26,682 research outputs found
Hydra tropomyosin TROP1 is expressed in head-specific epithelial cells and is a major component of the cytoskeletal structure that anchors nematocytes
A cDNA clone encoding a 253 amino acid tropomyosin was
isolated from
Hydra in a differential screen for headspecific
genes. The Hydra tropomyosin gene, designated
trop1, is a single copy gene, lacks introns and is strongly
expressed in tentacle-specific epithelial cells. Analysis of
protein synthesis in head and gastric tissue indicated a high
rate of tropomyosin synthesis in head tissue. Immunolocalization
of tropomyosin in tentacle tissue revealed a
cushion-like tropomyosin-containing structure within
battery cells at the base of nematocytes. The structure
appears to form part of the cytoskeletal anchor for nematocytes.
Tropomyosin cushions were also observed in
epithelial cells along the body column, which contain
mounted stenotele nematocytes
Wide-field weak lensing by RXJ1347-1145
We present an analysis of weak lensing observations for RXJ1347-1145 over a
43' X 43' field taken in B and R filters on the Blanco 4m telescope at CTIO.
RXJ1347-1145 is a massive cluster at redshift z=0.45. Using a population of
galaxies with 20<R<26, we detect a weak lensing signal at the p<0.0005 level,
finding best-fit parameters of \sigma_v=1400^{+130}_{-140} km s^{-1} for a
singular isothermal sphere model and r_{200} = 3.5^{+0.8}_{-0.2} Mpc with c =
15^{+64}_{-10} for a NFW model in an \Omega_m = 0.3, \Omega_\Lambda = 0.7
cosmology. In addition, a mass to light ratio M/L_R =90 \pm 20 M_\odot /
L_{R\odot} was determined. These values are consistent with the previous weak
lensing study of RXJ1347--1145 by Fischer and Tyson, 1997, giving strong
evidence that systemic bias was not introduced by the relatively small field of
view in that study. Our best-fit parameter values are also consistent with
recent X-ray studies by Allen et al, 2002 and Ettori et al, 2001, but are not
consistent with recent optical velocity dispersion measurements by Cohen and
Kneib, 2002.Comment: accepted to ApJ, tentative publication 10 May 2005, v624
Statistical Limits of Fourier Transform Imaging in the Gamma-ray Energy Range
Recent advances in imaging techniques and position-sensitive gamma-ray detectors
have made feasible hard x-ray and gamma-ray telescopes with arc-second resolution [ 1].
Above an energy of 100 keV, past instrumentation has been limited to a typical angular
resolution of a few degrees. A gamma-ray imaging device with 1 arc-second resolution
would be a dramatic improvement over conventional, non-imaging instrumentation
and have substantial new capabilities for observation of astrophysical gamma-ray sources.
The arc-second gamma-ray imager is based on the Fourier transform imaging technique
[2]. We briefly describe Fourier transform imaging and its application to hard x-ray
and gamma-ray imaging. This description is followed by an analysis of Fourier transform
imaging in the statistics limited regime. Computer simulations and laboratory
demonstrations of practical gamma-ray imaging systems are presented
A correlated-polaron electronic propagator: open electronic dynamics beyond the Born-Oppenheimer approximation
In this work we develop a theory of correlated many-electron dynamics dressed
by the presence of a finite-temperature harmonic bath. The theory is based on
the ab-initio Hamiltonian, and thus well-defined apart from any
phenomenological choice of collective basis states or electronic coupling
model. The equation-of-motion includes some bath effects non-perturbatively,
and can be used to simulate line- shapes beyond the Markovian approximation and
open electronic dynamics which are subjects of renewed recent interest. Energy
conversion and transport depend critically on the ratio of electron-electron
coupling to bath-electron coupling, which is a fitted parameter if a
phenomenological basis of many-electron states is used to develop an electronic
equation of motion. Since the present work doesn't appeal to any such basis, it
avoids this ambiguity. The new theory produces a level of detail beyond the
adiabatic Born-Oppenheimer states, but with cost scaling like the
Born-Oppenheimer approach. While developing this model we have also applied the
time-convolutionless perturbation theory to correlated molecular excitations
for the first time. Resonant response properties are given by the formalism
without phenomenological parameters. Example propagations with a developmental
code are given demonstrating the treatment of electron-correlation in
absorption spectra, vibronic structure, and decay in an open system.Comment: 25 pages 7 figure
Cauchy, infinitesimals and ghosts of departed quantifiers
Procedures relying on infinitesimals in Leibniz, Euler and Cauchy have been
interpreted in both a Weierstrassian and Robinson's frameworks. The latter
provides closer proxies for the procedures of the classical masters. Thus,
Leibniz's distinction between assignable and inassignable numbers finds a proxy
in the distinction between standard and nonstandard numbers in Robinson's
framework, while Leibniz's law of homogeneity with the implied notion of
equality up to negligible terms finds a mathematical formalisation in terms of
standard part. It is hard to provide parallel formalisations in a
Weierstrassian framework but scholars since Ishiguro have engaged in a quest
for ghosts of departed quantifiers to provide a Weierstrassian account for
Leibniz's infinitesimals. Euler similarly had notions of equality up to
negligible terms, of which he distinguished two types: geometric and
arithmetic. Euler routinely used product decompositions into a specific
infinite number of factors, and used the binomial formula with an infinite
exponent. Such procedures have immediate hyperfinite analogues in Robinson's
framework, while in a Weierstrassian framework they can only be reinterpreted
by means of paraphrases departing significantly from Euler's own presentation.
Cauchy gives lucid definitions of continuity in terms of infinitesimals that
find ready formalisations in Robinson's framework but scholars working in a
Weierstrassian framework bend over backwards either to claim that Cauchy was
vague or to engage in a quest for ghosts of departed quantifiers in his work.
Cauchy's procedures in the context of his 1853 sum theorem (for series of
continuous functions) are more readily understood from the viewpoint of
Robinson's framework, where one can exploit tools such as the pointwise
definition of the concept of uniform convergence.
Keywords: historiography; infinitesimal; Latin model; butterfly modelComment: 45 pages, published in Mat. Stu
Open problems in artificial life
This article lists fourteen open problems in artificial life, each of which is a grand challenge requiring a major advance on a fundamental issue for its solution. Each problem is briefly explained, and, where deemed helpful, some promising paths to its solution are indicated
Nod1 signaling overcomes resistance of S. pneumoniae to opsonophagocytic killing
Airway infection by the Gram-positive pathogen Streptococcus pneumoniae (Sp) leads to recruitment of neutrophils but
limited bacterial killing by these cells. Co-colonization by Sp and a Gram-negative species, Haemophilus influenzae (Hi),
provides sufficient stimulus to induce neutrophil and complement-mediated clearance of Sp from the mucosal surface
in a murine model. Products from Hi, but not Sp, also promote killing of Sp by ex vivo neutrophil-enriched peritoneal
exudate cells. Here we identify the stimulus from Hi as its peptidoglycan. Enhancement of opsonophagocytic killing
was facilitated by signaling through nucleotide-binding oligomerization domain-1 (Nod1), which is involved in
recognition of γ-D-glutamyl-meso-diaminopimelic acid (meso-DAP) contained in cell walls of Hi but not Sp. Neutrophils
from mice treated with Hi or compounds containing meso-DAP, including synthetic peptidoglycan fragments, showed
increased Sp killing in a Nod1-dependent manner. Moreover, Nod1-/- mice showed reduced Hi-induced clearance of Sp
during co-colonization. These observations offer insight into mechanisms of microbial competition and demonstrate
the importance of Nod1 in neutrophil-mediated clearance of bacteria in vivo
Intrazeolite chemistry of nickel(0) complexes and Ni(0,II) clusters studied by EXAFS, solid-state NMR and FT-IR spectroscopy
Inference of oxygen vacancies in hydrothermal Na0.5Bi0.5TiO3
A high-resolution x-ray powder diffraction study has been made of pseudo-rhombohedral and tetragonal phases in Na0.5Bi0.5TiO3 (NBT), produced via hydrothermal and conventional solidstate methods. Hydrothermal NBT exhibits significantly greater structural distortion at room
temperature than solid-state NBT. Peak widths and superstructure peak intensities show a phase
transition at 305 C, with trends suggesting that the structure tends towards cubic symmetry at this temperature. Structural refinements indicate that the transition occurs via a phase coexistence region with no clear intermediate phase. Piezoelectric data show evidence of polarisation pinning in hydrothermal NBT, interpreted as a high proportion of oxygen vacancies
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