This Week in Science

Science  15 Nov 2013:
Vol. 342, Issue 6160, pp. 776
  1. Quick-Dry Region


    The Sahara Desert is one of the driest places on Earth. However, 11,000 to 5000 years ago, it was a relatively lush region containing savannah grasslands and humid tropical forests. This interval, the Early Holocene African Humid Period, ended, of course, but whether the drying occurred rapidly or gradually is unclear. Tierney and deMenocal (p. 843, published online 10 October; see the Perspective by Bard) report results from the Horn of Africa that suggest that the transitions both into and out of the humid period were abrupt—occurring within centuries rather than over millennia.

  2. Monolayer Graphene via Two Transfers

    Oriented monolayers of graphene containing some bilayer regions can be formed on silicon carbide crystal surfaces, but, to be cost effective, the graphene needs to be exfoliated and transferred to other substrates so that the silicon carbide crystal can be reused. Kim et al. (p. 833, published online 31 October) used a nickel film grown to a thickness designed to impart a particular surface stress as a “handle” to exfoliate the graphene layer for transfer to a silica substrate. An additional gold layer was then used to remove the excess monolayer from the bilayer regions to create a monolayer suitable for electronics applications.

  3. Answers from Exomes

    Exome sequencing, which targets only the protein-coding regions of the genome, has the potential to identify the underlying genetic causes of rare inherited diseases. Angulo et al. (p. 866, published online 17 October; see Perspective by Conley and Fruman) performed exome sequencing of individuals from seven unrelated families with severe, recurrent respiratory infections. The patients carried the same mutation in the gene coding for the catalytic subunit of phosphoinositide 3-kinase δ (PI3Kδ). The mutation caused aberrant activation of this kinase, which plays a key role in immune cell signaling. Drugs inhibiting PI3Kδ are already in clinical trials for other disorders.

  4. Forests in Flux

    Forests worldwide are in a state of flux, with accelerating losses in some regions and gains in others. Hansen et al. (p. 850) examined global Landsat data at a 30-meter spatial resolution to characterize forest extent, loss, and gain from 2000 to 2012. Globally, 2.3 million square kilometers of forest were lost during the 12-year study period and 0.8 million square kilometers of new forest were gained. The tropics exhibited both the greatest losses and the greatest gains (through regrowth and plantation), with losses outstripping gains.

  5. Long-Lived Donors

    Quantum computing in materials such as silicon would simplify integration with existing electronic components; however, the coherence times of such qubits, especially at room temperature, are affected by the interaction with the busy environment of a solid. Eliminating isotopic impurities from the host material improves coherence times, as observed for qubits, based on the nuclear spin of neutral P donors in Si. Saeedi et al. (p. 830) modified this system by using charged P donors instead of neutral ones; by manipulating the states of the donors optically and using dynamical decoupling, the coherence time of the qubits was extended to 3 hours at cryogenic temperatures and 39 minutes at room temperature.

  6. Scaling Spindle Size


    The difficulty of modulating cell size in vivo has made it hard to test hypotheses for organelle size scaling during development. To this end, Hazel et al. (p. 853) and Good et al. (p. 856) developed microfluidic systems in which cytoplasmic extracts are encapsulated in compartments with definable size. The size of mitotic spindles assembled within cell-free extracts scaled with the volume of the compartment within which the spindle assembled. The findings suggest that the diminished availability of cytoplasmic components, notably tubulin, concomitant with cell size reduction, prescribes a smaller spindle size.

  7. Slipping the NET

    One spectacular response to bacterial infection is the release of NETs (neutrophil extracellular traps) of DNA from polymorphonuclear leukocytes that immobilize the pathogens and prevent phagocytosis by macrophages. Staphylococcus aureus evades NETs by degrading the DNA into deoxyadenosine (dAdo). Thammavongsa et al. (p. 863) found that dAdo also promotes immune cell death, which appears to ensure the exclusion of macrophages from the center of abscesses within which the bacteria survive.

  8. Dog Domestication

    The precise details of the domestication and origins of domestic dogs are unclear. Thalmann et al. (p. 871; see the cover) analyzed complete mitochondrial genomes from present-day dogs and wolves, as well as 18 fossil canids dating from 1000 to 36,000 years ago from the Old and New Worlds. The data suggest that an ancient, now extinct, central European population of wolves was directly ancestral to domestic dogs. Furthermore, several ancient dogs may represent failed domestication events.

  9. Stabilizing Silicon

    Solar-driven water splitting has potential as an energy storage mechanism to supplement the direct conversion of sunlight to electricity. A submersed integrated device has been proposed both to absorb the light and to catalyze the reaction, but stability has been a problem. Kenney et al. (p. 836; see the Perspective by Turner) found that a nickel coating, thin enough to let light through, could protect a silicon absorber in the alkaline environment of a lithium/potassium borate electrolyte. The nickel also functioned as the oxidation catalyst, and the lithium ions helped to establish a protective film structure in situ.

  10. Dead Wood

    Trees can be affected by a wide variety of diseases caused by insects, fungi, and other pathogens. Such diseases often make the headlines—particularly when iconic tree species are affected—for example, in the case of the ash dieback currently spreading through Europe, or the chestnut blight that devastated American chestnut trees. But what is the effect of these diseases on ecosystem services performed by trees in natural and managed ecosystems? Boyd et al. (10.1126/science.1235773) review the spread of tree diseases, as a result of globalization and climate change, and analyze the resulting damage to timber and fruit production, to climate regulation, and to parks and woodlands caused by tree diseases.

  11. The pKa of P450

    Cytochrome P450 enzymes oxidize hydrocarbons through activation of oxygen at heme iron centers. However, the protein backbone has various sites (particularly tyrosine residues) that are also sensitive to oxidation, so how can the enzyme rapidly transform substrates without attacking itself? Yosca et al. (p. 825) explored the energetics of the competition between substrate and self-oxidation by measuring the pKa of the enzyme's iron(IV)hydroxide motif. Cysteine thiolate coordination to iron in the P450 structure raised the pKa almost to 12—rendering the iron oxo far more basic than analogous motifs in other heme environments. Correspondingly, the electronic environment for H-atom transfer from the substrate was relatively favorable, compared to electron transfer from a backbone residue.

  12. [2+2] Asymmetrically

    Catalysts in thermal reactions operate by lowering energy barriers of bound substrates, and thereby increasing the proportion of reagents that can proceed to products at a given temperature. In photochemical reactions, light provides the energy to surmount the barrier. It is therefore challenging to alter selectivity through catalysis, because the catalyst may not be bound when a given reagent absorbs the light. Brimioulle and Bach (p. 840) surmounted this problem in the light-induced intramolecular [2+2] cycloaddition of enones by using a catalyst that shifted the absorption wavelength of the bound substrate. The light was thus predominantly absorbed by substrate-catalyst complexes, enabling asymmetric induction by the catalyst to provide enantiomerically enriched products.

  13. Getting the Dosage Right

    As sex chromosomes evolve, they must compensate for differential gene dosage, especially in non-sex-specific genes. However, owing to the relative rarity of chromosomes that are in the process of becoming sex chromosomes, the ongoing evolutionary processes resulting in dosage compensation are unclear. Ellison and Bachtrog (p. 846; see the Perspective by Chuong and Feschotte) identified a specific transposable element involved in dosage compensation in males on the evolutionarily new neo-X chromosome of Drosophila miranda. A particular copy of this transposable element has been inserted at chromatin entry sites on the neo-X chromosome that favorably binds to the male-specific lethal complex, which in turn facilitates the spreading of chromatin silencing on a single chromosome to provide dosage compensation.

  14. The Root of the Problem

    The quiescent center (QC) within the root meristem plays a key role as a stem cell organizer to sustain the root stem cell niche. The QC cells execute a dual role: prevention of the differentiation of neighboring stem cells, and maintenance of the root structure by undergoing only occasional cell division. The mechanisms that account for the low QC proliferation are unclear, although the anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase is known to suppress QC cell division. Through a systematic functional analysis of APC/C-copurifying proteins, Heyman et al. (p. 860) characterized a transcription factor ERF115 as a rate-limiting factor for QC cell division. ERF115 needs to be destroyed to retain QC cells in a resting state. ERF115 operates in a brassinosteroid-dependent manner and controls QC cell division through transcriptional activation of phytosulfokine signaling.

  15. Dissecting Initiation


    Ribosomes make proteins. The initiation step of protein synthesis involves initiation factors (IFs) that help to position the initiation codon of the messenger RNA (mRNA) and the initiator transfer RNA. The initiation factor eIF5B is universally conserved, and promotes recruitment of the large ribosomal subunit during the terminal step of initiation. Fernández et al. (10.1126/science.1240585, published online 8 November) used cryoelectron microscopy to determine the structure of the yeast eIF5B initiation complex and provide a snapshot of the final step of eukaryotic translation initiation.