This Week in Science

Science  19 Jun 2009:
Vol. 324, Issue 5934, pp. 1489
  1. Extinction Distinction


    The Triassic-Jurassic extinction approximately 200 million years ago is one of the five major extinctions in Earth's history. It has been primarily recognized through the loss of marine species, as well as the subsequent emergence of dinosaurs, but its pace, both on land and in sea, has been unclear. McElwain et al. (p. 1554) now provide evidence from the plant fossil record from rocks in East Greenland. The total number of taxa and the number of common taxa decreased across the extinction boundary. The decrease was fairly abrupt and seemed to coincide with a period with increased atmospheric CO2 levels.

  2. Sensorin Reporter

    Long-term memory and synaptic plasticity require changes in gene expression and yet can occur in a synapse-specific manner. Messenger RNA (mRNA) localization and regulated translation at synapses have been proposed as mechanisms for spatially restricting gene expression during transcription-dependent, synapse-specific forms of neuronal plasticity. In the sea slug Aplysia, which is a frequently used model system for studying learning and memory, sensorin is a sensory cell–specific peptide neurotransmitter. The mRNA encoding sensorin localizes to distal sensory neurites and further concentrates at sites of synaptic contact between sensory and motor neurons. Wang et al. (p. 1536, published online 14 May; see the Perspective by Korte) demonstrate, using translational reporters of sensorin mRNA expressed in individual cultured Aplysia sensory and motor neurons, that local translation occurs at synapses during transcription-dependent, learning-related forms of synaptic plasticity.

  3. Star Gazing

    Seismology of stars provides a unique insight into physical mechanisms taking place in their interior. Like the Sun, some stars have low-amplitude pulsations that are excited by turbulent convection in their outer layers. Belkacem et al. (p. 1540), using data gathered by the CoRoT satellite, report on low-amplitude, solar-like oscillations in a massive star that undergoes radial pulsations as a result of expanding and contracting layers in its interior. The finding opens the possibility of probing the interiors of this type of massive star, which can be the progenitors of supernovae.

  4. Minimally Acidic

    Acidity is usually construed in the context of a bulk liquid solvent: billions of trillions of molecules such as HCl, added to hundreds of billions of trillions of water molecules. What happens under sparser conditions, for example, in atmospheric or interstellar environments, when a single HCl molecule might interact with just three or four water molecules? Gutberlet et al. (p. 1545; see the Perspective by Zwier) explored this question using theoretical simulations together with vibrational spectroscopy in ultracold helium droplets that effectively isolated small aqueous HCl clusters. HCl remained intact upon solvation by one, two, or three water molecules. Dissociation into an ion pair, as occurs in bulk water, required the approach of a fourth water molecule and was facilitated by the geometry of the existing (H2O)3 cluster.

  5. Reaping Gain from Decay

    In photovoltaic devices, absorbed light excites electrons into a conduction band and thereby initiates electric current flow. Unfortunately, if the energy of the incident photons exceeds the threshold for this excitation (the bandgap), the excess tends to be wasted. Initially, a photon bearing several multiples of the bandgap energy may correspondingly promote several electrons, but before these can begin to travel through a circuit, most of them drop back down to the immobile state, transferring their packet of energy to a lone remaining carrier in a process termed “Auger decay.” Sukhovatkin et al. (p. 1542) show that a photoconductive device design can actually leverage the Auger decay process to improve sensitivity in ultraviolet detection. Their detector, a thin film assembled from lead sulfide quantum dots, improves its response by up to a factor of four when the incident light frequency rises to several multiples of the bandgap.

  6. Expanding Flatland


    Since its discovery and isolation some 5 years ago, research on graphene has exploded. The sheets of carbon atoms, just one atom thick, exhibit a vast array of properties—mechanical, optical, and electrical—that make it an ideal test bed to probe fundamental problems in physics, as well as lending itself to a wide portfolio of applications. Geim (p. 1530), the discoverer of this remarkable material, reviews recent advances and discusses what other developments may be in store for this material.

  7. A Change in the Air?

    Between around 1.2 million and 500,000 years ago, Earth's glacial cycle changed from one with a period of roughly 40,000 years to one with a period of about 100,000 years. Although there has been much speculation about why this transition may have occurred, no potential explanation seemed more likely than that it was caused by decreasing concentrations of atmospheric CO2. Hönisch et al. (p. 1551) present a record of atmospheric pCO2 for the past 2.1 million years, derived from the boron isotopic composition of planktonic foraminifera, and show that the amount of CO2 in the atmosphere has remained relatively constant over that period. While pCO2 was approximately 30 ppm higher before the start of the mid-Pleistocene transition than after the transition, atmospheric CO2 did not decrease gradually as would be expected were it to be the driver of the transition.

  8. Antiporter Antics


    Bacteria that survive in the acidic environment of the stomach have mechanisms to maintain a high intracellular pH. In Escherichia coli, glutamate (Glu) and arginine (Arg) are decarboxylated intracellularly and the reaction products are exchanged with extracellular Glu and Arg. Gao et al. (p. 1565, published online 28 May) now report a crystal structure of AdiC, an arginine:agmatine antiporter from E. coli. AdiC exhibits the same fold as that of the Na+-coupled symporters, including LeuT. It contains 12 transmembrane segments, forms a homodimer, and exists in an outward-facing, open conformation in the crystals. The structure, together with biochemical data, suggests how the antiporter senses the pH and responds to transport the reaction product agmatine out of the cell and Arg into the cell.

  9. Controlling Chronic Viral Infections

    Chronic viral infections such as HIV and hepatitis B and C viruses are major public health concerns. T cell—mediated immune responses are critical for controlling viral infections. In contrast to acute infections, chronic viral infections are characterized by “exhausted” cytotoxic CD8+ T cells, cells which exhibit reduced proliferative capacity, cytokine secretion, and cytotoxicity. Treatments that reverse exhaustion result in increased viral control. Despite their exhaustion, these CD8+ T cells eventually help to control chronic infections by killing virally infected cells, and require CD4+ T cell help to do so. How do CD4+ T cells provide help to CD8+ T cells during chronic infection (see the Perspective by Johnson and Jameson)? Elsaesser et al. (p. 1569, published online 7 May), Yi et al. (p. 1572, published online 14 May), and Fröhlich et al. (p. 1576, published online 28 May) now show that the cytokine, interleukin-21 (IL-21), known to be critical for the differentiation of certain CD4+ T cell effector subsets, is an essential factor produced by CD4+ T cells that helps CD8+ T cells to control chronic lymphocytic choriomeningitis virus infection in mice. Acute and chronic infections resulted in differing amounts of IL-21 production by virus-specific CD4+ T cells. CD8+ T cells required IL-21 directly, and when CD8+ T cells were unable to signal through IL-21 or IL-21 was not available, they were reduced in number, exhibited a more exhausted phenotype, and were not able to control the virus. In contrast, the absence of IL-21–dependent signaling did not affect primary CD8+ T cell responses to acute infection or responses to a viral rechallenge, suggesting that differentiation of memory CD8+ T cells is independent of IL-21.

  10. Mysterious Merkel cells

    Anatomists have known about the existence of Merkel cells in our skin for over a century. However, the function of these cells has been unclear and controversial. To solve this mystery, Maricich et al. (p. 1580) created a genetic deletion of Merkel cells. When Atoh1, a transcription factor expressed by Merkel cells, was conditionally deleted in the skin, Merkel cells were completely absent from Atoh1CKO mice. Using ex vivo skin and nerve preparations from the animals showed that Merkel cells are needed to properly encode light touch sensation in the skin.

  11. Addition to the Right, Subtraction to the Left

    High-level cognitive achievements, such as writing and mathematics, have appeared relatively recently in the evolutionary record in comparison to low-level skills, such as the perception of bright-dark boundaries. The latter have been demonstrated to arise from the coding properties of neurons in visual cortical centers, but what do the former map onto? Knops et al. (p. 1583, published online 7 May) provide evidence that addition and subtraction are encoded within the same cortical region that is responsible for eye movements to the right and left, such that the neural activity associated with addition could be distinguished from that associated with subtraction by a computational classifier trained to discriminate between rightward and leftward eye movements.

  12. 2 Be or Not 2 Be?

    The beryllium dimer is a rather simple-looking compound, bridging two atoms of the fourth-lightest element. Yet it has long posed a frustrating challenge to theorists, because, to a first approximation, chemical bonding models suggest that it should not exist. In recent decades, calculations of ever-increasing sophistication have offered rationales for the two atoms' puzzling mutual attraction. Now Merritt et al. (p. 1548, published online 21 May; see the Perspective by Bernath) have used high-resolution spectroscopy to assemble, at last, a full experimental potential energy function describing the ground electronic state of Be2. The curve is unusually shallow at long bond lengths, deviating from the contour of more traditional molecular Morse potentials, and provides a detailed basis of comparison for future theoretical bonding frameworks.

  13. Rapid Tree Building

    Phylogenetic reconstruction is used to determine the relationships between organisms and requires an accurate alignment and analysis of multiple sequences. Iterative rounds of alignment and tree building are often necessary to prevent errors in the phylogeny estimate. One such way to address this problem is to assess alignment and trees in a single step. However, efficient algorithms to analyze data sets of reasonable size have been lacking. Liu et al. (p. 1561; see the Perspective by Löytynoja and Goldman) describe an iterative approach that simultaneously incorporates both alignment and phylogeny and applies a fast maximum likelihood algorithm to the tree-building part. By assembling the components of the methods in this way, accurate results were obtained for up to 1000 sequences. Thus, it is possible to produce coestimation of sequence alignment and phylogeny that is both rapid and accurate.

  14. Swine Flu Benchmark

    The World Health Organization (WHO) announced on 29 April 2009, a level-5 pandemic alert for a strain of H1N1 influenza originating in pigs in Mexico and transmitting from human to human in several countries. Fraser et al. (p. 1557, published online 11 May; see the cover) amassed a team of experts in Mexico and WHO to make an initial assessment of the outbreak with a view to guiding future policy. The outbreak appears to have originated in mid-February in the village of La Gloria, Veracruz, where over half the population suffered acute respiratory illness, affecting more than 61% of children under 15 years old in the community. The basic reproduction number (the number of people infected per patient) is in the range of 1.5—similar or less than that of the pandemics of 1918, 1957, and 1968. There remain significant uncertainties about the severity of this outbreak, which makes it difficult to compare the economic and societal costs of intervention with lives saved and the risks of generating antiviral resistance.