bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics of ion beams from a Penning source for focused ion beam applications
Authors: Guharay S. K. , Sokolovsky E. , Orloff J.
Published: July 2002

Focused ion beam characteristics, measured by using PMMA resist exposures, show significant improvement of the previously reported results for hydrogen ion beams. A beam current density of >50 mA cm−2 at 6–7 keV has been obtained over a spot size of 2.3 μm, estimated from the full width at half maximum of the intensity distribution. A single Einzel lens with magnification of about 0.1 was used. A target current density of >1 A/cm2 can be achieved with this beam for a lens system with overall magnification of ∼0.01. In experiments with Ar, both the discharge current and extracted beam current increased by more than a factor of 2 when the discharge cell length was increased by about 25% with respect to the nominal cell length for operation with H. Ar ion beams were extracted with an angular beam intensity of >5 mA sr−1, brightness (90% beam) of ∼103 A sr−1 cm−2 at 10 keV, and energy spread of ∼4.5 eV. Greatly improved beam characteristics, with brightness >104 A sr−1 cm−2, are expected by optimization of the source for Ar beams.
High dielectric constant (Ba0.65Sr0.35)(Ti0.41Zr0.59)O3 capacitors for Gbit-scale dynamic random access memory devices
Authors: Kim Jae-Sun , Yoon Soon-Gil
Published: July 2002

High dielectric constant (Ba0.65Sr0.35)(Ti0.41Zr0.59)O3 (BSTZ) thin films were prepared on Pt/Ti/SiO2/Si substrates at 500 °C from the target composition of (Ba0.65Sr0.35)(Ti0.65Zr0.35)O3 by the rf magnetron sputtering technique and evaluated as a function of annealing temperature. The 95-nm-thick BSTZ films showed a dense morphology with a small grain size and an increase of the dielectric constant with increasing annealing temperature. The dielectric constant and dissipation factor of BSTZ films annealed at 750 °C were 610 and 0.025 at an applied frequency of 100 kHz, respectively. The leakage current density of films increased with increasing annealing temperature, and the value for films annealed at 750 °C was about 9.0×10−9 A/cm2 at 400 kV/cm. The dominant transport mechanism of BSTZ films was interface-limited Schottky emission. The BSTZ films formed by rf sputtering are attractive for Gbit-scale dynamic random access memory applications.
Dielectric properties of SiO2 thin films prepared by the sol–gel technique
Authors: Garcı́a-Cerda L. A. , Pérez-Roblez J. F. , González-Hernández J. , Mendoza-Galván A. , Vorobiev Yu. V. , Prokhorov E. F.
Published: July 2002

The low-temperature sol–gel method has been used to prepare two types of thin films on low-resistivity silicon substrates: one of pure SiO2 and the other containing 1% by weight of aluminum oxide particles embedded in the SiO2 matrix. The average size of the alumina particles was of 0.2 μm. The Al2O3 particles were added in the starting solution. Pure SiO2 dielectric films were prepared using four different solutions of tetraethyl orthosilicate, water, and ethanol, whereas the films with the aluminum oxide particles were prepared with only one composition. To study the film properties, electric impedance, atomic force microscopy, optical microscopy, ellipsometry, and infrared absorption measurements were used. The dependence of the dielectric properties of the films on the relative amount of the components in the starting solution (in particular, on the presence of the particles of Al2O3) and on the corresponding subsequent thermal treatment was investigated. A correlation was found between the film structure and its quality as a dielectric coating. It is shown that the method employed allows the preparation of films with dielectric characteristics close to those of the thermally grown silicon oxide.
Reduction of sidewall defect induced leakage currents by the use of nitrided field oxides in silicon selective epitaxial growth isolation for advanced ultralarge scale integration
Authors: Bashir R. , Su T. , Sherman J. M. , Neudeck G. W. , Denton J. , Obeidat A.
Published: July 2002

Defects in the near sidewall region in selective epitaxial growth of silicon have prevented its widespread use as a viable dielectric isolation technology. The main cause of these defects has been demonstrated to be thermal stress due to mismatch in the coefficient of thermal expansion between silicon and silicon dioxide. This article presents the detailed electrical characterization of these sidewall defects using P+/N junction diodes fabricated using silicon dioxide and thermally nitrided silicon dioxide as the field insulator. It is shown that the use of field oxide which was nitrided at 1100 °C for 60 min in ammonia gas ambient reduced the reverse saturation current density in the diodes by a factor of 6 and also improved the forward recombination and ideality factors when compared to standard thermal field oxide isolated diodes. The improvement of the sidewall quality was attributed to a reduction in thermal stress due to the modification of the coefficient of thermal expansion of nitrided silicon oxide.
Growth and characterization of metamorphic Inx(AlGa)1−xAs/InxGa1−xAs high electron mobility transistor material and devices with X=0.3–0.4
Authors: Hoke W. E. , Lyman P. S. , Whelan C. S. , Mosca J. J. , Torabi A. , Chang K. L. , Hsieh K. C.
Published: July 2002

High electron mobility transistor structures containing Inx(AlGa)1−xAs and InxGa1−xAs device layers with X=0.3–0.4 were grown on metamorphic buffer layers on GaAs substrates. The structures exhibited good flatness with a root mean square roughness of 9 Å. Cross-sectional transmission electron micrographs indicated that the threading dislocations from metamorphic growth were contained in the graded buffer layer. For double pulse doped structures, sheet densities up to 4×1012 cm−2 were readily achieved. Room temperature mobilities of 8600–8800 cm2/V s were obtained using In0.42Ga0.58As channel layers. Photoluminescence measurements of the InGaAs channel layer in metamorphic and pseudomorphic structures showed no significant reduction in room temperature luminescence intensity due to metamorphic growth. Transistor structures of the same device geometry were fabricated from both metamorphic and pseudomorphic GaAs high electron mobility transistor structures. At 25 GHz, the metamorphic device produced a 3 dB higher associated gain with a 0.4 dB lower noise figure. Device operation under current drive was demonstrated at 5–6 V bias which represents a significant improvement over InP-based devices. A state-of-the-art power density of 0.9 W/mm was obtained at 35 GHz.
Role of Importin-β in Coupling Ran to Downstream Targets in Microtubule Assembly
Authors: Wiese Christiane , Wilde Andrew , Moore Mary Shannon , Adam Stephen A. , Merdes Andreas , Zheng Yixian
Published: July 2002

The guanosine triphosphatase Ran stimulates assembly of microtubule asters and spindles in mitotic
Xenopus
egg extracts. A carboxyl-terminal region of the nuclear-mitotic apparatus protein (NuMA), a nuclear protein required for organizing mitotic spindle poles, mimics Ran's ability to induce asters. This NuMA fragment also specifically interacted with the nuclear transport factor, importin-β. We show that importin-β is an inhibitor of microtubule aster assembly in
Xenopus
egg extracts and that Ran regulates the interaction between importin-β and NuMA. Importin-β therefore links NuMA to regulation by Ran. This suggests that similar mechanisms regulate nuclear import during interphase and spindle assembly during mitosis.
Control of a Genetic Regulatory Network by a Selector Gene
Authors: Guss Kirsten A. , Nelson Craig E. , Hudson Angela , Kraus Mary Ellen , Carroll Sean B.
Published: July 2002

The formation of many complex structures is controlled by a special class of transcription factors encoded by selector genes. It is shown that SCALLOPED, the DNA binding component of the selector protein complex for the
Drosophila
wing field, binds to and directly regulates the cis-regulatory elements of many individual target genes within the genetic regulatory network controlling wing development. Furthermore, combinations of binding sites for SCALLOPED and transcriptional effectors of signaling pathways are necessary and sufficient to specify wing-specific responses to different signaling pathways. The obligate integration of selector and signaling protein inputs on cis-regulatory DNA may be a general mechanism by which selector proteins control extensive genetic regulatory networks during development.
RETRACTED: Hierarchical Organization of Guidance Receptors: Silencing of Netrin Attraction by Slit Through a Robo/DCC Receptor Complex
Authors: Stein Elke , Tessier-Lavigne Marc
Published: July 2002

Axonal growth cones that cross the nervous system midline change their responsiveness to midline guidance cues: They become repelled by the repellent Slit and simultaneously lose responsiveness to the attractant netrin. These mutually reinforcing changes help to expel growth cones from the midline by making a once-attractive environment appear repulsive. Here, we provide evidence that these two changes are causally linked: In the growth cones of embryonic
Xenopus
spinal axons, activation of the Slit receptor Roundabout (Robo) silences the attractive effect of netrin-1, but not its growth-stimulatory effect, through direct binding of the cytoplasmic domain of Robo to that of the netrin receptor DCC. Biologically, this hierarchical silencing mechanism helps to prevent a tug-of-war between attractive and repulsive signals in the growth cone that might cause confusion. Molecularly, silencing is enabled by a modular and interlocking design of the cytoplasmic domains of these potentially antagonistic receptors that predetermines the outcome of their simultaneous activation.
Why We Don't Get Sick: The Within-Host Population Dynamics of Bacterial Infections
Authors: Levin Bruce R. , Antia Rustom
Published: July 2002

To pathogenic microparasites (viruses, bacteria, protozoa, or fungi), we and other mammals (living organisms at large) are little more than soft, thin-walled flasks of culture media. Almost every time we eat, brush our teeth, scrape our skin, have sex, get bitten by insects, and inhale, we are confronted with populations of microbes that are capable of colonizing the mucosa lining our orifices and alimentary tract and proliferating in fluids and cells within us. Nevertheless, we rarely get sick, much less succumb to these infections. The massive numbers of bacteria and other micro- and not-so-micro organisms that abound and replicate in our alimentary tract and cover our skin and the mucosa lining our orifices normally maintain their communities in seemingly peaceful coexistence with the somatic cells that define us. Why don't these microbes invade and proliferate in the culture media within the soft, thin-walled flask that envelops us? Why don't they cause disease and lead to our rapid demise?
Modulation of Cell Proliferation by Heterotrimeric G Protein in
Arabidopsis
Authors: Ullah Hemayet , Chen Jin-Gui , Young Jeff C. , Im Kyung-Hoan , Sussman Michael R. , Jones Alan M.
Published: July 2002

The α subunit of a prototypical heterotrimeric GTP-binding protein (G protein), which is encoded by a single gene (
GPA1
) in
Arabidopsis
, is a modulator of plant cell proliferation.
gpa1
null mutants have reduced cell division in aerial tissues throughout development. Inducible overexpression of
GPA1
in
Arabidopsis
confers inducible ectopic cell division.
GPA1
overexpression in synchronized BY-2 cells causes premature advance of the nuclear cycle and the premature appearance of a division wall. Results from loss of function and ectopic expression and activation of
GPA1
indicate that this subunit is a positive modulator of cell division in plants.
Structure of Complement Receptor 2 in Complex with Its C3d Ligand
Authors: Szakonyi Gerda , Guthridge Joel M. , Li Dawei , Young Kendra , Holers V. Michael , Chen Xiaojiang S.
Published: July 2002

Complement receptor 2 (CR2/CD21) is an important receptor that amplifies B lymphocyte activation by bridging the innate and adaptive immune systems. CR2 ligands include complement C3d and Epstein-Barr virus glycoprotein 350/220. We describe the x-ray structure of this CR2 domain in complex with C3d at 2.0 angstroms. The structure reveals extensive main chain interactions between C3d and only one short consensus repeat (SCR) of CR2 and substantial SCR side-side packing. These results provide a detailed understanding of receptor-ligand interactions in this protein family and reveal potential target sites for molecular drug design.
Not "A Piece of Cake"!
Authors: Sigurdsson Haraldur
Published: July 2002

No Apparent Danger
The True Story of Volcanic Disaster at Galeras and Nevado del Ruiz. by Victoria Bruce. HarperCollins, New York, 2001. 251 pp. $26, C$39.50. ISBN 0-06-019920-2.

Surviving Galeras
. Stanley Williams and Fen Montaigne. Houghton Mifflin, Boston, 2001. 280 pp. $25. ISBN 0-618-03168-5.

The hazards of volcanic eruptions and risks they pose to researchers are highlighted in these two contrasting portrayals of the events that led to nine deaths at a Colombian volcano in 1993. Bruce claims that the dangers at Galeras were predictable and thus the fatalities could have been prevented. Offering a first-person account of the eruption and his own perspectives on the dangers and rewards of studying active volcanoes up close, Williams argues that the event could not have been forecast.
Effects of Helpers on Juvenile Development and Survival in Meerkats
Authors: Clutton-Brock T. H. , Russell A. F. , Sharpe L. L. , Brotherton P. N. M. , McIlrath G. M. , White S. , Cameron E. Z.
Published: July 2002

Although breeding success is known to increase with group size in several cooperative mammals, the mechanisms underlying these relationships are uncertain. We show that in wild groups of cooperative meerkats,
Suricata suricatta
, reductions in the ratio of helpers to pups depress the daily weight gain and growth of pups and the daily weight gain of helpers. Increases in the daily weight gain of pups are associated with heavier weights at independence and at 1 year of age, as well as with improved foraging success as juveniles and higher survival rates through the first year of life. These results suggest that the effects of helpers on the fitness of pups extend beyond weaning and that helpers may gain direct as well as indirect benefits by feeding pups.
Recombination in the Hemagglutinin Gene of the 1918 "Spanish Flu"
Authors: Gibbs Mark J. , Armstrong John S. , Gibbs Adrian J.
Published: July 2002

When gene sequences from the influenza virus that caused the 1918 pandemic were first compared with those of related viruses, they yielded few clues about its origins and virulence. Our reanalysis indicates that the hemagglutinin gene, a key virulence determinant, originated by recombination. The “globular domain” of the 1918 hemagglutinin protein was encoded by a part of a gene derived from a swine-lineage influenza, whereas the “stalk” was encoded by parts derived from a human-lineage influenza. Phylogenetic analyses showed that this recombination, which probably changed the virulence of the virus, occurred at the start of, or immediately before, the pandemic and thus may have triggered it.
Making Sense of Eukaryotic DNA Replication Origins
Authors: Gilbert David M.
Published: July 2002

DNA replication is the process by which cells make one complete copy of their genetic information before cell division. In bacteria, readily identifiable DNA sequences constitute the start sites or origins of DNA replication. In eukaryotes, replication origins have been difficult to identify. In some systems, any DNA sequence can promote replication, but other systems require specific DNA sequences. Despite these disparities, the proteins that regulate replication are highly conserved from yeast to humans. The resolution may lie in a current model for once-per-cell-cycle regulation of eukaryotic replication that does not require defined origin sequences. This model implies that the specification of precise origins is a response to selective pressures that transcend those of once-per-cell-cycle replication, such as the coordination of replication with other chromosomal functions. Viewed in this context, the locations of origins may be an integral part of the functional organization of eukaryotic chromosomes.
T Cell Death and Memory
Authors: Sprent Jonathan , Tough David F.
Published: July 2002

In typical immune responses, contact with antigen causes naı̈ve T cells to proliferate and differentiate into effector cells. After the pathogen is destroyed, most effector T cells are eliminated—thereby preserving the primary T cell repertoire—but some cells survive and form long-lived memory cells. During each stage of this process, the life or death fate of T cells is strictly regulated.
Priming of Memory But Not Effector CD8 T Cells by a Killed Bacterial Vaccine
Authors: Lauvau Gregoire , Vijh Sujata , Kong Philip , Horng Tiffany , Kerksiek Kristen , Serbina Natalya , Tuma Roman A. , Pamer Eric G.
Published: July 2002

Killed or inactivated vaccines targeting intracellular bacterial and protozoal pathogens are notoriously ineffective at generating protective immunity. For example, vaccination with heat-killed
Listeria monocytogenes
(HKLM) is not protective, although infection with live
L. monocytogenes
induces long-lived, CD8 T cell–mediated immunity. We demonstrate that HKLM immunization primes memory CD8 T lymphocyte populations that, although substantial in size, are ineffective at providing protection from subsequent
L. monocytogenes
infection. In contrast to live infection, which elicits large numbers of effector CD8 T cells, HKLM immunization primes T lymphocytes that do not acquire effector functions. Our studies show that it is possible to dissociate T cell–dependent protective immunity from memory T cell expansion, and that generation of effector T cells may be necessary for long-term protective immunity.
Cortical Neurons Encoding Path and Place: Where You Go Is Where You Are
Authors: Froehler Michael T. , Duffy Charles J.
Published: July 2002

We recorded neuronal activity in monkey medial superior temporal (MST) cortex during movement on a motorized sled. Most neurons showed a preferred heading direction, but some responded only when that heading was part of a particular path. Others responded only when the animal was at a certain place in the room, regardless of its path to that place. Video simulations of the self-movement scene evoked path, but not place, responses. Stationary positioning in the room revealed location preferences that matched place preferences recorded during movement. We conclude that MST encodes heading, path, and place information to support visuospatial orientation.
Pregnancy in a Polluted World
Authors: Woodwell George M.
Published: July 2002

Having Faith
An Ecologist's Journey to Motherhood. Sandra Steingraber. Perseus, Cambridge, MA, 2001. 351 pp. $26, C$39.50. ISBN 0-7382-0467-6.

Steingraber uses a chronicle of her pregnancy to discuss the potential influences of toxic contamination on human reproduction.
Trophic Control of Production in a Rocky Intertidal Community
Authors: Paine Robert T.
Published: July 2002

In the low intertidal zone at Tatoosh Island, Washington, United States, minimal estimates of primary production can vary from 0 to an average of 86 kilograms of wet mass per square meter per year when the grazing assemblage is manipulated. Highly productive annual kelps (Laminariales) replace less productive perennial species when macroscopic grazers are reduced or absent, resulting in monodominant assemblages of
Alaria marginata
. Experiments were repeated in seven consecutive years. Increased species richness makes no significant additional contribution to annual production. Rather, a competitively superior species is favored when its enemies are reduced, suggesting that terrestrial perspectives on the role of biodiversity that are developed without considering consumers may not be general.
Microbial Behavior in a Heterogeneous World
Authors: Fenchel Tom
Published: July 2002

Most microorganisms are motile during at least part of their life cycle, because they need to find optimal conditions in a patchy world. The sheer volume of microorganisms in the biosphere means that their motile sensory behavior also contributes to the global transformation and cycling of matter. How microorganisms move and how they orient themselves using environmental cues are integral to understanding the complex structure and function of microbial communities, but although motility in response to external stimuli was first described more than 120 years ago, understanding of the cellular and molecular mechanisms involved has only been achieved more recently.
Double Jeopardy for Gallo
Authors: Delaney Martin
Published: July 2002

Science Fictions
A Scientific Mystery, a Massive Cover-Up, and the Dark Legacy of Robert Gallo. John Crewdson. Little, Brown, New York, 2002. 688 pp. $27.95, C$39.95. ISBN 0-316-13476-7.

Expanding and filling in the claims he made in his 1989
Chicago Tribune
article, Crewdson follows the story of the Gallo case through 1995. He charges Gallo with a wide variety of unethical behavior during the discovery and isolation of HIV.
Goodness Gracious, Great Walls Afar
Authors: Vogel Gretchen
Published: July 2002

To cosmologists'surprise, some of the most distant galaxies ever seen are grouped in discrete structures--perhaps huge walls. Standard theories predict a smoother distribution of galaxies in the distant, early universe
Galileo's First Images of Jupiter and the Galilean Satellites
Authors: Belton M J. S. , Head J W. , Ingersoll A. P. , Greeley R. , McEwen A. S. , Klaasen K. P. , Senske D. , Pappalardo R. , Collins G. , Vasavada A. R. , Sullivan R. , Simonelli D. , Geissler P. , Carr M. H. , Davies M. E. , Veverka J. , Gierasch P. J. , Banfield D. , Bell M. , Chapman C. R. , Anger C. , Greenberg R. , Neukum G. , Pilcher C. B. , Beebe R. F. , Burns J. A. , Fanale F. , Ip W. , Johnson T. V. , Morrison D. , Moore J. , Orton G. S. , Thomas P. , West R. A.
Published: July 2002

The first images of Jupiter, Io, Europa, and Ganymede from the Galileo spacecraft reveal new information about Jupiter's Great Red Spot (GRS) and the surfaces of the Galilean satellites. Features similar to clusters of thunderstorms were found in the GRS. Nearby wave structures suggest that the GRS may be a shallow atmospheric feature. Changes in surface color and plume distribution indicate differences in resurfacing processes near hot spots on Io. Patchy emissions were seen while Io was in eclipse by Jupiter. The outer margins of prominent linear markings (triple bands) on Europa are diffuse, suggesting that material has been vented from fractures. Numerous small circular craters indicate localized areas of relatively old surface. Pervasive brittle deformation of an ice layer appears to have formed grooves on Ganymede. Dark terrain unexpectedly shows distinctive albedo variations to the limit of resolution.
Ice Rhythms—Core Reveals a Plethora of Climate Cycles
Authors: Kerr Richard A.
Published: July 2002

A record of climate extracted from Greenland's ice has laid out the whole spectrum of climate variations on scales of a few thousand years, from the well-known cycles of about 40,000 and 23,000 years, driven by the tilting and wobbling of Earth's axis, down the scale to cycles of 11,100, 6100, and 1450 years. Climate researchers have argued about the causes of these shorter cycles, but the crisp new record may be key to deciphering them.
Retroelements in Genome Organization
Authors: Voytas Daniel F.
Published: July 2002

Transposons, mobile pieces of DNA, were first described in maize by B. McClintock. Now maize yields another surprise, as shown by SanMiguel
et al
. (
p. 765
) in this week's issue. One particular class of transposon, retrotransposons, is present in the maize genome in numbers so vast that it may constitute half of the plant's genome. Voytas discusses in his Perspective the implications of these findings for genome organization and the strategies that these elements might use to avoid integrating into active genes.
Creation of Nanocrystals Through a Solid-Solid Phase Transition Induced by an STM Tip
Authors: Zhang Jian , Liu Jie , Huang Jin Lin , Kim Philip , Lieber Charles M.
Published: July 2002

A scanning tunneling microscope (STM) was used to fabricate T-phase tantalum diselenide (TaSe
2
) nanocrystals with sizes ranging from 7 to more than 100 nanometers within the surface layer of 2H-TaSe
2
crystals at liquid helium temperature. Atomic-resolution images elucidate the structural changes between T- and H-phase regions and were used to develop an atomic model that describes a pathway for the production of T-phase nanocrystals from the H-phase crystal precursor through a solid-solid phase transition. The size-dependent properties of these nanocrystals may lead to improved understanding of the physics of charge density waves in small structures.
Do Comets Get a Nudge From the Galaxy?
Authors: Seife Charles
Published: July 2002

What dislodges comets from their roost in the Oort Cloud, a vast spherical halo of icy objects that extends from the orbit of Pluto outward for more than a light-year? A new analysis points to the gravitational influence of the distant galaxy.
Hats Off to the Tricorn Protease
Authors: Schneider Christine , Hartl F. Ulrich
Published: July 2002

Cells regulate the destruction of proteins, just as they carefully regulate protein synthesis. Much of the proteolysis of cellular proteins occurs within a cylindrical organelle, the proteasome. Now Tamura
et al
. (
p. 1385
of this issue) report the existence of a second new proteolytic organelle, the tricorn protease, in the bacteria
Thermoplasma acidophilum
. In their Perspective, Schneider and Hartl compare the structure and function of the proteasome and the tricorn protease.
Heavy Ozone--A Difficult Puzzle to Solve
Authors: Krankowsky Dieter , Mauersberger Konrad
Published: July 2002

Although not abundant, ozone is one of the most important constituents of the atmosphere. The isotopic variants of ozone, which is a symmetric triangular molecule made of three oxygen atoms, are difficult to explain by theoretical analysis. In their Perspective, Krankowsky and Mauersberger discuss results published in the same issue by Gellene (
p. 1344
) that shed light on the mechanisms responsible for the heavier isotopes of ozone.
Diffuse Extreme-Ultraviolet Emission from the Coma Cluster: Evidence for Rapidly Cooling Gases at Submegakelvin Temperatures
Authors: Lieu Richard , Mittaz Jonathan P. D. , Bowyer Stuart , Breen Jeffrey O. , Lockman Felix J. , Murphy Edward M. , Hwang Chorng-yuan
Published: July 2002

The central region of the Coma cluster of galaxies was observed in the energy band from 0.065 to 0.245 kiloelectron volts by the Deep Survey telescope aboard the Extreme Ultraviolet Explorer. A diffuse emission halo of angular diameter ∼30 arc minutes was detected. The extreme-ultraviolet (EUV) emission level exceeds that expected from the x-ray temperature gas in Coma. This halo suggests the presence of two more phases in the emitting gas, one at a temperature of ∼2 × 10
6
kelvin and the other at ∼8 × 10
5
kelvin. The latter phase cools rapidly and, in steady state, would have produced cold matter with a mass of ∼10
14
solar masses within the EUV halo. Although a similar EUV enhancement was discovered in the Virgo cluster, this detection in Coma applies to a noncooling flow system.
Extreme-Ultraviolet Flux from the Virgo Cluster: Further Evidence for a 500,000-Kelvin Component
Authors: Bowyer Stuart , Lampton Michael , Lieu Richard
Published: July 2002

A surprising discovery in x-ray astronomy was that clusters of galaxies often contain vast quantities of hot (20 million kelvin) diffuse gas. Substantial diffuse extreme-ultraviolet (EUV) emission has recently been detected in the Virgo cluster of galaxies. Depending on the character of the interstellar medium in our galaxy, this emission could be either an aspect of the hot cluster gas or a previously undetected 500,000-kelvin component. Analysis of the observational data in combination with our current knowledge of the interstellar medium revealed that the EUV flux cannot be an effect of the interstellar medium. Hence, a warm cluster component appears likely.
Higher Yielding Perennials Point the Way to New Crops
Authors: Moffat Anne Simon
Published: July 2002

Today, almost all of our major crops are herbaceous annuals—plants such as corn and soybeans that have to be replanted every year. These plants have a big advantage: They produce masses of seeds. Recent results from a few agricultural pioneers suggest, however, that it is possible to boost dramatically the seed yields of at least some perennials, including cousins to corn and wheat. These studies may eventually lead to new, more environmentally friendly crops.
Ecological Controls on the Evolutionary Recovery of Post-Paleozoic Crinoids
Authors: Foote Mike
Published: July 2002

Analysis of morphological characters of a global sample of post-Paleozoic crinoid echinoderms shows that this group underwent a rapid diversification after the extinction at the end of the Permian to reach maximal morphological disparity by the Late Triassic, which is essentially the same evolutionary pattern seen during the group's early Paleozoic radiation. The accelerated morphological diversification of a single class implies that, even if clades surviving from the Paleozoic represented ecological incumbents that hindered the origin of new higher taxa, species within individual higher taxa rapidly exploited available ecological opportunities in the Mesozoic.
Reexpression of RAG-1 and RAG-2 Genes in Activated Mature Mouse B cells
Authors: Hikida Masaki , Mori Masaharu , Takai Toshiyuki , Tomochika Ken-ichi , Hamatani Kiyohiro , Ohmori Hitoshi
Published: July 2002

Recombination activating genes (RAG-1 and RAG-2), involved in V(D)J rearrangement of immunoglobulin genes, have been thought to be expressed only in immature stages of B-cell development. However, RAG-1 and RAG-2 transcripts were found to be reexpressed in mature mouse B cells after culture with interleukin-4 in association with several different co-stimuli. Reexpression was also detected in draining lymph nodes from immunized mice. RAG-1 and RAG-2 proteins could be detected by immunofluorescence microscopy in the nuclei of B cells cultured in vitro and in the germinal centers of draining lymph nodes. These findings suggest that RAG gene products play a heretofore unsuspected role in mature B cells.
Searching a Quantum Phone Book
Authors: Brassard Gilles
Published: July 2002

Finding a phone number in a directory when one knows the name is easy; much harder is finding the name given the number. On average, a search algorithm will have to scan half the total number of entries. If the database or directory is extremely large, the search may be impossible with classical computers. New possibilities emerge, however, with quantum computing. As Brassard discusses in his Perspective, a new algorithm developed by L. K. Grover enables much faster searching of large databases if one uses a quantum computer.
A Proficient Enzyme Revisited: The Predicted Mechanism for Orotidine Monophosphate Decarboxylase
Authors: Lee Jeehiun K. , Houk K. N.
Published: July 2002

A mechanism is proposed to explain the activity of orotidine 5′-monophosphate decarboxylase (ODCase). This enzyme is the one of the most proficient known, with a catalytic proficiency (
k
cat
/
K
m
)/
k
non
= 10
23
M
−1
. Quantum mechanical calculations predict a mechanism involving a stabilized carbene intermediate, which represents a previously unrecognized mode of enzymatic activity for ODCase. The proposed mechanism involves proton transfer from a weak acid (p
K
a
= 7, where
K
a
is the acid constant) concerted with decarboxylation, in a nonpolar enzyme environment. Such a mechanism makes possible different approaches to the design of ODCase inhibitors. Furthermore, the prediction that general acid catalysis may only be effective in low dielectric media is of general significance for understanding the activity of many enzymes.
Effectiveness of Anthracycline Against Experimental Prion Disease in Syrian Hamsters
Authors: Tagliavini F. , McArthur R. A. , Canciani B. , Giaccone G. , Porro M. , Bugiani M. , Lievens P. M.-J. , Bugiani O. , Peri E. , Dall’Ara P. , Rocchi M. , Poli G. , Forloni G. , Bandiera T. , Varasi M. , Suarato A. , Cassutti P. , Cervini M. A. , Lansen J. , Salmona M. , Post C.
Published: July 2002

Prion diseases are transmissible neurodegenerative conditions characterized by the accumulation of protease-resistant forms of the prion protein (PrP), termed PrPres, in the brain. Insoluble PrPres tends to aggregate into amyloid fibrils. The anthracycline 4′-iodo-4′-deoxy-doxorubicin (IDX) binds to amyloid fibrils and induces amyloid resorption in patients with systemic amyloidosis. To test IDX in an experimental model of prion disease, Syrian hamsters were inoculated intracerebrally either with scrapie-infected brain homogenate or with infected homogenate coincubated with IDX. In IDX-treated hamsters, clinical signs of disease were delayed and survival time was prolonged. Neuropathological examination showed a parallel delay in the appearance of brain changes and in the accumulation of PrPres and PrP amyloid.
Crystal Structure of Human BPI and Two Bound Phospholipids at 2.4 Angstrom Resolution
Authors: Beamer Lesa J. , Carroll Stephen F. , Eisenberg David
Published: July 2002

Bactericidal/permeability-increasing protein (BPI), a potent antimicrobial protein of 456 residues, binds to and neutralizes lipopolysaccharides from the outer membrane of Gram-negative bacteria. At a resolution of 2.4 angstroms, the crystal structure of human BPI shows a boomerang-shaped molecule formed by two similar domains. Two apolar pockets on the concave surface of the boomerang each bind a molecule of phosphatidylcholine, primarily by interacting with their acyl chains; this suggests that the pockets may also bind the acyl chains of lipopolysaccharide. As a model for the related plasma lipid transfer proteins, BPI illuminates a mechanism of lipid transfer for this protein family.
The Influence of Dominance Rank on the Reproductive Success of Female Chimpanzees
Authors: Pusey Anne , Williams Jennifer , Goodall Jane
Published: July 2002

Female chimpanzees often forage alone and do not display obvious linear dominance hierarchies; consequently, it has been suggested that dominance is not of great importance to them. However, with the use of data from a 35-year field study of chimpanzees, high-ranking females were shown to have significantly higher infant survival, faster maturing daughters, and more rapid production of young. Given the foraging behavior of chimpanzees, high rank probably influences reproductive success by helping females establish and maintain access to good foraging areas rather than by sparing them stress from aggression.
Correlations Between Ground and Excited State Spectra of a Quantum Dot
Authors: Stewart D. R. , Sprinzak D. , Marcus C. M. , Duruöz C. I. , Harris J. S.
Published: July 2002

The ground and excited state spectra of a semiconductor quantum dot with successive electron occupancy were studied with linear and nonlinear magnetoconductance measurements. A direct correlation was observed between the
m
th excited state of the
N
-electron system and the ground state of the (
N
+
m
)-electron system for
m
up to 4. The results are consistent with a single-particle picture in which a fixed spectrum of energy levels is successively filled, except for a notable absence of spin degeneracy. Further departures from the single-particle picture due to electron-electron interaction were also observed. Magnetoconductance fluctuations of ground states show anticrossings where wave function characteristics are exchanged between adjacent levels.
Structure of the HIV-1 Nucleocapsid Protein Bound to the SL3 Ψ-RNA Recognition Element
Authors: De Guzman Roberto N. , Wu Zheng Rong , Stalling Chelsea C. , Pappalardo Lucia , Borer Philip N. , Summers Michael F.
Published: July 2002

The three-dimensional structure of the human immunodeficiency virus–type 1 (HIV-1) nucleocapsid protein (NC) bound to the SL3 stem-loop recognition element of the genomic Ψ RNA packaging signal has been determined by heteronuclear magnetic resonance spectroscopy. Tight binding (dissociation constant, ∼100 nM) is mediated by specific interactions between the amino- and carboxyl-terminal CCHC-type zinc knuckles of the NC protein and the G
7
and G
9
nucleotide bases, respectively, of the G
6
-G
7
-A
8
-G
9
RNA tetraloop. A
8
packs against the amino-terminal knuckle and forms a hydrogen bond with conserved Arg
32
, and residues Lys
3
to Arg
10
of NC form a 3
10
helix that binds to the major groove of the RNA stem and also packs against the amino-terminal zinc knuckle. The structure provides insights into the mechanism of viral genome recognition, explains extensive amino acid conservation within NC, and serves as a basis for the development of inhibitors designed to interfere with genome encapsidation.
Mars Global Surveyor Mission: Overview and Status
Authors: Albee A. L. , Palluconi F. D. , Arvidson R. E.
Published: July 2002

The Mars Global Surveyor (MGS) spacecraft achieved a 45-hour elliptical orbit at Mars on 11 September 1997 after an 11-month cruise from Earth. The mission is acquiring high-quality global observations of the martian surface and atmosphere and of its magnetic and gravitational fields. These observations will continue for one martian year.
A Trivalent System from Vancomycin·
d
-Ala-
d
-Ala with Higher Affinity Than Avidin·Biotin
Authors: Rao Jianghong , Lahiri Joydeep , Isaacs Lyle , Weis Robert M. , Whitesides George M.
Published: July 2002

Tris(vancomycin carboxamide) binds a trivalent ligand derived from
d
-Ala-
d
-Ala with very high affinity: dissociation constant (
K
d
) ≈ 4 × 10
–17
± 1 × 10
–17
M. High-affinity trivalent binding and monovalent binding are fundamentally different. In trivalent (and more generally, polyvalent) binding, dissociation occurs in stages, and its rate can be accelerated by monovalent ligand at sufficiently high concentrations. In monovalent binding, dissociation is determined solely by the rate constant for dissociation and cannot be accelerated by added monomer. Calorimetric measurements for the trivalent system indicate an approximately additive gain in enthalpy relative to the corresponding monomers. This system is one of the most stable organic receptor-ligand pairs involving small molecules that is known. It illustrates the practicality of designing very high-affinity systems based on polyvalency.
Yeast Ku as a Regulator of Chromosomal DNA End Structure
Authors: Gravel Serge , Larrivée Michel , Labrecque Pascale , Wellinger Raymund J.
Published: July 2002

During telomere replication in yeast, chromosome ends acquire an S-phase–specific overhang of the guanosine-rich strand. Here it is shown that in cells lacking Ku, a heterodimeric protein involved in nonhomologous DNA end joining, these overhangs are present throughout the cell cycle. In vivo cross-linking experiments demonstrated that Ku is bound to telomeric DNA. These results show that Ku plays a direct role in establishing a normal DNA end structure on yeast chromosomes, conceivably by functioning as a terminus-binding factor. Because Ku-mediated DNA end joining involving telomeres would result in chromosome instability, our data also suggest that Ku has a distinct function when bound to telomeres.
Predatory Dinosaur Remains from Madagascar: Implications for the Cretaceous Biogeography of Gondwana
Authors: Sampson Scott D. , Witmer Lawrence M. , Forster Catherine A. , Krause David W. , O'Connor Patrick M. , Dodson Peter , Ravoavy Florent
Published: July 2002

Recent discoveries of fossil vertebrates from the Late Cretaceous of Madagascar include several specimens of a large theropod dinosaur. One specimen includes a nearly complete and exquisitely preserved skull with thickened pneumatic nasals, a median frontal horn, and a dorsal projection on the parietals. The new materials are assigned to the enigmatic theropod group Abelisauridae on the basis of a number of unique features. Fossil remains attributable to abelisaurids are restricted to three Gondwanan landmasses: South America, Madagascar, and the Indian subcontinent. This distribution is consistent with a revised paleogeographic reconstruction that posits prolonged links between these landmasses (via Antarctica), perhaps until late in the Late Cretaceous.
Perennial Antarctic Lake Ice: An Oasis for Life in a Polar Desert
Authors: Priscu John C. , Fritsen Christian H. , Adams Edward E. , Giovannoni Stephen J. , Paerl Hans W. , McKay Christopher P. , Doran Peter T. , Gordon Douglas A. , Lanoil Brian D. , Pinckney James L.
Published: July 2002

The permanent ice covers of Antarctic lakes in the McMurdo Dry Valleys develop liquid water inclusions in response to solar heating of internal aeolian-derived sediments. The ice sediment particles serve as nutrient (inorganic and organic)–enriched microzones for the establishment of a physiologically and ecologically complex microbial consortium capable of contemporaneous photosynthesis, nitrogen fixation, and decomposition. The consortium is capable of physically and chemically establishing and modifying a relatively nutrient- and organic matter–enriched microbial “oasis” embedded in the lake ice cover.
Type IV Pili, Transient Bacterial Aggregates, and Virulence of Enteropathogenic
Escherichia coli
Authors: Bieber David , Ramer Sandra W. , Wu Cheng-Yen , Murray William J. , Tobe Toru , Fernandez Rosemary , Schoolnik Gary K.
Published: July 2002

Type IV bundle-forming pili of enteropathogenic
Escherichia coli
are required for the localized adherence and autoaggregation phenotypes. Whether these pili are also required for virulence was tested in volunteers by inactivating
bfpA
or
bfpT
(
perA
) encoding, respectively, the pilus subunit and the
bfp
operon transcriptional activator. Both mutants caused significantly less diarrhea. Mutation of the
bfpF
nucleotide-binding domain caused increased piliation, enhanced localized adherence, and abolished the twitching motility–dispersal phase of the autoaggregation phenotype. The
bfpF
mutant colonized the human intestine but was about 200-fold less virulent. Thus, BfpF is required for dispersal from the bacterial aggregate and for full virulence.
Exploiting Chemical Libraries, Structure, and Genomics in the Search for Kinase Inhibitors
Authors: Gray Nathanael S. , Wodicka Lisa , Thunnissen Andy-Mark W. H. , Norman Thea C. , Kwon Soojin , Espinoza F. Hernan , Morgan David O. , Barnes Georjana , LeClerc Sophie , Meijer Laurent , Kim Sung-Hou , Lockhart David J. , Schultz Peter G.
Published: July 2002

Selective protein kinase inhibitors were developed on the basis of the unexpected binding mode of 2,6,9-trisubstituted purines to the adenosine triphosphate–binding site of the human cyclin-dependent kinase 2 (CDK2). By iterating chemical library synthesis and biological screening, potent inhibitors of the human CDK2–cyclin A kinase complex and of
Saccharomyces cerevisiae
Cdc28p were identified. The structural basis for the binding affinity and selectivity was determined by analysis of a three-dimensional crystal structure of a CDK2-inhibitor complex. The cellular effects of these compounds were characterized in mammalian cells and yeast. In the latter case the effects were characterized on a genome-wide scale by monitoring changes in messenger RNA levels in treated cells with high-density oligonucleotide probe arrays. Purine libraries could provide useful tools for analyzing a variety of signaling and regulatory pathways and may lead to the development of new therapeutics.
Activation of Apoptosis Signal-Regulating Kinase 1 (ASK1) by the Adapter Protein Daxx
Authors: Chang Howard Y. , Nishitoh Hideki , Yang Xiaolu , Ichijo Hidenori , Baltimore David
Published: July 2002

The Fas death receptor can activate the Jun NH
2
-terminal kinase (JNK) pathway through the receptor-associated protein Daxx. Daxx was found to activate the JNK kinase kinase ASK1, and overexpression of a kinase-deficient ASK1 mutant inhibited Fas- and Daxx-induced apoptosis and JNK activation. Fas activation induced Daxx to interact with ASK1, which consequently relieved an inhibitory intramolecular interaction between the amino- and carboxyl-termini of ASK1, activating its kinase activity. The Daxx-ASK1 connection completes a signaling pathway from a cell surface death receptor to kinase cascades that modulate nuclear transcription factors.
A Structural Explanation for the Recognition of Tyrosine-Based Endocytotic Signals
Authors: Owen David J. , Evans Philip R.
Published: July 2002

Many cell surface proteins are marked for endocytosis by a cytoplasmic sequence motif, tyrosine-X-X-(hydrophobic residue), that is recognized by the μ2 subunit of AP2 adaptors. Crystal structures of the internalization signal binding domain of μ2 complexed with the internalization signal peptides of epidermal growth factor receptor and the trans-Golgi network protein TGN38 have been determined at 2.7 angstrom resolution. The signal peptides adopted an extended conformation rather than the expected tight turn. Specificity was conferred by hydrophobic pockets that bind the tyrosine and leucine in the peptide. In the crystal, the protein forms dimers that could increase the strength and specificity of binding to dimeric receptors.
Macroscopic Quantum Interference from Atomic Tunnel Arrays
Authors: Anderson B. P. , Kasevich M. A.
Published: July 2002

Interference of atomic de Broglie waves tunneling from a vertical array of macroscopically populated traps has been observed. The traps were located in the antinodes of an optical standing wave and were loaded from a Bose-Einstein condensate. Tunneling was induced by acceleration due to gravity, and interference was observed as a train of falling pulses of atoms. In the limit of weak atomic interactions, the pulse frequency is determined by the gravitational potential energy difference between adjacent potential wells. The effect is closely related to the ac Josephson effect observed in superconducting electronic systems.
Defective T Cell Differentiation in the Absence of
Jnk1
Authors: Dong Chen , Yang Derek D. , Wysk Mark , Whitmarsh Alan J. , Davis Roger J. , Flavell Richard A.
Published: July 2002

The c-Jun NH
2
-terminal kinase (JNK) signaling pathway has been implicated in the immune response that is mediated by the activation and differentiation of CD4 helper T (T
H
) cells into T
H
1 and T
H
2 effector cells. JNK activity observed in wild-type activated T
H
cells was severely reduced in T
H
cells from
Jnk1
–/–
mice. The
Jnk1
–/–
T cells hyperproliferated, exhibited decreased activation-induced cell death, and preferentially differentiated to T
H
2 cells. The enhanced production of T
H
2 cytokines by
Jnk1
–/–
cells was associated with increased nuclear accumulation of the transcription factor NFATc. Thus, the JNK1 signaling pathway plays a key role in T cell receptor–initiated T
H
cell proliferation, apoptosis, and differentiation.
Coordinated Regulation of Iron-Controlling Genes, H-Ferritin and
IRP2
, by c-MYC
Authors: Wu Kou-Juey , Polack Axel , Dalla-Favera Riccardo
Published: July 2002

The protein encoded by the c-
MYC
proto-oncogene is a transcription factor that can both activate and repress the expression of target genes, but few of its transcriptional targets have been identified. Here, c-MYC is shown to repress the expression of the heavy subunit of the protein ferritin (H-ferritin), which sequesters intracellular iron, and to stimulate the expression of the iron regulatory protein–2 (IRP2), which increases the intracellular iron pool. Down-regulation of the expression of H-ferritin gene was required for cell transformation by c-MYC. These results indicate that c-MYC coordinately regulates genes controlling intracellular iron concentrations and that this function is essential for the control of cell proliferation and transformation by c-MYC.