Editors' Choice

Science  17 Jul 2015:
Vol. 349, Issue 6245, pp. 280
  1. Genome Defense

    Hunter RNAs seek and destroy parasitic DNAs

    1. Guy Riddihough

    Tetrahymena uses small RNAs to fend off transposable elements

    PHOTO: AARON J. BELL/SCIENCE SOURCE

    Transposable elements (TEs) are parasitic DNA sequences. They present a serious threat to the host genomes in which they reside. Like other eukaryotes, the single-celled protist Tetrahymena uses small RNAs to control parasitic TEs. Noto et al. show that a subset of these small RNAs recognize the TEs they are synthesized from as foreign, as well as sequence-related TEs in other places in the genome. These RNAs stimulate the production of a second wave of small RNAs at these distant sites. The second-wave, or “late,” RNAs can bind to still further TE sequences, ensuring a robust and heritable defense of the entire genome.

    Mol. Cell 59, 10.1016/j.molcel.2015.05.024 (2015).

  2. Astrophysics

    X-ray echoes used as a cosmic yardstick

    1. KTS

    Determining accurate distances to astronomical x-ray sources is notoriously difficult, but Heinz et al. have developed a new technique for doing so. They exploited “light echoes” from the object Cir X-1, in which x-rays emitted during a large flare bounced off foreground dust and appeared months later as delayed rings of emission around the source. By comparing the x-ray rings to radio data, they were able to identify the foreground clouds of dust and thereby measure the distance to Cir X-1 with an accuracy of 10%. Previous estimates had varied by almost a factor of 3.

    Astrophys. J. 806, 265 (2015).

  3. Physics

    A gap in a topological surface state

    1. Jelena Stajic

    Topological insulators (TIs) have, in theory, conducting surfaces and insulating bulks. In practice, TIs often have a considerable bulk conductance that masks the conduction from the topologically protected surface states. To make the surface state nonconducting—in other words, to open a gap in its dispersion—researchers usually resort to magnetic doping. Now, Weber et al. demonstrate that a topological surface state of the compound Bi4Se3 is naturally, although partially, gapped. To show this, the authors use a combination of photoemission measurements and calculations. The results may be more general within the subclass of TIs to which Bi4Se3 belongs.

    Phys. Rev. Lett. 10.1103/PhysRevLett.114.256401 (2015).

  4. Protein Folding

    Trapped on the wrong pathway

    1. Valda Vinson

    Protein folding can be described as diffusion over an energy landscape in conformational space to reach an energy minimum that represents a stable folded structure, and it is implicated in many diseases. A particularly dramatic example is the prion protein, PrP, that folds rapidly into a native monomeric structure but also has a stable oligomeric form that causes prion disease. Yu et al. used single-molecule force spectroscopy to monitor misfolding of a PrP dimer. The dimer misfolds along a single pathway involving several intermediates, one of which blocks native folding. Diffusion across the energy landscape is 1000 times slower than for the folding of native prion monomers, probably indicating that many unproductive interactions occur during misfolding.

    Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1419197112 (2015).

  5. Global Carbon Cycle

    Seeing the forest from the trees

    1. H. Jesse Smith

    Boreal forests contain nearly half of the carbon stored by the trees of the world,and thus are a critical component of the global carbon cycle. How much carbon is contained in this part of the ecosystem still is inadequately known, though. Chen and Luo studied the net annual above-ground biomass change of four major boreal forest types in western Canada over the period between 1958 and 2011 and found that it declined for all of those groups. They attribute the loss to increased tree mortality and reduced growth, caused mostly by persistent warming and decreasing water availability, trends that are expected to escalate in the future.

    Global Change Biol. 10.1111/gcb.12994 (2015).

  6. Neuroscience

    Contextual memory networks in monkey brains

    1. Peter Stern

    Memory tasks activate a number of areas in the prefrontal cortex of the brain. Lesions in this region cause memory deficits; however, lesions in only a fraction of the activated areas actually lead to severe memory loss. Osada et al. measured whole-brain activity during a memory task in monkeys. The activated areas and their task-specific functional connectivity formed a hierarchical network centered on a hub. This functional hub largely matched the documented sites where lesions had the most dramatic effect. Neighboring sites where lesions had no impact were much less affected. The functional connections of an area predicted the degree of memory loss much better than its role in the anatomical network.

    PLOS Biol. 10.1371/journal. pbio.1002177 (2015).

  7. Brain Evolution

    The genetic underpinnings of brain size

    1. Laura M. Zahn

    Angiopoietin-1 expression varies with brain size in guppies

    PHOTO: © ARCO IMAGES GMBH/ALAMY

    What genes and selective processes lead to an increase in brain size and intelligence is one of the greatest questions in human evolution. Chen et al. present findings in fish that may help us understand this process. When guppies were selected for brain size, expression differences were observed in the Angiopoietin-1 (Ang-1) gene. These differences seem to be due to noncoding variation; furthermore, lowering Ang-1 expression in the zebrafish also affected brain size. On the basis of these results, the authors suggest that further study of Ang-1 may help provide insight into the evolution of brain size and cognition in other species, including humans.

    Proc. R. Soc. London Ser. B 10.1098/rspb.2015.0872 (2015).

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