This Week in Science

Science  13 Jul 2012:
Vol. 337, Issue 6091, pp. 131
  1. Biofilms Up Close


    Many bacterial infections involve biofilm formation. Cells within a biofilm are significantly more resistant to immune clearance and antibiotics compared to unattached, planktonic cells. Berk et al. (p. 236) applied superresolution optical methods to image living bacteria with nanometer-scale precision as they form a biofilm. Vibrio cholerae biofilms were observed to have three distinct levels of spatial organization: cells, clusters of cells, and collections of clusters. Each cell cluster was wrapped in a flexible, elastic envelope. Several V. cholerae matrix proteins played complementary architectural roles during biofilm development. RbmA provided cell-cell adhesion, Bap1 allowed the developing biofilm to adhere to surfaces, and heterogeneous mixtures of VPS, RbmC, and Bap1 formed the dynamic, flexible, and ordered envelopes that encase the cell clusters.

  2. A Timely Structure

    The physiology and behavior of most organisms are inextricably aligned with the day/night cycle. In mammals, these daily rhythms are generated by a circadian clock encoded by transcriptional activators and repressors operating in a feedback loop that takes about 24 hours to complete. A key participant in this loop is a heterodimeric transcriptional activator consisting of the CLOCK and BMAL1 proteins. Huang et al. (p. 189, published online 31 May; see Perspective by Crane) determined the crystal structure of a complex containing the PAS domains (implicated in protein-protein interactions) and the basic helix-loop-helix domains (implicated in DNA binding) from each protein. CLOCK and BMAL1 were observed to be tightly intertwined in an unusual asymmetric conformation, which may contribute to the stability and activity of the complex.

  3. MMS19 Joins the CIA

    Iron-sulfur (Fe-S) proteins play a critical role in cell metabolism and particularly in DNA repair and replication. Mutants in eukaryotic gene MMS19 are particularly sensitive to DNA damaging agents, suggesting that it is involved in DNA repair, but the mutations can also have other wide-ranging effects on the cell (see the Perspective by Gottschling). Now, Stehling et al. (p. 195, published online 7 June) and Gari et al. (p. 243, published online 7 June) show that in both yeast and humans, MMS19 functions as part of the cytosolic Fe-S protein assembly (CIA) machinery. The MMS19 is part of a specialized CIA targeting complex that plays a role late in cytosolic Fe-S protein assembly to direct Fe-S cluster transfer from the CIA scaffold complex to a subset of Fe-S proteins, including a number associated with DNA metabolism.

  4. Influenza's Cryptic Constraint


    Because of the well-known pandemic potential of influenza viruses, it is important to understand the range of molecular interactions between the virus and its host. Despite years of intensive research on the virus, Jagger et al. (p. 199, published online 28 June; see the Perspective by Yewdell and Ince) have found that the influenza A virus has been hiding a gene in its small negative-sense RNA genome. An overlapping open reading frame was found contained in the PA viral RNA polymerase gene, which is accessed by ribosomal frameshifting to produce a fusion protein containing the N-terminal messenger RNA (mRNA) endonuclease domain of PA and an alternative C-terminal X domain. The resulting polypeptide, PA-X, selectively degrades host mRNAs and, in a mouse model of infection, modulated cellular immune responses, thus limiting viral pathogenesis.

  5. Spin-Dependent Light Emission

    Spintronic devices exploit electronic currents that are spin polarized, which have an excess of one spin current over the other. One way to detect this polarization would be to create a light-emitting diode that is sensitive to spin polarization. Along these lines, Nguyen et al. (p. 204) constructed a bipolar device in which an organic semiconductor was sandwiched between two ferromagnetic contacts whose relative polarization could be controlled by an applied magnetic field. Magneto-electroluminescence of the order of ∼1% was observed at a bias voltage of ∼3.5 V. The use of a deuterated organic polymer interlayer improved spin transport relative to polymers with hydrogen side groups, and a thin LiF buffer layer on the ferromagnetic cathode improved electron injection efficiency.

  6. Moving Dislocations

    The mechanical properties of crystalline materials are limited by the presence and motion of defects caused by extra or missing atoms in the crystal lattice. Plastic deformation of a material causes these defects, known as dislocations, to move and multiply. Much is known about the motion of dislocations in three dimensions but less so in two. Warner et al. (p. 209; see the Perspective by Bonilla and Carpio) used graphene as a model material to track dislocation dynamics in real time. The strain fields in the graphene sheet were mapped, which suggests that the dislocation motion is connected to the stretching, rotating, and breaking of individual carbon bonds.

  7. Abiotic Martian Organics

    Understanding the sources and the formation mechanisms of organic carbon compounds on Mars has implications for our understanding of the martian carbon cycle. Steele et al. (p. 212, published online 24 May) present measurements of organic material in 11 martian meteorites, including the Tissint meteorite, which fell in the Moroccan desert in July 2011. Ten of the meteorites contain complex hydrocarbons encased within igneous minerals. The results imply that the organics formed as the magma melt crystallized and are thus of abiotic origin.

  8. More Melting

    The last interglacial period, around 125,000 years ago, was 1° to 2°C warmer than the present, and the sea level was thought to be 4 to 6 meters higher. However, Dutton and Lambeck (p. 216), now suggest that sea level was possibly as much as 10 meters above current levels. Such a large excess of seawater would mean that the Greenland and Antarctic ice sheets melted much more than previously assumed, which has implications for how much sea-level rise we should expect with anthropogenic climate warming.

  9. Acidification Blues

    The increase in the concentration of atmospheric carbon dioxide threatens the health of the ocean's ecosystems because of the resulting acidification of the ocean and the decrease in its carbonate saturation state. Gruber et al. (p. 220, published online 14 June) used a regional ocean model to project how the saturation state of aragonite, a form of calcium carbonate that is produced by many marine organisms, will change in the California Current System through the year 2050. The sea floor along many parts of the California coast is likely to become exposed to year-round aragonite undersaturation within the next 20 to 30 years, a situation that could severely reduce the range of habitats for marine shellfish.

  10. They Walked Together

    Paisley Cave in Oregon provides some of the earliest evidence for humans in North America. Jenkins et al. (p. 223) provide a wide variety of additional evidence of early human occupation of this site, including a series of radiocarbon ages extending back to nearly 12,500 radiocarbon years ago (about 14,500 calendar years ago). The find includes examples of projectile points representative of the Western Stemmed Tradition dating to about 11,100 radiocarbon years ago. The Western Stemmed Tradition has been thought to have evolved after the dominant Clovis technology, but the find suggests that the two cultures overlapped in time.

  11. Growing Extinction Debt

    Predicting, and potentially preventing, extinction is a central goal of conservation biology. Wearn et al. (p. 228; see the Perspective by Rangel) describe a mathematical approach for predicting the time lags in extinction following habitat loss. The model was applied to the highly biodiverse Brazilian Amazon region, and used to reconstruct the spatial and temporal patterns of extinction and the accumulation of extinction debt from 1970 through to the present, and to extrapolate to 2050 under four deforestation scenarios. The Amazon basin sits at a critical point: Few species have been driven extinct to date, but an extinction debt is rapidly accumulating, which could lead to an increasing rate of extinction in the next four decades.

  12. GPCR Close-Up

    Structures of G protein–coupled receptors (GPCRs) determined in the past few years, have provided insight into the function of this important family of membrane proteins. Liu et al. (p. 232) used a protein-engineering strategy to produce a stabilized version of the human A2A adenosine receptor (A2AAR). The high-resolution structure reveals the position of about 60 internal waters, which suggests an almost continuous channel in the GPCR and can explain the allosteric effects of Na+ on ligand binding and how cholesterol may contribute to GPCR stabilization.

  13. Pole to Pole

    How do fission yeast cells decide when to grow at a single end (or pole) of the cell or whether to grow in a multipolar manner? Das et al. (p. 239, published online 17 May) found that accumulation of the active form of the small guanine nucleotide–binding protein Cdc42 at the growing tip of the cell oscillated with a period of a few minutes. In cells growing at one pole, the oscillations were primarily present at that pole and during bipolar growth symmetrical anticorrelated oscillations were observed. Dynamic competition for Cdc42 between multiple growth zones could represent a flexible mechanism to modulate cell growth asymmetry.