fi{}100. By a finite-size scaling analysis, the order-order interface tension per unit area is obtained. Our best infinite-volume extrapolation is in excellent agreement with Onsager's exact result.

"Was ist der Fall?" und "Was steckt dahinter?" - Die zwei Soziologien und die Gesellschaftstheorie

Abstract: Zusammenfassung Seit dem Beginn ihrer akademischen Karriere hat die Soziologie sich ihrem Gegenstand auf zwei verschiedene Weisen genähert: in einer positivistischen und einer kritischen Einstellung. Bedeutende Theorien, etwa die von Karl Marx oder die von Emile Dürkheim, haben jeweils eine Seite dieser Unterscheidung bevorzugt, konnten dabei aber die andere Seite nicht ignorieren. Als empirische Wissenschaft hat die Soziologie ein Interesse an latenten Strukturen entwickelt, als kritische Theorie ein Interesse an inkongruenten Perspektiven, die erklären konnten, daß die soziale Realität nicht das ist, als was sie erscheint. Jeder Versuch, auf der Basis dieser Unterscheidung eine einheitliche Theorie der Gesellschaft aufzubauen, mußte deshalb in ein Paradox führen: Vorderseite und Rückseite, manifeste und latente Strukturen hätten dann als Dasselbe dargestellt werden müssen. Unter diesen Vorgaben war es daher nicht möglich, eine Theorie der Gesellschaft zu entwickeln, die der fachlichen wie der öffentlichen Nachfrage hätte entsprechen können. Zur Zeit scheint sich diese Ausgangslage zu ändern in einer Radikalität, über die sich die Soziologie noch nicht im klaren zu sein scheint. Interdisziplinäre Diskussionen erörtern Theorien selbstreferentieller Systeme, autopoietische Schließung, Kybernetik zweiter Ordnung als Kybernetik beobachtender Systeme und konstruktivistische Voraussetzungen für Informationserarbeitung und Erkenntnis. Diese Anregungen könnten genutzt werden, um die Gesellschaft als ein sich selbst beobachtendes System zu begreifen, das seine eigene Identität definiert, aber in dieser Selbstbeschreibung zugleich einen imaginären „unmarked space“ erzeugt, der genutzt werden könnte, um das System in ganz anderer Weise zu unterscheiden und zu beschreiben.

Large deviations and the maximum entropy principle for marked point random fields

Abstract: We establish large deviation principles for the stationary and the individual empirical fields of Poisson, and certain interacting, random fields of marked point particles in ℝd. The underlying topologies are induced by a class of not necessarily bounded local functions, and thus finer than the usual weak topologies. Our methods yield further that the limiting behaviour of conditional Poisson distributions, as well as certain distributions of Gibbsian type, is governed by the maximum entropy principle. We also discuss various applications and examples.

Characterization of anf genes specific for the alternative nitrogenase and identification of nif genes required for both nitrogenases in Rhodobacter capsulatus

Abstract: To identify Rhodobacter capsulatus nif genes necessary for the alternative nitrogenase, strains carrying defined mutations in 32 genes and open reading frames of nif region A, B or C were constructed. The ability of these mutants to grow on nitrogen-free medium with molybdenum (Nif phenotype) or in a nifHDK deletion background on medium without molybdenum (Anf phenotype) was tested. Nine nif genes and nif-associated coding regions are absolutely essential for the alternative nitrogenase. These genes comprise nifV and nifB, the nif-specific ntr system (nifR1, R2, R4) and four open reading frames, which exhibit no homology to known genes. In addition, a significantly reduced activity of both the alternative nitrogenase and the molybdenum-dependent nitrogenase was found for fdxN mutants. By random Tn5 mutagenesis of a nifHDK deletion strain 42 Anf- mutants were isolated. Southern hybridization experiments demonstrated that 17 of these Tn5 mutants were localized in at least 13 different restriction fragments outside of known nif regions. Ten different Anf- Tn5 mutations are clustered on a 6 kb DNA fragment of the chromosome designated anf region A. DNA sequence analysis revealed that this region contained the structural genes of the alternative nitrogenase (anfHDGK). The identification of several Tn5 insertions mapping outside of anf region A indicated that at least 10 genes specific for the alternative nitrogenase are present in R. capsulatus.

A 3.9-kb DNA region of Xanthomonas campestris pv. campestris that is necessary for lipopolysaccharide production encodes a set of enzymes involved in the synthesis of dTDP-rhamnose.

Abstract: By mutational analysis it was found that a 3.9-kb SmaI-XhoII DNA fragment of Xanthomonas campestris pv. campestris is involved in lipopolysaccharide (LPS) biosynthesis. LPS samples isolated from different mutants carrying mutations in the 3.9-kb SmaI-XhoII DNA fragment exhibited banding patterns in silver-stained sodium dodecyl sulfate-polyacrylamide gels markedly different from that of the wild-type LPS. Moreover, comparison of the monosaccharide composition obtained by high-performance anion-exchange chromatography with pulsed amperometric detection of LPS purified from wild-type Xanthomonas campestris pv. campestris B100 and from mutants with mutations in the 3.9-kb SmaI-XhoII DNA fragment revealed a lack of rhamnose moieties in the mutant LPS. Sequence analysis of this DNA fragment revealed four open reading frames (ORFs), designated ORF302, ORF183, ORF295, and ORF351. The deduced amino acid sequences of these ORFs showed a high degree of homology to the deduced amino acid sequences of the rfbC, rfbD, rfbA, and rfbB genes of Salmonella typhimurium LT2, which have been shown to encode a set of enzymes responsible for conversion of glucose 1-phosphate to dTDP-rhamnose.