bluuebunny/crossref_metadata_2025 huggingface dataset:
The Accuracy of Reverse Transcriptase from HIV-1
Authors: Roberts John D. , Bebenek Katarzyna , Kunkel Thomas A.
Published: October 2006

A study was conducted to determine the fidelity of DNA synthesis catalyzed in vitro by the reverse transcriptase from a human immunodeficiency virus type 1 (HIV-1). Like other retroviral reverse transcriptases, the HIV-1 enzyme does not correct errors by exonucleolytic proofreading. Measurements with M13mp2-based fidelity assays indicated that the HIV-1 enzyme, isolated either from virus particles or from
Escherichia coli
cells infected with a plasmid expressing the cloned gene, was exceptionally inaccurate, having an average error rate per detectable nucleotide incorporated of 1/1700. It was, in fact, the least accurate reverse transcriptase described to date, one-tenth as accurate as the polymerases isolated from avian myeloblastosis or murine leukemia viruses, which have average error rates of ∼1/17,000 and ∼1/30,000, respectively. DNA sequence analyses of mutations generated by HIV-1 polymerase showed that base substitution, addition, and deletion errors were all produced. Certain template positions were mutational hotspots where the error rate could be as high as 1 per 70 polymerized nucleotides. The data are consistent with the notion that the exceptional diversity of the HIV-1 genome results from error-prone reverse transcription.
Identification of Synaptophysin as a Hexameric Channel Protein of the Synaptic Vesicle Membrane
Authors: Thomas Leo , Hartung Klaus , Langosch Dieter , Rehm Hubert , Bamberg Ernst , Franke Werner W. , Betz Heinrich
Published: October 2006

The quaternary structure and functional properties of synaptophysin, a major integral membrane protein of small presynaptic vesicles, were investigated. Cross-linking and sedimentation studies indicate that synaptophysin is a hexameric homo-oligomer, which in electron micrographs exhibits structural features common to channel-forming proteins. On reconstitution into planar lipid bilayers, purified synaptophysin displays voltage-sensitive channel activity with an average conductance of about 150 picosiemens. Because specific channels and fusion pores have been implicated in vesicular uptake and release of secretory compounds, synaptophysin may have a role in these processes.
Identification of a Platelet Membrane Glycoprotein as a Falciparum Malaria Sequestration Receptor
Authors: Ockenhouse Christian F. , Tandon Narendra N. , Magowan Cathleen , Jamieson G. A. , Chulay Jeffrey D.
Published: October 2006

Infections with the human malaria parasite
Plasmodium falciparum
are characterized by sequestration of erythrocytes infected with mature forms of the parasite. Sequestration of infected erythrocytes appears to be critical for survival of the parasite and to mediate immunopathological abnormalities in severe malaria. A leukocyte differentiation antigen (CD36) was previously suggested to have a role in sequestration of malaria-infected erythrocytes. CD36 was purified from platelets, where it is known as GPIV, and was shown to be a receptor for binding of infected erythrocytes. Infected erythrocytes adhered to CD36 immobilized on plastic; purified CD36 exhibited saturable, specific binding to infected erythrocytes; and purified CD36 or antibodies to CD36 inhibited and reversed binding of infected erythrocytes to cultured endothelial cells and melanoma cells in vitro. The portion of the CD36 molecule that reverses cytoadherence may be useful therapeutically for rapid reversal of sequestration in cerebral malaria.
Fluid Flow Stimulates Tissue Plasminogen Activator Secretion by Cultured Human Endothelial Cells
Authors: Diamond S. L. , Eskin S. G. , McIntire L. V.
Published: October 2006

Wall shear stress generated by blood flow may regulate the expression of fibrinolytic proteins by endothelial cells. Tissue plasminogen activator (tPA) and plasminogen activator inhibitor, type 1 (PAI-1) secretion by cultured human endothelial cells were not affected by exposure to venous shear stress (4 dynes/cm
2
). However, at arterial shear stresses of 15 and 25 dynes/cm
2
, the tPA secretion rate was 2.1 and 3.0 times greater, respectively, than the basal tPA secretion rate. PAI-1 secretion was unaffected by shear stress over the entire physiological range.
Indole-2-Carboxylic Acid: A Competitive Antagonist of Potentiation by Glycine at the NMDA Receptor
Authors: Huettner James E.
Published: October 2006

The
N
-methyl-D-aspartate (NMDA) class of excitatory amino acid receptors regulates the strength and stability of excitatory synapses and appears to play a major role in excitotoxic neuronal death associated with stroke and epilepsy. The conductance increase gated by NMDA is potentiated by the amino acid glycine, which acts at an allosteric site tightly coupled to the NMDA receptor. Indole-2-carboxylic acid (I2CA) specifically and competitively inhibits the potentiation by glycine of NMDA-gated current. In solutions containing low levels of glycine, I2CA completely blocks the response to NMDA, suggesting that NMDA alone is not sufficient for channel activation. I2CA will be useful for defining the interaction of glycine with NMDA receptors and for determining the in vivo role of glycine in excitotoxicity and synapse stabilization.
Sedimentary 12-
n
-Propylcholestanes, Molecular Fossils Diagnostic of Marine Algae
Authors: Moldowan J. Michael , Fago Frederick J. , Lee Cathy Y. , Jacobson Stephen R. , Watt David S. , Slougui Nacer-Eddine , Jeganathan Alwarsamy , Young Donald C.
Published: October 2006

Certain C
30
-steranes have been used for identifying sedimentary rocks and crude oils derived from organic matter deposited in marine environments. Analysis of a C
30
-sterane from Prudhoe Bay oil indicates that these C
30
-steranes are 24-
n
-propylcholestanes that apparently are derived from precursor sterols 24-
n
-propylidene-cholesterols and 24-
n
-propylcholesterol. These widely occurring sterols are biochemically synthesized in modern oceans by members of an order (Sarcinochrysidales) of chrysophyte algae. These data thus imply that C
30
-sterane biomarkers in sedimentary rocks and crude oils have a marine origin. Screening of a few organic-rich sedimentary rocks and oils from throughout the Phanerozoic suggests that these C
30
-steranes first appeared and, therefore, their source algae evolved between Early Ordovician and Devonian.
Effects on Carbon Storage of Conversion of Old-Growth Forests to Young Forests
Authors: Harmon Mark E. , Ferrell William K. , Franklin Jerry F.
Published: October 2006

Simulations of carbon storage suggest that conversion of old-growth forests to young fast-growing forests will not decrease atmospheric carbon dioxide (CO
2
) in general, as has been suggested recently. During simulated timber harvest, on-site carbon storage is reduced considerably and does not approach old-growth storage capacity for at least 200 years. Even when sequestration of carbon in wooden buildings is included in the models, timber harvest results in a net flux of CO
2
to the atmosphere. To offset this effect, the production of lumber and other long-term wood products, as well as the life-span of buildings, would have to increase markedly. Mass balance calculations indicate that the conversion of 5 × 10
6
hectares of old-growth forests to younger plantations in western Oregon and Washington in the last 100 years has added 1.5 × 10
9
to 1.8 × 10
9
megagrams of carbon to the atmosphere.
Regularization Algorithms for Learning That Are Equivalent to Multilayer Networks
Authors: Poggio T. , Girosi F.
Published: October 2006

Learning an input-output mapping from a set of examples, of the type that many neural networks have been constructed to perform, can be regarded as synthesizing an approximation of a multidimensional function (that is, solving the problem of hypersurface reconstruction). From this point of view, this form of learning is closely related to classical approximation techniques, such as generalized splines and regularization theory. A theory is reported that shows the equivalence between regularization and a class of three-layer networks called regularization networks or hyper basis functions. These networks are not only equivalent to generalized splines but are also closely related to the classical radial basis functions used for interpolation tasks and to several pattern recognition and neural network algorithms. They also have an interesting interpretation in terms of prototypes that are synthesized and optimally combined during the learning stage.
Amazon Deforestation and Climate Change
Authors: Shukla J. , Nobre C. , Sellers P.
Published: October 2006

A coupled numerical model of the global atmosphere and biosphere has been used to assess the effects of Amazon deforestation on the regional and global climate. When the tropical forests in the model were replaced by degraded grass (pasture), there was a significant increase in surface temperature and a decrease in evapotranspiration and precipitation over Amazonia. In the simulation, the length of the dry season also increased; such an increase could make reestablishment of the tropical forests after massive deforestation particularly difficult.
Sonochemistry
Authors: Suslick Kenneth S.
Published: October 2006

Ultrasound causes high-energy chemistry. It does so through the process of acoustic cavitation: the formation, growth and implosive collapse of bubbles in a liquid. During cavitational collapse, intense heating of the bubbles occurs. These localized hot spots have temperatures of roughly 5000°C, pressures of about 500 atmospheres, and lifetimes of a few microseconds. Shock waves from cavitation in liquid-solid slurries produce high-velocity interparticle collisions, the impact of which is sufficient to melt most metals. Applications to chemical reactions exist in both homogeneous liquids and in liquid-solid systems. Of special synthetic use is the ability of ultrasound to create clean, highly reactive surfaces on metals. Ultrasound has also found important uses for initiation or enhancement of catalytic reactions, in both homogeneous and heterogeneous cases.
Precise Monitoring of Global Temperature Trends from Satellites
Authors: Spencer Roy W. , Christy John R.
Published: October 2006

Passive microwave radiometry from satellites provides more precise atmospheric temperature information than that obtained from the relatively sparse distribution of thermometers over the earth's surface. Accurate global atmospheric temperature estimates are needed for detection of possible greenhouse warming, evaluation of computer models of climate change, and for understanding important factors in the climate system. Analysis of the first 10 years (1979 to 1988) of satellite measurements of lower atmospheric temperature changes reveals a monthly precision of 0.01°C, large temperature variability on time scales from weeks to several years, but no obvious trend for the 10-year period. The warmest years, in descending order, were 1987, 1988, 1983, and 1980. The years 1984, 1985, and 1986 were the coolest.
Commitment of Mouse Fibroblasts to Adipocyte Differentiation by DNA Transfection
Authors: Chen Suzie , Teicher Luz C. , Kazim Debora , Pollack Robert E. , Wise Leigh S.
Published: October 2006

Cells of the mouse cell line 3T3-F442A can be induced by various hormones to differentiate into adipocytes, whereas cells of 3T3-C2, a subclone of 3T3, cannot. However, transfection of DNA from uninduced 3T3-F422A cells into 3T3-C2 cells permits recovery of 3T3-C2 transfectants that differentiate into adipocytes in the presence of insulin. DNA isolated from human fat tissue, when transfected into 3T3-C2 mouse cells, also gives rise to mouse transfectants that are induced to differentiate into adipocytes by the addition of insulin. Apparently, transfection of a trans-regulatory gene (or genes) from 3T3-F442A or human fat cells into 3T3-C2 cells is sufficient to commit 3T3-C2 cells to adipocyte differentiation.
Subplate Neurons Pioneer the First Axon Pathway from the Cerebral Cortex
Authors: McConnell Susan K. , Ghosh Anirvan , Shatz Carla J.
Published: October 2006

During the development of the nervous system, growing axons must traverse considerable distances to find their targets. In insects, this problem is solved in part by pioneer neurons, which lay down the first axonal pathways when distances are at a minimum. Here the existence of a similar kind of neuron in the developing mammalian telencephalon is described. These are the subplate cells, the first postmitotic neurons of the cerebral cortex. Axons from subplate neurons traverse the internal capsule and invade the thalamus early in fetal life, even before the neurons of cortical layers 5 and 6, which will form the adult subcortical projections, are generated. During postnatal life, after the adult pattern of axonal projections is firmly established, most subplate neurons disappear. These observations raise the possibility that the early axonal scaffold formed by subplate cells may prove essential for the establishment of permanent subcortical projections.
Deforestation History of the Eastern Rain Forests of Madagascar from Satellite Images
Authors: Green Glen M. , Sussman Robert W.
Published: October 2006

Madagascar is biologically one of the richest areas on Earth, and its plants and animals are among the most endangered. Satellite images and vegetation maps based on earlier aerial photographs were used to determine the extent of eastern rain forests in Madagascar and to monitor the rate of deforestation over a 35-year period. In 1985, 3.8 million hectares of rain forest remained, representing only 50 percent of the 7.6 million hectares existing in 1950 and 34 percent of the estimated original extent (11.2 million hectares). Between 1950 and 1985, the rate of deforestation averaged 111,000 hectares per year. Deforestation was most rapid in areas with low topographic relief and high population density. If cutting of forests continues at the same pace, only forests on the steepest slopes will survive the next 35 years.
Theory of Spontaneous Vesicle Formation in Surfactant Mixtures
Authors: Safran S. A. , Pincus P. , Andelman David
Published: October 2006

The curvature elastic energy of bilayer vesicles formed by a mixture of two surfactants, which individually form either micelles or lamellar bilayer phases is described theoretically. In the limit of large bending elastic modulus
K
being much greater than the temperature
T,
the free energy is minimized by vesicles with different concentrations of the two surfactants in each monolayer of the bilayer. Vesicles are more stable than lameliar structures only when interactions or complexing of the two surfactants is taken into account.
Evidence for Magnetospheric Effects on the Sodium Atmosphere of Mercury
Authors: Potter A. E. , Morgan T. H.
Published: October 2006

Monochromatic images of Mercury at the sodium D
2
emission line showed excess sodium emission in localized regions at high northern and southern latitudes and day-to-day global variations in the distribution of sodium emission. These phenomena support the suggestion that magnetospheric effects could be the cause. Sputtering of surface minerals could produce sodium vapor in polar regions during magnetic substorms, when magnetospheric ions directly impact the surface. Another important process may be the transport of sodium ions along magnetic field lines toward polar regions, where they impact directly on the surface of Mercury and are neutralized to regenerate neutral sodium atoms. Day-to-day variations in planetary sodium distributions could result from changing solar activity, which can change the magnetosphere in time scales of a few hours. Observations of the sodium exosphere may provide a tool for remote monitoring of the magnetosphere of Mercury.
Image Reconstruction of the Interior of Bodies That Diffuse Radiation
Authors: Singer J. R. , Grünbaum F. Alberto , Kohn Philip , Zubelli Jorge Passamani
Published: October 2006

A method for reconstructing images from projections is described. The unique aspect of the procedure is that the reconstruction of the internal structure can be carried out for objects that diffuse the incident radiation. The method may be used with photons, phonons, neutrons, and many other kinds of radiation. The procedure has applications to medical imaging, industrial imaging, and geophysical imaging.
Explosive Deep Water Basalt in the Sumisu Backarc Rift
Authors: Gill J. , Torssander P. , Lapierre H. , Taylor R. , Kaiho K. , Koyama M. , Kusakabe M. , Aitchison J. , Cisowski S. , Dadey K. , Fujioka K. , Klaus A. , Lovell M. , Marsaglia K. , Pezard P. , Taylor B. , Tazaki K.
Published: October 2006

Eruption of 1-million-year-old tholeiitic basalt >1800 meters below sea level (>18 megapascals) in a backarc rift behind the Bonin arc produced a scoriaceous breccia similar in some respects to that formed during subaerial eruptions. Explosion of the magma is thought to have produced frothy agglutinate which welded either on the sea floor or in a submarine eruption column. The resulting 135-meter-thick pyroclastic deposit has paleomagnetic inclinations that are random at a scale of <2.5 meters. High magmatic water content, which is about 1.3 percent by weight after vesiculation, contributed to the explosivity.
Correlated Evolution of Female Mating Preferences and Male Color Patterns in the Guppy
Poecilia reticulata
Authors: Houde Anne E. , Endler John A.
Published: October 2006

Sexual selection may explain why secondary sexual traits of males are so strongly developed in some species that they seem maladaptive. Female mate choice appears to favor the evolution of conspicuous color patterns in male guppies (
Poecilia reticulata
) from Trinidad, but color patterns vary strikingly among populations. According to most theory, correlated evolution of female mating preferences and preferred male traits within populations could promote this kind of divergence between populations. But mating preferences could also constrain the evolution of male traits. In some guppy populations, females discriminate among males based on variation in the extent of orange pigment in male color patterns, and populations differ significantly in the degree of female preferences for orange area. In a comparison of seven populations, the degree of female preference based on orange is correlated with the population average orange area. Thus male traits and female preferences appear to be evolving in parallel.
The Fractional Quantum Hall Effect
Authors: Eisenstein J. P. , Stormer H. L.
Published: October 2006

Recent research has uncovered a fascinating quantum liquid made up solely of electrons confined to a plane surface. Found only at temperatures near absolute zero and in extremely strong magnetic fields, this liquid can flow without friction. The excited states of this liquid consist of peculiar particle-like objects that carry an exact fraction of an electron charge. Called quasiparticles, these excitations can themselves condense into new liquid states. Each such liquid is characterized by a fractional quantum number that is directly observable in a simple electrical measurement. This article attempts to convey the qualitative essence of this still unfolding phenomenon, known as the fractional quantum Hall effect.
Specific Interactions in RNA Enzyme-Substrate Complexes
Authors: Guerrier-Takada Cecilia , Lumelsky Nadya , Altman Sidney
Published: October 2006

Analysis of crosslinked complexes of M1 RNA, the catalytic RNA subunit of ribonuclease P from
Escherichia coli
, and transfer RNA precursor substrates has led to the identification of regions in the enzyme and in the substrate that are in close physical proximity to each other. The nucleotide in M1 RNA, residue C92, which participates in a crosslink with the substrate was deleted and the resulting mutant M1 RNA was shown to cleave substrates lacking the 3′ terminal CCAUCA sequence at sites several nucleotides away from the normal site of cleavage. The presence or absence of the 3′ terminal CCAUCA sequence in transfer RNA precursor substrates markedly affects the way in which these substrates interact with the catalytic RNA in the enzyme-substrate complex. The contacts between wild-type M1 RNA and its substrate are in a region that resembles part of the transfer RNA "E" (exit) site in 23
S
ribosomal RNA. These data demonstrate that in RNA's with very different cellular functions, there are domains with similar structural and functional properties and that there is a nucleotide in M1 RNA that affects the site of cleavage by the enzyme.
Seawater Strontium Isotopic Variations from 2.5 Million Years Ago to the Present
Authors: Capo R. C. , DePaolo D. J.
Published: October 2006

Measurements of marine carbonate samples indicate that during the past 2.5 million years the
87
Sr/
86
Sr ratio of seawater has increased by 14 × 10
-5
. The high average rate of increase of
87
Sr/
86
Sr indicates that continental weathering rates were exceptionally high. Nonuniformity in the rate of increase suggests that weathering rates fluctuated by as much as ±30 percent of present-day values. Some of the observed shifts in weathering rates are contemporaneous with climatic changes inferred from records of oxygen isotopes and carbonate preservation in deep sea sediments.
Precessional Forcing of Nutricline Dynamics in the Equatorial Atlantic
Authors: Molfino B. , McIntyre A.
Published: October 2006

Climate control of nutricline depth in the equatorial Atlantic can be monitored by variations in the abundance of the phytoplankton species
Florisphaera profunda
. A conceptual model, based on in situ evidence, associates high abundances of
F. profunda
with a deep nutricline and low abundances with a shallow nutricline. A 200,000-year record of
F. profunda
relative abundances, obtained from a deep-sea core sited beneath the region of maximum equatorial divergence at 10°W, has 52 percent of its variance centered on the 23,000-year precessional band. Cross-spectral analysis between the signals of
F. profunda
and sea-surface temperature, independently derived from zooplankton species, shows their 23,000-year cycles to be coherent and nearly in phase. Abundance minima of
F. profunda
coincide with times of December perihelion, whereas abundance maxima coincide with June perihelion. These relations indicate that nutricline dynamics in the divergence region of the equatorial Atlantic are controlled by variations in the tropical easterlies, forced by the precessional component of orbital insolation, on time scales greater than 10,000 years.
Thin Films of
n
-Si/Poly-(CH
3
)
3
Si-Cyclooctatetraene: Conducting-Polymer Solar Cells and Layered Structures
Authors: Sailor Michael J. , Ginsburg Eric J. , Gorman Christopher B. , Kumar Amit , Grubbs Robert H. , Lewis Nathan S.
Published: October 2006

The optical and electronic properties of thin films of the solution-processible polymer poly-(CH
3
)
3
Si-cyclooctatetraene are presented. This conjugated polymer is based on a polyacetylene backbone with (CH
3
)
3
Si side groups. Thin transparent films have been cast onto
n
-doped silicon (
n
-Si) substrates and doped with iodine to form surface-barrier solar cells. The devices produce photovoltages that are at the theoretical limit and that are much greater than can be obtained from
n
-Si contacts with conventional metals. Two methods for forming layered polymeric materials, one involving the spin-coating of preformed polymers and the other comprising the sequential polymerization of different monomers, are also described. An organic polymer analog of a metal/insulator/metal capacitor has been constructed with the latter method.
Surface-Induced X-Ray Reflection Visualization of Membrane Orientation and Fusion into Multibilayers
Authors: Cevc G. , Fenzl W. , Sigl L.
Published: October 2006

The fusion of lipid membranes at the air-water interface has been detected with the use of x-ray reflection as a high-resolution, surface-sensitive technique. The rate of this fusion for dimyristoylphosphatidylcholine (DMPC) bilayers is the highest at 29°C, which coincides with the chain-melting phase-transition temperature for the top membrane layers. After 6 hours of incubation a stack of at least ten surface-ordered membrane bilayers in equilibrium with the bulk vesicle suspension is formed. Such fusion is thus surface-catalyzed but not restricted to the first surface layer. The process involves partial membrane dehydration near the solution surface which decreases toward the bulk.
Kappa B-Specific DNA Binding Proteins: Role in the Regulation of Human Interleukin-2 Gene Expression
Authors: Hoyos Beatrice , Ballard Dean W. , Böhnlein Ernst , Siekevitz Miriam , Greene Warner C.
Published: October 2006

Transcriptional activation of the human interleukin-2 (IL-2) gene, like induction of the IL-2 receptor α (IL-2Rα) gene and the type 1 human immunodeficiency virus (HIV-1), is shown to be modulated by a κB-like enhancer element. Mutation of a κB core sequence identified in the IL-2 promoter (-206 to -195) partially inhibits both mitogen- and HTLV-I Tax-mediated activation of this transcription unit and blocks the specific binding of two inducible cellular factors. These κB-specific proteins (80 to 90 and 50 to 55 kilodaltons) similarly interact with the functional κB enhancer present in the IL-2Rα promoter. These data suggest that these κB-specific proteins have a role in the coordinate regulation of this growth factor-growth factor receptor gene system that controls T cell proliferation.
Expression of a Bacterial Gene in a Trypanosomatid Protozoan
Authors: Bellofatto Vivian , Cross George A. M.
Published: October 2006

A simple and reproducible assay for DNA-mediated transfection in the trypanosomatid protozoan
Leptomonas seymouri
has been developed. The assay is based on expression of the
Escherichia coli
chloramphenicol acetyl transferase (CAT) gene flanked by
Leptomonas
DNA fragments that are likely to contain necessary elements for gene expression in trypanosomes. After electroporation of cells in the presence of plasmid DNA, CAT activity was detected in crude cell lysates. No activity was detected when the orientation of the
L. seymouri
mini-exon sequence (placed upstream of the CAT gene) was reversed, or in additional control experiments. This system provides a method for defining transcriptional control elements in trypanosomes.
Free-Electron Lasers: Present Status and Future Prospects
Authors: Kim Kwang-Je , Sessler Andrew
Published: October 2006

Free-electron lasers as scientific instruments are reviewed. The present status and future prospects are delineated with attention drawn to the size, complexity, availability, and performance capability of this new tool.
Developmentally Regulated Systemic Endopolyploidy in Succulents with Small Genomes
Authors: De Rocher E. Jay , Harkins Kristi R. , Galbraith David W. , Bohnert Hans J.
Published: October 2006

Nuclei from
Mesembryanthemum crystallinum
(ice plant) exhibit multiple levels of ploidy in every tissue as revealed by flow microfluorometric analysis of isolated nuclei stained with mithramycin. Multiples of the haploid nuclear genome complement (1C) corresponding to 2C, 4C, 8C, 16C, 32C, and 64C were observed. The distribution of nuclei among the different ploidy levels is tissue-specific and in leaves is characteristic of the stage of development. This type of genome organization has been identified in eight other succulent CAM (crassulacean acid metabolism) plant species with small genomes. Multiploidy may be a common property of this type of plant.
Triton's Geyser-Like Plumes: Discovery and Basic Characterization
Authors: Soderblom L. A. , Kieffer S. W. , Becker T. L. , Brown R. H. , Cook A. F. , Hansen C. J. , Johnson T. V. , Kirk R. L. , Shoemaker E. M.
Published: October 2006

At least four active geyser-like eruptions were discovered in Voyager 2 images of Triton, Neptune's large satellite. The two best documented eruptions occur as columns of dark material rising to an altitude of about 8 kilometers where dark clouds of material are left suspended to drift downwind over 100 kilometers. The radii of the rising columns appear to be in the range of several tens of meters to a kilometer. One model for the mechanism to drive the plumes involves heating of nitrogen ice in a subsurface greenhouse environment; nitrogen gas pressurized by the solar heating explosively vents to the surface carrying clouds of ice and dark particles into the atmosphere. A temperature increase of less than 4 kelvins above the ambient surface value of 38 ± 3 kelvins is more than adequate to drive the plumes to an 8-kilometer altitude. The mass flux in the trailing clouds is estimated to consist of up to 10 kilograms of fine dark particles per second or twice as much nitrogen ice and perhaps several hundred or more kilograms of nitrogen gas per second. Each eruption may last a year or more, during which on the order of a tenth of a cubic kilometer of ice is sublimed.
Origin of Stick-Slip Motion in Boundary Lubrication
Authors: Thompson Peter A. , Robbins Mark O.
Published: October 2006

Molecular dynamics simulations of atomically thin, fluid films confined between two solid plates are described. For a broad range of parameters, a generic stick-slip motion is observed, consistent with the results of recent boundary lubrication experiments. Static plates induce crystalline order in the film. Stick-slip motion involves periodic shear-melting transitions and recrystallization of the film. Uniform motion occurs at high velocities where the film no longer has time to order. These results indicate that the origin of stick-slip motion is thermodynamic instability of the sliding state, rather than a dynamic instability as usually assumed.
Phase Change and the Regulation of Shoot Morphogenesis in Plants
Authors: Poethig R. Scott
Published: October 2006

The shoot system of higher plants passes through several different phases during its development. Each of these phases is characterized by a unique set of morphological and physiological attributes. The intermediate character of the structures produced during phase changes and the phenotypes of mutations that affect this process demonstrate that these phases are specified by independently regulated, overlapping developmental programs. Transitions between phases appear to be initiated by factors extrinsic to the shoot apical meristem; the ability of the shoot to respond to such factors and to remain in a particular phase of development is regulated by factors intrinsic to the meristem. The possibility that developmental phases are maintained by epigenetic cell states and the role of DNA methylation in this process are discussed.
Chiral Symmetry Breaking in Sodium Chlorate Crystallization
Authors: Kondepudi Dilip K. , Kaufman Rebecca J. , Singh Nolini
Published: October 2006

Sodium chlorate (NaClO
3
) crystals are optically active although the molecules of the compound are not chiral. When crystallized from an aqueous solution while the solution is not stirred, statistically equal numbers of levo (L) and dextro (D) NaClO
3
crystals were found. When the solution was stirred, however, almost all of the NaClO
3
crystals (99.7 percent) in a particular sample had the same chirality, either levo or dextro. This result represents an experimental demonstration of chiral symmetry breaking or total spontaneous resolution on a macroscopic level brought about by autocatalysis and competition between L- and D-crystals.
A Possible Younger Dryas Record in Southeastern Alaska
Authors: Engstrom D. R. , Hansen B. C. S. , Wright H. E.
Published: October 2006

A stratigraphic record of climatic cooling equal in timing and severity to the Younger Dryas event of the North Atlantic region has been obtained from lacustrine sediments in the Glacier Bay area of southeastern Alaska. Fossil pollen show that a late Wisconsin pine parkland was replaced about 10,800 years ago by shrub- and herb-dominated tundra, which lasted until about 9,800 years ago. This vegetational change is matched by geochemical evidence for loss of organic matter from catchment soils and increased mineral erosion. If this event represents the Younger Dryas, then an explanation for a hemisphere-wide propagation of a North Atlantic climatic perturbation must be sought.
Realized Reproductive Success of Polygynous Red-Winged Blackbirds Revealed by DNA Markers
Authors: Gibbs H. Lisle , Weatherhead Patrick J. , Boag Peter T. , White Bradley N. , Tabak Lisa M. , Hoysak Drew J.
Published: October 2006

Hypervariable genetic markers, including a novel locus-specific marker detected by a mouse major histocompatibility complex probe, reveal that multiple paternity is common in families of polygynous red-winged blackbirds (
Agelaius phoeniceus
). Almost half of all nests contained at least one chick resulting from an extra-pair fertilization, usually by a neighboring male. Genetically based measures of reproductive success show that individual males realize more than 20% of their overall success from extra-pair fertilizations, on average, and that this form of mating behavior confounds traditional measures of male success. The importance of alternative reproductive tactics in a polygynous bird is quantified, and the results challenge previous explanations for the evolution of avian polygny.
Hyperpolarizing Vasodilators Activate ATP-sensitive K
+
Channels in Arterial Smooth Muscle
Authors: Standen Nicholas B. , Quayle John M. , Davies Noel W. , Brayden Joseph E. , Huang Yu , Nelson Mark T.
Published: October 2006

Vasodilators are used clinically for the treatment of hypertension and heart failure. The effects of some vasodilators seem to be mediated by membrane hyperpolarization. The molecular basis of this hyperpolarization has been investigated by examining the properties of single K
+
channels in arterial smooth muscle cells. The presence of adenosine triphosphate (ATP)-sensitive K
+
channels in these cells was demonstrated at the single channel level. These channels were opened by the hyperpolarizing vasodilator cromakalim and inhibited by the ATP-sensitive K
+
channel blocker glibenclamide. Furthermore, in arterial rings the vasorelaxing actions of the drugs diazoxide, cromakalim, and pinacidil and the hyperpolarizing actions of vasoactive intestinal polypeptide and acetylcholine were blocked by inhibitors of the ATP-sensitive K
+
channel, suggesting that all these agents may act through a common pathway in smooth muscle by opening ATP-sensitive K
+
channels.
A Baculovirus Blocks Insect Molting by Producing Ecdysteroid UDP-Glucosyl Transferase
Authors: O'Reilly David R. , Miller Lois K.
Published: October 2006

The predicted amino acid sequence of a newly identified gene of the insect baculovirus
Autographa californica
nuclear polyhedrosis virus was similar to several uridine 5′-diphosphate (UDP)-glucuronosyl transferases and at least one UDP-glucosyl transferase. Genetic and biochemical studies confirmed that this gene encodes an ecdysteroid UDP-glucosyl transferase (egt). This enzyme catalyzes the transfer of glucose from UDP-glucose to ecdysteroids, which are insect molting hormones. Expression of the egt gene allowed the virus to interfere with normal insect development so that molting was blocked in infected larvae of fall armyworm (
Spodoptera frugiperda
).
Reduction of Deepwater Formation in the Greenland Sea During the 1980s: Evidence from Tracer Data
Authors: Schlosser Peter , Bönisch Gerhard , Rhein Monika , Bayer Reinhold
Published: October 2006

Hydrographic observations and measurements of the concentrations of chlorofluorocarbons (CFCs) have suggested that the formation of Greenland Sea Deep Water (GSDW) slowed down considerably during the 1980s. Such a decrease is related to weakened convection in the Greenland Sea and thus could have significant impact on the properties of the waters flowing over the Scotland-Iceland-Greenland ridge system into the deep Atlantic. Study of the variability of GSDW formation is relevant for understanding the impact of the circulation in the European Polar seas on regional and global deep water characteristics. New long-term multitracer observations from the Greenland Sea show that GSDW formation indeed was greatly reduced during the 1980s. A box model of deepwater formation and exchange in the European Polar seas tuned by the tracer data indicates that the reduction rate of GSDW formation was about 80 percent and that the start date of the reduction was between 1978 and 1982.
Evidence for an Inter-Organismic Heme Biosynthetic Pathway in Symbiotic Soybean Root Nodules
Authors: Sangwan Indu , O'Brian Mark R.
Published: October 2006

The successful symbiosis of soybean with
Bradyrhizobium japonicum
depends on their complex interactions, culminating in the development and maintenance of root nodules. A
B. japonicum
mutant defective in heme synthesis in culture was able to produce heme as a result of its symbiotic association with the soybean host. The bacterial mutant was incapable of synthesizing the committed heme precursor δ-aminolevulinic acid (ALA), but nodule plant cells formed ALA from glutamate. In addition, exogenous ALA was taken up by isolated nodule bacteria of the parent strain and of the mutant. It is proposed that bacterial heme found in nodules can be synthesized from plant ALA, hence segments of a single metabolic pathway are spatially separated into two organisms.
Variations in Terrigenous Input into the Deep Equatorial Atlantic During the Past 24,000 Years
Authors: Francois Roger , Bacon Michael P.
Published: October 2006

Estimates of terrigenous fluxes at three different water depths at two sites in the equatorial Atlantic by normalization against excess
230
Th flux indicate that the flux of terrigenous material to the seafloor was significantly higher during the last glacial period than it is today. Fluxes started to decrease during deglaciation and reached minimal values in the middle of the Holocene. From 15,000 to 5,000 years ago, there was a substantial increase in flux with increasing water depth below 2,800 meters; this increase may reflect resuspension and lateral transport of slope and rise sediment, possibly because of intensification of deepwater circulation during that period.
Growth Mechanism of Sputtered Films of YBa
2
Cu
3
O
7
Studied by Scanning Tunneling Microscopy
Authors: Hawley Marilyn , Raistrick Ian D. , Beery Jerome G. , Houlton Robert J.
Published: October 2006

The surface microstructures of
c
-axis-oriented films of YBa
2
Cu
3
O
7
, deposited by off-axis magnetron sputtering on MgO and SrTiO
3
single crystal (100) substrates, have been investigated with scanning tunneling microscopy and atomic force microscopy. There is strong evidence that the films nucleate as islands and grow by adding material to the edge of a spirally rising step. This results in columnar grains, each of which contains a screw dislocation at its center. This microstructure may be of significance in determining superconducting properties such as critical current, and represents a significant difference between thin films (especially those grown in situ) and bulk materials.
A Nondeletional Mechanism of Thymic Self Tolerance
Authors: Ramsdell Fred , Lantz Tracy , Fowlkes B. J.
Published: October 2006

T cells become tolerant of self antigens during their development in the thymus. Clonal deletion of thymocytes bearing self-reactive T cell receptors is a major mechanism for generating tolerance and occurs readily for antigens expressed by bone marrow-derived cells. Tolerance to antigens expressed on the radioresistant thymic stromal elements is demonstrated here to occur via a nondeletional mechanism. For minor lymphocyte stimulatory (Mls-1
a
) and major histocompatibility complex (MHC) antigens, this alternate form of tolerance induction results in clonal anergy.
Plant Lipids: Metabolism, Mutants, and Membranes
Authors: Somerville Chris , Browse John
Published: October 2006

The mechanisms that regulate plant lipid metabolism determine the dietary and industrial value of storage oils found in economically important species and may control the ability of many plants to survive exposure to temperature extremes. Many of the problems researchers have in defining the pathways, enzymes, and genes involved in plant lipid metabolism appear to be amenable to analysis by genetic approaches. Mutants with alterations in membrane lipid composition have also been used to study the structural and adaptive roles of lipids. The application of genetic engineering methods affords opportunities for researchers to apply knowledge gained about plant lipid metabolism toward enhanced use of plant oils as abundant and renewable sources of reduced carbon.
Fundamental Issues in the Geology and Geophysics of Venus
Authors: Solomon Sean C. , Head James W.
Published: October 2006

A number of important and currently unresolved issues in the global geology and geophysics of Venus will be addressable with the radar imaging, altimetry, and gravity measurements now forthcoming from the Magellan mission. Among these are the global volcanic flux and the rate of formation of new crust; the global heat flux and its regional variations; the relative importance of localized hot spots and linear centers of crustal spreading to crustal formation and tectonics; and the planform of mantle convection on Venus and the nature of the interactions among interior convective flow, near-surface deformation, and magmatism.
Magellan: Electrical and Physical Properties of Venus' Surface
Authors: Tyler G. Leonard , Ford Peter G. , Campbell Donald B. , Elachi Charles , Pettengill Gordon H. , Simpson Richard A.
Published: October 2006

Magellan probes Venus' surface by 12.6-cm-wavelength vertical and oblique radar scattering and measures microwave thermal emission. Emissivity and root-mean-square slope maps between 330° and 30°E and 90°N and 80°S are dissimilar, although some local features are exceptions. Inferred surface emissivities typically are ∼0.85, but vary from ∼0.35 at Maxwell to ∼0.95 northeast of Gula Mons and other locations. Lowest emissivities appear in topographically high areas; this relation suggests that a phase change or differences in chemical weathering occur at about 6055-kilometer radius. Initial results indicate that there are significant variations in the surface scattering function.
Magellan: Initial Analysis of Venus Surface Modification
Authors: Arvidson R. E. , Baker V. R. , Elachi C. , Saunders R. S. , Wood J. A.
Published: October 2006

Initial Magellan observations reveal a planet with high dielectric constant materials exposed preferentially in elevated regions with high slopes, ejecta deposits extending up to 1000 kilometers to the west of several impact craters, windblown deposits and features in areas where there are both obstacles and a source of particulate material, and evidence for slow, steady degradation by atmosphere-surface interactions and mass movements.
Dense Astrophysical Plasmas
Authors: Van Horn H. M.
Published: October 2006

Degenerate bodies composed primarily of dense hydrogen and helium plasmas range from giant planets to the so far hypothetical brown dwarfs. More massive objects begin their lives as nondegenerate stars and may end as white dwarfs, composed primarily of carbon and oxygen, or as neutron stars, with solid crusts of iron or heavier elements and cores of neutron matter. The physical properties of dense plasmas that are necessary to construct theoretical models of such degenerate stars include the equation of state, transport properties, and nuclear reaction rates.
Hot-Spot Evolution and the Global Tectonics of Venus
Authors: Phillips Roger J. , Grimm Robert E. , Malin Michael C.
Published: October 2006

The global tectonics of Venus may be dominated by plumes rising from the mantle and impinging on the lithosphere, giving rise to hot spots. Global sea-floor spreading does not take place, but direct convective coupling of mantle flow fields to the lithosphere leads to regional-scale deformation and may allow lithospheric transport on a limited scale. A hot-spot evolutionary sequence comprises (i) a broad domal uplift resulting from a rising mantle plume, (ii) massive partial melting in the plume head and generation of a thickened crust or crustal plateau, (iii) collapse of dynamic topography, and (iv) creep spreading of the crustal plateau. Crust on Venus is produced by gradual vertical differentiation with little recycling rather than by the rapid horizontal creation and consumption characteristic of terrestrial sea-floor spreading.
Arginine-Mediated RNA Recognition: the Arginine Fork
Authors: Calnan Barbara J. , Tidor Bruce , Biancalana Sara , Hudson Derek , Frankel Alan D.
Published: October 2006

Short peptides that contain the basic region of the HIV-1 Tat protein bind specifically to a bulged region in TAR RNA. A peptide that contained nine arginines (R
9
) also bound specifically to TAR, and a mutant Tat protein that contained R
9
was fully active for transactivation. In contrast, a peptide that contained nine lysines (K
9
) bound TAR poorly and the corresponding protein gave only marginal activity. By starting with the K
9
mutant and replacing lysine residues with arginines, a single arginine was identified that is required for specific binding and transactivation. Ethylation interference experiments suggest that this arginine contacts two adjacent phosphates at the RNA bulge. Model building suggests that the arginine η nitrogens and the ε nitrogen can form specific networks of hydrogen bonds with adjacent pairs of phosphates and that these arrangements are likely to occur near RNA loops and bulges and not within double-stranded A-form RNA. Thus, arginine side chains may be commonly used to recognize specific RNA structures.
Compressibility of Solid C
60
Authors: Fischer John E. , Heiney Paul A. , McGhie Andrew R. , Romanow William J. , Denenstein Arnold M. , McCauley John P. , Smith Amos B.
Published: October 2006

Room-temperature powder x-ray diffraction profiles have been obtained at hydrostatic pressures
P
= 0 and 1.2 gigapascals on the solid phase of cubic C
60
("fullerite"). Within experimental error, the linear compressibility
d
(ln
a
)/
dP
is the same as the interlayer compressibility
d
(ln
c
)/
dP
of hexagonal graphite, consistent with van der Waals intermolecular bonding. The volume compressibility -
d
(ln
V
)/
dP
is 7.0 ± 1 × 10
-12
square centimeter per dyne, 3 and 40 times the values for graphite and diamond, respectively.
Did the Breakout of Laurentia Turn Gondwanaland Inside-Out?
Authors: Hoffman Paul F.
Published: October 2006

Comparative geology suggests that the continents adjacent to northern, western, southern, and eastern Laurentia in the Late Proterozoic were Siberia, Australia-Antarctica, southern Africa, and Amazonia-Baltica, respectively. Late Proterozoic fragmentation of the supercontinent centered on Laurentia would then have been followed by rapid fan-like collapse of the (present) southern continents and eventual consolidation of East and West Gondwanaland. In this scenario, a pole of rotation near the Weddell Sea would explain the observed dominance of wrench tectonics in (present) east-west trending Pan-African mobile belts and subduction-accretion tectonics in north-south trending belts. In the process of fragmentation, rifts originating in the interior of the Late Proterozoic supercontinent became the external margins of Paleozoic Gondwanaland; exterior margins of the Late Proterozoic supercontinent became landlocked within the interior of Gondwanaland.
Molecular Beam Scattering from Liquid Surfaces
Authors: Saecker Mary E. , Govoni Steven T. , Kowalski Daniel V. , King MacKenzie E. , Nathanson Gilbert M.
Published: October 2006

By means of controlled collisions of atoms and molecules with liquid surfaces, molecular beam experiments can be used to probe how gases stick to, rebound from, and exchange energy with molecules in the liquid phase. This report describes measurements of energy exchange in collisions between gases (neon, xenon, and sulfur hexafluoride) and polyatomic liquids (squalane and perfluoropolyether). Energy transfer depends critically on liquid composition and is more efficient for the hydrocarbon than for the perfluorinated ether.