bluuebunny/crossref_metadata_2025 huggingface dataset:
Block Copolymer Lithography: Periodic Arrays of ~10
11
Holes in 1 Square Centimeter
Authors: Park Miri , Harrison Christopher , Chaikin Paul M. , Register Richard A. , Adamson Douglas H.
Published: July 2002

Dense periodic arrays of holes and dots have been fabricated in a silicon nitride–coated silicon wafer. The holes are 20 nanometers across, 40 nanometers apart, and hexagonally ordered with a polygrain structure that has an average grain size of 10 by 10. Spin-coated diblock copolymer thin films with well-ordered spherical or cylindrical microdomains were used as the templates. The microdomain patterns were transferred directly to the underlying silicon nitride layer by two complementary techniques that resulted in opposite tones of the patterns. This process opens a route for nanometer-scale surface patterning by means of spontaneous self-assembly in synthetic materials on length scales that are difficult to obtain by standard semiconductor lithography techniques.
Perturbed Equilibria and Statistical Energy Redistribution in a Gas-Phase S
N
2 Reaction
Authors: Craig Stephen L. , Brauman John I.
Published: July 2002

The relative translational energy of one set of reactants in the gas-phase equilibrium between chloride ion and chloroacetonitrile
35
Cl
−
+
37
ClCH
2
CN ⇌
37
Cl
−
+
35
ClCH
2
CN is varied without affecting the energy distribution of the other set. The steady-state ratio of ionic isotopes changes so that there is an increase in the concentration of energized reactants. The ratio provides a measure of the translational energy dependence of the rate constant for the bimolecular nucleophilic substitution (S
N
2) reaction. The observed energy dependence is indistinguishable from that predicted by Rice-Ramsperger-Kassel-Marcus theory, suggesting that the increased translational energy redistributes statistically in the collision complex.
A Nestling Bird from the Lower Cretaceous of Spain: Implications for Avian Skull and Neck Evolution
Authors: Sanz José L. , Chiappe Luis M. , Pérez-Moreno Bernardino P. , Moratalla José J. , Hernández-Carrasquilla Francisco , Buscalioni Angela D. , Ortega Francisco , Poyato-Ariza Francisco J. , Rasskin-Gutman Diego , Martı́nez-Delclòs Xavier
Published: July 2002

A feathered skeleton of a Lower Cretaceous enantiornithine bird from Spain indicates that the modified diapsid skull of modern birds did not evolve until late in their evolution: Basal birds retained an essentially primitive diapsid design. The fossil provides data clarifying long-standing debates on the cranial morphology of the basalmost bird,Archaeopteryx. It also reemphasizes the notion that the early morphological transformations of birds were focused on the flight apparatus. This fossil was a nestling and suggests that early postnatal developments in the Cretaceous enantiornithine birds and those in their extant counterparts are comparable.
Modulation of Hepatic Gene Expression by Hepatocyte Nuclear Factor 1
Authors: Ktistaki Eleni , Talianidis Iannis
Published: July 2002

Hepatocyte nuclear factors 1 and 4 (HNF-1 and HNF-4) are liver-enriched transcription factors that function in the regulation of several liver-specific genes. HNF-1 activates genes containing promoters with HNF-1 binding sites. However, this factor negatively regulates its own expression and that of other HNF-4–dependent genes that lack HNF-1 binding sites in their promoter region. This repression is exerted by a direct interaction of HNF-1 with AF2, the main activation domain of HNF-4. The dual functions of gene activation and repression suggest that HNF-1 is a global regulator of the transcriptional network involved in the maintenance of hepatocyte-specific phenotype.
The Ionosphere of Europa from Galileo Radio Occultations
Authors: Kliore A. J. , Hinson D. P. , Flasar F. M. , Nagy A. F. , Cravens T. E.
Published: July 2002

The Galileo spacecraft performed six radio occultation observations of Jupiter's Galilean satellite Europa during its tour of the jovian system. In five of the six instances, these occultations revealed the presence of a tenuous ionosphere on Europa, with an average maximum electron density of nearly 10
4
per cubic centimeter near the surface and a plasma scale height of about 240 ± 40 kilometers from the surface to 300 kilometers and of 440 ± 60 kilometers above 300 kilometers. Such an ionosphere could be produced by solar photoionization and jovian magnetospheric particle impact in an atmosphere having a surface density of about 10
8
electrons per cubic centimeter. If this atmosphere is composed primarily of O
2
, then the principal ion is O
2
+
and the neutral atmosphere temperature implied by the 240-kilometer scale height is about 600 kelvin. If it is composed of H
2
O, the principal ion is H
3
O
+
and the neutral temperature is about 340 kelvin. In either case, these temperatures are much higher than those observed on Europa's surface, and an external heating source from the jovian magnetosphere is required.
Effects of Rapid Intramolecular Electron Transfer on Vibrational Spectra
Authors: Ito Tasuku , Hamaguchi Tomohiko , Nagino Haruko , Yamaguchi Tadashi , Washington John , Kubiak Clifford P.
Published: July 2002

Single-electron reductions of linked triruthenium clusters of the general type Ru
3
-pyrazine-Ru
3
produced mixed valence systems showing spectroscopic characteristics of rapid intramolecular electron transfer. Reflectance infrared spectroelectrochemistry was used to characterize the vibrational spectra of mixed valence systems that contained one carbon monoxide ligand on each Ru
3
cluster. Infrared spectra in the CO stretching region showed two resolved, partially coalesced, and coalesced ν(CO) bands for clusters with rate constants for intramolecular electron transfer
k
e
increasing from = 1 × 10
9
s
–
1
up to 5 × 10
11
and 9 × 10
11
s
–
1
, respectively. These data provide a strong correlation between rates of intramolecular electron transfer and infrared spectral bandshape.
The Polio Eradication Effort: Should Vaccine Eradication Be Next?
Authors: Dove Alan W. , Racaniello Vincent R.
Published: July 2002

The World Health Organization (WHO) has implemented a plan to eradicate poliovirus that is widely viewed as having made enormous progress. If all goes as planned, polio will be eradicated on this planet by the year 2003. However, there is a debate, as highlighted in a pair of Policy Forums in this issue, over when vaccination against polio can be stopped. Dove and Racaniello believe that the reliance of the WHO on the live Sabin oral poliovirus vaccine (OPV) means that there will be a continuing threat of release of potentially pathogenic virus into the environment. They are also concerned that the planned destruction of all wild-type polio stocks will not be possible. They therefore recommend a switch to the inactivated polio vaccine (IPV). In response, Hull and Aylward explain why a switch from OPV is not necessary and describe the studies being sponsored by the WHO to determine how and when immunization can safely be ended.
Asymmetric Catalysis with Water: Efficient Kinetic Resolution of Terminal Epoxides by Means of Catalytic Hydrolysis
Authors: Tokunaga Makoto , Larrow Jay F. , Kakiuchi Fumitoshi , Jacobsen Eric N.
Published: July 2002

Epoxides are versatile building blocks for organic synthesis. However, terminal epoxides are arguably the most important subclass of these compounds, and no general and practical method exists for their production in enantiomerically pure form. Terminal epoxides are available very inexpensively as racemic mixtures, and kinetic resolution is an attractive strategy for the production of optically active epoxides, given an economical and operationally simple method. Readily accessible synthetic catalysts (chiral cobalt-based salen complexes) have been used for the efficient asymmetric hydrolysis of terminal epoxides. This process uses water as the only reagent, no added solvent, and low loadings of a recyclable catalyst (<0.5 mole percent), and it affords highly valuable terminal epoxides and 1,2-diols in high yield with high enantiomeric enrichment.
Genetic Restriction of AIDS Pathogenesis by an SDF-1 Chemokine Gene Variant
Authors: Winkler Cheryl , Modi William , Smith Michael W. , Nelson George W. , Wu Xueyun , Carrington Mary , Dean Michael , Honjo Tasaku , Tashiro Kai , Yabe D. , Buchbinder Susan , Vittinghoff Eric , Goedert James J. , O'Brien Thomas R. , Jacobson Lisa P. , Detels Roger , Donfield Sharyne , Willoughby Anne , Gomperts Edward , Vlahov David , Phair John ,   ALIVE Study,   Hemophilia Growth and Development Study (HGDS),   Multicenter AIDS Cohort Study (MACS),   Multicenter Hemophilia Cohort Study (MHCS),   San Francisco City Cohort (SFCC), O'Brien Stephen J.
Published: July 2002

Stromal-derived factor (SDF-1) is the principal ligand for CXCR4, a coreceptor with CD4 for T lymphocyte cell line–tropic human immunodeficiency virus–type 1 (HIV-1). A common polymorphism,SDF1-3′A, was identified in an evolutionarily conserved segment of the 3′ untranslated region of the SDF-1 structural gene transcript. In the homozygous state,SDF1-3′A/3′Adelays the onset of acquired immunodeficiency syndrome (AIDS), according to a genetic association analysis of 2857 patients enrolled in five AIDS cohort studies. The recessive protective effect ofSDF1-3′Awas increasingly pronounced in individuals infected with HIV-1 for longer periods, was twice as strong as the dominant genetic restriction of AIDS conferred byCCR5andCCR2chemokine receptor variants in these populations, and was complementary with these mutations in delaying the onset of AIDS.
Measurements of the Equation of State of Deuterium at the Fluid Insulator-Metal Transition
Authors: Collins G. W. , Da Silva L. B. , Celliers P. , Gold D. M. , Foord M. E. , Wallace R. J. , Ng A. , Weber S. V. , Budil K. S. , Cauble R.
Published: July 2002

A high-intensity laser was used to shock-compress liquid deuterium to pressures from 22 to 340 gigapascals. In this regime deuterium is predicted to transform from an insulating molecular fluid to an atomic metallic fluid. Shock densities and pressures, determined by radiography, revealed an increase in compressibility near 100 gigapascals indicative of such a transition. Velocity interferometry measurements, obtained by reflecting a laser probe directly off the shock front in flight, demonstrated that deuterium shocked above 55 gigapascals has an electrical conductivity characteristic of a liquid metal and independently confirmed the radiography.
Ocean Climate Change: Comparison of Acoustic Tomography, Satellite Altimetry, and Modeling
Authors:   The ATOC Consortium
Published: July 2002

Comparisons of gyre-scale acoustic and direct thermal measurements of heat content in the Pacific Ocean, satellite altimeter measurements of sea surface height, and results from a general circulation model show that only about half of the seasonal and year-to-year changes in sea level are attributable to thermal expansion. Interpreting climate change signals from fluctuations in sea level is therefore complicated. The annual cycle of heat flux is 150 ± 25 watts per square meter (peak-to-peak, corresponding to a 0.2°C vertically averaged temperature cycle); an interannual change of similar magnitude is also detected. Meteorological estimates of surface heat flux, if accurate, require a large seasonal cycle in the advective heat flux.
Imaging Electron Wave Functions of Quantized Energy Levels in Carbon Nanotubes
Authors: Venema Liesbeth C. , Wildöer Jeroen W. G. , Janssen Jorg W. , Tans Sander J. , Tuinstra Hinne L. J. Temminck , Kouwenhoven Leo P. , Dekker Cees
Published: July 2002

Carbon nanotubes provide a unique system for studying one-dimensional quantization phenomena. Scanning tunneling microscopy was used to observe the electronic wave functions that correspond to quantized energy levels in short metallic carbon nanotubes. Discrete electron waves were apparent from periodic oscillations in the differential conductance as a function of the position along the tube axis, with a period that differed from that of the atomic lattice. Wave functions could be observed for several electron states at adjacent discrete energies. The measured wavelengths are in good agreement with the calculated Fermi wavelength for armchair nanotubes.
The Generation and Trapping of Solitary Waves over Topography
Authors: Farmer David , Armi Laurence
Published: July 2002

Observations of tidal flow over a sill show that upstream solitary wave generation occurred in conjunction with the downstream movement of a streamline bifurcation over the obstacle crest. Solitary waves appeared upstream of the bifurcation, riding on the surface stratification. Shear flow instabilities on the plunging interface were also evident and may contribute to wave generation through subharmonic interaction. Nonlinear waves generated downstream of an internal control became trapped in the supercritical flow.
The Machinery of Mitochondrial Inheritance and Behavior
Authors: Yaffe Michael P.
Published: July 2002

The distribution of mitochondria to daughter cells during cell division is an essential feature of cell proliferation. Until recently, it was commonly believed that inheritance of mitochondria and other organelles was a passive process, a consequence of their random diffusion throughout the cytoplasm. A growing recognition of the reticular morphology of mitochondria in many living cells, the association of mitochondria with the cytoskeleton, and the coordinated movements of mitochondria during cellular division and differentiation has illuminated the necessity for a cellular machinery that mediates mitochondrial behavior. Characterization of the underlying molecular components of this machinery is providing insight into mechanisms regulating mitochondrial morphology and distribution.
Reconstitution of G
1
Cyclin Ubiquitination with Complexes Containing SCF
Grr1
and Rbx1
Authors: Skowyra Dorota , Koepp Deanna M. , Kamura Takumi , Conrad Michael N. , Conaway Ronald C. , Conaway Joan Weliky , Elledge Stephen J. , Harper J. Wade
Published: July 2002

Control of cyclin levels is critical for proper cell cycle regulation. In yeast, the stability of the G
1
cyclin Cln1 is controlled by phosphorylation-dependent ubiquitination. Here it is shown that this reaction can be reconstituted in vitro with an SCF E3 ubiquitin ligase complex. Phosphorylated Cln1 was ubiquitinated by SCF (Skp1-Cdc53–F-box protein) complexes containing the F-box protein Grr1, Rbx1, and the E2 Cdc34. Rbx1 promotes association of Cdc34 with Cdc53 and stimulates Cdc34 auto-ubiquitination in the context of Cdc53 or SCF complexes. Rbx1, which is also a component of the von Hippel–Lindau tumor suppressor complex, may define a previously unrecognized class of E3-associated proteins.
An Essential Role for DNA Adenine Methylation in Bacterial Virulence
Authors: Heithoff Douglas M. , Sinsheimer Robert L. , Low David A. , Mahan Michael J.
Published: July 2002

Salmonella typhimurium
lacking DNA adenine methylase (Dam) were fully proficient in colonization of mucosal sites but showed severe defects in colonization of deeper tissue sites. These Dam
–
mutants were totally avirulent and were effective as live vaccines against murine typhoid fever. Dam regulated the expression of at least 20 genes known to be induced during infection; a subset of these genes are among those activated by the PhoP global virulence regulator. PhoP, in turn, affected Dam methylation at specific genomic sites, as evidenced by alterations in DNA methylation patterns. Dam inhibitors are likely to have broad antimicrobial action, and Dam
–
derivatives of these pathogens may serve as live attenuated vaccines.
Activation of PPARγ Coactivator-1 Through Transcription Factor Docking
Authors: Puigserver Pere , Adelmant Guillaume , Wu Zhidan , Fan Melina , Xu Jianming , O'Malley Bert , Spiegelman Bruce M.
Published: July 2002

Transcriptional coactivators have been viewed as constitutively active components, using transcription factors mainly to localize their functions. Here, it is shown that PPARγ coactivator–1 (PGC-1) promotes transcription through the assembly of a complex that includes the histone acetyltransferases steroid receptor coactivator–1 (SRC-1) and CREB binding protein (CBP)/p300. PGC-1 has a low inherent transcriptional activity when it is not bound to a transcription factor. The docking of PGC-1 to peroxisome proliferator-activated receptor γ (PPARγ) stimulates an apparent conformational change in PGC-1 that permits binding of SRC-1 and CBP/p300, resulting in a large increase in transcriptional activity. Thus, transcription factor docking switches on the activity of a coactivator protein.
Structural Analysis of the Mechanism of Adenovirus Binding to Its Human Cellular Receptor, CAR
Authors: Bewley Maria C. , Springer Karen , Zhang Yian-Biao , Freimuth Paul , Flanagan John M.
Published: July 2002

Binding of virus particles to specific host cell surface receptors is known to be an obligatory step in infection even though the molecular basis for these interactions is not well characterized. The crystal structure of the adenovirus fiber knob domain in complex with domain I of its human cellular receptor, coxsackie and adenovirus receptor (CAR), is presented here. Surface-exposed loops on knob contact one face of CAR, forming a high-affinity complex. Topology mismatches between interacting surfaces create interfacial solvent-filled cavities and channels that may be targets for antiviral drug therapy. The structure identifies key determinants of binding specificity, which may suggest ways to modify the tropism of adenovirus-based gene therapy vectors.
Proapoptotic Bcl-2 Relative Bim Required for Certain Apoptotic Responses, Leukocyte Homeostasis, and to Preclude Autoimmunity
Authors: Bouillet Philippe , Metcalf Donald , Huang David C. S. , Tarlinton David M. , Kay Tom W. H. , Köntgen Frank , Adams Jerry M. , Strasser Andreas
Published: July 2002

Apoptosis can be triggered by members of the Bcl-2 protein family, such as Bim, that share only the BH3 domain with this family. Gene targeting in mice revealed important physiological roles for Bim. Lymphoid and myeloid cells accumulated, T cell development was perturbed, and most older mice accumulated plasma cells and succumbed to autoimmune kidney disease. Lymphocytes were refractory to apoptotic stimuli such as cytokine deprivation, calcium ion flux, and microtubule perturbation but not to others. Thus, Bim is required for hematopoietic homeostasis and as a barrier to autoimmunity. Moreover, particular death stimuli appear to activate apoptosis through distinct BH3-only proteins.
Posttranslational Quality Control: Folding, Refolding, and Degrading Proteins
Authors: Wickner Sue , Maurizi Michael R. , Gottesman Susan
Published: July 2002

Polypeptides emerging from the ribosome must fold into stable three-dimensional structures and maintain that structure throughout their functional lifetimes. Maintaining quality control over protein structure and function depends on molecular chaperones and proteases, both of which can recognize hydrophobic regions exposed on unfolded polypeptides. Molecular chaperones promote proper protein folding and prevent aggregation, and energy-dependent proteases eliminate irreversibly damaged proteins. The kinetics of partitioning between chaperones and proteases determines whether a protein will be destroyed before it folds properly. When both quality control options fail, damaged proteins accumulate as aggregates, a process associated with amyloid diseases.
Photic Induction of
mPer1
and
mPer2
in
Cry
-Deficient Mice Lacking a Biological Clock
Authors: Okamura Hitoshi , Miyake Shigeru , Sumi Yasuo , Yamaguchi Shun , Yasui Akira , Muijtjens Manja , Hoeijmakers Jan H. J. , van der Horst Gijsbertus T. J.
Published: July 2002

Mice lacking
mCry1
and
mCry2
are behaviorally arrhythmic. As shown here, cyclic expression of the clock genes
mPer1
and
mPer2
(mammalian
Period
genes 1 and 2) in the suprachiasmatic nucleus and peripheral tissues is abolished and
mPer1
and
mPer2
mRNA levels are constitutively high. These findings indicate that the biological clock is eliminated in the absence of both mCRY1 and mCRY2 (mammalian cryptochromes 1 and 2) and support the idea that mammalian CRY proteins act in the negative limb of the circadian feedback loop. The
mCry
double-mutant mice retain the ability to have
mPer1
and
mPer2
expression induced by a brief light stimulus known to phase-shift the biological clock in wild-type animals. Thus, mCRY1 and mCRY2 are dispensable for light-induced phase shifting of the biological clock.
Distal Initiation and Active Propagation of Action Potentials in Interneuron Dendrites
Authors: Martina Marco , Vida Imre , Jonas Peter
Published: July 2002

Fast and reliable activation of inhibitory interneurons is critical for the stability of cortical neuronal networks. Active conductances in dendrites may facilitate interneuron activation, but direct experimental evidence was unavailable. Patch-clamp recordings from dendrites of hippocampal oriens-alveus interneurons revealed high densities of voltage-gated sodium and potassium ion channels. Simultaneous recordings from dendrites and somata suggested that action potential initiation occurs preferentially in the axon with long threshold stimuli, but can be shifted to somatodendritic sites when brief stimuli are applied. After initiation, action potentials propagate over the somatodendritic domain with constant amplitude, high velocity, and reliability, even during high-frequency trains.
Clonal Propagation of Primate Offspring by Embryo Splitting
Authors: Chan A. W. S. , Dominko T. , Luetjens C. M. , Neuber E. , Martinovich C. , Hewitson L. , Simerly C. R. , Schatten G. P.
Published: July 2002

Primates that are identical in both nuclear and cytoplasmic components have not been produced by current cloning strategies, yet such identicals represent the ideal model for investigations of human diseases. Here, genetically identical nonhuman embryos were produced as twin and larger sets by separation and reaggregation of blastomeres of cleavage-stage embryos. A total of 368 multiples were created by the splitting of 107 rhesus embryos with four pregnancies established after 13 embryo transfers (31% versus 53% in vitro fertilization controls). The birth of Tetra, a healthy female cloned from a quarter of an embryo, proves that this approach can result in live offspring.
Late Cretaceous Polar Wander of the Pacific Plate: Evidence of a Rapid True Polar Wander Event
Authors: Sager William W. , Koppers and Anthony A. P.
Published: July 2002

We reexamined the Late Cretaceous–early Tertiary apparent polar wander path for the Pacific plate using 27 paleomagnetic poles from seamounts dated by40Ar/39Ar geochronology. The path shows little motion from 120 to 90 million years ago (Ma), northward motion from 79 to 39 Ma, and two groups of poles separated by 16 to 21 degrees with indistinguishable mean ages of 84 ± 2 Ma. The latter phenomenon may represent a rapid polar wander episode (3 to 10 degrees per million years) whose timing is not adequately resolved with existing data. Similar features in other polar wander paths imply that the event was a rapid shift of the spin axis relative to the mantle (true polar wander), which may have been related to global changes in plate motion, large igneous province eruptions, and a shift in magnetic field polarity state.
Surface Expression of HLA-E, an Inhibitor of Natural Killer Cells, Enhanced by Human Cytomegalovirus gpUL40
Authors: Tomasec Peter , Braud Veronique M. , Rickards Carole , Powell Martin B. , McSharry Brian P. , Gadola Stephan , Cerundolo Vincenzo , Borysiewicz Leszek K. , McMichael Andrew J. , Wilkinson Gavin W. G.
Published: July 2002

The nonclassical major histocompatibility complex (MHC) class I molecule HLA-E inhibits natural killer (NK) cell–mediated lysis by interacting with CD94/NKG2A receptors. Surface expression of HLA-E depends on binding of conserved peptides derived from MHC class I molecules. The same peptide is present in the leader sequence of the human cytomegalovirus (HCMV) glycoprotein UL40 (gpUL40). It is shown that, independently of the transporter associated with antigen processing, gpUL40 can up-regulate expression of HLA-E, which protects targets from NK cell lysis. While classical MHC class I molecules are down-regulated, HLA-E is up-regulated by HCMV. Induction of HLA-E surface expression by gpUL40 may represent an escape route for HCMV.
Neuroimaging Evidence for Dissociable Forms of Repetition Priming
Authors: Henson R. , Shallice T. , Dolan R.
Published: July 2002

Repetition priming has been characterized neurophysiologically as a decreased response following stimulus repetition. The present study used event-related functional magnetic resonance imaging to investigate whether this repetition-related response is sensitive to stimulus familiarity. A right fusiform region exhibited an attenuated response to the repetition of familiar stimuli, both faces and symbols, but exhibited an enhanced response to the repetition of unfamiliar stimuli. Moreover, both repetition effects were modulated by lag between successive presentations. Further experiments replicated the interactions between repetition, familiarity, and lag and demonstrated the persistence of these effects over multiple repetitions. Priming-related responses are therefore not unitary but depend on the presence or absence of preexisting stimulus representations.
Generation of Femtosecond Pulses of Synchrotron Radiation
Authors: Schoenlein R. W. , Chattopadhyay S. , Chong H. H. W. , Glover T. E. , Heimann P. A. , Shank C. V. , Zholents A. A. , Zolotorev M. S.
Published: July 2002

Femtosecond synchrotron pulses were generated directly from an electron storage ring. An ultrashort laser pulse was used to modulate the energy of electrons within a 100-femtosecond slice of the stored 30-picosecond electron bunch. The energy-modulated electrons were spatially separated from the long bunch and used to generate ∼300-femtosecond synchrotron pulses at a bend-magnet beamline, with a spectral range from infrared to x-ray wavelengths. The same technique can be used to generate ∼100-femtosecond x-ray pulses of substantially higher flux and brightness with an undulator. Such synchrotron-based femtosecond x-ray sources offer the possibility of applying x-ray techniques on an ultrafast time scale to investigate structural dynamics in condensed matter.
Gene Targeting by Homologous Recombination in
Drosophila
Authors: Rong Yikang S. , Golic Kent G.
Published: July 2002

Drosophila
offers many advantages as an experimental organism. However, in comparison with yeast and mouse, two other widely used eukaryotic model systems,
Drosophila
suffers from an inability to perform homologous recombination between introduced DNA and the corresponding chromosomal loci. The ability to specifically modify the genomes of yeast and mouse provides a quick and easy way to generate or rescue mutations in genes for which a DNA clone or sequence is available. A method is described that enables analogous manipulations of the
Drosophila
genome. This technique may also be applicable to other organisms for which gene-targeting procedures do not yet exist.
Group II Introns Designed to Insert into Therapeutically Relevant DNA Target Sites in Human Cells
Authors: Guo Huatao , Karberg Michael , Long Meredith , Jones J. P. , Sullenger Bruce , Lambowitz Alan M.
Published: July 2002

Mobile group II intron RNAs insert directly into DNA target sites and are then reverse-transcribed into genomic DNA by the associated intron-encoded protein. Target site recognition involves modifiable base-pairing interactions between the intron RNA and a >14-nucleotide region of the DNA target site, as well as fixed interactions between the protein and flanking regions. Here, we developed a highly efficient
Escherichia coli
genetic assay to determine detailed target site recognition rules for the
Lactococcus lactis
group II intron Ll.LtrB and to select introns that insert into desired target sites. Using human immunodeficiency virus–type 1 (HIV-1) proviral DNA and the human
CCR5
gene as examples, we show that group II introns can be retargeted to insert efficiently into virtually any target DNA and that the retargeted introns retain activity in human cells. This work provides the practical basis for potential applications of targeted group II introns in genetic engineering, functional genomics, and gene therapy.
Soluble Adenylyl Cyclase as an Evolutionarily Conserved Bicarbonate Sensor
Authors: Chen Yanqiu , Cann Martin J. , Litvin Tatiana N. , Iourgenko Vadim , Sinclair Meeghan L. , Levin Lonny R. , Buck Jochen
Published: July 2002

Spermatozoa undergo a poorly understood activation process induced by bicarbonate and mediated by cyclic adenosine 3′,5′-monophosphate (cAMP). It has been assumed that bicarbonate mediates its effects through changes in intracellular pH or membrane potential; however, we demonstrate here that bicarbonate directly stimulates mammalian soluble adenylyl cyclase (sAC) activity in vivo and in vitro in a pH-independent manner. sAC is most similar to adenylyl cyclases from cyanobacteria, and bicarbonate regulation of cyclase activity is conserved in these early forms of life. sAC is also expressed in other bicarbonate-responsive tissues, which suggests that bicarbonate regulation of cAMP signaling plays a fundamental role in many biological systems.
Selective Inhibition of NF-κB Activation by a Peptide That Blocks the Interaction of NEMO with the IκB Kinase Complex
Authors: May Michael J. , D'Acquisto Fulvio , Madge Lisa A. , Glöckner Judith , Pober Jordan S. , Ghosh Sankar
Published: July 2002

Activation of the transcription factor nuclear factor (NF)–κB by proinflammatory stimuli leads to increased expression of genes involved in inflammation. Activation of NF-κB requires the activity of an inhibitor of κB (IκB)-kinase (IKK) complex containing two kinases (IKKα and IKKβ) and the regulatory protein NEMO (NF-κB essential modifier). An amino-terminal α-helical region of NEMO associated with a carboxyl-terminal segment of IKKα and IKKβ that we term the NEMO-binding domain (NBD). A cell-permeable NBD peptide blocked association of NEMO with the IKK complex and inhibited cytokine-induced NF-κB activation and NF-κB–dependent gene expression. The peptide also ameliorated inflammatory responses in two experimental mouse models of acute inflammation. The NBD provides a target for the development of drugs that would block proinflammatory activation of the IKK complex without inhibiting basal NF-κB activity.
Subgroup of Reproductive Functions of Progesterone Mediated by Progesterone Receptor-B Isoform
Authors: Mulac-Jericevic Biserka , Mullinax Robert A. , DeMayo Francesco J. , Lydon John P. , Conneely Orla M.
Published: July 2002

Progesterone regulates reproductive function through two intracellular receptors, progesterone receptor–A (PR-A) and progesterone receptor–B (PR-B), that arise from a single gene and function as transcriptional regulators of progesterone-responsive genes. Although in vitro studies show that PR isoforms can display different transcriptional regulatory activities, their physiological significance is unknown. By selective ablation of PR-A in mice, we show that the PR-B isoform modulates a subset of reproductive functions of progesterone by regulation of a subset of progesterone-responsive target genes. Thus, PR-A and PR-B are functionally distinct mediators of progesterone action in vivo and should provide suitable targets for generation of tissue-selective progestins.
Oligodendrocyte Precursor Cells Reprogrammed to Become Multipotential CNS Stem Cells
Authors: Kondo Toru , Raff Martin
Published: July 2002

During animal development, cells become progressively more restricted in the cell types to which they can give rise. In the central nervous system (CNS), for example, multipotential stem cells produce various kinds of specified precursors that divide a limited number of times before they terminally differentiate into either neurons or glial cells. We show here that certain extracellular signals can induce oligodendrocyte precursor cells to revert to multipotential neural stem cells, which can self-renew and give rise to neurons and astrocytes, as well as to oligodendrocytes. Thus, these precursor cells have greater developmental potential than previously thought.
The Elemental Composition of Asteroid 433 Eros: Results of the NEAR-Shoemaker X-ray Spectrometer
Authors: Trombka J. I. , Squyres S. W. , Brückner J. , Boynton W. V. , Reedy R. C. , McCoy T. J. , Gorenstein P. , Evans L. G. , Arnold J. R. , Starr R. D. , Nittler L. R. , Murphy M. E. , Mikheeva I. , McNutt R. L. , McClanahan T. P. , McCartney E. , Goldsten J. O. , Gold R. E. , Floyd S. R. , Clark P. E. , Burbine T. H. , Bhangoo J. S. , Bailey S. H. , Petaev M.
Published: July 2002

We report major element composition ratios for regions of the asteroid 433 Eros imaged during two solar flares and quiet sun conditions during the period of May to July 2000. Low aluminum abundances for all regions argue against global differentiation of Eros. Magnesium/silicon, aluminum/silicon, calcium/silicon, and iron/silicon ratios are best interpreted as a relatively primitive, chondritic composition. Marked depletions in sulfur and possible aluminum and calcium depletions, relative to ordinary chondrites, may represent signatures of limited partial melting or impact volatilization.
Ubiquitin-Activating/Conjugating Activity of TAF
II
250, a Mediator of Activation of Gene Expression in
Drosophila
Authors: Pham Anh-Dung , Sauer Frank
Published: July 2002

Ubiquitination of histones has been linked to the complex processes that regulate the activation of eukaryotic transcription. However, the cellular factors that interpose this histone modification during the processes of transcriptional activation are not well characterized. A biochemical approach identified the
Drosophila
coactivator TAF
II
250, the central subunit within the general transcription factor TFIID, as a histone-specific ubiquitin-activating/conjugating enzyme (ubac). TAF
II
250 mediates monoubiquitination of histone H1 in vitro. Point mutations within the putative ubac domain of TAF
II
250 abolished H1-specific ubiquitination in vitro. In the
Drosophila
embryo, inactivation of the TAF
II
250 ubac activity reduces the cellular level of monoubiquitinated histone H1 and the expression of genes targeted by the maternal activator Dorsal. Thus, coactivator-mediated ubiquitination of proteins within the transactivation pathway may contribute to the processes directing activation of eukaryotic transcription.
Structure of Murine CTLA-4 and Its Role in Modulating T Cell Responsiveness
Authors: Ostrov David A. , Shi Wuxian , Schwartz Jean-Claude D. , Almo Steven C. , Nathenson Stanley G.
Published: July 2002

The effective regulation of T cell responses is dependent on opposing signals transmitted through two related cell-surface receptors, CD28 and cytotoxic T lymphocyte–associated antigen 4 (CTLA-4). Dimerization of CTLA-4 is required for the formation of high-avidity complexes with B7 ligands and for transmission of signals that attenuate T cell activation. We determined the crystal structure of the extracellular portion of CTLA-4 to 2.0 angstrom resolution. CTLA-4 belongs to the immunoglobulin superfamily and displays a strand topology similar to Vα domains, with an unusual mode of dimerization that places the B7 binding sites distal to the dimerization interface. This organization allows each CTLA-4 dimer to bind two bivalent B7 molecules and suggests that a periodic arrangement of these components within the immunological synapse may contribute to the regulation of T cell responsiveness.
X-Chromosome Inactivation in Cloned Mouse Embryos
Authors: Eggan Kevin , Akutsu Hidenori , Hochedlinger Konrad , Rideout William , Yanagimachi Ryuzo , Jaenisch Rudolf
Published: July 2002

To study whether cloning resets the epigenetic differences between the two X chromosomes of a somatic female nucleus, we monitored X inactivation in cloned mouse embryos. Both X chromosomes were active during cleavage of cloned embryos, followed by random X inactivation in the embryo proper. In the trophectoderm (TE), X inactivation was nonrandom with the inactivated X of the somatic donor being chosen for inactivation. When female embryonic stem cells with two active X chromosomes were used as donors, random X inactivation was seen in the TE and embryo. These results demonstrate that epigenetic marks can be removed and reestablished on either X chromosome during cloning. Our results also suggest that the epigenetic marks imposed on the X chromosomes during gametogenesis, responsible for normal imprinted X inactivation in the TE, are functionally equivalent to the marks imposed on the chromosomes during somatic X inactivation.
External Control of 20th Century Temperature by Natural and Anthropogenic Forcings
Authors: Stott Peter A. , Tett S. F. B. , Jones G. S. , Allen M. R. , Mitchell J. F. B. , Jenkins G. J.
Published: July 2002

A comparison of observations with simulations of a coupled ocean-atmosphere general circulation model shows that both natural and anthropogenic factors have contributed significantly to 20th century temperature changes. The model successfully simulates global mean and large-scale land temperature variations, indicating that the climate response on these scales is strongly influenced by external factors. More than 80% of observed multidecadal-scale global mean temperature variations and more than 60% of 10- to 50-year land temperature variations are due to changes in external forcings. Anthropogenic global warming under a standard emissions scenario is predicted to continue at a rate similar to that observed in recent decades.
Ultrahigh-Energy Cosmic Rays: Physics and Astrophysics at Extreme Energies
Authors: Sigl Günter
Published: July 2002

The origin of cosmic rays is one of the major unresolved questions in astrophysics. In particular, the highest energy cosmic rays observed have macroscopic energies up to several 10
20
electron volts and thus provide a probe of physics and astrophysics at energies unattained in laboratory experiments. Theoretical explanations range from astrophysical acceleration of charged particles, to particle physics beyond the established standard model, and processes taking place at the earliest moments of our universe. Distinguishing between these scenarios requires detectors with effective areas in the 1000-square-kilometer range, which are now under construction or in the planning stage. Close connections with γ-ray and neutrino astrophysics add to the interdisciplinary character of this field.
Charge Exchange-Induced X-Ray Emission from Comet C/1999 S4 (LINEAR)
Authors: Lisse C. M. , Christian D. J. , Dennerl K. , Meech K. J. , Petre R. , Weaver H. A. , Wolk S. J.
Published: July 2002

Using soft x-ray observations of the bright new comet C/1999 S4 (LINEAR) with the Chandra x-ray observatory, we have detected x-ray line emission created by charge exchange between highly ionized solar wind minor ions and neutral gases in the comet's coma. The emission morphology was symmetrically crescent shaped and extended out to 300,000 kilometers from the nucleus. The emission spectrum contains 6 lines at 320, 400, 490, 560, 600, and 670 electron volts, attributable to electron capture and radiative deexcitation by the solar wind species C
+5
, C
+6
, N
+7
, O
+7
, and O
+8
. A contemporaneous 7-day soft x-ray light curve obtained using the Extreme Ultraviolet Explorer demonstrates a large increase in the comet's emission coincident with a strong solar flare on 14 and 15 July 2000.
Integrin Connections Map: To Infinity and Beyond
Authors: Martin Karen H. , Slack Jill K. , Boerner Scott A. , Martin Clifford C. , Parsons J. Thomas
Published: July 2002

Integrins are transmembrane proteins that serve as primary sensors of the extracellular matrix (ECM) environment. In response to interactions with the ECM, integrins initiate signaling pathways that regulate cell migration, growth, and survival. Advances in imaging have contributed to the understanding of the dynamic nature of these cell-ECM interactions and the complexes that form at these sites and have provided insights into their regulation and signal organizing functions.
Single-Dose Intrapulmonary Pharmacokinetics of Rifapentine in Normal Subjects
Authors: Conte John E. , Golden Jeffrey A. , McQuitty Mari , Kipps Juliana , Lin Emil T. , Zurlinden Elisabeth
Published: July 2002

The intrapulmonary pharmacokinetics of rifapentine were studied in 30 volunteers who received a single, oral dose of rifapentine (600 mg). Subgroups of five subjects each underwent bronchoscopy and bronchoalveolar lavage (BAL) at timed intervals following drug administration. Drug concentrations, including the concentration of the primary metabolite 25-desacetyl rifapentine, were determined in plasma, BAL fluid, and alveolar cells (AC) by high-pressure liquid chromatography. The concentrations in epithelial lining fluid (ELF) were calculated by the urea diffusion method. The concentration-time data were fit to two-compartment (plasma) or one-compartment (AC and ELF) models. The peak concentrations in plasma, ELF, and AC, 26.2, 3.7, and 5.3 μg/ml, respectively, occurred at 5, 5, and 7 h after drug administration, respectively. The half-lives and areas under the curve for plasma, ELF, and AC were 18.3 h and 520 μg · h/ml, 20.8 h and 111 μg · h/ml, and 13.0 h and 133 μg · h/ml, respectively. Although the intrapulmonary rifapentine concentrations were less than the plasma rifapentine concentrations at all time periods, they remained above the proposed breakpoint forM. tuberculosis(0.5 μg/ml) for the 48-h observation period. These data provide a pharmacokinetic rationale for extended-interval dosing. The optimum dosing regimen for rifapentine will have to be determined by controlled clinical trials.
Bactericidal Activity of Gentamicin against
Enterococcus faecalis
In Vitro and In Vivo
Authors: Lefort Agnès , Arthur Michel , Garry Louis , Carbon Claude , Courvalin Patrice , Fantin Bruno
Published: July 2002

The activity of gentamicin at various concentrations against two strains of
Enterococcus faecalis
was investigated in vitro and in a rabbit model of aortic endocarditis. In vitro, gentamicin at 0.5 to 4 times the MIC failed to reduce the number of bacteria at 24 h. Rabbit or human serum dramatically increased gentamicin activity, leading to a ≥3-log
10
CFU/ml decrease in bacterial counts when the drug concentration exceeded the MIC. Susceptibility testing in the presence of serum was predictive of in vivo activity, since gentamicin alone significantly reduced the number of surviving bacteria in the vegetations if the peak-to-MIC ratio was greater than 1. However, gentamicin selected resistant mutants in rabbits. The intrinsic activity of gentamicin should be taken into account in evaluation of combinations of gentamicin and cell wall-active agents against enterococci.
In Vitro Activity and Fecal Concentration of Rifaximin after Oral Administration
Authors: Jiang Zhi-Dong , Ke Shi , Palazzini Ernesto , Riopel Lise , Dupont Herbert
Published: July 2002

Rifaximin showed moderately high MICs (the MIC at which 90% of the isolates tested were inhibited = 50 μg/ml) for 145 bacterial enteropathogens from patients with traveler's diarrhea acquired in Mexico during the summers of 1997 and 1998. Rifaximin concentrations in stool the day after oral administration (800 mg daily for 3 days) were high (average, 7,961 μg/g), proving the value of the drug.
Clinical Isolation and Resistance Patterns of and Superinfection with 10 Nosocomial Pathogens after Treatment with Ceftriaxone versus Ampicillin-Sulbactam
Authors: Carmeli Yehuda , Castro Julio , Eliopoulos George M. , Samore Matthew H.
Published: July 2002

Isolation of pathogens from clinical cultures and their resistance patterns may be altered by antecedent antibiotic treatment. The objective of this study was to assess the influence of treatment with ceftriaxone versus that with ampicillin-sulbactam on recovery and superinfections with 10 nosocomial pathogens. The study was designed as a historical cohort study, using a propensity score to adjust for confounding by indication and multivariate survival analyses to adjust for other confounding. Two thousand four hundred forty-five patients were treated with ampicillin-sulbactam, and 1,308 were treated with ceftriaxone. The study analyzed two outcomes: (i) recovery of pathogens from clinical cultures and (ii) microbiologically documented infections. Data were obtained from administrative, pharmacy, clinical, and laboratory databases and by chart extraction. Following treatment, new isolation of at least 1 of the 10 target pathogens occurred for 244 patients. After adjustment, more infections occurred in the ampicillin-sulbactam group (hazard ratio [HR], 1.55;P= 0.009). This was observed with all gram-negative rods combined (HR, 3.6;P< 0.001) and with each genus of the familyEnterobacteriaceae. No differences in isolation of gram-positive bacteria were evident (P= 0.33). Microbiologically documented superinfections occurred in 172 patients and were less frequent in the ceftriaxone group (3.8% versus 5%; HR, 1.6;P= 0.015). All theEscherichia coliandKlebsiellaspp. isolates were susceptible to ceftriaxone, but half were resistant to ampicillin-sulbactam. The prevalence of oxacillin resistance amongStaphylococcus aureusisolates was higher in the ceftriaxone group (63% versus 31%; odds ratio, 3.8;P= 0.08). Differences in the rates of superinfections and the likely causative organisms following treatment with ceftriaxone or ampicillin-sulbactam were evident. This may guide clinicians in empirical choices of antibiotics to treat superinfection.
Precision and Accuracy of Fluconazole Susceptibility Testing by Broth Microdilution, Etest, and Disk Diffusion Methods
Authors: Barry A. L. , Pfaller M. A. , Rennie R. P. , Fuchs P. C. , Brown S. D.
Published: July 2002

Interpretive agreements among the results of fluconazole broth microdilution tests, Etests, and disk diffusion tests were documented by evaluating 495
Candida
spp. Microdilution reference test results were in agreement with 96% of the Etest results; most discrepancies were minor differences. Fluconazole resistance of
Candida krusei
strains often required a full 48 h of incubation in order to be observed by the standard method. For the disk diffusion tests that were performed on Mueller-Hinton agar with glucose and methylene blue, 97% of results were in agreement with those of the reference test, especially when zones of inhibition were measured after the first 24 h of incubation. Some
Candida glabrata
isolates failed to grow satisfactorily until a full 48 h of incubation was completed. Precision was determined by testing 50 selected isolates in triplicate in each of three laboratories. The reproducibility of results of disk diffusion tests was comparable to that of the reference method. With all procedures, determination of test results was particularly challenging with some strains, and new methods are needed in order to improve endpoint definition.