This Week in Science

Science  05 Nov 2004:
Vol. 306, Issue 5698, pp. 937
  1. The Metallic Secret of Deinococcus radiodurans' Success


    The bacterium D. radiodurans is extremely resistant to both ionizing radiation and desiccation. The mechanisms that allow the bacterium to survive are unclear, although each cell does contain four to eight genomic copies and the chromosome has an unusual ringlike structure. Daly et al. (p. 1025, published online 30 September 2004) now show that the ringlike organization of the chromosomes is unlikely to play a role in radiation resistance. Instead, it is the very high intracellular concentration of Mn ions relative to iron that is critical. Rather than providing protection against the initial burst of radiation, high intracellular Mn might act against sudden increases in damaging reactive oxygen species during the recovery from radiation insult.

  2. Black Hole Noise Deciphered

    An active galactic nucleus (AGN) can be modeled as a spinning black hole that accretes matter and energizes itself from a disk of gravitationally trapped material. An AGN is characterized as radio-loud if there are radio emissions that might be produced by relativistic jets spurting from the nucleus. Grandi and Palumbo (p. 998) studied the spectral variability of a radio-loud AGN, 3C 273, for 6 years in order to distinguish the jet emission from the disk emission. The two components show a significant amount of variability compared to the total flux, and the jet dominates the higher energy emissions.

  3. Distant Energy Transfer Between Molecules

    The transfer of energy from a donor molecule to an acceptor is usually thought of as a short-distance effect. Typically, the nonradiative transfer of energy requires molecules to be less than 10 nanometers apart. Although radiative transfer can have further reach, the emission and reabsorption of a photon at particular sites is not an efficient process unless that energy can be directed and controlled. Andrew and Barnes (p. 1002; see the Perspective by Van Duyne) show that surface-plasmon polaritons can be effective couriers of energy between molecules on either side of a thin film of silver and that the modified radiative energy transfer can extend to distances of 100 nanometers or more. The effect should prove useful in nanophotonics, where optical energy will be routed around a circuit.

  4. Straining to Improve

    Strained BaTiO3 films have been fabricated that have ferroelectric transition temperatures and remanent polarizations that are substantially enhanced compared to pure BaTiO3 and approach those of commonly used lead-based materials. Choi et al. (p. 1005) fabricated such films on substrates whose lattice mismatch to BaTiO3 was optimized. The strain induced in the film could be used to enhance or degrade its ferroelectric properties. This method could prove useful in the design of ferroelectric random-access memory.

  5. Faster Trading

    Nanosized particles can be more reactive than their bulk counterparts, but this difference is often thought of as a surface property. Son et al. (p. 1009) show that ion exchange of monovalent silver ion reactions can reversibly occur throughout cadmium selenide nanoparticles. The reaction rates are orders of magnitude faster than in the bulk. Surprisingly, although the crystal structure changes, the overall shape of the particles does not, even for complex geometries.

  6. How Dry It Has Been

    Much of the western United States continues to experience a multiyear drought, and planners are wondering if there will be adequate water available to fill the growing thirst of an expanding population. How unusual is such a drought? Cook et al. (p. 1015, published online 7 October 2004) used a large set of tree ring data for the past 1200 years to compile a regional summer drought reconstruction for the western United States. The current dry period is far less severe than those that occurred repeatedly during most of the last millennium. A 400-year-long period with long intervals of extreme aridity, which began in about 900 A.D. and is roughly coincident with the Medieval Warm Period, suggests that aridity may be a natural consequence of higher temperatures in the warmer world of the near future.

  7. Ins and Outs of Hydrogen Storage


    Hydrogen storage is still a challenging materials science problem. Zhao et al. (p. 1012, published online 14 October 2004) describe metal-organic frameworks with relatively small channels and find that some of them exhibit hysteresis in the adsorption and desorption of hydrogen at −196°C. They argue that hysteresis arises from kinetic trapping of the gas caused by the flexibility of the organic ligands (4,4'-bipyridine). Such materials allow hydrogen to be loaded at high pressures but then stored at lower pressures. Applications such as fuel storage in vehicles will require improvements in the amount of hydrogen that these materials store by weight.

  8. Functional Diversity and Composting

    The relation between species diversity and ecosystem functioning has been keenly debated. As well as the absolute number and relative abundance of species, the functional dissimilarity of the species in a community may influence the overall functioning of an ecosystem. Heemsbergen et al. (p. 1019) filled tubes with soil, topped them with alder leaves, and added combinations of different numbers of species of annelids, isopods, and millipedes. They then tracked three “ecosystem function” variables over subsequent weeks. The number of species per se did not determine assemblage performance, but functional groups were the key to performance: The more dissimilar in function its members, the better the assemblage at decomposing the leaves.

  9. Autophagy in Health and Disease

    Autophagy, the process by which cells sequester and degrade organelles and cytoplasm in times of stress, is being recognized as playing a role in a variety of disease processes. Shintani and Klionsky (p. 990; see the cover) review the sometimes paradoxical roles of autophagy in human health, disease, and aging. Nakagawa et al. (p. 1037) describe how cells can clear an invading pathogen, group A Streptococcus, by targeting the cytosolic bacteria for destruction by the cellular autophagy machinery.

  10. The Molecules Behind Nicotine Addiction

    Identification of the nicotinic acetylcholine receptor (nAChR) subtypes that are critical for nicotine dependence will provide insights into addiction mechanisms and should also help to identify potential smoking-cessation targets. Tapper et al. (p. 1029; see the Perspective by Hogg and Bertrand) engineered genetically modified mice in which nicotinic α4 receptors were hypersensitive to nicotine. A range of cellular assays and simple behavioral procedures showed that nicotinic activation of α4 nAChRs is sufficient to explain the development of sensitization and tolerance and also to explain the rewarding effects of nicotine.

  11. Immunology and Olfaction

    Although absent in humans, the vomeronasal organ (VNO) plays a central role in controlling reproductive and social behaviors in most mammals. Vomeronasal sensory neurons detect pheromones and other molecules that carry information about gender, sexual and social status, dominance hierarchy, and individuality, but it has been very difficult to define the molecular nature of these signals. Leinders-Zufall et al. (p. 1033) show that major histocompatibility complex (MHC) class I peptides can activate selective VNO neurons in the basal layer of the VNO. Recognition of peptides by vomeronasal sensory neurons is independent of MHC haplotype. Peptides specific for different MHC molecules generate distinctive neural activation patterns that provide the basis for the neural representation of the structural diversity of this family of chemosignals.

  12. Examining the Needle


    Type III secretion systems (TTSSs) are used by bacterial pathogens to inject target cells with bacterial effector proteins. The central component of these systems is the needle complex, composed of a multiring base that is anchored in the bacterial envelope and a protruding needle-like structure that is thought to serve as a conduit facilitating effector protein transport. Marlovits et al. (p. 1040) present detailed maps of the TTSS needle complex, isolated from Salmonella typhimurium, that were derived from cryoelectron microscopy analysis.

  13. Candid Camera on Secretion

    The interaction of SNARE proteins, like SNAP25, syntaxin, or synaptobrevin, is a key event in the fusion of intracellular membranes, such as when secretory vesicles fuse with the plasma membrane during regulated secretion. An and Almers (p. 1042) engineered a series of fluorescent SNAP25 analogs that change color when SNAREs interact in living cells. In time-resolved recordings, SNARE complexes could be observed assembling and disassembling when single cells were stimulated to secrete.

  14. Dynamic Instability in Plasmid Segregation

    The dynamic instability that microtubules exhibit is critical for their role in chromosome segregation. Recently, it has been suggested that actin-related proteins may be involved in chromosome segregation in prokaryotes. Garner et al. (p. 1021; see the Perspective by Møller-Jensen and Gerdes) show that the prokaryotic actin homolog ParM displays dynamic instability and bidirectional polymerization. ParM assembly is required for the segregation of large, low-copy drug-resistance plasmids found in many enteric pathogens. Filaments with plasmid bound at both ends could thus be protected from catastrophic disassembly, and bidirectional elongation could drive plasmid segregation.

  15. Triggering RNA Interference

    In many cases of RNA interference (RNAi), the silencing signal is double-stranded (ds)RNA, generated either by self-complementary sequences or antisense transcripts. For single-copy transgenes, it has been suggested that defects in the transcript itself might trigger an RNA-dependent RNA-polymerase (RdRp) to generate the dsRNA required for RNAi. However, the nature of the “aberrant” transcripts used by RdRps in the in vivo RNAi reaction has been unknown. Gazzani et al. (p. 1046) provide evidence that RdRps in Arabidopsis can generate dsRNA from decapped mRNA, removal of which by an RNase involved in messenger RNA turnover reduces the efficacy of RdRp-dependent cosuppression of several transgenes.