29,025 research outputs found
cis-acting sequences and trans-acting factors in the localization of mRNA for mitochondrial ribosomal proteins
mRNA localization is a conserved post-transcriptional process crucial for a variety of systems. Although several mechanisms have been identified, emerging evidence suggests that most transcripts reach the protein functional site by moving along cytoskeleton elements. We demonstrated previously that mRNA for mitochondrial ribosomal proteins are asymmetrically distributed in the cytoplasm, and that localization in the proximity of mitochondria is mediated by the 3′-UTR. Here we show by biochemical analysis that these mRNA transcripts are associated with the cytoskeleton through the microtubule network. Cytoskeleton association is functional for their intracellular localization near the mitochondrion, and the 3′-UTR is involved in this cytoskeleton-dependent localization. To identify the minimal elements required for localization, we generated DNA constructs containing, downstream from the GFP gene, deletion mutants of mitochondrial ribosomal protein S12 3′-UTR, and expressed them in HeLa cells. RT-PCR analysis showed that the localization signals responsible for mRNA localization are located in the first 154 nucleotides. RNA pulldown assays, mass spectrometry, and RNP immunoprecipitation assay experiments, demonstrated that mitochondrial ribosomal protein S12 3′-UTR interacts specifically with TRAP1 (tumor necrosis factor receptor-associated protein1), hnRNPM4 (heterogeneous nuclear ribonucleoprotein M4), Hsp70 and Hsp60 (heat shock proteins 70 and 60), and α-tubulin in vitro and in vivo
Strong Magnetic Limit of String Theory
We show that there exists a certain limit in type I and type II superstring
theory in the presence of a suitable configuration of magnetic U(1) fields
where all string excitations get an infinite mass, except for the neutral
massless sector and for the boson and fermion string states lying on the
leading Regge trajectory. For a supersymmetric configuration of magnetic fields
in internal directions, the resulting theory after the limit is a 3+1 Lorentz
invariant supersymmetric theory. Supersymmetry can be broken by introducing
extra components of the magnetic field or else by finite temperature. In both
cases we compute the one-loop partition function for the type I string model
after taking the limit, which turns out to be different from the Yang-Mills
result that arises by a direct limit. In the case of finite
temperature, no Hagedorn transition appears, in consistency with the reduction
of the string spectrum. In type II superstring theory, the analogous limit is
constructed by starting with a configuration of Melvin twists in two or more
complex planes. The resulting theory contains gravitation plus an infinite
number of states of the leading Regge trajectory.Comment: 10 pages. Minor correction
Different domains cooperate to target the human ribosomal L7a protein to the nucleus and to the nucleoli.
The human ribosomal protein L7a is a component of the major ribosomal subunit. We transiently expressed in HeLa cells L7a-β-galactosidase fusion proteins and studied their subcellular localization by indirect immunofluorescence staining with anti-β-galactosidase antibodies. We have identified three distinct domains responsible for the nuclear targeting of the protein: domain I, amino acids 23-51; domain II, amino acids 52-100; domain III, amino acids 101-220, each of which contains at least one nuclear localization signal (NLS). Through subcellular localization analysis of deletion mutants of L7a-β-galactosidase chimeras, we demonstrate that domain II plays a special role because it is necessary, although not sufficient, to target the chimeric β-galactosidase to the nucleoli. In fact, we demonstrate that the nucleolar targeting process requires the presence of domain II plus an additional basic domain that can be represented by an NLS or a basic stretch of amino acids without NLS activity. Thus, when multiple NLS are present, each NLS exerts distinct functions. Domain II drives nucleolar accumulation of a reporter protein with the cooperative action of a short basic amino acid sequence, suggesting a mechanism requiring protein-protein or protein-nucleic acid interactions
Computer program for analysis of split-Stirling-cycle cryogenic coolers
A computer program for predicting the detailed thermodynamic performance of split-Stirling-cycle refrigerators has been developed. The mathematical model includes the refrigerator cold head, free-displacer/regenerator, gas transfer line, and provision for modeling a mechanical or thermal compressor. To allow for dynamic processes (such as aerodynamic friction and heat transfer) temperature, pressure, and mass flow rate are varied by sub-dividing the refrigerator into an appropriate number of fluid and structural control volumes. Of special importance to modeling of cryogenic coolers is the inclusion of real gas properties, and allowance for variation of thermo-physical properties such as thermal conductivities, specific heats and viscosities, with temperature and/or pressure. The resulting model, therefore, comprehensively simulates the split-cycle cooler both spatially and temporally by reflecting the effects of dynamic processes and real material properties
Using Event Calculus to Formalise Policy Specification and Analysis
As the interest in using policy-based approaches for systems management grows, it is becoming increasingly important to develop methods for performing analysis and refinement of policy specifications. Although this is an area that researchers have devoted some attention to, none of the proposed solutions address the issues of analysing specifications that combine authorisation and management policies; analysing policy specifications that contain constraints on the applicability of the policies; and performing a priori analysis of the specification that will both detect the presence of inconsistencies and explain the situations in which the conflict will occur. We present a method for transforming both policy and system behaviour specifications into a formal notation that is based on event calculus. Additionally it describes how this formalism can be used in conjunction with abductive reasoning techniques to perform a priori analysis of policy specifications for the various conflict types identified in the literature. Finally, it presents some initial thoughts on how this notation and analysis technique could be used to perform policy refinement
Coupling of sedimentation and liquid structure: influence on hard sphere nucleation
The discrepancy in nucleation rate densities between simulated and
experimental hard spheres remains staggering and unexplained. Suggestively,
more strongly sedimenting colloidal suspensions of hard spheres nucleate much
faster than weakly sedimenting systems. In this work we consider firstly the
effect of sedimentation on the structure of colloidal hard spheres, by tuning
the density mismatch between solvent and colloidal particles. In particular we
investigate the effect on the degree of five fold symmetry present. Secondly we
study the size of density fluctuations in these experimental systems in
comparison to simulations. The density fluctuations are measured by assigning
each particle a local density, which is related to the number of particles
within a distance of 3.25 particle diameters. The standard deviation of these
local densities gives an indication of the fluctuations present in the system.
Five fold symmetry is suppressed by a factor of two when sedimentation is
induced in our system. Density fluctuations are also increased by a factor of
two in experiments compared to simulations. The change in five fold symmetry
makes a difference to the expected nucleation rates, but we demonstrate that it
is ultimately too small to resolve the discrepancy between experiment and
simulation, while the fluctuations are shown to be an artefact of 3d particle
tracking.Comment: 8 page
Energy-momentum tensor for scalar fields coupled to the dilaton in two dimensions
We clarify some issues related to the evaluation of the mean value of the
energy-momentum tensor for quantum scalar fields coupled to the dilaton field
in two-dimensional gravity. Because of this coupling, the energy-momentum
tensor for the matter is not conserved and therefore it is not determined by
the trace anomaly. We discuss different approximations for the calculation of
the energy-momentum tensor and show how to obtain the correct amount of Hawking
radiation. We also compute cosmological particle creation and quantum
corrections to the Newtonian potential.Comment: 18 pages, RevTex, no figures. Some changes have been added. To appear
in Physical Review
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