bluuebunny/crossref_metadata_2025 huggingface dataset:
Semiclassical electron capture probabilities for proton–hydrogenic ion collisions in dense plasmas
Authors: Kim Chang-Geun , Jung Young-Dae
Published: July 2002

Electron capture processes by protons from hydrogenic ions in dense plasmas are investigated in accordance with a semiclassical version of the Bohr and Lindhard model using the straight-line trajectory method. The screened electron capture radius and screened bound state wave function are obtained by using the Debye–Hückel interaction potential. The scaled semiclassical electron capture probability is obtained as a function of the impact parameter, Debye length, and projectile velocity. The plasma screening effects on the target system is found to be quite small so that the plasma screening effects on the capture probability is mainly determined by the screened capture radius. The plasma screening effects on the scaled semiclassical capture probability for the intermediate energy projectile is more significant than those for the high-energy projectile. The maximum position of the capture probability approaches the target nucleus as the projectile energy increases.
Higher-order terms in mode conversion
Authors: Swanson D. G.
Published: July 2002

The development of model equations to describe mode conversion and tunneling in an inhomogeneous plasma usually uses expansions of the Modified Bessel functions in the parameter λ=12k⊥2ρL2, keeping only the lowest possible order to demonstrate tunneling. With the high temperatures of fusion parameters, especially for fusion alpha particles, this parameter may not be small. It is shown here that the effective expansion parameter is really βj=2μ0njkTj/B02, the ratio of plasma pressure to the magnetic pressure.
Parametric decay of an electromagnetic pump wave in a two-dimensionally inhomogeneous plasma
Authors: Borisov N. , Stubbe P. , Gorbunov L.
Published: July 2002

The decay instability of an ordinary hf electromagnetic pump wave into quasitransversal upper hybrid (UH) and lower hybrid (LH) waves in the ionosphere is discussed. The longitudinal and transversal inhomogeneity of the medium is taken into account. It is shown that, due to the decay instability within weak plasma density enhancements, a rather broad spectrum of UH oscillations, similar to the spectrum of the downshifted maximum (DM) in stimulated electromagnetic emission (SEE) spectra, is formed. Estimates of the frequency range and characteristic k numbers of the DM spectrum are given.
Characterization of high frequency oscillations in a small Hall-type thruster
Authors: Guerrini G. , Michaut C.
Published: July 2002

The linear theory for plasma stability in a Hall-type thruster shows that high frequency oscillations are generated inside the thruster. These oscillations generate waves propagating along the channel. The frequencies depend on the value of different discharge parameters and the direction of propagation depends on the studied region. Here, two approaches will be proposed according to the value of the radial magnetic field and the sign of its gradient. An experimental detection performed outside the channel shows agreement between the models and the experiments. These waves appear as being “quasilongitudinal” inside the channel and azimuthal outside the channel.
A self-consistent global model of neutral gas depletion in pulsed helicon plasmas
Authors: Cho Suwon
Published: July 2002

The time-dependent global model is employed to examine the temporal behavior of the electron density and temperature in helicon plasmas. The power absorption calculated from the solutions of the Maxwell equations is used in solving the power balance equation and a balance model for the neutral gas is considered to find its density self-consistently. The numerical results successfully explain neutral gas depletion and the occurrence of two distinct modes of pulsed helicon discharge, which have been observed experimentally.
Steady state drift vortices in plasmas with shear flow in equilibrium
Authors: Chakrabarti Nikhil
Published: July 2002

The Hasegawa–Mima equation in the presence of sheared poloidal flow is solved for two-dimensional steady state vortex. It is shown that when the phase velocity of the vortex is the same as the diamagnetic drift velocity, an exact solution in the form of counter-rotating vortices may appear. In the small amplitude limit near the origin this solution reduces to a localized Lamb-dipole.
Added discussion of “Observations of fast anisotropic ion heating, ion cooling, and ion recycling in large-amplitude drift waves”
Authors: Sanders S. J. , Boivin R. F. , Bellan P. M. , Stern R. A.
Published: July 2002

A recent paper [S. J. Sanders, P. M. Bellan, and R. A. Stern, Phys. Plasmas 5, 716, (1998)] identified neutral particle recycling as one important aspect of severe heating and cooling cycles observed in large-amplitude drift waves. An apparent inconsistency in the ionization mean free path of these neutrals, left as an open question in the original paper, is resolved in this comment.
Evidence for second-order oscillations at the Best frequency in direct numerical simulations of the Vlasov equation
Authors: Nocera L. , Mangeney A.
Published: July 2002

Quantitative evidence for second-order oscillations occurring at the frequency predicted by Best [Physica 74, 183 (1974)] and by Sedláček and Nocera [J. Plasma Phys. 48, 367 (1992)] is provided by direct, ab initio numerical simulations of the Vlasov equation. These oscillations are relevant both for their intrinsic connection to the more customary plasma echo (also retrieved), for their diagnostic applications to the study of turbulence, and for the high accuracy needed to reproduce them. This latter fact is used as a sensitive test for the numerical integration, which indeed reproduces both known and new features of the oscillations, including nonlinear Landau damping, particle trapping, and the oscillations’ frequencies, in excellent agreement with theory.
Nonmodal energetics of electromagnetic drift waves
Authors: Camargo Suzana J. , Tippett Michael K. , Caldas Iberê L.
Published: July 2002

The linear properties of an electromagnetic drift-wave model are examined. The linear system is non-normal in that its eigenvectors are not orthogonal with respect to the energy inner product. The non-normality of the linear evolution operator can lead to enhanced finite-time growth rates compared to modal growth rates. Previous work with an electrostatic drift-wave model found that nonmodal behavior is important in the hydrodynamic limit. Here, similar behavior is seen in the hydrodynamic regime even with the addition of magnetic fluctuations. However, unlike the results for the electrostatic drift-wave model, nonmodal behavior is also important in the adiabatic regime with moderate to strong magnetic fluctuations.
Convective instability of radiatively cooling self-similar implosions
Authors: Murakami Masakatsu , Nishihara Katsunobu
Published: July 2002

The linear convective instability of imploding gaseous masses is investigated with a self-similar solution, which takes radiation heat conduction into account. The solution shows that the implosion process continuously transits from the initial adiabatic regime to the consequent nonadiabatic regime, where the mechanical compression work and the radiation loss balance such that the Péclet number of the system is kept constant. The transition accompanies the decrease in the polytropic index, Γ≡d(log p)/d(log ρ) where p and ρ are, respectively, the pressure and density, with the adiabatic index γ (⩾ Γ) as its initial value. As a result of the radiative cooling, the fluid becomes unstable to convective modes, when the criterion for instability, d(p/ρΓ)/dr<0, is fulfilled in the core. The spatial and temporal dependence of the perturbations are presented.
Alfvénicity of fluctuations associated with the Kelvin–Helmholtz instability
Authors: Parhi S. , Suess S. T.
Published: July 2002

The velocity shear which exists between three layers in an ideal plasma is studied. This configuration is modeled as a jet (or, strictly speaking, a wake) embedded in a uniform medium using a magnetohydrodynamics (MHD) code developed for astrophysical jet simulations. Weak and strong magnetic fields are considered both inside and outside the jet with a shear Mach number of 6. The shear can be Kelvin–Helmholtz (KH) unstable and evolve into a new less sheared pattern. There exists extensive literature on the KH instability which is extended by quantitatively describing the MHD properties of the fluctuations associated with the instability. To do so, a time series analysis of the fluctuations at various points inside and adjacent to the jet is performed. Specifically, points either in the center of the jet or just outside the transition layer—the initial location of the shear layer are considered. In the nonlinear stage, the perturbation is found to be a sum of the fast magnetosonic mode, slow magnetosonic mode, and the Alfvén component. To quantitatively evaluate the fluctuations, the normalized cross-helicity and Elsässer ratios are calculated, which in turn measure the degree of Alfvénicity. Fully nonlinear fluctuations are found to be more Alfvénic than magnetosonic in the low β case (β≈0.833) as compared with high β case (β≈13.3). This is in contrast to linear modes generated by the KH instability, which are magnetosonic modes.
Acceleration of electrons by a laser pulse in a tube
Authors: Rau B. , Cairns R. A.
Published: July 2002

In this paper we consider a scheme for laser driven electron acceleration in which a short intense laser pulse travels along a hollow tube, ionizing and heating the walls as it goes. Hot electrons expanding off the walls produce a large negative potential behind the pulse and the resulting potential gradient along the tube can be used to accelerate electrons. Computer simulations of this process suggest that accelerating fields in excess of 10 GeV/m can be reached with currently available technology and that accelerated electron bunches are well-focused on the axis of the tube. It is suggested that this scheme may have advantages over beat wave and wake field schemes, in terms of the controllability of the speed and phasing of the accelerating potential.
Response to “Comment on ‘Alfvén wave current drive in tokamak rotating plasma with negative magnetic shear’ ” [Phys. Plasmas 7, 3119 (2000)]
Authors: Ding N. , Huang L. , Qiu X. M.
Published: July 2002

The current drive due to low-frequency waves—Alfvén waves, ω≪Ωi (Ωi is the ion cyclotron frequency) in tokamak rotating plasma with negative magnetic shear has been considered in the paper [Phys. Plasma 6, 4633 (1999)]. In that paper, both the poloidal rotation frequency Ω and the magnetic shear parameter δ̄ have been assumed to be the first-order corrections, that is, Ω/ω≪1 and δ̄≪1. The single-fluid magnetohydrodynamic (MHD) model with a scalar resistivity and viscosity, and an Ohm’s law, nj=E+(1/c)V×B, is adequate for our first analysis in the linear theory.
Evidence for bond strengthening in Cd1−xZnxTe (x=0.04)
Authors: Qadri S. B. , Skelton E. F. , Webb A. W. , Kennedy J.
Published: July 2002

Hydrostatic compression data have been obtained for Cd1−xZnxTe(x=0.04±0.01) using energy dispersive x-ray diffraction techniques with heterochromatic, synchrotron produced radiation. The B3 to B1 structural transition pressure is observed to occur at 3.25±0.15 GPa, in agreement with published work on CdTe. The volume discontinuity at the transition is observed to be 15±1%, which is also in agreement with that found for CdTe. However, compression data for each of the two phases indicate that Cd1−xZnxTe is about 2% stiffer than CdTe, suggesting that the CdTe bond is strengthened by the addition of Zn. This is in agreement with recent bond strength calculations and provides the first microscopic, experimental evidence of this strengthening effect.
Room-temperature laser operation of AlGaAs/GaAs double heterostructures fabricated on Si substrates by metalorganic chemical vapor deposition
Authors: Sakai Shiro , Soga Tetsuo , Takeyasu Masanari , Umeno Masayoshi
Published: July 2002

AlGaAs/GaAs double heterostructure laser diodes have been fabricated on Si substrates using GaP/(GaP/GaAsP) superlattice/(GaAsP/GaAs) superlattice intermediate layers grown by metalorganic chemical vapor deposition. A threshold current density at 16.5 °C and a characteristic temperature T0 of 4.9 kA/cm2 and 179 K respectively have been obtained for the diode on Si substrate.
Graphoepitaxy of platinum on sawtooth profile gratings
Authors: Kushida Keiko , Takeuchi Hiroshi , Kobayashi Toshio , Takagi Kazumasa
Published: July 2002

Graphoepitaxy of Pt films on sawtooth profile gratings is realized. Gratings with 0.4 μm period are fabricated in (100)Si wafers using laser holographic lithography and Si anisotropic etching. Pt films evaporated at 200 °C on the gratings and annealed at 800 °C have (100) planes parallel to the substrate. From scanning electron micrograph it is found that the 100-nm-thick Pt film consists of crystallites embedded in the gratings. The grain size is about one grating period wide and 0.5–5 μm long. Continuous films with (100) orientation are produced after further deposition and annealing.
Stability of multilayers for synchrotron optics
Authors: Ziegler E. , Lepetre Y. , Schuller Ivan K. , Spiller E.
Published: July 2002

The temperature stability of metal (W, WRe, Co, Cr)-carbon multilayers has been studied using x-ray diffraction (θ–2θ and Debye–Scherrer) and electron microscopy. The results show that in all cases a crystallization occurs in the temperature range 650–750 °C. As a consequence of this crystallization, the layered structure is destroyed, the surface of the film becomes rough, and the x-ray reflectivity is considerably reduced. These results imply that efficient cooling or new multilayer structures will have to be developed for use at high temperatures or under high x-ray incident flux.
Absolutely measured x-ray spectra from laser plasmas with targets of different elements
Authors: Eidmann K. , Kishimoto T.
Published: July 2002

Soft x-ray spectra emitted by laser-produced plasmas in the wavelength range 10 Å<λ<250 Å were absolutely measured with a transmission grating and calibrated Kodak 101-01 film. Laser pulses with a length of 3 ns and a wavelength of 0.53 μm were focused at an intensity of 3×1013 W/cm2 on planar targets of various elements with atomic numbers ranging from 4 to 82. The overall conversion of incident laser energy into x-ray energy increases from 2% for Be to nearly 50% for the heaviest elements.
Low threshold pulsed and continuous laser oscillation from AlGaAs/GaAs double heterostructures grown by metalorganic chemical vapor deposition on Si substrates
Authors: van der Ziel J. P. , Dupuis R. D. , Logan R. A. , Mikulyak R. M. , Pinzone C. J. , Savage A.
Published: July 2002

Using graded refractive index, single quantum well GaAs lasers grown by metalorganic chemical vapor deposition on Si substrates, we have obtained injection lasers with broad area pulsed thresholds of 3.5 kA/cm2 at 300 K. With the current flow restricted to 5-μm-wide stripes, pulsed thresholds of 130 mA and differential quantum efficiencies of 70% were obtained at 300 K. Pulsed oscillation has been obtained up to 375 K and continuous oscillation up to 160 K. The rapid degradation of the lasers suggests the growth of nonradiative regions in the active layer.
Nonequilibrium electron transport in bipolar devices
Authors: Levi A. F. J. , Yafet Y.
Published: July 2002

The dynamics of nonequilibrium electron transport in bipolar devices have been investigated by calculating minority-carrier elastic and inelastic scattering rates as a function of energy for different majority-carrier concentrations in typical p-type III-V semiconductors. Scattering rates depend on the majority-carrier concentration and a constraint involving the ratio of electron and heavy-hole effective mass.
Narrow-linewidth, electro-optically tunable InGaAsP-Ti:LiNbO3 extended cavity laser
Authors: Heismann F. , Alferness R. C. , Buhl L. L. , Eisenstein G. , Korotky S. K. , Veselka J. J. , Stulz L. W. , Burrus C. A.
Published: July 2002

We report an electro-optically tunable, single-frequency extended cavity laser with a linewidth of less than 60 kHz. The laser consists of a 1.5-μm InGaAsP gain medium and an electro-optically tunable, narrow-band Ti:LiNbO3 wavelength filter (Δλ≊12 Å). Electro-optic tuning over at least 70 Å and single-frequency operation with output power of more than 1 mW have been demonstrated. The laser linewidth was measured by beating the laser against a 1.523-μm HeNe laser.
Ultrasonic excitation and detection of capillary waves for the measurement of surface film properties
Authors: Khuri-Yakub B. T. , Reinholdtsen P. A. , Chou C-H. , Vesecky J. F. , Teague C. C.
Published: July 2002

A 10 MHz focused ultrasonic transducer is used to excite capillary waves by placing its focus at the air-water interface and using a tone burst to excite the ultrasonic wave. The radiation pressure associated with the upward propagating ultrasonic pulse lifts the surface of the water which then relaxes by exciting a radially propagating capillary wave. An amplitude and phase measuring acoustic microscope operating at 10 MHz is used to detect the amplitude and slope of the capillary wave as it propagates over the focused transducer of the acoustic microscope. This arrangement allows us to make a noncontacting measurement of surface tension and surface viscosity which will be used for characterizing surface films, such as the marine microlayer.
Violet cw neodymium upconversion laser
Authors: Macfarlane R. M. , Tong F. , Silversmith A. J. , Lenth W.
Published: July 2002

We have observed cw laser action on a new transition (4D3/2→4I11/2) of the Nd3+ ion at 380 nm. This was demonstrated in Nd:LaF3 using an upconversion pumping scheme in which near infrared (788 nm) and visible (591 nm) pump photons from cw dye lasers produce stepwise excitation of the Nd3+ ion. In addition, lasing was observed with a single pump source at wavelengths around 578 nm using doubly resonant, sequential absorption of two yellow photons. Single-mode, cw operation with an output power of 12 mW was measured at 20 K with 1% output coupling and pump powers of several hundred mW. At 77 K the maximum power dropped to 4 mW.
Characterizations of the dc discharge plasma during chemical vapor deposition for diamond growth
Authors: Suzuki Kazuhiro , Sawabe Atsuhito , Inuzuka Tadao
Published: July 2002

During the course of diamond growth by dc plasma chemical vapor deposition, characteristics of the plasma have been measured by means of the Langmuir single probe method and emission spectrometry. For the source gas system of hydrogen and methane (gas ratio: CH4/H2=2/100, total gas pressure: 2.66×104 Pa), it is revealed that the statistical temperatures of hydrogen atoms and electrons in the positive column of the plasma are obtained to be 4.8×103–5.3×103 K and 1×105–1.1×105 K, respectively. The amount of ionized species is fairly small. By calculating the equilibrium constant of gas molecules in these gas temperature ranges, it is found that molecules over 99% H2 and CH4 are decomposed to the neutral H and C atoms.
2′,3′-Cyclic nucleotide 3′-phosphodiesterase: A membrane-bound, microtubule-associated protein and membrane anchor for tubulin
Authors: Bifulco Maurizio , Laezza Chiara , Stingo Stefania , Wolff J.
Published: July 2002

2′,3′-Cyclic nucleotide-3′-phosphodiesterase (CNP) is firmly associated with tubulin from brain tissue and FRTL-5 thyroid cells as demonstrated by copolymerization with microtubules through several warm/cold cycles, the presence of CNP activity in purified tubulin preparations, and identical behavior during various extraction procedures. CNP acts as a microtubule-associated protein in promoting microtubule assembly at low mole ratios. This activity resides in the C terminus of the enzyme, which, by itself, promotes microtubule assembly at higher mole ratios. Phosphorylation of CNP interferes with its assembly-promoting activity, as does deletion of the C terminus, which leads to abnormal microtubule distribution in the cell. Submembranous colocalization of the proteins and CNP-dependent microtubule organization suggest that CNP is a membrane-bound microtubule-associated protein that can link tubulin to membranes and may regulate cytoplasmic microtubule distribution.
Structure of spinach chloroplast F
1
-ATPase complexed with the phytopathogenic inhibitor tentoxin
Authors: Groth Georg
Published: July 2002

Tentoxin, a natural cyclic tetrapeptide produced by phytopathogenic fungi from the
Alternaria
species affects the catalytic function of the chloroplast F
1
-ATPase in certain sensitive species of plants. In this study, we show that the uncompetitive inhibitor tentoxin binds to the αβ-interface of the chloroplast F
1
-ATPase in a cleft localized at βAsp-83. Most of the binding site is located on the noncatalytic α-subunit. The crystal structure of the tentoxin-inhibited CF
1
-complex suggests that the inhibitor is hydrogen bonded to Asp-83 in the catalytic β-subunit but forms hydrophobic contacts with residues Ile-63, Leu-65, Val-75, Tyr-237, Leu-238, and Met-274 in the adjacent α-subunit. Except for minor changes around the tentoxin-binding site, the structure of the chloroplast α
3
β
3
-core complex is the same as that determined with the native chloroplast ATPase. Tentoxin seems to act by inhibiting inter-subunit contacts at the αβ-interface and by blocking the interconversion of binding sites in the catalytic mechanism.
X-ray structure of the human hyperplastic discs protein: An ortholog of the C-terminal domain of poly(A)-binding protein
Authors: Deo Rahul C. , Sonenberg Nahum , Burley Stephen K.
Published: July 2002

The poly(A)-binding protein (PABP) recognizes the 3′ mRNA poly(A)
tail and plays an essential role in eukaryotic translation initiation
and mRNA stabilization/degradation. PABP is a modular protein, with
four N-terminal RNA-binding domains and an extensive C terminus. The
C-terminal region of PABP is essential for normal growth in yeast and
has been implicated in mediating PABP homo-oligomerization and
protein–protein interactions. A small, proteolytically stable, highly
conserved domain has been identified within this C-terminal segment.
Remarkably, this domain is also present in the hyperplastic discs
protein (HYD) family of ubiquitin ligases. To better understand the
function of this conserved region, an x-ray structure of the PABP-like
segment of the human HYD protein has been determined at 1.04-Å
resolution. The conserved domain adopts a novel fold resembling a
right-handed supercoil of four α-helices. Sequence profile searches
and comparative protein structure modeling identified a small ORF from
the
Arabidopsis thaliana
genome that encodes a
structurally similar but distantly related PABP/HYD domain.
Phylogenetic analysis of the experimentally determined (HYD) and
homology modeled (PABP) protein surfaces revealed a conserved feature
that may be responsible for binding to a PABP interacting protein,
Paip1, and other shared interaction partners.
Neuropeptide Y: A hypothalamic brake restraining the onset of puberty in primates
Authors: Majdoubi M. El , Sahu A. , Ramaswamy S. , Plant T. M.
Published: July 2002

The adult reproductive axis is driven by an intermittent discharge of gonadotropin-releasing hormone (GnRH) generated by a network of hypothalamic neurons known as the GnRH pulse generator. Although this signal generator is operational in infant primates, puberty in these species is delayed by activation shortly after birth of a central neural mechanism that holds GnRH release in check during juvenile development. Here, we show that, in the male rhesus monkey, the postnatal pattern in GnRH pulse generator activity is inversely related to that in neuropeptide Y (NPY) gene and protein expression in the mediobasal hypothalamus and that central administration of an NPY Y
1
receptor antagonist to juvenile animals elicits precocious GnRH release. Cell imaging indicated that the developmentally regulated NPY neurons may be located in regions dorsal to the arcuate nucleus. These findings lead us to propose that NPY is a fundamental component of the neurobiological brake restraining the onset of puberty in primates.
Synaptic pattern formation during cellular recognition
Authors: Qi S. Y. , Groves Jay T. , Chakraborty Arup K.
Published: July 2002

Cell–cell recognition often requires the formation of a highly organized pattern of receptor proteins (a synapse) in the intercellular junction. Recent experiments [e.g., Monks, C. R. F., Freiberg, B. A., Kupfer, H., Sciaky, N. & Kupfer, A. (1998)Nature (London)395, 82–86; Grakoui, A., Bromley, S. K., Sumen, C., Davis, M. M., Shaw, A. S., Allen, P. M. & Dustin, M. L. (1999)Science285, 221–227; and Davis, D. M., Chiu, I., Fassett, M., Cohen, G. B., Mandelboim, O. & Strominger, J. L. (1999)Proc. Natl. Acad. Sci. USA96, 15062–15067] vividly demonstrate a complex evolution of cell shape and spatial receptor–ligand patterns (several microns in size) in the intercellular junction during immunological synapse formation. The current view is that this dynamic rearrangement of proteins into organized supramolecular activation clusters is driven primarily by active cytoskeletal processes [e.g., Dustin, M. L. & Cooper, J. A. (2000)Nat. Immunol.1, 23–29; and Wulfing, C. & Davis, M. M. (1998)Science282, 2266–2269]. Here, aided by a quantitative analysis of the relevant physico-chemical processes, we demonstrate that the essential characteristics of synaptic patterns observed in living cells can result from spontaneous self-assembly processes. Active cellular interventions are superimposed on these self-organizing tendencies and may also serve to regulate the spontaneous processes. We find that the protein binding/dissociation characteristics, protein mobilities, and membrane constraints measured in the cellular environment are delicately balanced such that the length and time scales of spontaneously evolving patterns are in near-quantitative agreement with observations for synapse formation between T cells and supported membranes [Grakoui, A., Bromley, S. K., Sumen, C., Davis, M. M., Shaw, A. S., Allen, P. M. & Dustin, M. L. (1999)Science285, 221–227]. The model we present provides a common way of analyzing immunological synapse formation in disparate systems (e.g., T cell/antigen-presenting cell junctions with different MHC-peptides, natural killer cells, etc.).
New roles forcis-jasmone as an insect semiochemical and in plant defense
Authors: Birkett Michael A. , Campbell Colin A. M. , Chamberlain Keith , Guerrieri Emilio , Hick Alastair J. , Martin Janet L. , Matthes Michaela , Napier Johnathan A. , Pettersson Jan , Pickett John A. , Poppy Guy M. , Pow Eleanor M. , Pye Barry J. , Smart Lesley E. , Wadhams George H. , Wadhams Lester J. , Woodcock Christine M.
Published: July 2002

cis-Jasmone, or (Z)-jasmone, is well known as a component of plant volatiles, and its release can be induced by damage, for example during insect herbivory. Using the olfactory system of the lettuce aphid to investigate volatiles from plants avoided by this insect, (Z)-jasmone was found to be electrophysiologically active and also to be repellent in laboratory choice tests. In field studies, repellency from traps was demonstrated for the damson-hop aphid, and with cereal aphids numbers were reduced in plots of winter wheat treated with (Z)-jasmone. In contrast, attractant activity was found in laboratory and wind tunnel tests for insects acting antagonistically to aphids, namely the seven-spot ladybird and an aphid parasitoid. When applied in the vapor phase to intact bean plants, (Z)-jasmone induced the production of volatile compounds, including the monoterpene (E)-β-ocimene, which affect plant defense, for example by stimulating the activity of parasitic insects. These plants were more attractive to the aphid parasitoid in the wind tunnel when tested 48 h after exposure to (Z)-jasmone had ceased. This possible signaling role of (Z)-jasmone is qualitatively different from that of the biosynthetically related methyl jasmonate and gives a long-lasting effect after removal of the stimulus. Differential display was used to compare mRNA populations in bean leaves exposed to the vapor of (Z)-jasmone and methyl jasmonate. One differentially displayed fragment was cloned and shown by Northern blotting to be up-regulated in leaf tissue by (Z)-jasmone. This sequence was identified by homology as being derived from a gene encoding an α-tubulin isoform.
Crystal structure of
Escherichia coli
CyaY protein reveals a previously unidentified fold for the evolutionarily conserved frataxin family
Authors: Cho Seung-Je , Lee Myong Gyong , Yang Jin Kuk , Lee Jae Young , Song Hyun Kyu , Suh Se Won
Published: July 2002

Friedreich ataxia is an autosomal recessive neurodegenerative disease caused by defects in the
FRDA
gene, which encodes a mitochondrial protein called frataxin. Frataxin is evolutionarily conserved, with homologs identified in mammals, worms, yeast, and bacteria. The CyaY proteins of γ-purple bacteria are believed to be closely related to the ancestor of frataxin. In this study, we have determined the crystal structure of the CyaY protein from
Escherichia coli
at 1.4-Å resolution. It reveals a protein fold consisting of a six-stranded antiparallel β-sheet flanked on one side by two α-helices. This fold is likely to be shared by all members of the conserved frataxin family. This study also provides a framework for the interpretation of disease-associated mutations in frataxin and for understanding the possible functions of this protein family.
Visualizing recycling synaptic vesicles in hippocampal neurons by FM 1-43 photoconversion
Authors: Harata Nobutoshi , Ryan Timothy A. , Smith Stephen J , Buchanan JoAnn , Tsien Richard W.
Published: July 2002

Exo–endocytotic turnover of synaptic vesicles (SVs) at synapses between hippocampal neurons in culture was examined by electron microscopy (EM). We carried out photoconversion (PC) of the fluorescent endocytotic marker FM 1-43 by using 3,3′-diaminobenzidine to convert the dye signal into an electron-dense product. Electron-dense products were located almost exclusively in SVs, whose densities were bimodally distributed in two sharply demarcated populations, PC-positive (PC+) and PC-negative (PC−). The median densities of these populations did not vary with the proportion of vesicles stained within a presynaptic terminal (bouton). The proportion of PC+ SVs remained constant across consecutive thin sections of single boutons, but varied greatly from one bouton to another, indicating marked heterogeneity in exo-endocytotic activity. Our experiments indicated that only a minority of SVs were stained in most boutons after stimuli known to cause complete turnover of the functional vesicular pool. A direct spatial correlation was found between FM 1-43 fluorescent spots seen with light microscopy and PC+ boutons by EM. The correlation was clearer in isolated boutons than in clusters of boutons. Photoconversion in combination with FM dyes allows clarification of important aspects of vesicular traffic in central nervous system nerve terminals.
The synthetic enantiomer of pregnenolone sulfate is very active on memory in rats and mice, even more so than its physiological neurosteroid counterpart: Distinct mechanisms?
Authors: Akwa Yvette , Ladurelle Nathalie , Covey Douglas F. , Baulieu Etienne-Emile
Published: July 2002

The demonstration that the neurosteroid pregnenolone sulfate (PREGS) is active on memory function at both the physiological and pharmacological levels led to us examining in detail the effects of the steroid on spatial working memory by using a two-trial recognition task in a Y-maze, a paradigm based on the natural drive in rodents to explore a novel environment. Dose–response studies in young male adult Sprague–Dawley rats and Swiss mice, after the postacquisition intracerebroventricular injection of steroid, showed an U-inverted curve for memory performance and indicated a greater responsiveness in rats compared with mice. Remarkably, the synthetic (−) enantiomer of PREGS not only also displayed promnesiant activity, but its potency was 10 times higher than that of the natural steroid. Intracerebroventricular coadministration experiments with
dl
-2-amino-5-phosphonovaleric acid, a competitive selective antagonist of the
N
-methyl-
d
-aspartate receptor, abolished the memory-enhancing effect of PREGS, but not that of the PREGS enantiomer, evoking enantiomeric selectivity at the
N
-methyl-
d
-aspartate receptor and/or different mechanisms for the promnestic function of the two enantiomers.
Nest predation by cowbirds and its consequences for passerine demography.
Authors: Arcese P , Smith J N , Hatch M I
Published: July 2002

Brood parasitism by brown-headed cowbirds (Molothrus ater) reduces reproductive success in many passerines that nest in fragmented habitats and ecological edges, where nest predation is also common. We tested the hypothesis that parasitism and predation are often linked because cowbirds depredate nests discovered late in the host's nesting cycle to enhance future opportunities for parasitism. Over a 20-year study period, brood parasitism by cowbirds was a prerequisite to observing marked inter- and intraannual variation in the rate of nest failure in an insular song sparrow (Melospiza melodia) population. Nest failure increased with the arrival and laying rate of cowbirds and declined when cowbirds ceased laying. The absence or removal of cowbirds yielded the lowest nest failure rates recorded in the study. The absence of cowbirds also coincided with the absence of an otherwise strong positive correlation between host numbers and the annual rate of nest failure. Host numbers, cowbird parasitism, and nest failure may be correlated because cowbirds facilitate nest failure rather than cause it directly. However, an experiment mimicking egg ejection by cowbirds did not affect nest failure, and, contrary to the main prediction of the predation facilitation hypothesis, naturally parasitized nests failed less often than unparasitized nests. Higher survival of parasitized nests is expected under the cowbird predation hypothesis when female cowbirds defend access to hosts because cowbirds should often depredate unparasitized nests but should not depredate nests they have laid in. Where female cowbirds have overlapping laying areas, we expect parasitized nests to fail more often than others if different cowbirds often discover the same nests. We suggest that nest predation by cowbirds represents an adaptation for successful parasitism and that cowbirds influence host demography via nest predation.
A meta-analysis of the freshwater trophic cascade.
Authors: Brett M T , Goldman C R
Published: July 2002

The generality of the trophic cascade has been an intensely debated topic among ecologists. We conducted a meta-analysis of 54 separate enclosure and pond experiments that measured the response of the zooplankton and phytoplankton to zooplanktivorous fish treatments. These results provide unequivocal support for the trophic cascade hypothesis in freshwater food webs. Zooplanktivorous fish treatments resulted in reduced zooplankton biomass and increased phytoplankton biomass. The trophic cascade was weakly dampened at the level of the phytoplankton. However, the response of the phytoplankton to the trophic cascade was highly skewed, with very strong responses in slightly more than one-third of the cases and weak responses in the others.
Ergodic theorems along sequences and Hardy fields.
Authors: Boshernitzan M , Wierdl M
Published: July 2002

Let a(x) be a real function with a regular growth as x --> infinity. [The precise technical assumption is that a(x) belongs to a Hardy field.] We establish sufficient growth conditions on a(x) so that the sequence ([a(n)])(infinity)(n=1) is a good averaging sequence in L2 for the pointwise ergodic theorem. A sequence (an) of positive integers is a good averaging sequence in L2 for the pointwise ergodic theorem if in any dynamical system (Omega, Sigma, m, T) for f [symbol, see text] in L2(Omega) the averages [equation, see text] converge for almost every omicron in. Our result implies that sequences like ([ndelta]), where delta > 1 and not an integer, ([n log n]), and ([n2/log n]) are good averaging sequences for L2. In fact, all the sequences we examine will turn out to be good averaging for Lp, p > 1; and even for L log L. We will also establish necessary and sufficient growth conditions on a(x) so that the sequence ([a(n)]) is good averaging for mean convergence. Note that for some a(x) (e.g., a(x) = log2 x), ([a(n)]) may be good for mean convergence without being good for pointwise convergence.
Determinants of RNA polymerase alpha subunit for interaction with beta, beta', and sigma subunits: hydroxyl-radical protein footprinting.
Authors: Heyduk T , Heyduk E , Severinov K , Tang H , Ebright R H
Published: July 2002

Escherichia coli RNA polymerase (RNAP) alpha subunit serves as the initiator for RNAP assembly, which proceeds according to the pathway 2 alpha-->alpha 2-->alpha 2 beta-->alpha 2 beta beta'-->alpha 2 beta beta' sigma. In this work, we have used hydroxyl-radical protein footprinting to define determinants of alpha for interaction with beta, beta', and sigma. Our results indicate that amino acids 30-75 of alpha are protected from hydroxyl-radical-mediated proteolysis upon interaction with beta (i.e., in alpha 2 beta, alpha 2 beta beta', and alpha 2 beta beta' sigma), and amino acids 175-210 of alpha are protected from hydroxyl-radical-mediated proteolysis upon interaction with beta' (i.e., in alpha 2 beta beta' and alpha 2 beta beta' sigma). The protected regions are conserved in the alpha homologs of prokaryotic, eukaryotic, archaeal, and chloroplast RNAPs and contain sites of substitutions that affect RNAP assembly. We conclude that the protected regions define determinants of alpha for direct functional interaction with beta and beta'. The observed maximal magnitude of protection upon interaction with beta and the observed maximal magnitude of protection upon interaction with beta' both correspond to the expected value for complete protection of one of the two alpha protomers of RNAP (i.e., 50% protection). We propose that only one of the two alpha protomers of RNAP interacts with beta and that only one of the two alpha protomers of RNAP interacts with beta'.
A novel nonneuronal catecholaminergic system: exocrine pancreas synthesizes and releases dopamine.
Authors: Mezey E , Eisenhofer G , Harta G , Hansson S , Gould L , Hunyady B , Hoffman B J
Published: July 2002

Cells of the exocrine pancreas produce digestive enzymes potentially harmful to the intestinal mucosa. Dopamine has been reported to protect against mucosal injury. In looking for the source of dopamine in the small intestine, we found that the duodenal juice contains high levels of dopamine and that the pancreas itself has a high dopamine [and dihydroxyphenylalanine (dopa)] content that does not change significantly after chemical sympathectomy. Furthermore, we were able to demonstrate tyrosine hydroxylase (TH) activity in control pancreas as well as in pancreas from rats after chemical sympathectomy. Immunostaining and in situ hybridization histochemistry confirmed both the presence of TH, dopamine, and the dopamine transporter, and the mRNAs encoding TH and dopamine transporter, and the presence of both types of vesicular monoamine transporters in the exocrine cells of the pancreas. Since there are no catecholaminergic enteric ganglia in the pancreas, the above results indicate that pancreatic cells have all the characteristics of dopamine-producing cells. We suggest that the pancreas is an important source of nonneuronal dopamine in the body, and that this dopamine has a role in protecting the intestinal mucosa and suggests that dopamine D1b receptor agonists might be used to help mucosal healing in the gastrointestinal tract.
Amino-terminal protein-protein interaction motif (POZ-domain) is responsible for activities of the promyelocytic leukemia zinc finger-retinoic acid receptor-alpha fusion protein.
Authors: Dong S , Zhu J , Reid A , Strutt P , Guidez F , Zhong H J , Wang Z Y , Licht J , Waxman S , Chomienne C
Published: July 2002

Promyelocytic leukemia zinc finger-retinoic acid receptor a (PLZF-RARalpha), a fusion receptor generated as a result of a variant t(11;17) chromosomal translocation that occurs in a small subset of acute promyelocytic leukemia (APL) patients, has been shown to display a dominant-negative effect against the wild-type RARalpha/retinoid X receptor alpha (RXRalpha). We now show that its N-terminal region (called the POZ-domain), which mediates protein-protein interaction as well as specific nuclear localization of the wild-type PLZF and chimeric PLZF-RARalpha proteins, is primarily responsible for this activity. To further investigate the mechanisms of PLZF-RARalpha action, we have also studied its ligand-receptor, protein-protein, and protein-DNA interaction properties and compared them with those of the promyelocytic leukemia gene (PML)-RARalpha, which is expressed in the majority of APLs as a result of t(15;17) translocation. PLZF-RARalpha and PML-RARalpha have essentially the same ligand-binding affinities and can bind in vitro to retinoic acid response elements (RAREs) as homodimers or heterodimers with RXRalpha. PLZF-RARalpha homodimerization and heterodimerization with RXRalpha were primarily mediated by the POZ-domain and RARalpha sequence, respectively. Despite having identical RARalpha sequences, PLZF-RARalpha and PML-RARalpha homodimers recognized with different affinities distinct RAREs. Furthermore, PLZF-RARalpha could heterodimerize in vitro with the wild-type PLZF, suggesting that it may play a role in leukemogenesis by antagonizing actions of not only the retinoid receptors but also the wild-type PLZF and possibly other POZ-domain-containing regulators. These different protein-protein interactions and the target gene specificities of PLZF-RARalpha and PML-RARalpha may underlie, at least in part, the apparent resistance of APL with t(11;17) to differentiation effects of all-trans-retinoic acid.
A calcium responsive element that regulates expression of two calcium binding proteins in Purkinje cells
Authors: Arnold Donald B. , Heintz Nathaniel
Published: July 2002

Calbindin D28 encodes a calcium binding protein that is expressed in the cerebellum exclusively in Purkinje cells. We have used biolistic transfection of organotypic slices of P12 cerebellum to identify a 40-bp element from the calbindin promoter that is necessary and sufficient for Purkinje cell specific expression in this transient
in situ
assay. This element (PCE1) is also present in the calmodulin II promoter, which regulates expression of a second Purkinje cell Ca
2+
binding protein. Expression of high levels of exogenous calbindin or calretinin decreased transcription mediated by PCE1 in Purkinje cells 2.5- to 3-fold, whereas the presence of 1 μM ionomycin in the extracellular medium increased expression. These results demonstrate that PCE1 is a component of a cell-specific and Ca
2+
-sensitive transcriptional regulatory mechanism that may play a key role in setting the Ca
2+
buffering capacity of Purkinje cells.
Parallel-up structure evidences the molecular directionality during biosynthesis of bacterial cellulose
Authors: Koyama Makiko , Helbert William , Imai Tomoya , Sugiyama Junji , Henrissat Bernard
Published: July 2002

The “parallel-up” packing in cellulose I
α
and I
β
unit cells was experimentally demonstrated by a combination of direct-staining the reducing ends of cellulose chains and microdiffraction-tilting electron crystallographic analysis. Microdiffraction investigation of nascent bacterial cellulose microfibrils showed that the reducing end of the growing cellulose chains points away from the bacterium, and this provides direct evidence that polymerization by the cellulose synthase takes place at the nonreducing end of the growing cellulose chains. This mechanism is likely to be valid also for a number of processive glycosyltransferases such as chitin synthases, hyaluronan synthases, and proteins involved in the synthesis of nodulation factor backbones.
A phylogenetic perspective on
P
transposable element evolution in
Drosophila
Authors: Clark Jonathan B. , Kidwell Margaret G.
Published: July 2002

The
P
element, originally described in
Drosophila melanogaster
, is one of the best-studied eukaryotic transposable elements. In an attempt to understand the evolutionary dynamics of the
P
element family, an extensive phylogenetic analysis of 239 partial
P
element sequences has been completed. These sequences were obtained from 40 species in the
Drosophila
subgenus
Sophophora
. The phylogeny of the
P
element family is examined in the context of a phylogeny of the species in which these elements are found. An interesting feature of many of the species examined is the coexistence in the same genome of
P
sequences belonging to two or more divergent subfamilies. In general,
P
elements in
Drosophila
have been transmitted vertically from generation to generation over evolutionary time. However, four unequivocal cases of horizontal transfer, in which the element was transferred between species, have been identified. In addition, the
P
element phylogeny is best explained in numerous instances by horizontal transfer at various times in the past. These observations suggest that, as with some other transposable elements, horizontal transfer may play an important role in the maintenance of
P
elements in natural populations.
Hormones, genes, and behavior
Authors: Pfaff Donald W.
Published: July 2002

With assays of hormone-sensitive behaviors, it is possible to
demonstrate both direct and indirect actions of genes on mammalian
social behaviors. Direct effects of estrogen receptor gene expression
and progesterone receptor gene expression figure prominently in well
analyzed neuroendocrine mechanisms for sex behavior, operating through
a neural circuit that has been delineated. Indirect effects, notably
the consequences of sexual differentiation, display complex
dependencies. In a human condition, Kallmann syndrome, the data show a
clear, indirect genetic influence on an important human social
behavior, in which damage at chromosome Xp-22.3 works through at least
six discrete steps to affect libido. Altogether, simplistic
extrapolations from lower animals, especially during brief summaries
for nonscientists, do not appear justified as we discover and
conceptualize genetic influences on mammalian brain and behavior.
Apparent mtDNA heteroplasmy in Alzheimer’s disease patients and in normals due to PCR amplification of nucleus-embedded mtDNA pseudogenes
Authors: Hirano Michio , Shtilbans Alexander , Mayeux Richard , Davidson Mercy M. , DiMauro Salvatore , Knowles James A. , Schon Eric A.
Published: July 2002

In an unprecedented finding, Davis
et al.
[Davis, R. E., Miller, S., Herrnstadt, C., Ghosh, S. S., Fahy, E., Shinobu, L. A., Galasko, D., Thal, L. J., Beal, M. F., Howell, N. & Parker, W. D., Jr. (1997)
Proc. Natl. Acad. Sci. USA
94, 4526–4531] used an unusual DNA isolation method to show that healthy adults harbor a specific population of mutated mitochondrial cytochrome
c
oxidase (
COX
) genes that coexist with normal mtDNAs. They reported that this heteroplasmic population was present at a level of 10–15% in the blood of normal individuals and at a significantly higher level (20–30%) in patients with sporadic Alzheimer’s disease. We provide compelling evidence that the DNA isolation method employed resulted in the coamplification of authentic mtDNA-encoded
COX
genes together with highly similar
COX
-like sequences embedded in nuclear DNA (“mtDNA pseudogenes”). We conclude that the observed heteroplasmy is an artifact.