This Week in Science

Science  11 Nov 2011:
Vol. 334, Issue 6057, pp. 737
  1. Young for Their Age


    Cycads—sometimes called living fossils—are an ancient group of nonflowering plants that likely originated in the Permian more than 250 million years ago. However, despite the cycads' ancient origin and relative lack of recently derived characteristics, Nagalingum et al. (p. 796, published online 20 October; see the Perspective by Renner) show through a phylogenetic analysis, calibrated with fossils, that the majority of extant taxa diversified approximately 5 to 10 million years ago. Thus, despite the antiquity of cycad groups, much more recent radiations have generated their current diversity.

  2. An Iron Cleaver

    The century-old Haber-Bosch process is still used to produce ammonia for fertilizer. Although surface science studies have mapped out the contours of the mechanism operating at the surface of the iron-based catalyst, molecular model compounds could shed additional light on the specifics of bond breaking and forming steps. Rodriguez et al. (p. 780) show that a dinuclear iron coordination compound in tandem with potassium can cleave the triple bond in N2, yielding a well-characterized product that subsequently liberates ammonia on reaction with H2.

  3. Fiery Forecast

    Carbon emissions from deforestation and forest degradation in South America, particularly in the Amazon, accounted for nearly 25% of the total global flux from land use and land cover change in the 1990s; about half of these emissions were due to fires. More effective ways to manage fires would help in climate mitigation and adaptation strategies. The ability to forecast dry season fire activity in advance would provide extra time to decide upon management options such as allocation of fire-fighting resources and targeted burning. Now, Chen et al. (p. 787) have been able to use satellite-derived estimates of fire activity in South America by using sea surface temperature anomalies to predict regional annual fire season severity 3 to 5 months in advance.

  4. Genome-Wide Demethylation

    In the mammalian genome, CpG dinucleotides are often methylated, providing an epigenetic mark associated with transcriptional silencing. Twice in early development, in the pre-implantation embryo and in primordial germ cells, there is a genome-wide loss of methylation that is thought to contribute to the “reprogramming” of the genome. Shearstone et al. (p. 799) now describe a third instance of genome-wide loss of DNA methylation that takes place in association with normal red blood cell differentiation.

  5. A Difficult Break-Up


    As the distribution of tectonic plates on Earth's surface evolves over geologic time, forces from the convecting mantle drive some plates together and pull others apart. Over time, the extensional process—known as rifting—can break up entire continents. In the present-day southwestern United States, rifting is pulling regions of California's lithosphere apart. Lekic et al. (p. 783, published online 6 October) directly imaged the lithosphere below Southern California by analyzing seismicwave receiver functions collected from a number of dense seismic networks. The images show an extremely thin lithosphere in rift zones, suggesting that stretching forces are compensated by efficient localization of accumulated strain.

  6. Toxin Identified

    The Gram-negative bacterium Burkholderia pseudomallei, the causative agent of melioidosis, is endemic in Southeast Asia and northern Australia and is often associated with stagnant water and rice paddy fields. Clinical manifestations of melioidosis include subclinical infections, acute septicemia, and subacute and chronic disease. There is no licensed vaccine against B. pseudomallei, which can infect almost any tissues of its hosts and is resistant to a number of antibiotics. Cruz-Migoni et al. (p. 821) report the identification and molecular characterization of a B. pseudomallei protein that can act as a potent toxin in mice and human cells and can inhibit protein translation.

  7. Viral Control

    Chronic viral infections, like the human immunodeficiency virus and the hepatitis C virus, result in altered immune responses that help to keep the infection under control and limit virus-related pathology. Harker et al. (p. 825, published online 29 September) now show that the cytokine interleukin-6 (IL-6) is required for resolving a chronic viral infection in mice. Surprisingly, IL-6 was not required during the acute stages of viral infection, but it was required once a chronic infection was established.

  8. Awareness and Attention

    There has been a long-standing controversy of whether activity in the primary visual cortex is necessary for perceptual awareness. In human brain-imaging experiments, Watanabe et al. (p. 829) were able to dissociate perceptual awareness from simple attention. Awareness was manipulated in a binocular flash suppression paradigm, and attention was manipulated by using standard attentional instructions. Activity in the primary visual cortex varied little, whether a target was visible or not. However, activity in the human primary visual cortex varied when subjects attended to the target or ignored it. Thus, humans do not need primary visual cortical activity in order to be aware of seeing something.

  9. Beating the Noise

    Interferometers use waves for the measurement of small displacements, with the wavelength determining the accuracy of the measurement. Besides electromagnetic interferometers, there are also matter wave interferometers, in which the atoms' small de Broglie wavelength increases the accuracy, but the shot noise—a consequence of their particle nature—limits the precision. In order to beat the shot noise limit but still have a practical device, entanglement must be achieved among a large number of atoms. Lücke et al. (p. 773, published online 13 October) used spin dynamics in a Bose-Einstein condensate to create an entangled state consisting of up to 10,000 atoms, with roughly half the atoms having an up and half a down spin; this input state achieved interferometric precision slightly beyond the shot noise limit. This proof-of-principle demonstration should point the way toward a new generation of atom interferometers.

  10. Asymmetry at the Surface

    Because biologically derived compounds are homochiral (exhibiting only one of two possible mirror-image geometries), it is critical for pharmaceutical synthesis to generate an asymmetric product distribution. Solution phase metal complexes bearing chiral ligands can bias products toward one or the other mirror-image isomer with a high degree of selectivity. In principle, heterogeneous asymmetric catalysis could leverage chiral agents adsorbed to metal surfaces as analogs of coordinated ligands, but in practice, very few such systems have yielded impressive results. Demers-Carpentier et al. (p. 776) demonstrate progress toward this end by rigorously analyzing the interactions of substrate molecules and chiral adsorbates on a platinum surface, using a synergistic combination of scanning tunneling microscopy and density functional theory calculations.

  11. Convecting Clues

    The hot, convecting liquid that comprises Earth's outer core controls Earth's magnetic field. Fluid dynamic models of the outer core, however, depend on relatively unknown structural and compositional properties of the iron, nickel, and other lighter elements that are common at that depth. Ozawa et al. (p. 792) performed x-ray diffraction experiments at the temperature and pressure conditions of the outer core to decipher the structure of iron monoxide (FeO), one of the main phases of iron bonded to a light element. Based on this analysis, FeO takes on new structure with modified Fe–O bond distances at pressures and temperatures that correspond to the middle of the outer core. Such a phase transition, although not detectable through seismology, may stratify the outer core into two convective regimes that are responsible for the present-day magnetic field.

  12. Another Aerosol Effect

    Anthropogenic aerosols can affect climate in various ways, through what are called direct effects (in which the particles themselves absorb or reflect radiation) and indirect effects (by which they influence clouds, which in turn affect the radiative balance of the atmosphere). Mahowold (p. 794) suggests that there is another way that these particles can affect climate—by their slow impact on biogeochemical cycles potentially affecting the fluxes of CO2 into and out of the atmosphere.

  13. Inositol Pyrophosphates in Metabolism Control

    Roles of certain inositol polyphosphates as intracellular signaling molecules are well known. However, the functions of more recently described inositol pyrophosphates, such as diphosphoinositol pentakisphosphate, which contain high-energy phosphate bonds, are less clear. Szijgyarto et al. (p. 802; see the Perspective by Irvine and Denton) propose that these molecules may help to control metabolism and mitochondrial function. Yeasts engineered to lack inositol pyrophosphates grew more slowly. Such yeasts, and mouse embryo fibroblasts similarly depleted of inositol pyrophosphates, showed decreased mitochondrial function and increased glycolysis.

  14. Unraveling Sirt5 Activity

    Sirtuins, which are known as nicotinamide adenine dinucleotide (NAD)–dependent deacetylases, are involved in aging, transcription, apoptosis, and metabolism. Sirt5 is one of seven mammalian sirtuins. Du et al. (p. 806) now report that Sirt5 is a NAD-dependent desuccinylase and demalonylase. In vitro, Sirt5 specifically catalyzed the hydrolysis of succinyl and malonyl groups much more efficiently than acetyl groups.

  15. A New Pathway for Prostaglandins

    Prostaglandins play a major role in many physiological and pathological processes, often as proinflammatory mediators. The proinflammatory effects of prostaglandins may be beneficial when fighting an infection; however, when produced chronically, they can lead to irreparable damage. Prostaglandins are synthesized from arachadonic acid by cycloxygenase enzymes. The major source of arachidonic acid is thought to be its synthesis by the enzyme phospholipase A2. Nomura et al. (p. 809, published online 20 October; see the Perspective by Stella) now demonstrate an alternative pathway for the production of arachidonic acid: the hydrolysis of the endocannabinoid 2-arachidonoylglycerol by monoacylglycerol lipase.

  16. Desperately Seeking XMRV

    A report that patients with chronic fatigue syndrome (CFS) are infected with a retrovirus called XMRV [Science 326, 585 (2009)] attracted considerable attention, but follow-up work by other investigators failed to confirm the finding. In a study by Simmons et al. (p. 814, published online 22 September; see the 23 September News story by Cohen and Enserink), nine laboratories—including the authors of the original report—independently analyzed blind-coded blood samples from 15 individuals previously found to be positive for the virus (14 with CFS) and 15 healthy controls previously found to be negative. Only the two laboratories associated with the original report detected XMRV. However, in these laboratories, the virus was found in healthy controls as often as in CFS patients and replicate samples yielded inconsistent results. In addition to showing that current assays for detecting XMRV are unreliable, these data support previous studies that have questioned the association between XMRV and CFS.

  17. RNA Endings

    The last three bases in many transfer RNAs (tRNAs) and tRNA-like small RNAs (CCA) are critical for tRNA function but are not encoded in the genome. Instead, they are added on by the CCA-adding enzyme. Wilusz et al. (p. 817) show that the CCA-adding enzyme from all domains of life can add CCACCA (or CCACC) to tRNAs and tRNA-like molecules in vitro if they have an unstable acceptor stem and guanosines at the first and second positions in the RNA. The tRNAs ending in CCACCA are rapidly degraded in HeLa nuclear extracts. For many tRNAs, sensitivity to the rapid tRNA decay surveillance pathway in yeast correlated with stability of the acceptor stem and CCACCA addition, the latter generating a single-stranded tail that is sufficiently long to be recognized by 3′-5′ exonucleases.