bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of smoothing by spectral dispersion on flow induced laser beam deflection: The random phase modulation scheme
Authors: Rose Harvey A. , Ghosal Sandip
Published: July 2002

Analytical results are presented for the effect of random phase modulated smoothing by spectral dispersion on flow induced laser beam deflection. It is shown that in the limit of a large number of color cycles, Ncc, the effect is identical to that of the induced spatial incoherence method of temporal smoothing. For small Ncc, the beam deflection rate may be significantly larger in the direction perpendicular to the dispersion, than in the parallel direction.
Conductivity and ion density of a plasma channel induced by a mildly relativistic electron beam from a gas-filled diode
Authors: Choi E. H. , Ko J. J. , Choi M. C. , Cho T. S. , Jung Y. , Kim D. I. , Seo Y. , Cho G. S. , Kang S. O. , Shin H. M. , Uhm H. S.
Published: July 2002

Conductivity and ion density of a plasma channel induced by a mildly relativistic electron beam (300 kV, ∼2 kA, 10–50 ns) have been experimentally investigated under various gas pressures. Pressures of filling gas (air) in this experiment ranged from 10 mTorr to 100 mTorr. The net currents of the beam-induced plasma channel were measured by four Rogowski coils located along the propagating region, while the electron beam currents were measured by a Faraday cup. The inductive plasma currents observed at the above pressure regimes have been characterized by magnetic decay time. Plasma-channel conductivity and ion density induced by the beam are measured along the propagating axial positions under various gas pressures. The numerical result of the ion density is also obtained at the charge neutralization time when the ion density is just the same as the electron beam density, and the digitizing experimental data of the beam current Ib(t) and voltage Vd(t) have been used. As expected, in both numerical and experimental results the ion density increases to a peak value of about 3.0×1011  cm−3 and 3.3×1011  cm−3, respectively, at 50 mTorr and slowly decreases for both cases as the gas pressure increases from 50 mTorr to 100 mTorr. Moreover, the results of ion density predicted by the theoretical model developed here are also found to be in remarkably good agreement with experimental and numerical results at pressure regimes from 10 mTorr to 100 mTorr.
An influence of frozen-in law violation effects on the turbulent equipartition in tokamaks
Authors: Ivonin Igor A. , Pavlenko Vladimir P. , Persson Hans
Published: July 2002

Analytical investigations and numerical simulations of the influence of frozen-in law violation effects on the turbulent equipartition (TEP) of plasma density n∼1/q through the safety factor q in tokamaks are performed. Namely, the changes of the frozen-in field topology are taken into account due to strong turbulence. The found influence leads to flatter than 1/q density and temperature profiles. For a moderate level of turbulence, another reason for flat equipartition profiles was found analytically, if turbulent mixing of passing electrons takes place. This possibility arises due to conservation of the helicity (AB) integral over frozen-in field lines and produces a flat TEP density profile n∼AB of passing electrons (A is the vector potential of the magnetic field B). These influences have been tested in numerical experiments and the results were compared with experimental data in tokamaks. So, the numerical scaling of combined TEP profile of both trapped and passing electrons is n∼1/q0.5–0.6, which is in good agreement with the experiments.
Drift waves in plasmas with sheared flows
Authors: Vranješ J. , Jovanović D. , Shukla P. K.
Published: July 2002

In the standard approach to the problem of shear stabilization of drift waves the diamagnetic drift velocity v* is regarded as a constant, which is usually not satisfied at the edge of a tokamak plasma. In configurations of this type, with a steep density gradient and radially sheared transverse velocity field, the magnetic shear stabilization criteria are severely restricted, and the velocity profile curvature v0″(x) is found to play a crucial role. In the strongly nonlinear regime, unstable drift waves may saturate into stationary coherent vortex-type structures. The latter include a tripolar vortex.
Coherent nonlinear inertial-Alfvén structures driven by parallel sheared flows
Authors: Jovanović D. , Shukla P. K.
Published: July 2002

It is shown that the nonlinear equations governing the dynamics of the parallel sheared flow driven inertial Alfvén waves admit stationary solutions in the form of a vortex street. The latter may be associated with the coherent nonlinear structures that are observed in the auroral region of the Earth’s ionosphere, and a very good quantitative agreement with the corresponding Freja satellite measurements is found.
Computer studies on the spontaneous fast reconnection model as a nonlinear instability
Authors: Ugai M.
Published: July 2002

The spontaneous fast reconnection model is examined for a current-driven anomalous resistivity model where the threshold for resistivity occurrence is assumed to increase with the temperature (thermal velocity). It is demonstrated that the fast reconnection mechanism can effectively evolve even if the threshold notably increases with the temperature increase near an X neutral point. The evolutionary process can be characterized by two distinct phases. In the initial phase, coupled to the anomalous resistivity, the reconnection process grows rather slowly. In the subsequent explosive phase, as soon as the plasma and the reconnected magnetic flux are effectively ejected from near the X point, it drastically grows due to the powerful positive feedback between the localized enhancement of anomalous resistivity and the growth of reconnection flow. For the larger resistivity threshold, the localization of anomalous resistivity near the X point becomes more effective, so that the fast reconnection development becomes more drastic. On the nonlinear saturation level, the (quasi-steady) fast reconnection mechanism is set up, where the peak fast reconnection rate is sustained, and standing slow shocks, attached to the localized diffusion region, are extended outwards with time. On the other hand, for uniform resistivity, the explosive phase does not occur, so that the fast reconnection mechanism cannot build up. Hence, it is concluded that the spontaneous fast reconnection model describes a new-type nonlinear instability in a long current sheet system, leading to drastic collapse of the overall magnetic field system.
Magnetic field stochasticity in the presence of current sheets
Authors: Bulanov S. , Farina D. , Pozzoli R.
Published: July 2002

The stochasticity of field lines around a current sheet induced by static magnetic perturbations is investigated. By means of Hamiltonian analysis, it is pointed out that the system exhibits two different chaotic regimes, depending on the wavelength of the perturbation: At short wavelengths (shorter than the length of the separatrix), resonant processes determine the onset of stochasticity and can be modeled by a simple mapping; at long wavelengths of the perturbation, stochasticity is mainly due to the breaking of adiabatic invariants.
The pseudo-symmetric optimization of the National Compact Stellarator Experiment
Authors: Isaev M. Yu. , Mikhailov M. I. , Monticello D. A. , Mynick H. E. , Subbotin A. A. , Ku L. P. , Reiman A. H.
Published: July 2002

A new experiment, the National Compact Stellarator Experiment (NCSX) [Monticello et al. “Physics Consideration for the Design of NCSX,” Proceedings of 25th EPS Conference on Controlled Fusion and Plasma Physics, Prague, 1998 (European Physical Society, Petit-Lancy), paper 1.187], hopes to overcome the deleterious ripple transport usually associated with stellarators by creating a quasi-axisymmetric configuration. A quasi-axisymmetric configuration is one in which the Fourier spectrum of the magnetic field strength in so-called Boozer coordinates is dominated by the toroidal angle averaged (n=0) components. In this article the concept of pseudosymmetry is used to improve ripple transport in a four-period variant of NCSX. By definition, pseudosymmetric magnetic configurations have no locally trapped particles. To obtain a pseudosymmetric configuration, different target functions are considered. It is found that a target function equal to the area of ripple of the magnetic field magnitude along the field line is very effective in reducing the neoclassical transport coefficient.
Limitations to the plasma energy and density in electron cyclotron resonance ion sources
Authors: Perret C. , Girard A. , Khodja H. , Melin G.
Published: July 2002

Electron Cyclotron Resonance (ECR) plasmas are generated by the resonant interaction of a high-frequency (hf) wave with the electrons of the plasma close to the electron Larmor frequency. At high hf power the collisional relaxation is not sufficient to thermalize the electrons and a complex Electron Distribution Function (EDF) is generated. In ECR Ion Sources (ECRIS), dedicated to the production of Highly Charged Ions (HCI), such a hot electron component is required to ionize the atoms to very high charged states; in this paper experiments on the electron population are presented. It is shown that ECRIS plasmas are very far from thermal equilibrium; which is necessary for the production of HCI; the observed saturation of the performances (in electron energy and density, and ion charge state) can be explained by the wave–plasma interaction itself: low-energy electrons interact primarily with the electric field of the wave, whereas the high-energy component experiences mainly pitch-angle scattering due to the interaction with the magnetic field of the wave, which becomes dominant. A simple model, consistent with the results obtained in the domain of strong hf power, is developed. This model explains the saturation phenomena encountered.
Effect of ambient density fluctuations on Langmuir wave collapse and strong turbulence
Authors: Robinson P. A. , de Oliveira G. I.
Published: July 2002

The effect of ambient density fluctuations on Langmuir wave collapse and strong Langmuir turbulence is investigated. Hamiltonian analysis of the collapse threshold implies that fluctuations with scales near those of nucleating wave packets can disrupt them before they can accumulate enough energy to collapse, provided the ambient fluctuation level is greater than that generated ponderomotively by the Langmuir waves. If packet disruption is effective, Langmuir energy cannot be dissipated via wave collapse and burnout, but must be scattered off density fluctuations directly to high wave numbers, as predicted by previous analyses. Numerical simulations of strong Langmuir turbulence confirm these predictions, with sudden transitions occurring from a strong-turbulence regime to one dominated by scattering or one with relatively rare wave collapses as a result of disruption of nascent wave packets. A corresponding sudden drop in Langmuir energy density is observed. Simulations of individual wave packets near the threshold for collapse show that such packets are easily disrupted by fluctuations with wavelengths near their linear scale, and confirm previous analytic disruption criteria.
Nonlinear neoclassical transport in a rotating impure plasma with large gradients
Authors: Fülöp T. , Helander P.
Published: July 2002

The theory of neoclassical transport in an impure toroidal plasma is extended to allow for larger pressure and temperature gradients and faster toroidal rotation than are usually considered. Under these conditions, the density of heavy impurities is not constant on flux surfaces, and the neoclassical transport becomes a nonlinear function of the gradients. Rapid toroidal rotation increases the transport, which can significantly exceed the conventional Pfirsch–Schlüter value if the impurity Mach number is of order unity. In a plasma with steep density or temperature profile, the transport is severely reduced and can even be a nonmonotonic function of the gradients. Finally, in the presence of both rapid toroidal rotation and steep gradients, the transport becomes sensitive to the geometry of the magnetic equilibrium. For instance, in a single-null diverted magnetic field the ion particle flux is typically inward if the ion drift is toward the X-point and changes direction if the toroidal field is reversed.
Numerical simulation of magnetic compression on a field-reversed configuration plasma
Authors: Kanki T. , Suzuki Y. , Okada S. , Goto S.
Published: July 2002

A two-dimensional magnetohydrodynamic (MHD) simulation of an axial magnetic compression on a field-reversed configuration (FRC) plasma is carried out for the parameter range of a corresponding experiment conducted on the FRC Injection Experiment (FIX) [S. Okada et al., 17th IAEA Fusion Energy Conference 1998 (International Atomic Energy Agency, Vienna) (in press)]. The simulation results show that during the initial stage of the magnetic compression the front part of the FRC plasma is mainly compressed radially, and that after this stage, the compression is primarily axial. Of particular interest is expected that the closed magnetic flux surfaces of the FRC can be retained without any degradation during the magnetic compression process. Further, it is observed in the simulation that the axial magnetic compression enables a transition of the MHD equilibrium from a long and thin to a short and fat FRC. The effects of this magnetic compression on FRC plasmas are discussed.
On the maximum current for a self-focusing relativistic electron beam
Authors: Honda M.
Published: July 2002

The maximum possible current has been calculated in a relativistic electron beam-plasma equilibrium. The self-consistent analysis reads Imax<(m0 c3/e)βbγb, where γb=(1−βb2)−1/2 for the charge-neutralized relativistic beam having a large radius compared with the beam thermal Debye length. The special case found in the literature is reproduced [H. Alfvén, Phys. Rev 55, 425 (1939)]. The present work is essential for relativistic electron transport in fast ignition of targets for laser fusion.
Kinetic simulation on ion acoustic wave in gas discharge plasma with convective scheme
Authors: Matsunaga Yasushi , Hatori Tadatsugu , Kato Tomokazu
Published: July 2002

In a one-dimensional plasma-sheath system representing a concave quasistationary electric potential typical of a negatively charged system, oscillations of ion are simulated by the aid of a convective scheme useful for weakly ionized plasma, and are theoretically investigated. The frequency spectra of the ion current through a cathode reveal to us that two modes of ion acoustic waves are dominant; a high and a low frequency mode. By deriving a linear differential equation with a dissipation and an ion flow, and taking for granted the sheath width and the distribution of ion flow velocity, the dispersion relation for a finite length system can be calculated. The simulation results, such as the reinforcement of the low frequency mode and the suppression of the high frequency mode, are satisfactorily corroborated by the linear theory. The instabilities of the waves are caused by the asymmetry of boundary conditions and by the dissipative effect.
Self-consistent distributions in collisional absorption of intense laser fields
Authors: Ersfeld B. , Bell A. R.
Published: July 2002

The influence of the shape of the electron velocity distribution on collisional absorption of laser irradiation in a plasma is considered, describing its self-consistent evolution by a Boltzmann equation, and treating collisions with (immobile) ions and among electrons in Fokker–Planck approximation. In order to facilitate numerical solutions for intensities where the distribution is displaced by several times its width it is approximated as isotropic in a reference frame moving with the average electron velocity. In the limit of high ion charges the resulting non-Maxwellian radial shape fairly reproduces those features of the complete solution relevant for absorption. For singly charged ions the shape is closer to Maxwellian, as expected, but the remaining differences still affect the absorption. For low collision frequency, averaging over the laser time scale yields a generalization of Langdon’s analysis for low intensities [Phys. Rev. Lett. 44, 575 (1980)].
The compressible plane current-vortex sheet
Authors: Dahlburg R. B. , Einaudi G.
Published: July 2002

The plane current-vortex sheet is a magnetohydrodynamic configuration in which a vortex sheet lies at the location of an electric current sheet. Most previous research on this structure has focused on the incompressible situation. In this paper some effects of compressibility on the linear stability properties of the plane current-vortex sheet are examined. The relevant compressible equations are derived and then solved by a new magnetohydrodynamic extension of the SPEctral Compressible Linear Stability (SPECLS) algorithm, a Chebyshev collocation code. Of particular interest is an investigation of how the properties of the low sonic Mach number (M) analogs of previously investigated incompressible unstable modes vary as M is increased to supersonic values. It is found that, in general, the growth rates of these modes decrease as M increases. However, new unstable modes are found to appear at high M. These new modes, which have a finite phase velocity, are also found to be weakly evanescent and oscillatory in the cross-stream direction. Further data is presented on the influence of the streamwise and spanwise wave numbers, and also the Alfvén number. The morphology of the perturbations is also discussed, with an emphasis on the temperature and mass density structure. A short discussion is also given of the effect of spatial variation of the zeroth-order temperature and mass density fields, a situation that would arise when magnetofluids with different thermodynamic properties are brought into contact with each other.
Gyrokinetic simulations in general geometry and applications to collisional damping of zonal flows
Authors: Lin Z. , Hahm T. S. , Lee W. W. , Tang W. M. , White R. B.
Published: July 2002

A fully three-dimensional gyrokinetic particle code using magnetic coordinates for general geometry has been developed and applied to the investigation of zonal flows dynamics in toroidal ion-temperature-gradient turbulence. Full torus simulation results support the important conclusion that turbulence-driven zonal flows significantly reduce the turbulent transport. Linear collisionless simulations for damping of an initial poloidal flow perturbation exhibit an asymptotic residual flow. The collisional damping of this residual causes the dependence of ion thermal transport on the ion–ion collision frequency, even in regimes where the instabilities are collisionless.
Surface plasmon polariton enhanced photoconductivity of tunnel junctions in the visible
Authors: Berthold K. , Höpfel R. A. , Gornik E.
Published: July 2002

We have investigated the influence of surface plasmon polaritons (SPP’s) on the reflection spectra of Ag-Al2O2-Al and Al-AL2O2-Al junctions in the visible range. The SPP’s were excited via sinusoidal grating structures. The SPP photon coupling efficiency was optimized as a function of the grating modulation height and dielectric coatings. For optimum coupling conditions a narrowband photoresponse of up to 60 mV/W into a 100-Ω impedance at 632,8 nm was achieved. The narrowband (3–6 nm wide) photosignal is more than an order of magnitude higher than a broadband background signal. The color sensitivity is varied over the whole visible range with different dielectric coatings.
Intrinsic gettering in oxygen-free silicon
Authors: Nauka N. , Lagowski J. , Gatos H. C. , Li C.-J.
Published: July 2002

We found that a three-step high-temperature annealing leads to intrinsic gettering in oxygen-lean Czochralski silicon grown in a magnetic field as well as in oxygen-free float zone silicon. Such thermal treatment has been used thus far for achieving intrinsic gettering only in oxygen-rich Czochralski silicon. We present experimental characteristics of this new intrinsic gettering process, and we propose a phenomenological explanation involving outdiffusion and precipitation of silicon interstitials rather than oxygen which is involved in the standard intrinsic gettering.
Enhancement of the plasma density and deposition rate in rf discharges
Authors: Overzet L. J. , Verdeyen J. T.
Published: July 2002

The peak and time averaged electron density in rf excited silane-helium mixtures increased significantly above the cw value by square wave modulating the source. The deposition rate of amorphous hydrogenated silicon films is also enhanced and apparently follows the electron density. Attachment to the discharge products appears to be responsible.
Rapid solidification studies of a model alloy system
Authors: Thompson Michael O. , Peercy P. S. , Tsao J. Y. , Aziz M. J.
Published: July 2002

Silicon-germanium alloys are found to be simple model systems for studying the effects of melting temperature changes on the liquid-solid interface dynamics during pulsed laser melting. The melting temperature of these alloys can be continuously varied from 1210 to 1685 K. An alloy of Si50Ge50 is shown to closely model the melting kinetics of amorphous silicon. By tailoring the melting temperature versus depth with multilayer structures, the effects of superheating, undercooling, and thermal conductivity on the solidification dynamics have been studied.
n-InAs/GaAs heterostructure superconducting weak links with Nb electrodes
Authors: Kleinsasser A. W. , Jackson T. N. , Pettit G. D. , Schmid H. , Woodall J. M. , Kern D. P.
Published: July 2002

We report on the fabrication and characterization of planar superconductor-normal-superconductor (SNS) weak links in which the normal region is deposited n-InAs. The InAs is part of a heterostructure consisting of 100 nm of n-InAs grown on an undoped GaAs buffer layer on a semi-insulating GaAs substrate. The superconductor is Nb, patterned by electron beam lithography with interelectrode spacings as small as 260 nm. Device behavior is well explained by SNS weak link theory, with coherence lengths calculated from measured material parameters. These heterostructure weak links can be the basis for superconducting field-effect devices. They have the significant advantage of allowing simple device isolation compared with bulk InAs, which has been used in previous attempts to make such devices.
Piezoelectricity of c-axis oriented PbTiO3 thin films
Authors: Kushida Keiko , Takeuchi Hiroshi
Published: July 2002

Through seeded lateral overgrowth, c-axis oriented PbTiO3 films are formed on patterned Pt electrode films that are on a SrTiO3 single crystal. The impedance characteristics are then measured for this sample as if it were a composite bulk wave resonator. A series of resonance and antiresonance characteristics is observed. Using a conventional circuit model, the electrical impedance is analyzed to evaluate the electromechanical coupling constant kt of the PbTiO3 films. It is found that kt is as large as 0.8.
Partial epitaxial growth of cobalt germanides on (111)Ge
Authors: Hsieh Y. F. , Chen L. J. , Marshall E. D. , Lau S. S.
Published: July 2002

Localized  epitaxial  Co5Ge7  and  CoGe2  have  been  grown  in  cobalt  thin  films  on (111)Ge  in  the solid phase epitaxy regime. The orientation relationships between epitaxial germanides and the substrates as well as the configuration of the interfacial dislocations were analyzed by transmission  electron  microscopy (TEM) in detail.  Surface morphology was examined by scanning  electron  microscopy.  The  results  obtained  from  Read  camera  glancing  angle  x-ray diffraction and Rutherford backscattering channeling analysis were found to corroborate with those from TEM examinations.
Fine structure of the alloy-broadened thermal emission spectra from D
X centers in GaAlAs
Authors: Calleja E. , Gomez A. , Muñoz E. , Cámara P.
Published: July 2002

Deep level transient spectroscopy in Si- and Sn-doped GaAlAs reveals a fine structure of the DX center thermal emission spectra under adequate filling pulse and sampling window times. This structure is reproducible in samples with Al mode fractions near 30% but it is not detectable in samples with 85% Al content. All resolved peaks of this fine structure have the same thermal emission energy but quite different capture cross section (σ∞n). This fact indicates that the origin of the fine structure and of the nonexponential behavior of the thermal emission processes is the discrete broadening of σ∞n due to the alloy effect.
Exciton line broadening in CdxZn1−xTe/ZnTe multiple quantum wells
Authors: Stanley R. P. , Hegarty J. , Feldman R. D. , Austin R. F.
Published: July 2002

We have investigated Cdx Zn1−xTe/ZnTe multiple quantum wells using absorption techniques. We have observed sharp excitonic features at low temperatures which strongly broaden at room temperature. The strength of the exciton-phonon coupling is determined from linewidth analysis. The large measured coupling explains the lack of well defined exciton resonances at room temperature, an important consequence for their use as optoelectronic devices.
Changes in subcellular distribution of pituitary receptors for luteinizing hormone-releasing hormone (LH-RH) after treatment with the LH-RH antagonist cetrorelix
Authors: Halmos Gabor , Schally Andrew V.
Published: July 2002

Treatment with antagonists of luteinizing hormone-releasing hormone (LH-RH) leads to down-regulation of pituitary LH-RH receptors. Thus, the effect of LH-RH antagonists is similar to that of the LH-RH agonists, but the mode of action of antagonists is not completely understood. The aim of this study was to investigate the effects of LH-RH antagonist cetrorelix on the binding characteristics and subcellular localization of receptors for LH-RH in rat pituitaries. Radioligand binding studies, performed afterin vitrodesaturation, revealed that a single s.c. injection of cetrorelix at a dose of 100 μg per rat significantly decreased the number of pituitary membrane receptors for LH-RH in a time-dependent manner with the nadir occurring at 6 h. In contrast, 2–6 h after cetrorelix treatment, the concentration of binding sites for LH-RH in the nuclei of rat pituitaries was significantly higher (P< 0.01) than in controls. Chronic administration of cetrorelix also decreased the level of membrane receptors for LH-RH by 83% (P< 0.01) after 7 days, and 86% (P< 0.01) after 14 days. The number of LH-RH binding sites in the nuclear pellet was increased 3-fold (P< 0.01) by days 7 and 14 after the initiation of treatment with cetrorelix. A single injection or prolonged treatment with LH-RH antagonist also decreased the mRNA expression of pituitary receptors for LH-RH. Our results demonstrate that the down-regulation of LH-RH receptors on the cell membranes of rat pituitaries after therapy with antagonist cetrorelix is associated with an increase in receptor concentration in the nuclei. These phenomena could be related to the internalization and subcellular translocation of LH-RH receptors.
A critical residue for isoform difference in tetrodotoxin affinity is a molecular determinant of the external access path for local anesthetics in the cardiac sodium channel
Authors: Sunami Akihiko , Glaaser Ian W. , Fozzard Harry A.
Published: July 2002

Membrane-impermeant quaternary derivatives of lidocaine (QX222 and QX314) block cardiac Na
+
channels when applied from either side of the membrane, but they block neuronal and skeletal muscle channels poorly from the outside. To find the molecular determinants of the cardiac external QX access path, mutations of adult rat skeletal muscle (μ1) and rat heart (rH1) Na
+
channels were studied by two-electrode voltage clamp in
Xenopus
oocytes. Mutating the μ1 domain I P-loop Y401, which is the critical residue for isoform differences in tetrodotoxin block, to the heart sequence (Y401C) allowed outside QX222 block, but its mutation to brain type (Y401F) showed little block. μ1-Y401C accelerated recovery from block by internal QX222. Block by external QX222 in μ1-Y401C was diminished by chemical modification with methanethiosulfonate ethylammonium (MTSEA) to the outer vestibule or by a double mutant (μ1-Y401C/F1579A), which altered the putative local anesthetic binding site. The reverse mutation in heart rH1-C374Y reduced outside QX314 block and slowed dissociation of internal QX222. Mutation of μ1-C1572 in IVS6 to Thr, the cardiac isoform residue (C1572T), allowed external QX222 block, and accelerated recovery from internal QX222 block, as reported. Blocking efficacy of outside QX222 in μ1-Y401C was more than that in μ1-C1572T, and the double mutant (μ1-Y401C/C1572T) accelerated internal QX recovery more than μ1-Y401C or μ1-C1572T alone. We conclude that the isoform-specific residue (Tyr/Phe/Cys) in the P-loop of domain I plays an important role in drug access as well as in tetrodotoxin binding. Isoform-specific residues in the IP-loop and IVS6 determine outside drug access to an internal binding site.
Chlorophyll and carotenoid binding in a simple red algal light-harvesting complex crosses phylogenetic lines
Authors: Grabowski Beatrice , Cunningham Francis X. , Gantt Elisabeth
Published: July 2002

The membrane proteins of peripheral light-harvesting complexes
(LHCs) bind chlorophylls and carotenoids and transfer energy to the
reaction centers for photosynthesis. LHCs of chlorophytes,
chromophytes, dinophytes, and rhodophytes are similar in that they have
three transmembrane regions and several highly conserved Chl-binding
residues. All LHCs bind Chl
a,
but in specific taxa
certain characteristic pigments accompany Chl
a
: Chl
b
and lutein in chlorophytes, Chl
c
and
fucoxanthin in chromophytes, Chl
c
and peridinin in
dinophytes, and zeaxanthin in rhodophytes. The specificity of pigment
binding was examined by
in vitro
reconstitution of
various pigments with a simple light-harvesting protein (LHCaR1), from
a red alga (
Porphyridium cruentum
), that normally has
eight Chl
a
and four zeaxanthin molecules. The pigments
typical of a chlorophyte (
Spinacea oleracea
), a
chromophyte (
Thallasiosira fluviatilis
), and a dinophyte
(
Prorocentrum micans
) were found to functionally bind to
this protein as evidenced by their participation in energy transfer to
Chl
a,
the terminal pigment. This is a demonstration of
a functional relatedness of rhodophyte and higher plant LHCs. The
results suggest that eight Chl-binding sites per polypeptide are an
ancestral trait, and that the flexibility to bind various Chl and
carotenoid pigments may have been retained throughout the evolution of
LHCs.
Differential effects of a centrally acting fatty acid synthase inhibitor in lean and obese mice
Authors: Kumar Monica V. , Shimokawa Teruhiko , Nagy Tim R. , Lane M. Daniel
Published: July 2002

C75 is a potent inhibitor of fatty acid synthase that acts centrally to reduce food intake and body weight in mice; a single dose causes a rapid (>90%) decrease of food intake. These effects are associated with inhibition of fasting-induced up-regulation and down-regulation, respectively, of the expression of orexigenic (NPY and AgRP) and anorexigenic (POMC and CART) neuropeptide messages in the hypothalamus. Repeated administration of C75 at a submaximal level, however, differentially affected food intake of lean and obese mice. With lean mice, C75 suppressed food intake by ≈50% and, with obese mice (
ob/ob
and dietary-induced obesity), by 85–95% during the first day of treatment. Lean mice, however, became tolerant/resistant to C75 over the next 2–5 days of treatment, with food intake returning to near normal and rebound hyperphagia occurring on cessation of treatment. In contrast,
ob/ob
obese mice responded to C75 with a >90% suppression of food intake throughout the same period with incipient tolerance becoming evident only after substantial weight loss had occurred. Dietary-induced obese mice exhibited intermediate behavior. In all cases, a substantial loss of body weight resulted. Pair-fed controls lost 24–50% less body weight than C75-treated mice, indicating that, in addition to suppressing food intake, C75 may increase energy expenditure. The decrease in body weight by
ob/ob
mice was due primarily to loss of body fat. In contrast to the short-term effects of C75 on “fasting-induced” changes of hypothalamic orexigenic and anorexigenic neuropeptide mRNAs, repeated administration of C75 either had the inverse or no effect as tolerance developed.
Detection of leukemia-associated
MLL-GAS7
translocation early during chemotherapy with DNA topoisomerase II inhibitors
Authors: Megonigal Maureen D. , Cheung Nai-Kong V. , Rappaport Eric F. , Nowell Peter C. , Wilson Robert B. , Jones Douglas H. , Addya Kathakali , Leonard Debra G. B. , Kushner Brian H. , Williams Terence M. , Lange Beverly J. , Felix Carolyn A.
Published: July 2002

Leukemias with
MLL
gene translocations are a
complication of primary cancer treatment with DNA topoisomerase II
inhibitors. How early translocations appear during primary cancer
treatment has not been investigated. We tracked the leukemic clone with
an
MLL
gene translocation during neuroblastoma therapy
in a child who developed acute myeloid leukemia. The karyotype of the
leukemic clone showed del(11)(q23). We used panhandle PCR-based methods
to isolate the breakpoint junction involving
MLL
and an
unknown partner gene. Marrow DNA from neuroblastoma diagnosis and DNA
and RNA from serial preleukemic marrows were examined for the
translocation. The karyotypic del(11)(q23) was a cryptic t(11;17).
GAS7,
a growth arrest-specific gene at chromosome band
17p13, was the partner gene of
MLL
. Two different
MLL-GAS7
fusion transcripts were expressed. The
translocation was already detectable by 1.5 months after the start of
neuroblastoma treatment. The translocation was not detectable in the
marrow at neuroblastoma diagnosis or in peripheral blood lymphocyte
DNAs of six normal subjects.
GAS7
is a new partner gene
of
MLL
in treatment-related acute myeloid leukemia.
MLL
gene translocations can be present early during
anticancer treatment at low cumulative doses of DNA topoisomerase II
inhibitors. Although
MLL
has many partner genes and most
have not been characterized, panhandle PCR strategies afford new means
for detecting
MLL
gene translocations early during
therapy when the partner gene is unknown.
Herpes simplex virus DNA packaging sequences adopt novel structures that are specifically recognized by a component of the cleavage and packaging machinery
Authors: Adelman Karen , Salmon Brandy , Baines Joel D.
Published: July 2002

The product of the herpes simplex virus type 1 U
L
28
gene is essential for cleavage of concatemeric viral DNA into
genome-length units and packaging of this DNA into viral procapsids. To
address the role of U
L
28 in this process, purified
U
L
28 protein was assayed for the ability to recognize
conserved herpesvirus DNA packaging sequences. We report that DNA
fragments containing the
pac1
DNA packaging motif can be
induced by heat treatment to adopt novel DNA conformations that migrate
faster than the corresponding duplex in nondenaturing gels.
Surprisingly, these novel DNA structures are high-affinity substrates
for U
L
28 protein binding, whereas double-stranded DNA of
identical sequence composition is not recognized by U
L
28
protein. We demonstrate that only one strand of the
pac1
motif is responsible for the formation of novel DNA structures that are
bound tightly and specifically by U
L
28 protein. To
determine the relevance of the observed U
L
28
protein–
pac1
interaction to the cleavage and packaging
process, we have analyzed the binding affinity of U
L
28
protein for
pac1
mutants previously shown to be
deficient in cleavage and packaging
in vivo
. Each of the
pac1
mutants exhibited a decrease in DNA binding by
U
L
28 protein that correlated directly with the reported
reduction in cleavage and packaging efficiency, thereby supporting a
role for the U
L
28 protein–
pac1
interaction
in vivo
. These data therefore suggest that the formation
of novel DNA structures by the
pac1
motif confers added
specificity on recognition of DNA packaging sequences by the
U
L
28-encoded component of the herpesvirus cleavage and
packaging machinery.
Single-molecule spectroscopy of the β
2
adrenergic receptor: Observation of conformational substates in a membrane protein
Authors: Peleg Gadi , Ghanouni Pejman , Kobilka Brian K. , Zare Richard N.
Published: July 2002

Single-molecule studies of the conformations of the intact β
2
adrenergic receptor were performed in solution. Photon bursts from the fluorescently tagged adrenergic receptor in a micelle were recorded. A photon-burst algorithm and a Poisson time filter were implemented to characterize single molecules diffusing across the probe volume of a confocal microscope. The effects of molecular diffusion and photon number fluctuations were deconvoluted by assuming that Poisson distributions characterize the molecular occupation and photon numbers. Photon-burst size histograms were constructed, from which the source intensity distributions were extracted. Different conformations of the β
2
adrenergic receptor cause quenching of the bound fluorophore to different extents and hence produce different photon-burst sizes. An analysis of the photon-burst histograms shows that there are at least two distinct substates for the native adrenergic membrane receptor. This behavior is in contrast to one peak observed for the dye molecule, rhodamine 6G. We test the reliability and robustness of the substate number determination by investigating the application of different binning criteria. Conformational changes associated with agonist binding result in a marked change in the distribution of photon-burst sizes. These studies provide insight into the conformational heterogeneity of G protein-coupled receptors in the presence and absence of a bound agonist.
The mechanism of tryptophan induction of tryptophanase operon expression: Tryptophan inhibits release factor-mediated cleavage of TnaC-peptidyl-tRNA
Pro
Authors: Gong Feng , Ito Koichi , Nakamura Yoshikazu , Yanofsky Charles
Published: July 2002

Expression of the tryptophanase (
tna
) operon of
Escherichia coli
is regulated by catabolite repression and tryptophan-induced transcription antitermination. In a previous study, we reproduced the regulatory features of this operon observed
in vivo
by using an
in vitro
S-30 system. We also found that, under inducing conditions, the leader peptidyl-tRNA (TnaC-peptidyl-tRNA
Pro
) is not cleaved; it accumulates in the S-30 reaction mixture. In this paper, we examine the requirements for TnaC-peptidyl-tRNA
Pro
accumulation and cleavage,
in vitro
. We show that this peptidyl-tRNA remains bound to the translating ribosome. Removal of free tryptophan and addition of release factor 1 or 2 leads to hydrolysis of TnaC-peptidyl-tRNA
Pro
and release of TnaC from the ribosome-mRNA complex. Release factor-mediated cleavage is prevented by the addition of tryptophan. TnaC of the ribosome-bound TnaC-peptidyl-tRNA
Pro
was transferable to puromycin. This transfer was also blocked by tryptophan. Tests with various tryptophan analogs as substitutes for tryptophan revealed the existence of strict structural requirements for tryptophan action. Our findings demonstrate that the addition of tryptophan to ribosomes bearing nascent TnaC-peptidyl-tRNA
Pro
inhibits both TnaC peptidyl-tRNA
Pro
hydrolysis and TnaC peptidyl transfer. The associated translating ribosome therefore remains attached to the leader transcript where it blocks Rho factor binding and subsequent transcription termination.
Molecular emergence of acute myeloid leukemia during treatment for acute lymphoblastic leukemia
Authors: Blanco Javier G. , Dervieux Thierry , Edick Mathew J. , Mehta Perdeep K. , Rubnitz Jeffrey E. , Shurtleff Sheila , Raimondi Susana C. , Behm Frederick G. , Pui Ching-Hon , Relling Mary V.
Published: July 2002

Therapy-related acute myeloid leukemias (t-AML) with translocations of the
MLL
gene are associated with the use of topoisomerase II inhibitors. We established the emergence of the malignant clone in a child who developed t-AML with a t(11;19) (q23;p13.3) during treatment for acute lymphoblastic leukemia (ALL). The
MLL-ENL
and the reciprocal
ENL-MLL
genomic fusions and their chimeric transcripts were characterized from samples collected at the time of t-AML diagnosis. We used PCR with patient-specific genomic primers to establish the emergence of the
MLL-ENL
fusion in serially obtained DNA samples. The
MLL-ENL
fusion was not detectable in bone marrow at the time of ALL diagnosis or after 2 months of chemotherapy (frequency <8.3 × 10
−7
cells
−1
). The genomic fusion was first detected in bone marrow after 6 months of treatment at a frequency of one in 4,000 mononuclear bone marrow cells; the frequency was one in 70 cells after 20 months of therapy. At the first detection of
MLL-ENL
, the only topoisomerase II inhibitors the patient had received were one dose of daunorubicin and two doses of etoposide. The
MLL-ENL
fusion was not detectable in blood at the time of ALL diagnosis or after 0.7, 2, 8, 10, and 12 months of therapy but was detectable in blood at 16 months (one in 2.3 × 10
4
cells). Recombinogenic
Alu
sequences bracketed the breakpoints in both fusions. These data indicate that the malignant clone was not present before therapy, arose early during chemotherapy, and was able to proliferate even during exposure to antileukemic therapy.
Induction of multiple signaling loops by MuSK during neuromuscular synapse formation
Authors: Moore Chris , Leu Marco , Müller Ulrich , Brenner Hans Rudolf
Published: July 2002

At the neuromuscular junction, two motor neuron-derived
signals have been implicated in the regulation of synaptogenesis.
Neuregulin-1 is thought to induce transcription of acetylcholine
receptor (AChR) genes in subsynaptic muscle nuclei by activating ErbB
receptors. Neural agrin aggregates AChRs by activating the receptor
tyrosine kinase MuSK. Here, we show that these two signals act
sequentially. Agrin, by activating MuSK, induces the synthesis and
aggregation of both MuSK and ErbB receptors. ErbB acts downstream of
MuSK in synapse formation. In this way, MuSK activation leads to the
establishment of a neuregulin-1-dependent signaling complex that
maintains MuSK, ErbB, and AChR expression at the synapse of
electrically active muscle fibers.
Tetrahydrofolate biosynthesis in plants: Molecular and functional characterization of dihydrofolate synthetase and three isoforms of folylpolyglutamate synthetase in
Arabidopsis
thaliana
Authors: Ravanel Stéphane , Cherest Hélène , Jabrin Samuel , Grunwald Didier , Surdin-Kerjan Yolande , Douce Roland , Rébeillé Fabrice
Published: July 2002

Tetrahydrofolate coenzymes involved in one-carbon (C1) metabolism
are polyglutamylated. In organisms that synthesize tetrahydrofolate
de novo
, dihydrofolate synthetase (DHFS) and
folylpolyglutamate synthetase (FPGS) catalyze the attachment of
glutamate residues to the folate molecule. In this study we isolated
cDNAs coding a DHFS and three isoforms of FPGS from
Arabidopsis
thaliana
. The function of each enzyme was demonstrated
by complementation of yeast mutants deficient in DHFS or FPGS activity,
and by measuring
in vitro
glutamate incorporation into
dihydrofolate or tetrahydrofolate. DHFS is present exclusively in the
mitochondria, making this compartment the sole site of synthesis of
dihydrofolate in the plant cell. In contrast, FPGS is present as
distinct isoforms in the mitochondria, the cytosol, and the
chloroplast. Each isoform is encoded by a separate gene, a situation
that is unique among eukaryotes. The compartmentation of FPGS isoforms
is in agreement with the predominance of γ-glutamyl-conjugated
tetrahydrofolate derivatives and the presence of serine
hydroxymethyltransferase and C1-tetrahydrofolate interconverting
enzymes in the cytosol, the mitochondria, and the plastids. Thus, the
combination of FPGS with these folate-mediated reactions can supply
each compartment with the polyglutamylated folate coenzymes required
for the reactions of C1 metabolism. Also, the multicompartmentation of
FPGS in the plant cell suggests that the transported forms of folate
are unconjugated.
A novel function for Myc: inhibition of C/EBP-dependent gene activation.
Authors: Mink S , Mutschler B , Weiskirchen R , Bister K , Klempnauer K H
Published: July 2002

We have investigated the effect of the v-Myc oncoprotein on gene expression in myelomonocytic cells. We find that v-Myc dramatically down-regulates the expression of myelomonocytic-specific genes, such as the chicken mim-1 and lysozyme genes, both of which are known targets for C/EBP transcription factors. We present evidence that Myc downregulates these genes by inhibiting the function of C/EBP transcription factors. Detailed examination of the inhibitory mechanism shows that amino-terminal sequences of v-Myc, but not its DNA-binding domain, are required for the suppression of C/EBP-dependent transactivation. Our findings identify a new function for Myc and reveal a novel mechanism by which Myc affects the expression of other genes.
Long-term potentiation increases tyrosine phosphorylation of the N-methyl-D-aspartate receptor subunit 2B in rat dentate gyrus in vivo.
Authors: Rosenblum K , Dudai Y , Richter-Levin G
Published: July 2002

Long-term potentiation (LTP) is a form of synaptic memory that may subserve developmental and behavioral plasticity. An intensively investigated form of LTP is dependent upon N-methyl-D-aspartate (NMDA) receptors and can be elicited in the dentate gyrus and hippocampal CA1. Induction of this type of LTP is triggered by influx of Ca2+ through activated NMDA receptors, but the downstream mechanisms of induction, and even more so of LTP maintenance, remain controversial. It has been reported that the function of NMDA receptor channel can be regulated by protein tyrosine kinases and protein phosphatases and that inhibition of protein tyrosine kinases impairs induction of LTP. Herein we report that LTP in the dentate gyrus is specifically correlated with tyrosine phosphorylation of the NMDA receptor subunit 2B in an NMDA receptor-dependent manner. The effect is observed with a delay of several minutes after LTP induction and persists in vivo for several hours. The potential relevance of this post-translational modification to mechanisms of LTP and circuit plasticity is discussed.
Correction of hypertension by normalization of endothelial levels of fibroblast growth factor and nitric oxide synthase in spontaneously hypertensive rats.
Authors: Cuevas P , García-Calvo M , Carceller F , Reimers D , Zazo M , Cuevas B , Muñoz-Willery I , Martínez-Coso V , Lamas S , Giménez-Gallego G
Published: July 2002

Acidic and basic fibroblast growth factors (FGFs) share a wide range of diverse biological activities. To date, low levels of FGF have not been correlated with a pathophysiologic state. We report that blood vessels of spontaneously hypertensive rats are shown to be associated with a marked decrement in endothelial basic FGF content. This decrement correlates both with hypertension and with a decrease in the endothelial content of nitric oxide synthase. Restoration of FGF to physiological levels in the vascular wall, either by systemic administration or by in vivo gene transfer, significantly augmented the number of endothelial cells with positive immunostaining for nitric oxide synthase, corrected hypertension, and ameliorated endothelial-dependent responses to vasoconstrictors. These results suggest an important role for FGFs in blood pressure homeostasis and open new avenues for the understanding of the etiology and treatment of hypertension.