Editors' Choice

Science  14 Jun 2013:
Vol. 340, Issue 6138, pp. 1267
  1. Astronomy

    Cosmic Correlation

    1. Maria Cruz

    The cosmic infrared background, the integrated infrared light produced by all extragalactic sources in the universe, has been found to exceed the expected emissions from known galaxies, including the most distant ones. To understand the nature of the populations responsible for this excess, Cappelluti et al. cross-correlated the fluctuations in the infrared and x-ray backgrounds. The infrared background is sensitive to stellar populations, whereas the x-ray background probes radiation from accreting black holes and thermal x-ray emission from hot ionized gas. The detected correlations indicate that at least 15 to 20% of the cosmic infrared background is produced by sources that are powerful x-ray emitters. Based on theoretical calculations, Yue et al. propose in a different study that the first cosmic black holes, which formed from direct collapse of the gas in the first galaxy halos, are responsible for the infrared background fluctuations. This hypothesis is consistent with the observed correlation between the infrared and x-ray backgrounds.

    Astrophys. J. 769, 68; (2013) Mon. Not. R. Astron. Soc. 10.1093/mnras/stt826 (2013).

  2. Physics

    Unexpected Conductivity

    1. Jelena Stajic

    Interfacing two dissimilar crystals can lead to unexpected phenomena, as best illustrated by the formation of a highly mobile metallic two-dimensional electron gas (2DEG) between two insulators, SrTiO3 (STO) and LaAlO3. The interface is usually formed using the (100) crystallographic orientation of the substrate, STO, and the conductivity of the 2DEG is explained through the polarization catastrophe model, in which the discontinuity of the polarization at the interface leads to charge transfer. Annadi et al. studied what happens when the (110)-oriented STO is used as a substrate instead. Naïvely, one may expect that in this case the interface would always be insulating, because there is no polarization discontinuity; however, the researchers found that the transport properties of such interfaces are similar to those formed on (100) surfaces. One exception to that similarity was the appearance of anisotropic conductivity, attributed to the differing properties of the Ti-O-Ti chains along the two in-plane directions. First-principles calculations suggested that the unexpected conductivity could be explained through the model of an energetically stable buckled interface, which, contrary to expectations, did induce a polarization discontinuity.


    Nat. Comm. 4, 1838 (2013).

  3. Virology

    A Careful Escape

    1. Pamela J. Hines

    Viruses gain entry to host cells through binding to specific receptors, which subsequently mutate to block virus binding. This can be problematic, however, because many viral receptors perform essential functions for their hosts. One such example is transferrin receptor 1, which regulates iron uptake by host cells. How are such mutations selected so as to not disrupt the essential function of the receptor? Demogines et al. now show how a small number of mutations in transferrin receptor 1 can alter viral host specificity. Analysis of the ratio of synonymous to nonsynonymous DNA mutations in the rodent transferrin receptor identified a handful of residues that were under positive selection. These residues corresponded to the region of the receptor that interacts with the virus rather than those residues necessary for iron uptake. A search for human single-nucleotide polymorphisms identified transferrin receptor variants that reduced viral uptake in cultured human cells but maintained iron regulation. These results demonstrate that in the constant arms race between viruses and the cells they infect, positive selection of residues involved in viral entry can be divorced from regions of the receptor that are essential for host function.

    PLoS Biol. 11, e1001571 (2013).

  4. Biochemistry

    Handily Handling Nitrite

    1. Gilbert Chin

    Nitrogen is, of course, an essential element for life; fortunately, humans have no need to worry about its various oxidation states because we acquire it in readily usable forms (such as amino acids) in our diet. Microbes do the hard work of reducing dinitrogen and handling ammonium, nitrite, and nitrate ions. Crystal structures of some of the key enzymes and transporters have provided insights into the biochemistry of these small molecules, and Zheng et al. offer the latest step forward by presenting the 2.6 Å structure of Escherichia coli nitrate/nitrite transport protein NarK. They find the familiar 12-transmembrane helices of a major facilitator superfamily transporter, and by soaking in nitrite, they established the location of the active site, which almost certainly is also where nitrate binds. The anionic substrates are locked into place by two opposing arginine residues, and phenylalanines constitute the floor and ceiling of this cage. How reciprocal conformational changes are triggered by the binding of cytoplasmic NO2 versus the binding of extracellular NO3 is as yet unclear, but these two anions differ in their shape (vee versus trigonal) and their distribution of partial charge, so there are lots of ways for NarK to do what proteins do best.

    Nature 497, 647 (2013).

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