Editors' Choice

Science  15 Mar 2019:
Vol. 363, Issue 6432, pp. 1188
  1. Pathogen Metabolism

    Equal opportunity nitrogen sourcing

    1. Michael A. Funk

    Color-enhanced scanning electron micrograph of Mycobacterium tuberculosis, which multiplies slowly and evades immune detection.


    Mycobacterium tuberculosis (Mtb) has an unusual growth profile and metabolism, requiring around 20 hours to double in rich culture medium. A better understanding of Mtb metabolic requirements, particularly fundamental transformations such as nitrogen assimilation, may reveal weaknesses that can be exploited in drug design or other therapies. Agapova et al. cultured Mtb with each of the 20 proteinogenic amino acids, looking for differences in how nitrogen was incorporated into metabolites in the growing cells. Intracellular pools of amino acids varied in every case, suggesting a complex and flexible metabolism in which nitrogen is ideally obtained from amino acids rather than free ammonium. The enzyme alanine dehydrogenase exclusively liberated ammonium from alanine, suggesting a possible role for this enzyme and amino acid in nitrogen storage in Mtb.

    eLife 8, e41129 (2019).

  2. Paleoecology

    Disturbance and diversity through deep time

    1. Andrew M. Sugden

    Sedimentary pollen records are reliable indicators of patterns of plant diversity through time. Kuneš et al. show that they can also be used to track patterns of habitat disturbance. Known niche preferences of plant species can often be associated with different disturbance regimes. Pollen records from central Europe throughout the 12,000 years of the Holocene show a consistent increase in species diversity over time in this region. The greatest levels of disturbance occurred during the earliest and most recent millennia, and the least in between. Early Holocene high disturbance and low diversity were associated with rapid postglacial climate change, whereas more recent disturbance and greater diversity reflected the increasing habitat diversity and patchiness caused by the spread of human influence.

    J. Ecol. 10.1111/1365-2745.13136 (2019).

  3. Circadian Rhythm

    Circadian muscle mastery

    1. Beverly A. Purnell

    Circadian rhythms exist in almost all mammalian cells. The molecular clock is composed of transcriptional and translational components that show oscillatory behavior and are entrained by external cues, such as mealtimes, and thus keep time and regulate gene expression downstream. Core molecular-clock components can also be expressed in a tissue-specific way. For instance, the transcription factor MYOD1 serves as the muscle master gene regulator. It mediates gene-expression responses to external cues, such as nutrient surges after mealtimes. Hodge et al. observed that MYOD1 can also bind to an enhancer within the clock gene Bmal1 to regulate its expression. In turn, BMAL1 protein interacts with molecularclock components to increase muscle-specific gene expression in a daily cyclical manner. Hence, MYOD1 also acts as a “clock amplifier” in muscle to increase gene expression at particular times during the diurnal cycle.

    eLife 8, e43017 (2019).

  4. Human Impact

    Hunting and population decline

    1. Sacha Vignieri

    More than a quarter of the population decline seen among larger species of animals is caused by hunting.


    Human activities are driving population declines across species, but identifying actual sources of individual mortality from our activities is ch allenging. Hill et al. conducted a large meta-analysis based on radio-telemetry data for more than 120,000 individuals across 305 vertebrate species. They found that 28% of individual mortalities were caused directly by hunting, with larger species of birds and mammals most affected. Although most of the mortality was due to natural processes, that humans directly cause more than a quarter of the mortality for animals in the wild emphasizes the degree to which we affect natural systems.

    Glob. Ecol. Biogeogr. 10.1111/geb.12881 (2019).

  5. Cancer

    Modeling a pediatric brain tumor

    1. Paula A. Kiberstis

    Childhood cancers often harbor somatic mutations in genes encoding epigenetic regulators such as histones. For example, about 80% of diffuse intrinsic pontine gliomas (DIPGs), which are aggressive pediatric brainstem tumors, have a specific mutation (lysine-27 substituted with methionine, or K27M) in histone H3.3. This mutation impairs the repressive effect of the histone on transcription, but how it contributes to tumor development has been unclear. By analyzing mouse models, Larson et al. found that the H3.3 K27M mutation transiently stimulated self-renewal of neural stem cells on its own. When this mutation was combined with two other cancer genes, mice developed brainstem gliomas resembling human DIPG. These tumors showed increased expression of genes associated with neural development, supporting the hypothesis that pediatric gliomas are developmental disorders.

    Cancer Cell 35, 140 (2019).

  6. Hydrogen Bonding

    Right in the middle

    1. Jake Yeston

    Hydrogen bonding is a deceptively subtle concept. Typically, there is a donor with a strong covalent bond to hydrogen and an acceptor that attracts the hydrogen much more weakly. But what if the donor and acceptor are equivalent? Is equal sharing possible? Perrin and Wu explored this question in the intramolecular hydrogen bonds of keto-enol tautomers by nuclear magnetic resonance spectroscopy of oxygen-18 isotopologues. Whereas hydrogen typically seems to oscillate in its affinity for competing sites, nitromalonamide trapped it right in between both oxygens.

    J. Am. Chem. Soc. 141, 4103 (2019).

  7. Space Physics

    Earth's extensive hydrogen corona

    1. Keith T. Smith

    Dissociation of water in Earth's atmosphere produces hydrogen atoms, which escape into space through a combination of thermal and nonthermal processes. These hydrogen atoms resonantly scatter light from the Sun, producing an ultraviolet glow around Earth called the geocorona. Baliukin et al. analyzed ultraviolet observations taken by the Solar and Heliospheric Observatory spacecraft. They found that the geocoronal emission extends to at least 100 Earth radii, almost twice the distance to the Moon. This greater-than-expected extent of the geocorona indicates that nonthermal processes are launching hydrogen atoms from Earth's upper atmosphere at high speeds.

    J. Geophys. Res. Space Phys. 10.1029/2018JA026136 (2019).