This Week in Science

Science  15 Aug 2008:
Vol. 321, Issue 5891, pp. 888
  1. Cloud Transitions


    Aerosols can produce changes in the number, size, and size distribution of cloud drops, thereby impacting climate by affecting how clouds change the distributions and fluxes of energy and water. There are two major pathways by which aerosols act on clouds, the microphysical and the radiative, and (depending on the conditions) the net result can be either warming or cooling. Koren et al. (p. 946) focus on the Amazon to show that there exists a smooth transition between these two opposing effects and that a feedback between the optical properties of aerosols and cloud fraction can change the distribution of energy within the atmosphere.

  2. Suffocating the Oceans

    In many coastal regions of the world during the past 60 years, the concentration of dissolved oxygen has declined to levels anathema to life and the number and extent of listed hypoxic areas has increased from 46 in 1995 to more than 400. Loss of dissolved oxygen is linked to the release of nutrients when organic waste or fertilizer runs off into river outflows. Hypoxia poses a grave threat to the viability of coastal marine and estuarine ecosystems and can quickly lead to the elimination of the sea bed organisms and fish. Diaz and Rosenberg (p. 926) review how the issue of dissolved oxygen may become the most important factor controlling man's use of the sea.

  3. From Storm to Aurora

    Where do explosive auroral displays and their space-counterpart, magnetospheric substorms, which release energy from the solar wind stored in Earth's magnetosphere, originate? Angelopoulos et al. (p. 931, published online 24 July; cover; see the Perspective by Petrukovich) have used a series of satellites and ground networks to time the development of a substorm in detail and identify its source. Magnetic reconnection in Earth's magnetotail started the event, triggering an aurora display 1.5 minutes later.

  4. Small Pillars and Blocks


    Block copolymers, which are made from chemically dissimilar polymers covalently bonded together, will phase-segregate into a range of ordered patterns, and provide valuable tools for making lithographic patterns at the nanometer scale through a self-assembly process. However, a significant challenge is to make patterns over large distances owing to the formation of boundary regions or defects where the ordering is defective (see the Perspective by Segalman). Ruiz et al. (p. 936), through a judicious choice of substrate pattern, could multiply the resolution of the resulting block copolymer by a factor of four, allowing for patterning over large areas without substantial numbers of defects. Bita et al. (p. 939) created a sparse array of pillars that chemically mimicked the minority component of their block copolymer. The pillars disrupt the uniformity of the substrate and act as nucleation sites for the self-assembly, thus aiding in the creation of large-area-patterned templates.

  5. Ironing Out Ancient Ocean Chemistry

    The Neoproterozoic Era, which lasted from approximately 1 billion to 540 million years ago, was distinguished by a phenomenal diversification of organisms and a transition from an anoxic to an oxic atmosphere. How did ocean chemistry change during that time? Canfield et al. (p. 949; published online 17 July; see the Perspective by Lyons) report that for most of the mid- and upper Neoproterozoic, the deep ocean was enriched in ferrous iron (ferruginous), sometimes sulfidic, and finally oxic. The observed return of ocean chemical conditions to the ferruginous ones not seen for more than 1 billion years probably was because of the long preceding interval of a sulfidic marine environment.

  6. CRISPR Virus Defenses

    Like eukaryotes, bacteria must defend themselves against viruses and transposons. A system has evolved in prokaryotes where fragments of these pathogenic species are collected into special genomic regions known as clusters of regularly interspaced short palindromic repeats (CRISPRs). CRISPRs provide a heritable memory of previous infections and a means to fend off subsequent infections. Brouns et al. (p. 960; see the Perspective by Young) show that the CRISPR region in Escherichia coli is transcribed and the CRISPR-associated (cas) gene casE is required for cleavage of the transcript into small, ∼57-nucleotide CRISPR-RNAs (crRNAs). A complex of cas genes, including casE, form the Cascade complex, which uses the crRNAs to target the DNA of invading species and prevent infection.

  7. Arsenic and Old Organisms

    Mat-forming purple bacteria and cyanobacteria that couple arsenite oxidation to the reduction of carbon dioxide in the absence of oxygen have been found in hot brine springs of Mono Lake, California. The advent of photosynthesis was a key moment in the evolution of the Earth because the reaction split water to release oxygen and promoted the diversification of life and our planet's characteristic geochemistry. But photosynthesis evolved under anoxic conditions, and one alternative route is that light-driven carbon fixation was based on arsenic as an electron donor. In a series of biochemical investigations on the Mono Lake organisms, Kulp et al. (p. 967) have confirmed the phylogenetic hints that this scenario was indeed the case. Increasingly, arsenic is implicated in a complex round of redox transformations mediated by microorganisms, to the extent that examples have been discovered of entire microbial communities supported by a metalloid that is toxic to most other forms of life.

  8. RNA Interference and Plant Defenses


    RNA interference plays an important role in innate immunity in plants and in animals. Specific microRNAs have also been implicated in pathways that sense pathogen-associated molecular patterns (PAMPs). Now Navarro et al. (p. 964) examine in more detail the role of microRNAs in innate immunity in Arabidopsis. MicroRNAs were found to be more broadly required for PAMP sensing. Pathogenic bacteria appear to have evolved various effectors that are secreted into the host that suppress the microRNA pathway at various points. Infection with Turnip Mosaic virus, which produces a suppressor of both the small interfering RNA and microRNA pathways, promotes infection by nonpathogenic bacteria, which may explain the observed synergy between viral and bacterial pathogens seen in the field.

  9. Unwitting Accomplices

    Many parasitic diseases are transmitted via the bite of an infected insect vector. The host response at the early subsequent stages is likely to influence the course of disease. Peters et al. (p. 970; see the Perspective by John and Hunter) use intravital imaging to visualize the dynamics of the initial events in mice following transmission of the intracellular parasite Leishmania, which normally infects macrophages. Unexpectedly, neutrophils were among the first major arrivals at the site of the insect bite and were seen to engulf parasites, which remained viable and infective. Rather than helping the host deal with the parasite, this behavior made these innate immune cells unwitting accomplices in the ongoing process of infection.

  10. Tethering Therapeutic T Cells

    Considerable effort has been made in cancer immunotherapy in elaborating robust T cell responses to tumors. However, focusing a T cell's attention on its tumor target is difficult, often because tumor cells do not present sufficient distinguishing features from normal human cells for the immune system to detect. Bargou et al. (p. 974) overcome this by using a modified bi-specific antibody that simultaneously binds two different cell surface proteins: one on a killer T cell and one on the target tumor cells—in this case, non‐Hodgkin's lymphoma B cells. By tethering the T cell to its intended target, the modified antibody forces direct killing of the lymphoma cells and, even at very low doses, could achieve measurable, or even complete, regression of cancer in a small number of patients who had proven refractory to existing therapies. Although the durability of this treatment needs careful follow-up, it offers further patient-based evidence that T cell‐based immunotherapy may yet offer a viable means of treating cancer in the clinic.

  11. Synaptic Coding Capacity

    What is the contribution of single excitatory synaptic events to the representation of sensory stimuli? In vitro preparations have provided theoretical limits on single-input coding. However, analysis of stimulus-evoked unitary synaptic activity with physiologically relevant stimuli in vivo has been hampered by compound synaptic responses and poor stimulus control. Taking advantage of cerebellar granule cells as a model system with very few synaptic inputs and a well-controlled quantifiable vestibular stimulus, Arenz et al. (p. 977) explored sensory encoding at single synapses in vivo in real time over a broad range of stimuli. Unitary, direction-sensitive synapses report motion velocity by using a frequency code that is modulated around a tonic rate. The reliability of the synaptic signal ensures that velocity is represented linearly by charge transfer. Only 100 synapses were required for realistic velocity resolution, well within the number of inputs received by many neurons in dedicated sensory processing brain regions. Single-cell computation can thus easily achieve fine-scale reconstruction of sensory stimulus features.

  12. Sending a Surface Structure Packing

    The structure of self-assembled monoloayers formed by the chemisorption of alkane thiol molecules onto a gold surface is fairly simple in principle—gold-sulfur bonds form and the alkane tails can pack together in domains of different orientation, with defects between the domains. In detail, this system is highly complex, in that the interactions between the gold atoms in the surface layer, and gold-sulfur bonds, and the tail packings are somewhat comparable, so the structures for short and long alkyl chains may differ. Cossaro et al. (p. 943) now show that the structure they have determined for hexyl thiols differs from that for methyl thiols, based on grazing-incidence x-ray diffraction and molecular dynamics studies. Compared to the methyl structure, the packing of the hexyl chains is much more ordered and creates not only two types of gold-sulfur bonding arrangements (bridge bonds by sulfur between gold atoms and attachment of two thiols to one gold atom) but also vacancies of gold atoms in the first surface layer.

  13. Nuts and Bolts of Plant Pathogen Response

    Changes in oxidation status somehow regulate pathogen resistance in plants. The Arabidopsis NPR1, a master regulator of salicylic acid (SA)‐mediated defense genes, is held in an inactive multimeric state in the absence of SA and, upon SA release, is converted into a monomer before it is transported into the nucleus where it acts. Tada et al. (p. 952; published online 17 July) show that NPR1 is sequestered in the cytoplasm as an oligomer through S-nitrosylation at residue Cys156, which facilitates the oligomerization. Conversely, the SA-induced NPR1 monomerization is catalyzed by reduced thioredoxins. Mutants in both NPR1 Cys156 and thioredoxins compromised NPR1-mediated gene expression and disease resistance providing a missing link between pathogen-triggered cellular redox changes and gene regulation in plant immunity.

  14. Improving Gene Expression Analysis

    Analysis of the complexity and dynamics of gene expression has been limited by the costs and technical limitations of microarrays and other approaches. Sultan et al. (p. 956, published online 3 July) have used a second-generation sequencing approach, short read high-throughput sequencing, for the global digital analysis of gene expression and the detection of splicing junctions in a human embryonic kidney and a B cell line. This approach was able to detect 25% more genes than microarrays.