33,954 research outputs found
Overexpression of beta-carotene hydroxylase enhances stress tolerance in Arabidopsis
Plant stress caused by extreme environmental conditions is already a principal reason for yield reduction in crops. The threat of global environment change makes it increasingly important to generate crop plants that will withstand such conditions. Stress, particularly stress caused by increased sunlight, leads to the production of reactive oxygen species that cause photo-oxidative cell damage. Carotenoids, which are present in the membranes of all photosynthetic organisms, help protect against such light-dependent oxidative damage. In plants, the xanthophyll cycle (the reversible interconversion of two carotenoids, violaxanthin and zeaxanthin) has a key photoprotective role and is therefore a promising target for genetic engineering to enhance stress tolerance. Here we show that in Arabidopsis thaliana overexpression of the chyB gene that encodes -carotene hydroxylase—an enzyme in the zeaxanthin biosynthetic pathway—causes a specific twofold increase in the size of the xanthophyll cycle pool. The plants are more tolerant to conditions of high light and high temperature, as shown by reduced leaf necrosis, reduced production of the stress indicator anthocyanin and reduced lipid peroxidation. Stress protection is probably due to the function of zeaxanthin in preventing oxidative damage of membranes
Citizenship, community, and counter-terrorism : UK security discourse, 2001-2011
This paper analyses a corpus of UK policy documents which sets out national security policy as an exemplar of the contemporary discourse of counter-terrorism in Europe, the USA and worldwide. A corpus of 148 documents (c. 2.8 million words) was assembled to reflect the security discourse produced by the UK government before and after the 7/7 attacks on the London Transport system. To enable a chronological comparison, the two sub-corpora were defined: one relating to a discourse of citizenship and community cohesion (2001-2006); and one relating to the ‘Preventing Violent Extremism’ discourse (2007-2011). Wordsmith Tools (Scott 2008) was used to investigate keywords and patterns of collocation. The results present
themes emerging from a comparative analysis of the 100 strongest keywords in each sub- corpus; as well as a qualitative analysis of related patterns of the collocation, focusing inparticular on features of connotation and semantic prosody
How Efficient is Rotational Mixing in Massive Stars ?
The VLT-Flames Survey for Massive Stars (Evans05,Evans06) provides recise
measurements of rotational velocities and nitrogen surface abundances of
massive stars in the Magellanic Clouds. Specifically, for the first time, such
abundances have been estimated for stars with significant rotational
velocities. This extraordinary data set gives us the unique possibility to
calibrate rotationally and magnetically induced mixing processes. Therefore, we
have computed a grid of stellar evolution models varying in mass, initial
rotational velocity and chemical composition. In our models we find that
although magnetic fields generated by the Spruit-Taylor dynamo are essential to
understand the internal angular momentum transport (and hence the rotational
behavior), the corresponding chemical mixing must be neglected to reproduce the
observations. Further we show that for low metallicities detailed initial
abundances are of prime importance, as solar-scaled abundances may result in
significant calibration errors.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 3 figure
How Efficient is Rotational Mixing in Massive Stars ?
The VLT-Flames Survey for Massive Stars (Evans05,Evans06) provides recise
measurements of rotational velocities and nitrogen surface abundances of
massive stars in the Magellanic Clouds. Specifically, for the first time, such
abundances have been estimated for stars with significant rotational
velocities. This extraordinary data set gives us the unique possibility to
calibrate rotationally and magnetically induced mixing processes. Therefore, we
have computed a grid of stellar evolution models varying in mass, initial
rotational velocity and chemical composition. In our models we find that
although magnetic fields generated by the Spruit-Taylor dynamo are essential to
understand the internal angular momentum transport (and hence the rotational
behavior), the corresponding chemical mixing must be neglected to reproduce the
observations. Further we show that for low metallicities detailed initial
abundances are of prime importance, as solar-scaled abundances may result in
significant calibration errors.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 3 figure
Inference for mixtures of symmetric distributions
This article discusses the problem of estimation of parameters in finite
mixtures when the mixture components are assumed to be symmetric and to come
from the same location family. We refer to these mixtures as semi-parametric
because no additional assumptions other than symmetry are made regarding the
parametric form of the component distributions. Because the class of symmetric
distributions is so broad, identifiability of parameters is a major issue in
these mixtures. We develop a notion of identifiability of finite mixture
models, which we call k-identifiability, where k denotes the number of
components in the mixture. We give sufficient conditions for k-identifiability
of location mixtures of symmetric components when k=2 or 3. We propose a novel
distance-based method for estimating the (location and mixing) parameters from
a k-identifiable model and establish the strong consistency and asymptotic
normality of the estimator. In the specific case of L_2-distance, we show that
our estimator generalizes the Hodges--Lehmann estimator. We discuss the
numerical implementation of these procedures, along with an empirical estimate
of the component distribution, in the two-component case. In comparisons with
maximum likelihood estimation assuming normal components, our method produces
somewhat higher standard error estimates in the case where the components are
truly normal, but dramatically outperforms the normal method when the
components are heavy-tailed.Comment: Published at http://dx.doi.org/10.1214/009053606000001118 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
The Outer Edges of Dwarf Irregular Galaxies: Stars and Gas
We have in recent years come to view dwarf galaxy evolution in the broader
context of the cosmic evolution of large-scale structure. Dwarf galaxies, as
the putative building blocks of hierarchical galaxy formation, and also as the
most numerous galaxies in the Universe, play a central role in cosmic
evolution. In particular, the interplay of galactic and intergalactic material
around dwarf irregulars must be more extensive than in more massive disk
galaxies because of their lower gravitational potential and lower interstellar
pressures. The outer regions of dwarf irregular galaxies therefore yield vital
clues to the dominant processes in this interaction zone.
The Workshop addressed a number of questions related to the role of the outer
regions in the evolution of dwarf galaxies and broader consequences. On-line
Workshop Proceedings are at http://www.lowell.edu/Workshops/Lowell02/Comment: Summary of the 2002 Lowell Observatory Workshop, to appear in PASP
Conference Highlights; 6 pp, uses aaspp4.sty. On-line Proceedings at
http://www.lowell.edu/Workshops/Lowell02
Experimental study of a free turbulent shear flow at Mach 19 with electron-beam and conventional probes
An experimental study of the initial development region of a hypersonic turbulent free mixing layer was made. Data were obtained at three stations downstream of a M = 19 nozzle over a Reynolds range of 1.3 million to 3.3 million per meter and at a total temperature of about 1670 K. In general, good agreement was obtained between electron-beam and conventional probe measurements of local mean flow parameters. Measurements of fluctuating density indicated that peak root-mean-square (rms) levels are higher in the turbulent free mixing layer than in boundary layers for Mach numbers less than 9. The intensity of rms density fluctuations in the free stream is similar in magnitude to pressure fluctuations in high Mach number flows. Spectrum analyses of the measured fluctuating density through the shear layer indicate significant fluctuation energy at the lower frequencies (0.2 to 5 kHZ) which correspond to large-scale disturbances in the high-velocity region of the shear layer
Lorentz Violation and Short-Baseline Neutrino Experiments
A general discussion is given of signals for broken Lorentz symmetry in
short-baseline neutrino experiments. Among the effects that Lorentz violation
can introduce are a dependence on energy differing from that of the usual
massive-neutrino solution and a dependence on the direction of neutrino
propagation. Using the results of the LSND experiment, explicit analysis of the
effects of broken Lorentz symmetry yields a nonzero value (3+/-1) x 10^{-19}
GeV for a combination of coefficients for Lorentz violation. This lies in the
range expected for effects originating from the Planck scale in an underlying
unified theory.Comment: 4 pages REVTe
Proteomics of Carbon Fixation Energy Sources in Halothiobacillus neapolitanus
Through the use of proteomics, it was uncovered that the autotrophic, aerobic purple sulfur bacterium Halothiobacillus neapolitanus displays changes in cellular levels of portions of its carbon dioxide uptake and fixation mechanisms upon switch from bicarbonate to CO2(g) as carbon source. This includes an increase in level of a heterodimeric bicarbonate transporter along with a potential switch between form I and form II of RubisCO. Additional changes are seen in several sulfur oxidation pathways, which may indicate a link between sulfur oxidation pathways as an energy source and carbon uptake/fixation mechanisms
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