This Week in Science

Science  15 Apr 2005:
Vol. 308, Issue 5720, pp. 321
  1. Open Access to Sea Levels


    Sea level has varied by amounts well in excess of 100 meters during recent 100,000-year-long glacial cycles. However, smaller but still substantial variations of tens of meters occurred on time scales of only tens of thousands of years. Corals are the best absolute indicators of sea level, but they often exchange uranium with seawater after they have died, which makes it difficult to perform the uranium-thorium radiometric dating needed to establish their ages and the timing of associated sea-level changes. Thompson and Goldstein (p. 401; see the Perspective by Henderson) have circumvented this problem by developing an analytical method that allows them to correct for the open system behavior of U-series nuclides in corals. They generated a sea-level curve for the last glacial period with sufficient temporal resolution to reveal variations that were not previously clear.

  2. Damming Analysis

    Many large river systems that support a wide variety of ecosystems have been impacted by human needs. Nilsson et al. (p. 405) present a global overview of how dams have fragmented the world's largest river systems. Nearly half of the world's large river systems have major dams or diversions that either fragment ecosystems or reduce or regulate flow. Syvitski et al. (p. 376) describe a method for quantifying the impacts of anthropogenic activity, such as building dams, on the delivery of sediments to the coasts. They present an analysis of how sediment fluxes have changed between the past, when human influence was negligible, and the present. Their quantitative, global, river-by-river survey of the majority of the world's rivers reveals that human activities, like irrigation or agriculture, have increased fluvial sediment erosion, but that the rate of sediment delivery to the coasts has decreased because of trapping in artificial reservoirs.

  3. Tracking a Solid-Liquid Transition

    A number of studies have examined the ultrafast or nonthermal melting of crystals induced by ultrafast excitation. Shortening of the excitation and probe pulses should allow for greater mechanistic insights into the disordering process. Lindenberg et al. (p. 392) studied the optically induced melting of an indium antimonide crystal with sub-100-femtosecond x-ray pulses from an accelerator-based source. They modeled the decreases in diffraction intensity of (111) and (220) reflections at the crystal melted. At short times after the optical excitation, the atoms appear to move along a barrierless potential, with their velocities determined by their initial conditions.

  4. Limits on Spin Entanglement?


    Many schemes for quantum information processing are based on spin manipulation, but could the interactions between spins place limitations on processing capabilities? Rønnow et al. (p. 389) look to a solid-state system that may provide some answers. Tuning the magnetic insulator LiHoF4 to a quantum critical point, they monitor the dispersion relation by neutron scattering and show that there is coupling between the electronic and nuclear spins of the ensemble. Such coupling, they suggest, may place limitations on quantum information processing, such as the distance over which spin excitations can be entangled.

  5. Tetracyclines from Scratch

    Pharmaceutical chemists try constantly to modify the structures of antibiotic compounds as bacteria develop resistance to the drugs currently in use. In the case of tetracycline, which treats a broad range of infections including pneumonia, efficient synthetic routes to derivatives have proven hard to develop. Charest et al. (p. 395; see the Perspective by Khosla and Tang) have now found a strategy to access a broad range of structural variants (all of them 6-deoxytetracyclines) in sufficient quantity for bacterial testing in culture. Tetracyclines consist of four consecutively fused carbon rings, labeled A through D, and D-ring variations have shown particular promise against resistant bacteria. The authors prepared the AB fragment first, and then use the same reaction sequence to attach any of six distinctly modified D rings, forming the C ring in the process. The overall routes proceed in 5 to 7% net yield in 14 steps from benzoic acid.

  6. Last Gasps

    The cause of the end-Permian extinctions has remained unclear. Reconstructions show that oxygen levels, which were extreme earlier, may have declined markedly around as Earth's overall climate warmed. Huey and Ward (p. 398; see the news story by Kerr) present a physiological model of the likely effects of such low oxygen levels and show that the only habitable zone may have been at or near sea level.

  7. Blinded by a Complement

    Age-related macular degeneration (AMD) is the leading cause of blindness in the developed world and is characterized by a breakdown of light-sensitive cells in the retina. Both genetic and environmental factors are thought to contribute to the disorder, but its molecular pathogenesis has been unclear. Three research groups [Klein et al. (p. 385), see the cover; Edwards et al. (p. 421); and Haines et al. (p. 419)—all published online 3 March 2005] have identified a sequence-specific variant in the genome that increases an individual's risk of developing AMD by three-to sevenfold and that may explain 20 to 60% of AMD incidence in older adults (see the Perspective by Daiger). The culprit gene, located on chromosome 1q32, encodes complement factor H, a protein involved in inflammation. This finding opens the door for the development of presymptomatic tests that would allow earlier detection of AMD, which in turn may lead to better treatments.

  8. Let's Get Together

    Cells fuse in a great variety of circumstances during normal development, such as during the fertilization of the egg by sperm or the formation of mature muscle fibers from individual precursor cells. Cell fusion has also complicated interpretations of experiments involving stem cells. Chen and Olson (p. 369) review the mechanics of cell fusion and the variety of circumstances where cell fusion is normally seen, and comment on some of the circumstances surrounding aberrant cell fusion.

  9. Keeping Up Defenses


    Protective barriers in animals, whether the skin of mammals or cuticle in insects, help prevent dehydration and protect against injury. A conserved innate immune system functions in both vertebrates and invertebrates to combat infectious microbes introduced by epidermal injury. However, less is known about the mechanisms for the aseptic wounding response (see the Perspective by Harden). Mace et al. (p. 381) now describe a wound response pathway in Drosophila, which is mediated by the factor grainyhead, and which senses aseptic breaks in the epidermis. The grainyhead mediated response provides cross-linking molecules to fix the cuticular barrier. Complementary work by Ting et al. (p. 411) suggests that this type of barrier wound response pathway is conserved—mice with a mutation in a mouse grainyhead ortholog show defects in epidermal wound repair.

  10. Malaria Membrane Protein Structure

    Apical membrane antigen 1 (AMA1) is an integral membrane protein in malaria-causing Plasmodium parasites and is currently in clinical trials against P. falciparum, the species that causes the most serious forms of malaria in humans. Although AMA1 is essential for host cell invasion, its molecular function is unknown. Pizarro et al. (p. 408, published online 24 February 2005) have solved the crystal structure of the three-domain ectoplasmic region of AMA1 from P. vivax at 1.8 angstrom resolution. Domains I and II belong to the PAN motif, a protein fold that functions in receptor binding.

  11. The Making of a Simple Timepiece

    Cyanobacteria operate under a circadian clock unlike those found in other organisms. It is driven by periodic phosphorylation of a core clock protein, rather than by periodic transcription or translation. Nakajima et al. (p. 414) now show that this oscillator can be reconstituted in vitro with only three clock proteins and a phosphate source, adenosine triphosphate. This supports the notion that biological time measurement in this simple organism is not rooted in the control of gene or protein expression, but on the dynamics of a complex of three proteins in a mechanism that requires little energy.

  12. Keep Your Eyes on the Ball

    Under normal conditions, we are generally not consciously aware of how stimuli arriving via multiple input pathways (such as sight and sound) are integrated into a single percept; this kind of processing can be uncovered when illusory stimuli are presented (for instance, in the McGurk effect: seeing one word being spoken while hearing a related one). Indovina et al. (p. 416) have adapted this approach to explore the interaction between visual and vestibular systems. Although superb at all sorts of tasks, our visual processing centers do not work quite so well in estimating the accelerations of objects. However, our vestibular system learns to cope with gravity at an early age. Behavioral and brain imaging data suggest that the vestibular system relies on an internal model of how the motions of objects are influenced by gravity and passes that information on to the visual processing centers when subjects estimate the time to collision of a falling ball.

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