This Week in Science

Science  17 Nov 2017:
Vol. 358, Issue 6365, pp. 882
  1. Plant Science

    How the snapdragon chooses its color

    1. Pamela J. Hines

    A bee entering a magenta snapdragon, guided by a yellow highlight

    CREDIT: ANNABEL WHIBLEY

    In some snapdragons, a yellow spot in a field of magenta shows the bee the best place to go. Flowers of a related subspecies are mainly yellow with magenta veins marking the target. Bradley et al. analyzed a locus that regulates the pattern of color. The locus contains an inverted gene duplication that encodes small RNAs that repress pigment biosynthesis. Analysis of flowers derived from a region of the Pyrenees where the subspecies coexist indicates that natural selection is operating upon the locus.

    Science, this issue p. 925

  2. Transformation Optics

    How to hide a dimension from view

    1. Ian S. Osborne

    Compacted dimensions are essential ingredients of advanced string theories; the extra dimensions used to describe the world are hidden from view, or compacted. Pendry et al. used concepts of transformation optics, in which metamaterials are designed to produce specific optical properties, to show theoretically that compacted dimensions could be possible in plasmonic structures. For example, simulations with a metallic grating and periodically doped graphene revealed that two-dimensional structures exhibit optical properties of a bulk material. To all intents and purposes, it looks as if one of the dimensions is compacted. The results could have practical application in designing broadband optical absorbers.

    Science, this issue p. 915

  3. Particle Astrophysics

    Exotic origin for cosmic positrons

    1. Keith T. Smith

    Several cosmic-ray detectors have found more positrons arriving at Earth than expected. Some researchers interpret this as a signature of exotic physics, such as the annihilation of dark matter particles. Others prefer a more mundane explanation that involves positron generation at pulsars followed by diffusion to Earth. Abeysekara et al. detected extended emission of gamma rays around two nearby pulsars, generated by high-energy electrons and positrons. The size of the extended emission was used to calculate how far positrons generated by the pulsars diffuse through space—which turns out to be insufficient to reach Earth. The excess positrons detected on Earth must therefore have a more exotic origin than nearby pulsars.

    Science, this issue p. 911

  4. Pigeon Genomics

    Genetics of the passenger pigeon

    1. Laura M. Zahn

    The now-extinct passenger pigeon used to be one of the most numerous vertebrates on Earth. Murray et al. examined the genomes of four passenger pigeon samples from different locales within its range. They describe the interplay between passenger pigeon population size, genome structure and recombination, and natural selection. They conclude that a reduction in genetic diversity provided few avenues for the bird to respond to human pressures, which ultimately drove it to extinction.

    Science, this issue p. 951

  5. Transcription

    Structural basis for transcription activation

    1. Steve Mao

    Bacteria can initiate transcription through two independent classes of recruitment mechanisms. Liu et al. determined the cryo–electron microscopy structure of an intact class I transcription activation complex. The positions and orientations of all the components and the detailed protein-protein and protein–nucleic acid interactions reveal how an activator interacts with the promoter DNA and recruits RNA polymerase through the class I mechanism. Together with a recently reported class II transcription activation complex, the findings complete our structural understanding of bacterial transcription activation.

    Science, this issue p. 947

  6. Catalysis

    Hydrogen from methane in molten metal

    1. Phil Szuromi

    The hydrogen used in making ammonia and other industrial reactions is produced mainly through steam reformation of methane over nickel catalysts. This high-temperature process also releases carbon dioxide, a greenhouse gas. Upham et al. used nickel dissolved in molten bismuth to pyrolyze methane to release hydrogen and form carbon, which floats to the surface of the melt, where it can be removed. Carbon formation on steam-reforming catalysts is usually a deactivating side reaction, but in the new process, the carbon can be stored or incorporated into composite materials.

    Science, this issue p. 917

  7. Tumor Immunology

    Lighting up antitumor responses

    1. Christiana N. Fogg

    A melanin-loaded transdermal microneedle patch

    PHOTO: Y. YE/JOINT DEPARTMENT OF BIOMEDICAL ENGINEERING, UNC–NC STATE

    Dendritic cell (DC)–based and whole-tumor antigen vaccines have shown promise as cancer therapies but have been constrained by suboptimal antigen presentation and T cell activation. Ye et al. treated mice transdermally with a microneedle patch loaded with whole-tumor lysate from B16F10 melanoma combined with melanin and exposed the patch to near-infrared light irradiation. This approach promoted heat generation by melanin and enhanced antigen uptake by DCs. The light-triggered heat response enhanced T cell migration, localized proinflammatory cytokine production, and was effective at targeting both primary and distal secondary tumors.

    Sci. Immunol. 2, eaan5692 (2017).

  8. Climatology

    Predicting local sea level rise

    1. Kip Hodges

    The melting of ice sheets on land, particularly in the Arctic and polar regions, contributes substantially to sea level rise. What has been less well understood is how the localization of melting may relate to the rate of sea level rise in specific places. Larour et al. developed a way to make such predictions based on geographical mapping of the drainage basins of continental ice sheets and glaciers. Their work aids in the identification of especially sensitive regions of ice that, if melted, would inundate major port cities and more generally informs better coastal planning.

    Sci. Adv. 10.1126/sciadv.1700537 (2017).

  9. Structural Biology

    How protons power rotation

    1. Valda Vinson

    Synthesis of adenosine triphosphate (ATP) in mitochondria is accomplished by a large molecular machine, the F1FO ATP synthase. Proton translocation across the FO region that spans the mitochondrial inner membrane drives ATP synthesis in the F1 region through a rotational mechanism. Guo et al. present a high-resolution structure of the dimeric FO complex from Saccharomyces cerevisiae, determined by electron microscopy. The structure gives insights into how proton translocation powers rotation and suggests how FO dimers bend the membrane to give mitochondria their characteristic cristae.

    Science, this issue p. 936

  10. Materials Science

    Peptide-based semiconductors

    1. Marc S. Lavine

    For semiconductors, one often thinks of inorganic materials, such as doped silicon, or aromatic organic polymers and small molecules. Tao et al. review progress in making semiconductors based on self-assembling short peptides. The structures that form show extensive π and hydrogen bonding leading to a range of semiconductor properties, which can be tuned through doping or functionalization of the peptide sequences. These materials may shed light on biological semiconductors or provide an alternative for constructing biocompatible and therapeutic materials.

    Science, this issue p. eaam9756

  11. Network Science

    The domino effect in power failure

    1. Barbara R. Jasny

    Sometimes a power failure can be fairly local, but other times, a seemingly identical initial failure can cascade to cause a massive and costly breakdown in the system. Yang et al. built a model for the North American power grid network based on samples of data covering the years 2008 to 2013 (see the Perspective by D'Souza). Although the observed cascades were widespread, a small fraction of all network components, particularly the ones that were most cohesive within the network, were vulnerable to cascading failures. Larger cascades were associated with concurrent triggering events that were geographically closer to each other and closer to the set of vulnerable components.

    Science, this issue p. eaan3184; see also p. 860

  12. Planar Optics

    From spins to spirals

    1. Ian S. Osborne

    The polarization state of light can be used in imaging applications and optical communications. Light can also be structured into vortices that carry optical angular momentum, which can be used for micromanipulation and enhancing the capacity of optical communication channels. Devlin et al. present a metasurface converter for optical states that transforms polarization states into optical angular momentum states. The coupling between arbitrary spin and optical angular momentum states of light in a compact planar structure may find applications in producing complex structured light fields.

    Science, this issue p. 896

  13. Human Evolution

    African genomics and skin color

    1. Laura M. Zahn

    Skin color varies among human populations and is thought to be under selection, with light skin maximizing vitamin D production at higher latitudes and dark skin providing UV protection in equatorial zones. To identify the genes that give rise to the palette of human skin tones, Crawford et al. applied genome-wide analyses across diverse African populations (see the Perspective by Tang and Barsh). Genetic variants were identified with likely function in skin phenotypes. Comparison to model organisms verified a conserved function of MFSD12 in pigmentation. A global genetic panel was used to trace how alleles associated with skin color likely moved across the globe as humans migrated, both within and out of Africa.

    Science, this issue p. eaan8433; see also p. 867

  14. Solid-State Physics

    Delving deep into electronic properties

    1. Jelena Stajic

    The spectral function of a material, which reflects the distribution of its electronic states as a function of momentum and energy, carries a wealth of information on its properties. However, measuring the spectral function directly is tricky, particularly in systems inaccessible to surface probes or in insulators. Jang et al. introduce a method dubbed momentum- and energy-resolved tunneling spectroscopy, in which electrons tunnel from a probe layer to unoccupied states in a target layer deep in a heterostructure. Because the momentum and energy of the electrons are tightly controlled, the measured tunneling probability is proportional to the spectral function of the target system.

    Science, this issue p. 901

  15. Immunology

    Another spin at the wheel

    1. Seth Thomas Scanlon

    The NLR (nucleotide-binding domain leucine-rich repeat) proteins are a key intracellular component of the early innate immune response to pathogens. After binding microbial ligands, assorted NLR family members assemble to form enormous signaling complexes (inflammasomes), which promote proinflammatory cytokine secretion and cell death. Tenthorey et al. used cryo–electron microscopy to visualize an assembled ligand-bound inflammasome. They find that when NAIP5 binds flagellin, it changes conformation, which triggers a rotation in monomeric NLRC4, catalyzing further NLRC4 recruitment. Steric clash results in a partially open structure, in contrast with previous descriptions of a closed symmetrical “wheel.” Furthermore, NAIP5 recognizes multiple regions of its ligand, and mutations of flagellin that allow for NAIP5 evasion compromise bacterial fitness.

    Science, this issue p. 888

  16. Chemical Physics

    Tracking a trio of rubidium atoms

    1. Jake Yeston

    Crossed molecular beams have provided decades' worth of knowledge into how quantum mechanics governs chemical reactivity. Nonetheless, the technique is generally limited to the collision of two partners. Wolf et al. report on a three-body process with full quantum state resolution. By cooling rubidium atoms to ultralow temperatures in an optical trap, they were able to observe dimer formation, stabilized by collision with a third atom, and extract the precise dependence of product states on the initial states of the atoms involved.

    Science, this issue p. 921

  17. Attosecond Optics

    Resetting the clock on photoemission

    1. Ian S. Osborne

    The ability to produce attosecond pulses of light provides access to some of the fastest electronic processes occurring within atoms. Tracking the temporal dynamics of the photoemission process in which an atom absorbs a high-energy photon and the electron escapes has exposed a discrepancy between the initial experimental findings and subsequent theoretical modeling. Isinger et al. present an ultrafast process that can account for and distinguish the different contributions to the photoemission processes in neon atoms. The findings reveal an “electron shake-up” process that may explain the discrepancy, bringing closure to a 7-year discussion.

    Science, this issue p. 893

  18. Graphene

    Pairing up electrons and holes in bilayer graphene

    1. Jelena Stajic

    Excitons—bound pairs of electron and holes in solids—can be harnessed for optoelectronic applications. Being able to tune the exciton energy would bring functional flexibility to such devices. Although tunable excitons have been predicted to form in bilayer graphene, observing them experimentally has been difficult. Ju et al. used high-quality bilayer graphene samples sandwiched between layers of hexagonal boron nitride to observe excitons in this material. Exciton energy was tuned across a large range by controlling the gate voltages.

    Science, this issue p. 907

  19. Virology

    Too much or too little—better than some

    1. Caroline Ash

    Dengue fever is caused by a mosquito-transmitted flavivirus resembling Zika virus. Both viruses can cause severe diseases in humans with catastrophic sequelae. It has been suspected in humans, and shown in animal models, that the host's immune responses can make disease worse. Katzelnick et al. examined data from a long-term study of Nicaraguan children exposed to dengue virus (see the Perspective by Feinberg and Ahmed). They confirmed that antibody-dependent enhancement of disease occurs at a specific range of antibody concentrations. Low levels of antibody did not enhance disease, intermediate levels exacerbated disease, and high antibody titers protected against severe disease. These findings have major implications for vaccines against flaviviruses. Indeed, recent vaccine trials have shown evidence of severe disease in some recipients who were previously exposed to virus.

    Science, this issue p. 929; see also p. 865

  20. Metabolism

    Cancer cells put ammonia back to work

    1. L. Bryan Ray

    Ammonia, often considered a metabolic waste product, can be recycled to build new amino acids. Rapidly proliferating cells produce extracellular nitrogen. Spinelli et al. used metabolic tracing of 15N to follow the fate of extracellular ammonia and its incorporation into more than 200 components of the nitrogen metabolome (see the Perspective by Dang). Accumulation of ammonia enabled glutamate dehydrogenase to function in reductive amination, which allowed incorporation of nitrogen from ammonia back into amino acids. Experiments in mice also showed incorporation of ammonia into glutamate, aspartate, and proline.

    Science, this issue p. 941; see also p. 862

  21. Zika

    Mutation for microcephaly

    1. Caroline Ash

    Zika virus infections in humans have been known since 1947. Microcephaly and neuropathologies associated with Zika have only been reported recently, most prevalently in the Americas. Yuan et al. investigated recent stable mutations in the virus genome and engineered them into a low-virulence ancestral strain (see the Perspective by Screaton and Mongkolsapaya). A single amino acid substitution (serine to asparagine, S139N) in the viral precursor membrane protein exacerbated symptoms in pregnant mice. The reverse mutation (N139S) was less virulent. The S139N mutation arose in 2013 in French Polynesia before the virus jumped to Brazil in 2015. In vitro, this amino acid change made the virus more infectious for mouse and human neural progenitor cells and promoted apoptosis. The terrible sequelae of infection during pregnancy could thus be the result of a simple viral mutation.

    Science, this issue p. 933; see also p. 863

  22. Leukemia

    New target, better leukemia mouse model

    1. Leslie K. Ferrarelli

    T cell acute lymphoblastic leukemias (T-ALLs) are often caused by mutations in the gene encoding Notch1, which mediates cell-cell contact signaling. However, mice expressing these mutants frequently fail to develop T-ALL. Pajcini et al. found that the transcription factor MAFB increased the expression of Notch1 target genes in mouse and human T-ALL cells. Expressing MAFB enhanced the development of Notch1-mutant T-ALL in mice. Because Notch1 helps to maintain various adult tissues, developing a way to inhibit MAFB may provide a more targeted therapy for leukemia patients.

    Sci. Signal. 10, eaam6846 (2017).

  23. Polymer Chemistry

    Routes to greener plastics

    1. Julia Fahrenkamp-Uppenbrink

    Plastics are mostly produced from fossil fuel resources and are often discarded after short-term use (see the Editorial by MacArthur). In a Perspective, Hillmyer discusses two routes toward more sustainable plastics: (i) production of the monomer building blocks of commodity polymers from plant resources and (ii) creation of new plant-based polymers that can compete with existing fossil fuel–based products. In a related Perspective, Garcia and Robertson look at why only a small percentage of mixed plastic waste is recycled. They highlight three main steps to more recycling: advances in mixed-plastics recycling, reducing the energy requirements for chemical recycling methods, and exploding the range of plastics that can be recycled. In a third Perspective, Albertsson and Hakkarainen look at the possible roles of degradable polymers in reducing plastic waste.

    Science, this issue p. 868, p. 870, p. 872; see also p. 843

  24. Transplantation

    MSC sacrifice for immunosuppression

    1. Lindsey Pujanandez

    Transfer of mesenchymal stromal cells (MSCs) induces immunosuppression, even though the cells are undetectable shortly after transfer. The immunosuppressive mechanism of MSCs has been something of a mystery. Galleu et al. now suggest that MSC apoptosis is crucial. In a mouse model of graft versus host disease, cytotoxic cells rendered the MSCs apoptotic shortly after transfer. Moreover, cells from patients that responded to MSC therapy had more cytotoxic activity against MSCs. These findings suggest that patients could be screened for likely responsiveness to MSC therapy before transfer.

    Sci. Transl. Med. 9, eaam7828 (2017).