6,044 research outputs found
Uncharged tRNA Activates GCN2 by Displacing the Protein Kinase Moiety from a Bipartite tRNA-Binding Domain
Protein kinase GCN2 regulates translation in amino acid–starved cells by phosphorylating eIF2. GCN2 contains a regulatory domain related to histidyl-tRNA synthetase (HisRS) postulated to bind multiple deacylated tRNAs as a general sensor of starvation. In accordance with this model, GCN2 bound several deacylated tRNAs with similar affinities, and aminoacylation of tRNAPhe weakened its interaction with GCN2. Unexpectedly, the C-terminal ribosome binding segment of GCN2 (C-term) was required in addition to the HisRS domain for strong tRNA binding. A combined HisRS+C-term segment bound to the isolated protein kinase (PK) domain in vitro, and tRNA impeded this interaction. An activating mutation (GCN2c-E803V) that weakens PK–C-term association greatly enhanced tRNA binding by GCN2. These results provide strong evidence that tRNA stimulates the GCN2 kinase moiety by preventing an inhibitory interaction with the bipartite tRNA binding domain
Non-linear effects on Turing patterns: time oscillations and chaos.
We show that a model reaction-diffusion system with two species in a monostable regime and over a large region of parameter space, produces Turing patterns coexisting with a limit cycle which cannot be discerned from the linear analysis. As a consequence, Turing patterns oscillate in time, a phenomenon which is expected to occur only in a three morphogen system. When varying a single parameter, a series of bifurcations lead to period doubling, quasi-periodic and chaotic oscillations without modifying the underlying Turing pattern. A Ruelle-Takens-Newhouse route to chaos is identified. We also examined the Turing conditions for obtaining a diffusion driven instability and discovered that the patterns obtained are not necessarily stationary for certain values of the diffusion coefficients. All this results demonstrates the limitations of the linear analysis for reaction-diffusion systems
Influence of Zn excess on compositional, structural and vibrational properties of Cu2ZnSn0.5Ge0.5Se4 thin films and their effect on solar cell efficiency
This Accepted Manuscript will be available for reuse under a CC BY-NC-ND licence after 24 months of embargo periodThe effect of Zn content on compositional, structural and vibrational properties of Cu2ZnSn1-xGexSe4 (CZTGSe, x ~ 0.5) thin films is studied. Kesterite layer is deposited by co-evaporation onto 5 × 5 cm2 Mo/SLG substrate followed by a thermal treatment at maximum temperature of 480 °C, obtaining areas with different composition and morphology which are due to the sample position in the co-evaporation system and to the non-uniform temperature distribution across the substrate. Kesterite layers with higher Zn amounts are characterized by lower Cu and Ge contents; however, a uniform Ge distribution through the absorber layer is detected in all cases. The excess Zn concentration leads to the formation of ZnSe secondary phase on the surface and in the bulk of the absorber as determined by Raman spectroscopy. When higher Ge content and no ZnSe are present in the absorber layer, a compact structure is formed with larger grain size of kesterite. This effect could explain the higher Voc of the solar cell. The Zn content does not affect the bandgap energy significantly (Eg near 1.3 eV), although the observed effect of Zn excess in CZTGSe results in a decreased device performance from 6.4 to 4.2%. This investigation reveals the importance of the control of the off-stoichiometric CZTGSe composition during the deposition process to enhance solar cells propertiesThis work was supported by Spanish Ministry of Science, Innovation and Universities Project WINCOST (ENE2016-80788-C5-2-R) and European Project INFINITE CELL (H2020-MSCA-RISE-2017-777968). ARP also acknowledges financial support from Community of Madrid within Youth Employment Program (PEJD-2017-PRE/IND-4062). MG acknowledges the financial support from ACCIÓ-Generalitat de Catalunya within the TECNIOspring Plus fellowship (TECSPR18-1-0048
Semi-Parametric Seasonal Unit Root Tests
It is well known that (seasonal) unit root tests can be seriously affected by the presence of weak dependence in the driving shocks when this is not accounted for. In the non-seasonal case both parametric (based around augmentation of the test regression with lagged dependent variables) and semi-parametric (based around an estimator of the long run variance of the shocks) unit root tests have been proposed. Of these, the M class of unit root tests introduced by Stock (1999), Perron and Ng (1996) and Ng and Perron (2001), appear to be particularly successful, showing good finite sample size control even in the most problematic (near-cancellation) case where the shocks contain a strong negative moving average component. The aim of this paper is threefold. First we show the implications that neglected weak dependence in the shocks has on lag un-augmented versions of the well known regression based seasonal unit root tests of Hylleberg et al. (1990). Second, in order to complement extant parametrically augmented versions of the tests of Hylleberg et al. (1990), we develop semi-parametric seasonal unit root test procedures, generalising the methods developed in the non-seasonal case to our setting. Third, we compare the finite sample size and power properties of the parametric and semi-parametric seasonal unit root tests considered. Our results suggest that the superior size/power trade-off offered by the M approach in the non-seasonal case carries over to the seasonal case
MAGIC sensitivity to millisecond-duration optical pulses
The MAGIC telescopes are a system of two Imaging Atmospheric Cherenkov
Telescopes (IACTs) designed to observe very high energy (VHE) gamma rays above
~50 GeV. However, as IACTs are sensitive to Cherenkov light in the UV/blue and
use photo-detectors with a time response well below the ms scale, MAGIC is also
able to perform simultaneous optical observations. Through an alternative
system installed in the central PMT of MAGIC II camera, the so-called central
pixel, MAGIC is sensitive to short (1ms - 1s) optical pulses. Periodic signals
from the Crab pulsar are regularly monitored. Here we report for the first time
the experimental determination of the sensitivity of the central pixel to
isolated 1-10 ms long optical pulses. The result of this study is relevant for
searches of fast transients such as Fast Radio Bursts (FRBs).Comment: Proceedings of the 35th International Cosmic Ray Conference (ICRC
2017), Bexco, Busan, Korea (arXiv:1708.05153
Variable selection with LASSO regression for complex survey data
Variable selection is an important step to end up with good prediction models. LASSO regression
models are one of the most commonly used methods for this purpose, for which
cross-validation is the most widely applied validation technique to choose the tuning parameter
(λ). Validation techniques in a complex survey framework are closely related to
“replicate weights”. However, to our knowledge, they have never been used in a LASSO
regression context. Applying LASSO regression models to complex survey data could be
challenging. The goal of this paper is two-fold. On the one hand, we analyze the performance
of replicate weights methods to select the tuning parameter for fitting LASSO regression
models to complex survey data. On the other hand, we propose new replicate weights methods
for the same purpose. In particular, we propose a new design-based cross-validation
method as a combination of the traditional cross-validation and replicate weights. The performance
of all these methods has been analyzed and compared by means of an extensive
simulation study to the traditional cross-validation technique to select the tuning parameter
for LASSO regression models. The results suggest a considerable improvement when
the new proposal design-based cross-validation is used instead of the traditional crossvalidation.IT1456-22
PIF18/21
Precompact convergence of the nonconvex Primal–Dual Hybrid Gradient algorithm
The Primal–Dual Hybrid Gradient (PDHG) algorithm is a powerful algorithm used quite frequently in recent years for solving saddle-point optimization problems. The classical application considers convex functions, and it is well studied in literature. In this paper, we consider the convergence of an alternative formulation of the PDHG algorithm in the nonconvex case under the precompact assumption. The proofs are based on the Kurdyka–L ojasiewic functions, that cover a wide range of problems. A simple numerical experiment illustrates the convergence properties
Pulsar Prospects for the Cherenkov Telescope Array
In the last few years, the Fermi-LAT telescope has discovered over a 100
pulsars at energies above 100 MeV, increasing the number of known gamma-ray
pulsars by an order of magnitude. In parallel, imaging Cherenkov telescopes,
such as MAGIC and VERITAS, have detected for the first time VHE pulsed
gamma-rays from the Crab pulsar. Such detections have revealed that the Crab
VHE spectrum follows a power-law up to at least 400 GeV, challenging most
theoretical models, and opening wide possibilities of detecting more pulsars
from the ground with the future Cherenkov Telescope Array (CTA). In this
contribution, we study the capabilities of CTA for detecting Fermi pulsars. For
this, we extrapolate their spectra with "Crab-like" power-law tails in the VHE
range, as suggested by the latest MAGIC and VERITAS results.Comment: 4 pages, 3 figures. In Proceedings of the 2012 Heidelberg Symposium
on High Energy Gamma-Ray Astronomy. All CTA contributions at arXiv:1211.184
Spin dependent D-brane interactions and scattering amplitudes in matrix theory
Spin interactions beteween two moving Dp-branes are analyzed using the
Green-Schwarz formalism of boundary states. This approach turns out to be
extremely efficient to compute all the spin effects related by supersymmetry to
the leading v^4/r^7-p term. All these terms are shown to be scale invariant,
supporting a matrix model description of supergravity interactions. By
employing the LSZ reduction formula for matrix theory and the mentioned
supersymmetric effective potential for D0-branes, we compute the t-pole of
graviton-graviton and three form-three form scattering in matrix theory. The
results are found to be in complete agreement with tree level supergravity in
the corresponding kinematical regime and provide, moreover, an explicit map
between these degrees of freedom in both theories.Comment: 8 pages, no figures, talk presented at the conference "Quantum
aspects of gauge theories, supergravity and unification", Corfu, Greece, to
appear in the proceeding
Symbolic Toolkit for Chaos Explorations
New computational technique based on the symbolic description utilizing
kneading invariants is used for explorations of parametric chaos in a two
exemplary systems with the Lorenz attractor: a normal model from mathematics,
and a laser model from nonlinear optics. The technique allows for uncovering
the stunning complexity and universality of the patterns discovered in the
bi-parametric scans of the given models and detects their organizing centers --
codimension-two T-points and separating saddles.Comment: International Conference on Theory and Application in Nonlinear
Dynamics (ICAND 2012
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