Subscribe: Science: Current Issue
http://www.sciencemag.org/rss/current.xml
Added By: Feedage Forager Feedage Grade C rated
Language: English
Tags:
assembly  bond  brain  egg shape  egg  ground state  human  microglia  mitotic chromosome  protein  quantum  shape  shell  spicules  state  time 
Rate this Feed
Rate this feedRate this feedRate this feedRate this feedRate this feed
Rate this feed 1 starRate this feed 2 starRate this feed 3 starRate this feed 4 starRate this feed 5 star

Comments (0)

Feed Details and Statistics Feed Statistics
Preview: Science: Current Issue

Science current issue



Science RSS feed -- current issue



 



Anniversaries for particle physics

2017-06-22T10:20:57-07:00




News at a glance

2017-06-22T10:20:57-07:00













East Africa turmoil imperils giraffes

2017-06-22T10:20:57-07:00













The footprints of giants

2017-06-22T10:20:57-07:00




Trial balloons

2017-06-22T10:20:57-07:00




Fast exoskeleton optimization

2017-06-22T10:20:57-07:00




Ambient quantum optomechanics

2017-06-22T10:20:57-07:00




Building chromosomes without bricks

2017-06-22T10:20:57-07:00




The most perfect thing, explained

2017-06-22T10:20:57-07:00




Locked and loaded for apoptosis

2017-06-22T10:20:57-07:00







Human nature

2017-06-22T10:20:57-07:00




Imagine this

2017-06-22T10:20:57-07:00




Benefits of trees in tropical cities

2017-06-22T10:20:57-07:00




Time to codify scientific integrity

2017-06-22T10:20:57-07:00







Understanding the formation of spicules

2017-06-22T10:20:57-07:00




Lnc-ing fibroblasts to cardiac fibrosis

2017-06-22T10:20:57-07:00




Quantum effects in ambient conditions

2017-06-22T10:20:57-07:00




How to make a protein-based nanocontainer

2017-06-22T10:20:57-07:00




The influence of flying

2017-06-22T10:20:57-07:00




Instant tough bonding of hydrogels

2017-06-22T10:20:57-07:00




PKA-activation mechanism revised

2017-06-22T10:20:57-07:00




Optimum human input

2017-06-22T10:20:57-07:00




One person's meat is another's poison

2017-06-22T10:20:57-07:00




Broadening the immune spectrum

2017-06-22T10:20:57-07:00




Neuronal basis of lethargy in worms

2017-06-22T10:20:57-07:00




Packaging without nucleosomes

2017-06-22T10:20:57-07:00




Resonant systems with high bandwidth

2017-06-22T10:20:57-07:00




Sulfur's balancing act in cytochrome c

2017-06-22T10:20:57-07:00




Lithium gets a new ground state

2017-06-22T10:20:57-07:00




Kiss-and-run or a full commitment?

2017-06-22T10:20:57-07:00




Of mice and men's microglia

2017-06-22T10:20:57-07:00




Trifluoromethylation via broken C-F bonds

2017-06-22T10:20:57-07:00




Timing the life cycle of molecular clouds

2017-06-22T10:20:57-07:00




How much does a virus cost?

2017-06-22T10:20:57-07:00







Monitoring inflammation from within

2017-06-22T10:20:57-07:00




Small RNA retunes photosynthesis

2017-06-22T10:20:57-07:00




Nailing down the elusive statistics

2017-06-22T10:20:57-07:00




Experimentation on the Tube

2017-06-22T10:20:57-07:00




Avian egg shape: Form, function, and evolution

2017-06-22T10:20:57-07:00

Avian egg shape is generally explained as an adaptation to life history, yet we currently lack a global synthesis of how egg-shape differences arise and evolve. Here, we apply morphometric, mechanistic, and macroevolutionary analyses to the egg shapes of 1400 bird species. We characterize egg-shape diversity in terms of two biologically relevant variables, asymmetry and ellipticity, allowing us to quantify the observed morphologies in a two-dimensional morphospace. We then propose a simple mechanical model that explains the observed egg-shape diversity based on geometric and material properties of the egg membrane. Finally, using phylogenetic models, we show that egg shape correlates with flight ability on broad taxonomic scales, suggesting that adaptations for flight may have been critical drivers of egg-shape variation in birds.




Quantum and isotope effects in lithium metal

2017-06-22T10:20:57-07:00

The crystal structure of elements at zero pressure and temperature is the most fundamental information in condensed matter physics. For decades it has been believed that lithium, the simplest metallic element, has a complicated ground-state crystal structure. Using synchrotron x-ray diffraction in diamond anvil cells and multiscale simulations with density functional theory and molecular dynamics, we show that the previously accepted martensitic ground state is metastable. The actual ground state is face-centered cubic (fcc). We find that isotopes of lithium, under similar thermal paths, exhibit a considerable difference in martensitic transition temperature. Lithium exhibits nuclear quantum mechanical effects, serving as a metallic intermediate between helium, with its quantum effect–dominated structures, and the higher-mass elements. By disentangling the quantum kinetic complexities, we prove that fcc lithium is the ground state, and we synthesize it by decompression.




Breaking Lorentz reciprocity to overcome the time-bandwidth limit in physics and engineering

2017-06-22T10:20:57-07:00

A century-old tenet in physics and engineering asserts that any type of system, having bandwidth , can interact with a wave over only a constrained time period t inversely proportional to the bandwidth (t· ~ 2). This law severely limits the generic capabilities of all types of resonant and wave-guiding systems in photonics, cavity quantum electrodynamics and optomechanics, acoustics, continuum mechanics, and atomic and optical physics but is thought to be completely fundamental, arising from basic Fourier reciprocity. We propose that this "fundamental" limit can be overcome in systems where Lorentz reciprocity is broken. As a system becomes more asymmetric in its transport properties, the degree to which the limit can be surpassed becomes greater. By way of example, we theoretically demonstrate how, in an astutely designed magnetized semiconductor heterostructure, the above limit can be exceeded by orders of magnitude by using realistic material parameters. Our findings revise prevailing paradigms for linear, time-invariant resonant systems, challenging the doctrine that high-quality resonances must invariably be narrowband and providing the possibility of developing devices with unprecedentedly high time-bandwidth performance.




Quantum correlations from a room-temperature optomechanical cavity

2017-06-22T10:20:57-07:00

The act of position measurement alters the motion of an object being measured. This quantum measurement backaction is typically much smaller than the thermal motion of a room-temperature object and thus difficult to observe. By shining laser light through a nanomechanical beam, we measure the beam’s thermally driven vibrations and perturb its motion with optical force fluctuations at a level dictated by the Heisenberg measurement-disturbance uncertainty relation. We demonstrate a cross-correlation technique to distinguish optically driven motion from thermally driven motion, observing this quantum backaction signature up to room temperature. We use the scale of the quantum correlations, which is determined by fundamental constants, to gauge the size of thermal motion, demonstrating a path toward absolute thermometry with quantum mechanically calibrated ticks.




On the generation of solar spicules and Alfvenic waves

2017-06-22T10:20:57-07:00

In the lower solar atmosphere, the chromosphere is permeated by jets known as spicules, in which plasma is propelled at speeds of 50 to 150 kilometers per second into the corona. The origin of the spicules is poorly understood, although they are expected to play a role in heating the million-degree corona and are associated with Alfvénic waves that help drive the solar wind. We compare magnetohydrodynamic simulations of spicules with observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. Spicules are shown to occur when magnetic tension is amplified and transported upward through interactions between ions and neutrals or ambipolar diffusion. The tension is impulsively released to drive flows, heat plasma (through ambipolar diffusion), and generate Alfvénic waves.




A catalytic fluoride-rebound mechanism for C(sp3)-CF3 bond formation

2017-06-22T10:20:57-07:00

The biological properties of trifluoromethyl compounds have led to their ubiquity in pharmaceuticals, yet their chemical properties have made their preparation a substantial challenge, necessitating innovative chemical solutions. We report the serendipitous discovery of a borane-catalyzed formal C(sp3)-CF3 reductive elimination from Au(III) that accesses these compounds by a distinct mechanism proceeding via fluoride abstraction, migratory insertion, and C-F reductive elimination to achieve a net C-C bond construction. The parent bis(trifluoromethyl)Au(III) complexes tolerate a surprising breadth of synthetic protocols, enabling the synthesis of complex organic derivatives without cleavage of the Au-C bond. This feature, combined with the "fluoride-rebound" mechanism, was translated into a protocol for the synthesis of 18F-radiolabeled aliphatic CF3-containing compounds, enabling the preparation of potential tracers for use in positron emission tomography.




Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy

2017-06-22T10:20:57-07:00

The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe–S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe–S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.




Human-in-the-loop optimization of exoskeleton assistance during walking

2017-06-22T10:20:57-07:00

Exoskeletons and active prostheses promise to enhance human mobility, but few have succeeded. Optimizing device characteristics on the basis of measured human performance could lead to improved designs. We have developed a method for identifying the exoskeleton assistance that minimizes human energy cost during walking. Optimized torque patterns from an exoskeleton worn on one ankle reduced metabolic energy consumption by 24.2 ± 7.4% compared to no torque. The approach was effective with exoskeletons worn on one or both ankles, during a variety of walking conditions, during running, and when optimizing muscle activity. Finding a good generic assistance pattern, customizing it to individual needs, and helping users learn to take advantage of the device all contributed to improved economy. Optimization methods with these features can substantially improve performance.




Mitotic chromosome assembly despite nucleosome depletion in Xenopus egg extracts

2017-06-22T10:20:57-07:00

The nucleosome is the fundamental structural unit of eukaryotic chromatin. During mitosis, duplicated nucleosome fibers are organized into a pair of rod-shaped structures (chromatids) within a mitotic chromosome. However, it remains unclear whether nucleosome assembly is indeed an essential prerequisite for mitotic chromosome assembly. We combined mouse sperm nuclei and Xenopus cell-free egg extracts depleted of the histone chaperone Asf1 and found that chromatid-like structures could be assembled even in the near absence of nucleosomes. The resultant "nucleosome-depleted" chromatids contained discrete central axes positive for condensins, although they were more fragile than normal nucleosome-containing chromatids. Combinatorial depletion experiments underscored the central importance of condensins in mitotic chromosome assembly, which sheds light on their functional cross-talk with nucleosomes in this process.




Local protein kinase A action proceeds through intact holoenzymes

2017-06-22T10:20:57-07:00

Hormones can transmit signals through adenosine 3',5'-monophosphate (cAMP) to precise intracellular locations. The fidelity of these responses relies on the activation of localized protein kinase A (PKA) holoenzymes. Association of PKA regulatory type II (RII) subunits with A-kinase–anchoring proteins (AKAPs) confers location, and catalytic (C) subunits phosphorylate substrates. Single-particle electron microscopy demonstrated that AKAP79 constrains RII-C subassemblies within 150 to 250 angstroms of its targets. Native mass spectrometry established that these macromolecular assemblies incorporated stoichiometric amounts of cAMP. Chemical-biology– and live cell–imaging techniques revealed that catalytically active PKA holoenzymes remained intact within the cytoplasm. These findings indicate that the parameters of anchored PKA holoenzyme action are much more restricted than originally anticipated.




Assembly principles and structure of a 6.5-MDa bacterial microcompartment shell

2017-06-22T10:20:57-07:00

Many bacteria contain primitive organelles composed entirely of protein. These bacterial microcompartments share a common architecture of an enzymatic core encapsulated in a selectively permeable protein shell; prominent examples include the carboxysome for CO2 fixation and catabolic microcompartments found in many pathogenic microbes. The shell sequesters enzymatic reactions from the cytosol, analogous to the lipid-based membrane of eukaryotic organelles. Despite available structural information for single building blocks, the principles of shell assembly have remained elusive. We present the crystal structure of an intact shell from Haliangium ochraceum, revealing the basic principles of bacterial microcompartment shell construction. Given the conservation among shell proteins of all bacterial microcompartments, these principles apply to functionally diverse organelles and can inform the design and engineering of shells with new functionalities.




New Products

2017-06-22T10:20:57-07:00







My lessons in mentorship

2017-06-22T10:20:57-07:00




Chemical transformation of xenobiotics by the human gut microbiota

2017-06-22T10:20:57-07:00

The human gut microbiota makes key contributions to the metabolism of ingested compounds (xenobiotics), transforming hundreds of dietary components, industrial chemicals, and pharmaceuticals into metabolites with altered activities, toxicities, and lifetimes within the body. The chemistry of gut microbial xenobiotic metabolism is often distinct from that of host enzymes. Despite their important consequences for human biology, the gut microbes, genes, and enzymes involved in xenobiotic metabolism are poorly understood. Linking these microbial transformations to enzymes and elucidating their biological effects is undoubtedly challenging. However, recent studies demonstrate that integrating traditional and emerging technologies can enable progress toward this goal. Ultimately, a molecular understanding of gut microbial xenobiotic metabolism will guide personalized medicine and nutrition, inform toxicology risk assessment, and improve drug discovery and development.




An environment-dependent transcriptional network specifies human microglia identity

2017-06-22T10:20:57-07:00

Microglia play essential roles in central nervous system (CNS) homeostasis and influence diverse aspects of neuronal function. However, the transcriptional mechanisms that specify human microglia phenotypes are largely unknown. We examined the transcriptomes and epigenetic landscapes of human microglia isolated from surgically resected brain tissue ex vivo and after transition to an in vitro environment. Transfer to a tissue culture environment resulted in rapid and extensive down-regulation of microglia-specific genes that were induced in primitive mouse macrophages after migration into the fetal brain. Substantial subsets of these genes exhibited altered expression in neurodegenerative and behavioral diseases and were associated with noncoding risk variants. These findings reveal an environment-dependent transcriptional network specifying microglia-specific programs of gene expression and facilitate efforts to understand the roles of microglia in human brain diseases.




A global brain state underlies C. elegans sleep behavior

2017-06-22T10:20:57-07:00

How the brain effectively switches between and maintains global states, such as sleep and wakefulness, is not yet understood. We used brainwide functional imaging at single-cell resolution to show that during the developmental stage of lethargus, the Caenorhabditis elegans brain is predisposed to global quiescence, characterized by systemic down-regulation of neuronal activity. Only a few specific neurons are exempt from this effect. In the absence of external arousing cues, this quiescent brain state arises by the convergence of neuronal activities toward a fixed-point attractor embedded in an otherwise dynamic neural state space. We observed efficient spontaneous and sensory-evoked exits from quiescence. Our data support the hypothesis that during global states such as sleep, neuronal networks are drawn to a baseline mode and can be effectively reactivated by signaling from arousing circuits.