1,155 research outputs found
Association of DRG1 and DRG2 with Ribosomes from Pea, Arabidopsis and Yeast
DRGs are highly conserved GTP binding proteins.All eukaryotes examined contain DRG1 and DRG2 orthologs.
The first experimental evidence for GTP binding by a plant DRG1 protein and by DRG2 from any organism is
presented. DRG1 antibodies recognized a single ~43-kDa band in plant tissues, whereas DRG2 antibodies
recognized ~45-,43-,and 30-kDa bands.An in vitro transcription and translation assay suggested that the 45-kDa
band represents full-length DRG2 and that the smaller bands are specific proteolytic products. Homogenates from
pea roots and root apices were used to produce fractions enriched in cytosolic and microsomal monosomes and
polysomes. DRG1 and the 45- and 43-kDa DRG2 bands occurred in the cytosol and associated with cytosolic
monosomes.I n contrast,the 30-kDa form of DRG2 was strongly enriched in polysome fractions.Thus,DRG1and
the larger forms of DRG2 may be involved in translational initiation, and the 30-kDa form of DRG2 may be
involved in translational elongation.DRG1 and the 45-and43-kDa forms of DRG2 can reassociate with ribosomes
in vitro, a process that is partially inhibited by GTP-y-S Cells expressing FLAG-tagged ribosomal proteins from
transgenic lines of
Arabidopsis
and yeast also demonstrated DRG-ribosome interactions
Crystallization of a classical two-dimensional electron system: Positional and orientational orders
Crystallization of a classical two-dimensional one-component plasma
(electrons interacting with the Coulomb repulsion in a uniform neutralizing
positive background) is investigated with a molecular dynamics simulation. The
positional and the orientational correlation functions are calculated for the
first time. We have found an indication that the solid phase has a
quasi-long-range (power-law) positional order along with a long-range
orientational order. This indicates that, although the long-range Coulomb
interaction is outside the scope of Mermin's theorem, the absence of ordinary
crystalline order at finite temperatures applies to the electron system as
well. The `hexatic' phase, which is predicted between the liquid and the solid
phases by the Kosterlitz-Thouless-Halperin-Nelson-Young theory, is also
discussed.Comment: 3 pages, 4 figures; Corrected typos; Double columne
Supersymmetric effects in top quark decay into polarized W-boson
We investigate the one-loop supersymmetric QCD (SUSY-QCD) and electroweak
(SUSY-EW) corrections to the top quark decay into a b-quark and a longitudinal
or transverse W-boson. The corrections are presented in terms of the
longitudinal ratio \Gamma(t-->W_L b)/\Gamma(t--> W b) and the transverse ratio
\Gamma(t-->W_- b)/\Gamma(t--> W b). In most of the parameter space, both
SUSY-QCD and SUSY-EW corrections to these ratios are found to be less than 1%
in magnitude and they tend to have opposite signs. The corrections to the total
width \Gamma(t-->W b) are also presented for comparison with the existing
results in the literature. We find that our SUSY-EW corrections to the total
width differ significantly from previous studies: the previous studies give a
large correction of more than 10% in magnitude for a large part of the
parameter space while our results reach only few percent at most.Comment: Version in PRD (explanation and refs added
Experimental pulse technique for the study of microbial kinetics in continuous culture
A novel technique was developed for studying the growth kinetics of microorganisms in continuous culture. The method is based on following small perturbations of a chemostat culture by on-line measurement of the dynamic response in oxygen consumption rates. A mathematical model, incorporating microbial kinetics and mass transfer between gas and liquid phases, was applied to interpret the data. Facilitating the use of very small disturbances, the technique is non-disruptive as well as fast and accurate. The technique was used to study the growth kinetics of two cultures, Methylosinus trichosporium OB3b growing on methane, both in the presence and in the absence of copper, and Burkholderia (Pseudomonas) cepacia G4 growing on phenol. Using headspace flushes, gas blocks and liquid substrate pulse experiments, estimates for limiting substrate concentrations, maximum conversion rates Vmax and half saturation constants Ks could rapidly be obtained. For M. trichosporium OB3b it was found that it had a far higher affinity for methane when particulate methane monooxygenase (pMMO) was expressed than when the soluble form (sMMO) was expressed under copper limitation. While for B. cepacia G4 the oxygen consumption pattern during a phenol pulse in the chemostat indicated that phenol was transiently converted to an intermediate (4-hydroxy-2-oxovalerate), so that initially less oxygen was used per mole of phenol.
Universality in the Screening Cloud of Dislocations Surrounding a Disclination
A detailed analytical and numerical analysis for the dislocation cloud
surrounding a disclination is presented. The analytical results show that the
combined system behaves as a single disclination with an effective fractional
charge which can be computed from the properties of the grain boundaries
forming the dislocation cloud. Expressions are also given when the crystal is
subjected to an external two-dimensional pressure. The analytical results are
generalized to a scaling form for the energy which up to core energies is given
by the Young modulus of the crystal times a universal function. The accuracy of
the universality hypothesis is numerically checked to high accuracy. The
numerical approach, based on a generalization from previous work by S. Seung
and D.R. Nelson ({\em Phys. Rev A 38:1005 (1988)}), is interesting on its own
and allows to compute the energy for an {\em arbitrary} distribution of
defects, on an {\em arbitrary geometry} with an arbitrary elastic {\em energy}
with very minor additional computational effort. Some implications for recent
experimental, computational and theoretical work are also discussed.Comment: 35 pages, 21 eps file
Phase Transitions of Hard Disks in External Periodic Potentials: A Monte Carlo Study
The nature of freezing and melting transitions for a system of hard disks in
a spatially periodic external potential is studied using extensive Monte Carlo
simulations. Detailed finite size scaling analysis of various thermodynamic
quantities like the order parameter, its cumulants etc. are used to map the
phase diagram of the system for various values of the density and the amplitude
of the external potential. We find clear indication of a re-entrant liquid
phase over a significant region of the parameter space. Our simulations
therefore show that the system of hard disks behaves in a fashion similar to
charge stabilized colloids which are known to undergo an initial freezing,
followed by a re-melting transition as the amplitude of the imposed, modulating
field produced by crossed laser beams is steadily increased. Detailed analysis
of our data shows several features consistent with a recent dislocation
unbinding theory of laser induced melting.Comment: 36 pages, 16 figure
Hexatic-Herringbone Coupling at the Hexatic Transition in Smectic Liquid Crystals: 4- Renormalization Group Calculations Revisited
Simple symmetry considerations would suggest that the transition from the
smectic-A phase to the long-range bond orientationally ordered hexatic
smectic-B phase should belong to the XY universality class. However, a number
of experimental studies have constantly reported over the past twenty years
"novel" critical behavior with non-XY critical exponents for this transition.
Bruinsma and Aeppli argued in Physical Review Letters {\bf 48}, 1625 (1982),
using a renormalization-group calculation, that short-range
molecular herringbone correlations coupled to the hexatic ordering drive this
transition first order via thermal fluctuations, and that the critical behavior
observed in real systems is controlled by a `nearby' tricritical point. We have
revisited the model of Bruinsma and Aeppli and present here the results of our
study. We have found two nontrivial strongly-coupled herringbone-hexatic fixed
points apparently missed by those authors. Yet, those two new nontrivial
fixed-points are unstable, and we obtain the same final conclusion as the one
reached by Bruinsma and Aeppli, namely that of a fluctuation-driven first order
transition. We also discuss the effect of local two-fold distortion of the bond
order as a possible missing order parameter in the Hamiltonian.Comment: 1 B/W eps figure included. Submitted to Physical Review E. Contact:
[email protected]
Photoconductance Quantization in a Single-Photon Detector
We have made a single-photon detector that relies on photoconductive gain in
a narrow electron channel in an AlGaAs/GaAs 2-dimensional electron gas. Given
that the electron channel is 1-dimensional, the photo-induced conductance has
plateaus at multiples of the quantum conductance 2e/h. Super-imposed on
these broad conductance plateaus are many sharp, small, conductance steps
associated with single-photon absorption events that produce individual
photo-carriers. This type of photoconductive detector could measure a single
photon, while safely storing and protecting the spin degree of freedom of its
photo-carrier. This function is valuable for a quantum repeater that would
allow very long distance teleportation of quantum information.Comment: 4 pages, 4 figure
Dynamic renormalization group study of a generalized continuum model of crystalline surfaces
We apply the Nozieres-Gallet dynamic renormalization group (RG) scheme to a
continuum equilibrium model of a d-dimensional surface relaxing by linear
surface tension and linear surface diffusion, and which is subject to a lattice
potential favoring discrete values of the height variable. The model thus
interpolates between the overdamped sine-Gordon model and a related continuum
model of crystalline tensionless surfaces. The RG flow predicts the existence
of an equilibrium roughening transition only for d = 2 dimensional surfaces,
between a flat low-temperature phase and a rough high-temperature phase in the
Edwards-Wilkinson (EW) universality class. The surface is always in the flat
phase for any other substrate dimensions d > 2. For any value of d, the linear
surface diffusion mechanism is an irrelevant perturbation of the linear surface
tension mechanism, but may induce long crossovers within which the scaling
properties of the linear molecular-beam epitaxy equation are observed, thus
increasing the value of the sine-Gordon roughening temperature. This phenomenon
originates in the non-linear lattice potential, and is seen to occur even in
the absence of a bare surface tension term. An important consequence of this is
that a crystalline tensionless surface is asymptotically described at high
temperatures by the EW universality class.Comment: 22 pages, 5 figures. Accepted for publication in Physical Review
Melting as a String-Mediated Phase Transition
We present a theory of the melting of elemental solids as a
dislocation-mediated phase transition. We model dislocations near melt as
non-interacting closed strings on a lattice. In this framework we derive simple
expressions for the melting temperature and latent heat of fusion that depend
on the dislocation density at melt. We use experimental data for more than half
the elements in the Periodic Table to determine the dislocation density from
both relations. Melting temperatures yield a dislocation density of (0.61\pm
0.20) b^{-2}, in good agreement with the density obtained from latent heats,
(0.66\pm 0.11) b^{-2}, where b is the length of the smallest
perfect-dislocation Burgers vector. Melting corresponds to the situation where,
on average, half of the atoms are within a dislocation core.Comment: 18 pages, LaTeX, 3 eps figures, to appear in Phys. Rev.
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