Showing papers by "Utrecht University published in 1988"
TL;DR: In this paper, the authors studied conformal field theories with a finite number of primary fields with respect to some chiral algebra and showed that the fusion rules are completely determined by the behavior of the characters under the modular group.
1,506 citations
TL;DR: The supermembrane can be viewed as a limiting case of a class of models in supersymmetric quantum mechanics and its ground state can be examined in this article, where the complexity of the ground state and various truncations of these models are examined.
939 citations
TL;DR: To characterize the critical M. tuberculosis epitope, AA T-cell clones were used to screen mycobacterial antigens expressed in Escherichia coli and genetically engineered truncated proteins and synthetic peptides and recognized an epitope formed by the amino acids at positions 180–188 in the sequence of a Mycobacterium bovis BCG antigen.
Abstract: Adjuvant arthritis (AA) is a chronic disease inducible in rats by immunization with an antigen of Mycobacterium tuberculosis1. After the isolation of arthritogenic T-cell lines2 and clones3, it became possible to demonstrate that the critical M. tuberculosis antigen contained an epitope cross-reactive with a self-antigen in joint cartilage4–6. Like AA rats, patients suffering from rheumatoid arthritis demonstrated specific T-lymphocyte reactivity to the M. tuberculosis fraction containing the cross-reactive epitope7. To characterize the critical M. tuberculosis epitope we used AA T-cell clones to screen mycobacterial antigens expressed in Escherichia coli and genetically engineered truncated proteins and synthetic peptides. The AA T-cell clones recognized an epitope formed by the amino acids at positions 180–188 in the sequence of a Mycobacterium bovis BCG antigen8. Administration of this antigen to rats induced resistance to subsequent attempts to produce AA.
858 citations
Journal Article•
556 citations
TL;DR: The present article is based on the large amount of sequence data accumulated in these years and focuses on the viral nucleic acids and proteins and their function.
Abstract: Introduction. Progress in coronavirology is illustrated by the number of workshops convened and reviews written. International meetings have been held in Germany (1980), the Netherlands (1983) and the U.S.A. (1986), and the Fourth Coronavirus Symposium will be organized by one of us (D.C.) in Cambridge, U.K. in July 1989. In addition, reviews have appeared which highlighted particularly interesting characteristics of the family, e.g. the replication strategy (Lai, 1986) and the glycoproteins (Sturman & Holmes, 1985). As the last general accounts were published some 5 years ago (Siddell et al., 1983; Sturman & Holmes, 1983) an update is timely. The present article is based on the large amount of sequence data accumulated in these years and focuses on the viral nucleic acids and proteins and their function.
Coronaviruses cause infections in man, other mammals and birds. Most experimental data have been obtained from studies of mouse hepatitis virus (MHV) and infectious bronchitis virus of chickens (IBV).
542 citations
TL;DR: In this article, the behaviour of 38 major and trace elements as well as changes in the mineralogy have been examined in 10 weathering profiles developing on some Portuguese granitic rocks, where element mobilities are calculated from geochemical data normalized with respect to Ti in the fresh parent rock.
512 citations
TL;DR: In this paper, the concentrations of dissolved and suspended particulate rare earth elements in the Cariaco Trench are reported, and the relative turnover rates at the interface are ranked as ======Mn > Ce = Ce-anomaly > Nd, Sm, Eu, La, Dy, Er��.
464 citations
TL;DR: The activity of motor units of the important muscles acting across the elbow joint during combinations of voluntary isometric torques in flexion/extension direction and supination/pronation direction at different angles of the elbow joints is recorded.
Abstract: 1. In this study we have recorded the activity of motor units of the important muscles acting across the elbow joint during combinations of voluntary isometric torques in flexion/extension direction and supination/pronation direction at different angles of the elbow joint. 2. Most muscles are not activated homogeneously; instead the population of motor units of muscles can be subdivided into several subpopulations. Inhomogeneous activation of the population of motor units in a muscle is a general finding and is not restricted to some multifunctional muscles. 3. Muscles can be activated even if their mechanical action does not contribute directly to the external torque. For example, m. triceps is activated during supination torques and thus compensates for the flexion component of the m. biceps. On the other hand, motor units in muscles are not necessarily activated if their mechanical action contributes to a prescribed torque. For example, there are motor units in the m. biceps that are activated during flexion torques, but not during supination torques. 4. The relative activation of the muscles depends on the elbow angle. Changing the elbow angle affects the mechanical advantage of different muscles differently. In general, muscles with the larger mechanical advantage receive the larger input. 5. We have calculated the relative contributions of some muscles to isometric torques. These contributions depend on the combination of the torques exerted. 6. Existing theoretical models on muscle coordination do not incorporate subpopulations of motor units and therefore need to be amended.
395 citations
TL;DR: In this article, the authors derived minimum time estimates for the duration of the Hellenic subduction zone that range from 26 to 40 Ma, which is considerably longer than earlier estimates which vary between 5 and about 13 Ma.
Abstract: New tomographic images of the Hellenic subduction zone demonstrate slab penetration in the Aegean Upper Mantle to depths of at least 600 km. Beneath Greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern Aegean. Schematically we derive minimum time estimates for the duration of the Hellenic subduction zone that range from 26 to 40 Ma. This is considerably longer than earlier estimates which vary between 5 and about 13 Ma.
390 citations
TL;DR: In this paper, the authors study the theory of c=1 torus and ℤ2-orbifold models on general Riemann surfaces and give an algebraic construction of the operator formulation of these nonchiral theories on higher genus surfaces.
Abstract: We study the theory ofc=1 torus and ℤ2-orbifold models on general Riemann surfaces. The operator content and occurrence of multi-critical points in this class of theories is discussed. The partition functions and correlation functions of vertex operators and twist fields are calculated using the theory of double covered Riemann surfaces. It is shown that orbifold partition functions are sensitive to the Torelli group. We give an algebraic construction of the operator formulation of these nonchiral theories on higher genus surfaces. Modular transformations are naturally incorporated as canonical transformations in the Hilbert space.
373 citations
TL;DR: In this article, the authors constructed the algebra of a primary field of dimension 3 constructed from the 3rd order Casimir of AN−1 for N = 3 and compared their results to the Z3-extended Virasoro algebra proposed by Fateev and Zamolodchikov.
11 Jul 1988
TL;DR: For several NP-complete problems, and subclasses of the graphs with bounded treewidth, polynomial algorithms have been obtained.
Abstract: In this paper we study the complexity of graph decision problems, restricted to the class of graphs with treewidth k, (or equivalently, the class of partial k-trees), for fixed k. We introduce two classes of graph decision problems, LCC and ECC, and subclasses C-LCC, and C-ECC. We show that each problem in LCC (or C-LCC) is solvable in polynomial (O(n )) time, when restricted to graphs with fixed upper bounds on the treewidth and degree; and that each problem in ECC (or C-ECC) is solvable in polynomial (O(n )) time, when restricted to graphs with a fixed upper bound on the treewidth (with given corresponding tree-decomposition). Also, problems in C-LCC and C-ECC are solvable in polynomial time for graphs with a logarithmic treewidth, and given corresponding tree-decomposition, and in the case of C-LCC-problems, a fixed upper bound on the degree of the graph. Also, we show for a large number of graph decision problems, their membership in LCC, ECC, C-LCC and/or C-ECC, thus showing the existence of O(n ) or polynomial algorithms for these problems, restricted to the graphs with bounded tree width (and bounded degree). In several cases, C = 1, hence our method gives in these cases linear algorithms. For several NP-complete problems, and subclasses of the graphs with bounded treewidth, polynomial algorithms have been obtained. In a certain sense, the results in this paper unify these results.
TL;DR: It is suggested that with increasing phosphorus or nitrogen availability Molinia outcompetes Erica because the former invests more biomass in leaves which in turn permits more carbon to be allocated to the root system, which thereupon leads to a higher nutrient uptake.
Abstract: A three year fertilization experiment was conducted in which nitrogen (N series: 20 g N m−2 yr−1), phosphorus (P series: 4 g P m−2 yr−1) and potassium (K series: 20 g K m−2 yr−1) were added to a mixed vegetation of Erica tetralix and Molinia caerulea. At the end of each growing season the percentage cover of each species was determined. At the end of the experiment percentage cover of each species was found to be positively correlated with the harvested biomass. In the unfertilized control series the cover of Erica and Molinia did not change significantly during the experiment. In all fertilized series however, especially in the P series, cover of Erica decreased significantly. The cover of Molinia increased significantly in the P series only.
TL;DR: In this article, it was shown that hard spherocylinders with both translational and orientational freedom can form a thermodynamically stable smectic phase in a fluid of hard rod-like particles.
Abstract: One of the most remarkable phenomena exhibited by colloidal suspensions of monodisperse rod-like particles is the spontaneous formation of smectic liquid crystals1–5. In these smectic phases, the particles order in periodic layers; on average, the axes of the rods are perpendicular to the layers. Smectics are distinct from crystals in that there is no long-range positional order within the layers. Because the spacing of the smectic layers is of the order of optical wavelengths, white light is separated into colours when scattered, giving rise to beautiful iridescence as in the colour photographs of ref. 2. As early as 1949, Onsager6 showed that nematic ordering may arise from hard-core repulsions between anisometric particles. However, it appears to have been generally accepted in the literature that smectic liquid crystalline ordering demands that attractive forces also operate7. There is nevertheless experimental evidence that smectic ordering does occur in colloidal systems where the particles interact predominantly through repulsive electrostatic interactions. This observation raises the fundamental question of whether smectic ordering can occur in a system of particles with purely repulsive interactions. Inspired by the seminal work of Alder and Wainwright8 on the freezing of a system of hard spheres, we have explored the possibility that smectic ordering occurs in a fluid of hard rod-like particles. Earlier computer simulations on hard parallel spherocylinders9,10 (cylinders with length L and diameter D, capped at each end with hemispheres of the same diameter) indicated that, in this somewhat artificial system, smectic order was indeed possible. Here we present numerical evidence that hard spherocylinders with both translational and orientational freedom can form a thermodynamically stable smectic phase.
TL;DR: In this article, a cross-national comparison involving West German, Dutch, and United States data was made on the use of social ties in the job-search process, showing that there is more use of informal sources in the U.S. than in West Germany and the Netherlands, and that informal sources do not generally lead to higher occupational prestige and income.
Abstract: This paper gives a cross-national comparison, involving West German, Dutch, and United States data, on the use of social ties in the job-search process. Data for West Germany and the Netherlands are compared with the Lin et al. research on Albany-Schenectady and Troy. The findings show that (1) there is more use of informal sources in the U.S. than in West Germany and the Netherlands; (2) informal sources do not generally lead to higher occupational prestige and income; (3) for the Dutch data (no West German data are available for this question) greater social resources, that is, a contact person with relatively high prestige, do lead to a job with a higher prestige, though effects are less pronounced than found by Lin et al.; and (4) an extension of the Lin et al. analysis shows that these social resources do not have a significant impact on income. This last finding is consistent with the recent results of Marsden and Hurlbert's analysis of Detroit Area Study data. Institutional differences that might be relevant to the explanation of crossnational differences are discussed.
TL;DR: In this paper, a modification of the tectonic evolution for the Cretan segment of the Hellenic arc is proposed, which is based on the idea that compressional tectonics is of much greater importance than current models imply.
TL;DR: The cellular localization of mineralocorticoid receptor (MR) and glucocorting receptor (GR) gene expression in the rat hippocampus was studied by in situ hybridization using 35S‐labeled RNA‐probes.
Abstract: The cellular localization of mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) gene expression in the rat hippocampus was studied by in situ hybridization using 35S-labeled RNA-probes, complementary to either 513 bases of the rat brain mineralocorticoid receptor (MR)-mRNA or 500 bases of the rat liver glucocorticoid receptor (GR)-mRNA. Neurons in CA1, CA2, and the dentate gyrus expressed both receptor genes at high levels. The MR-mRNA was demonstrated in all pyramidal cell fields (CA1-4) of the hippocampal formation and the granular neurons of the dentate gyrus. In contrast, GR-mRNA was mainly restricted to CA1 and CA2 pyramidal cell fields and the dentate gyrus. This pattern of hybridization was found to agree with the cellular distribution of the two types of corticosteroid receptors detected previously in the hippocampus by autoradiography of the radio-labeled receptors and by immunocytochemistry of the receptor protein. These observations suggest that the corticosteroid receptors described previously as type 1 and type 2 are encoded by MR- and GR-mRNA, respectively. Although both the MR and GR genes are co-expressed in some hippocampal neurons, the unique patterns of distribution of the two receptor mRNAs in the hippocampal formation suggest that the genes for these receptors are differentially regulated. Moreover, the microanatomy of MR and GR expression provides insight into molecular mechanisms underlying the characteristic action of various steroids on behaviors involved in stress and circadian regulation.
TL;DR: The development and the reproductive output of 26 Bombus terrestris colonies were investigated and the loss of dominance by the queen was expressed by the beginning of aggression on the part of queen and workers, worker oviposition, oophagy and the functional elimination of the queen.
Abstract: The development and the reproductive output of 26 Bombus terrestris colonies were investigated. Four important points in colony development are distinguished. These are: a) the start of egg laying by the queen, leading to the beginning of the eusocial phase, the emergence of the first workers; b) the moment which we term the switch point, at which the queen switches from laying diploid eggs (producing workers or queens) to the laying of haploid eggs (producing males); c) the onset of queen production reared from diploid eggs; d) the loss of dominance by the queen, expressed by the beginning of aggression on the part of queen and workers, worker oviposition, oophagy and the functional elimination of the queen. This we call the competition point. The onset of queen production is highly correlated with the competition point but not correlated with the switch point. The pattern in time of the egg laying by a queen differed from the rate of increase of worker numbers published by previous authors. After an initial and slow phase, in which two broods of egg cells are produced, the rate of egg laying of a queen becomes constant, whether or not the eggs are fertilized. The transformation from this rate of egg laying to the earlier descriptions of increase in number is only possible if one neglects differences between colonies in time spent in the production of the two broods as well as the highly variable time needed for development into adults. The time at which the competition point occurs is much more predictable (at day 30.8 ± 4.9 after the emergence of the first worker) than the switch point. The latter ranges from day 6 to day 32. In our colonies two groups can be discerned, one of early switching colonies (at day 9.8 ± 2.4), the other of late switching colonies (at day 23.4 ± 4.6). The occurrence of the switch cannot be predicted from preceding behavioural or demographic data. As a consequence of the early switch such colonies produce mainly males. Partly due to the unfavourable larva/worker ratio only a few queens were reared from the last laid diploid eggs. In contradiction the late switching colonies produce on average sexuals at a 1.3:1 sex ratio (1:1.7 investment ratio). Remarkable however, biomass of sexuals is equal in both types of colonies. Males are about half the weight of queens. Certain colonies invest in males only ("3 males : 0 queens"). Since the two types of colonies occurred in about equal numbers, our local population is characterized by a 4:1 1(♂:♀) sex ratio. This male biased sex ratio, also observed for other bumblebees species (B. terricola and B. melanopygus, OWEN et al., 1980; OWEN & PLOWRIGHT, 1982), contrasts with existing theoretical models based on kin selection (TRIVERS & HARE, 1976) or local mate competition (ALEXANDER & SHERMAN, 1977) arguments. A model is proposed in which the males of the early switching colonies monopolize the matings due to their early appearance in the field. In fact, the early switching colonies, by investing mainly in males, parasitize on the reproductive strategy of the late switching colonies, which are characterized by a 1:1 sex ratio. At population level the two reproductive strategies are in equilibrium at a 1:1 frequency.
TL;DR: A 65 kD-protein-specific reactivity of synovial T cells, mainly present in an early stage of joint inflammation, may be responsible for triggering chronic arthritis.
TL;DR: In this article, extensions of the Virasoro algebra obtained by generalizing the Sugawara construction to higher order Casimir invariants of a Lie algebra g are discussed, with branching rules and generalizations to all simple, simply-laced g.
TL;DR: It is argued that the long‐latency reflex activity reflects the co‐ordinated activation of muscles which is necessary for an adequate response to torque perturbations, and this hypothesis is supported by the fact that the distribution of long-latency Reflex activity over muscles was found to be similar to the Distribution of activation found during a voluntary exerted torque in the direction opposite to the applied perturbation.
Abstract: 1. Reflex activity in human arm muscles has been measured in response to torque perturbations around the elbow joint in the flexion-extension direction and/or the supination-pronation direction. Intramuscular fine-wire electrodes were used to record electromyographic (EMG) activity in the muscles. A pre-load was applied in the same direction as or in a different direction to the perturbation. The subjects were instructed to 'hold on', which means that they had to actively resist the perturbation without unduly co-activating their muscles. 2. The EMG activity showed segmented reflex responses with short-latency (25-50 ms) and long-latency (50-75 ms) components, followed by other bursts of activity which probably originated from the subject's voluntary or triggered reactions. 3. Motor units in m. triceps gave short-latency and long-latency responses to imposed elbow extension but only long-latency responses were seen to imposed pronation, which does not stretch m. triceps. Motor units in m. brachialis gave short-latency and long-latency excitatory (inhibitory) responses to imposed extension (flexion) torques. However, only long-latency inhibitory responses were observed to imposed pronation torques. Motor units in m. biceps gave short-latency and long-latency excitatory responses to imposed pronation torques. 4. These results show that the long-latency reflex cannot be the result of a simple feed-back mechanism that controls muscle length only. We argue that the long-latency reflex activity reflects the co-ordinated activation of muscles which is necessary for an adequate response. This hypothesis is supported by the fact that the distribution of long-latency reflex activity over muscles was found to be similar to the distribution of activation found during a voluntary exerted torque in the direction opposite to the applied perturbation.
01 Jun 1988
TL;DR: A method for modeling herbaceous plants, suitable for generating realistic plant images and animating developmental processes, is presented using the formalism of L-systems.
Abstract: In this paper we present a method for modeling herbaceous plants, suitable for generating realistic plant images and animating developmental processes. The idea is to achieve realism by simulating mechanisms which control plant growth in nature. The developmental approach to the modeling of plant architecture is extended to the modeling of leaves and flowers. The method is expressed using the formalism of L-systems.
TL;DR: A new UVB fluorescent sunlamp, the Philips TL‐01 lamp, which emits a narrow peak around 311–312 nm, with the currently used PhilipsTL‐I2 lamp, is compared in patients with psoriasis.
Abstract: We have compared the therapeutic effectiveness of a new UVB fluorescent sunlamp, the Philips TL-01 lamp, which emits a narrow peak around 311-312 nm, with the currently used Philips TL-12 lamp, in 10 patients with psoriasis. We also compared the tumour inducing capacity of the two lamps in hairless mice. The therapeutic effect of the TL-01 lamp was superior to that of the TL-12 lamp in nine of the 10 patients. In the mice, the median tumour induction time was significantly longer in animals exposed to the TL-01 lamp. Phototherapy with the new type of lamp requires a higher dose than phototherapy with the usual broadband UVB sources. In practice this means that more lamps are needed in the light cabinet. However, the new lamps appear to provide more effective and safer phototherapy for psoriasis.
TL;DR: An overview of the current understanding of the dynamics of the hypothalamuspituitary-adrenal (HPA) system in the adult rat is provided and the implications of animal studies for the field of human functional teratology are considered.
Abstract: Publisher Summary This chapter provides an overview of the current understanding of the dynamics of the hypothalamuspituitary-adrenal (HPA) system in the adult rat The chapter particularly emphasizes on the role of glucocorticoid receptors in this process In the adult organism, glucocorticoids (GCs) serve a wide variety of regulatory and permissive functions, aimed basically at controlling the organism's responses to stress, and at regulating circadian-driven activities The principal GC synthesized by the rat adrenal cortex is corticosterone (CORT) CORT levels display a pronounced circadian rhythmicity: they are highest immediately preceding the animal's active period, and lowest at the end of this period During development, however, GCs have also been shown to produce, in experimental animals, permanent effects on growth and differentiation of a number of systems, including the central nervous system In rats, for example, high doses of GCs administered neonatally cause a decrease in mitosis and myelination, as well as altered neural morphogenesis In addition, the chapter also reviews some of the relevant human literature and considers the implications of animal studies for the field of human functional teratology
TL;DR: Growth and morphology of seedlings of ten tropical rain forest species showed a complex growth response in which they resembled the shade intolerant extreme in some aspects of the response, and the shade tolerant extreme in other aspects.
Abstract: Growth and morphology of seedlings of ten tropical rain forest species were studied at Los Tuxtlas, Mexico. Seedlings were grown in three environmental conditions: the shaded forest understorey (FU, receiving 0.9-2.3% of the daily photosynthetic photon flux, PF, above the canopy), a small canopy gap of approx. 50 m2 (SG, receiving 2.1-6.1% of daily PF), and a large canopy gap of approx. 500 m2 (LG, receiving 38.6-53.4% of daily PF). The growth of all species was enhanced in gaps, and in LG the effect was stronger than in SG. Plants grown in LG had a sunplant morphology, with a high root-shoot ratio (R/S), a high specific leaf weight (SLW) and a low leaf area ratio (LAR). Plants grown in SG or FU showed a shade-plant morphology, with a low R/S, a low SLW and a high LAR. Growth responses varied from species unable to grow in the shade but with strong growth in the sun, to species with relatively high growth rates in both shade and sun conditions. Shade tolerant species were able to grow in the shade because of a relatively high unit leaf rate. The pioneerCecropia had a high growth rate in LG because of a high LAR. Most species showed a complex growth response in which they resembled the shade intolerant extreme in some aspects of the response, and the shade tolerant extreme in other aspects.
TL;DR: It is concluded that MCF7 cells require insulin (or insulin‐like growth factors) for proliferation, and at suboptimal insulin concentrations, E2 acts synergistically with insulin, possibly by inducing autocrine production of polypeptide growth factors by these cells.
Abstract: The cooperative action of 17 beta-estradiol (E2) and polypeptide growth factors in stimulating proliferation of human breast cancer cells in vitro was investigated. To prevent background estrogenic stimulation, only phenol red-free media were used. When cultured in media supplemented with steroid-stripped serum in which all polypeptide growth factor activity had been chemically inactivated, MCF7 cells were unable to proliferate and became virtually quiescent. In the additional presence of insulin, epidermal growth factor (EGF), and E2, however, cells proliferated as rapidly as did cells cultured in media supplemented with fetal calf serum. Analysis by DNA flow cytometry showed that in the absence of external growth factors, MCF7 cells became arrested predominantly in the G1/G0 phase of the cell cycle. Upon addition of insulin in combination with EGF and E2, however, cells reentered the cell cycle with a high degree of synchrony. When added alone, E2 induced only slight mitogenic effects under these growth factor-defined conditions. In contrast, this steroid induced optimal proliferation in conventional steroid-stripped serum, which in itself contained considerable mitogenic activity. Insulin (at 10 micrograms/ml) was the most potent stimulator of MCF7 cell proliferation under growth factor-defined conditions, resulting in a more than sixfold increase in cell number after 96 hours. Other growth factors such as platelet-derived growth factor (PDGF), transforming growth factor beta (TGF beta), and EGF had little effect by themselves and only slightly influenced insulin-induced proliferation. At suboptimal concentrations of insulin (10-100 ng/ml), however, strong synergism was observed between E2 and insulin in inducing MCF7 proliferation. Using the CG5 cell line, a highly E2-sensitive MCF7 variant, synergism with E2 was already observed at 1 ng/ml insulin. It is concluded that MCF7 cells require insulin (or insulin-like growth factors) for proliferation. At suboptimal insulin concentrations, E2 acts synergistically with insulin, possibly by inducing autocrine production of polypeptide growth factors by these cells.
TL;DR: A high-resolution integrated biostratigraphy of the Cretaceous/Tertiary boundary interval at El Haria (northwest Tunisia) is presented in this paper.
TL;DR: It is demonstrated here, using mutants of Escherichia coli defective in the synthesis of the major anionic membrane phospholipids, that phosphatidylglycerol is involved in the translocation of newly synthesized outer-membrane proteins across the inner membrane.
Abstract: Newly synthesized proteins to be exported out of the cytoplasm of bacterial cells have to pass across the inner membrane. In Gram-negative bacteria ATP, a membrane potential, the products of the sec genes and leader peptidases (enzymes which cleave the N-terminal signal peptides of the precursor proteins) are required. The mechanism of translocation, however, remains elusive. Important additional roles for membrane lipids have been repeatedly suggested both on theoretical grounds and on the basis of experiments with model systems but no direct evidence had been obtained. We demonstrate here, using mutants of Escherichia coli defective in the synthesis of the major anionic membrane phospholipids, that phosphatidylglycerol is involved in the translocation of newly synthesized outer-membrane proteins across the inner membrane.
AD - Department of Biochemistry, University of Utrecht, The Netherlands.