This Week in Science

Science  03 Sep 2004:
Vol. 305, Issue 5689, pp. 1365
  1. Plug-In Photonics


    Photonic crystals confine light by the periodicity of their structure. When defects are introduced at specific positions in their lattice, light can be guided out of the structure, and this capability has resulted in optical devices with spatial volumes on the size scale of the wavelength of light. The smaller devices so far, however, have been optically pumped. For practical application and ready integration into optoelectronic technology, electrically driven devices are required. Park et al. (p. 1444) have developed a defect-mode photonic crystal laser that allows the carriers to be injected electrically. The devices have low current thresholds and operate in pulsed mode at room temperature.

  2. Clumps and Bumps in a Dusty Disk

    Debris disks around young stars are full of dust and gas, created when objects in the disk collide. The young, nearby star Beta Pictoris (β Pic) has a well-studied, dust-rich and relatively large disk, and by studying such disks, astronomers may find evidence of extrasolar planets. Recently, a disk was discovered around the young star, AU Microscopii (AU Mic), which is near to β Pic and about the same age. Using the Keck II 10-meter telescope and adaptive optics, Liu (p. 1442, published online 12 August 2004) has now found clumps, an asymmetric variation in disk thickness, and some bending of the inner disk around AU Mic. These substructures may be attributed to perturbations of the disk by extrasolar planets.

  3. Neat Nanotube Fibers

    Single-walled carbon nano- tubes (SWNTs) can be difficult to process because they are insoluble in most solvents. The addition of surfactants can improve SWNT solubility, but the surfactants tend to poison the outstanding nanotube properties. Ericson et al. (p. 1447), building on previous work in which they showed that SWNTs can dissolve in fuming sulfuric acid, have developed a process for spinning the SWNTs into highly oriented fibers without having to debundle the as-formed nanotubes. They show how the superacids interact with the nanotubes and nanotube bundles to make them soluble.

  4. Bone Supports Bipedal Contention


    One candidate for an extremely early hominid is Orronin tugenensis, found in 2001 in Kenya. The fossils included several limb bone fragments, including several parts of three femora. These fossils were interpreted as representing a bipedal hominid dating to 6 million years ago, although this interpretation has been widely debated and disputed. Galik et al. (p. 1450) have now used computerized tomography to analyze the internal structure of the most complete left femur. The structure of the femur, which reflects the loads placed on it, matches closely that of humans and is distinct from those of gorillas and chimps, and confirms a bipedal origin.

  5. Slicer Steps into the Limelight

    During RNA interference, small interfering (si)RNAs generated by Dicer (or provided exogenously) are loaded onto the RNA- induced silencing complex (RISC), which then binds homologous target RNAs, cleaving and inactivating them. The major constituents of RISC are the single-stranded siRNA and any one of a number of different proteins of the Argonaute (Ago) family. Until now, the identity of the nuclease in RISC, nicknamed “Slicer,” has remained a mystery (see the Perspective by Sontheimer and Carthew). Song et al. (p. 1434, published online 29 July 2004) present the structure of the Ago protein from Pyrococcus furiosus, which consists of four domains; the PAZ and PIWI domains being the defining characteristics of Ago. The PfAgo PIWI domain is homologous to RNase H, including conserved catalytic residues, and the juxtaposition of PAZ and PIWI domains suggests a mechanism by which Ago might load and cleave target RNAs. Liu et al. (p. 1437, published online 29 July 2004) show that, unlike other mouse Agos, only Ago2 can form a cleavage-competent RISC. Ago2 is also essential in vivo for RNAi, and is required for normal mouse development. Because the conserved catalytic residues in the RNaseH-like PIWI domain are critical for RISC cleavage activity, it is likely that Ago2 is “Slicer.”

  6. Methane Counter-Production

    The anaerobic oxidation of methane that takes place in anoxic sediments has long been attributed to sulfate-reducing bacteria, but none has been found that oxidize methane. More recently, it has been suggested that the methanogens themselves can consume methane. Hallam et al. (p. 1457) have discovered methane-oxidizing archaeans that have most of the methanogenesis machinery, and suggest these organisms also consume methane by reversing the methanogenesis pathway. This process is apparently thermodynamically coupled with the activities of sulfate-reducing bacteria in microbial consortia that develop in anoxic sediments.

  7. Molecular Beak Tweaking

    Two studies explore the molecular origin of beak variation (see the news story by Pennisi). Abzhanov et al. (p. 1462) examined the genus Geospiza, or “Darwin's finches,” to explain the molecular events in specifying the bird beak. The correlation of the morphology of the beak and expression of bone morphogenic protein 4 (Bmp4) among six species of finches supports the hypothesis that the expression of this factor accounts for differences in beak morphology between species. Wu et al. (p. 1465) looked at differences in chicken and duck beaks and note variations in the respective zones of cell proliferation and Bmp4 expression.

  8. Sending a Cell-Death Sentence

    Cancer cells proliferate because they evade programmed cell-death pathways, and much effort is being devoted to finding ways to activate apoptotic pathways in such cells (see the Perspective by Denicourt and Dowdy). Key interactions that determine whether cells live or die are mediated by so-called BH3 (BCL-2 homology 3) domains, which are found in proteins that regulate apoptosis. Such signals can be mimicked or disrupted by peptides that resemble the interaction domains, but such molecules have major shortcomings as experimental or therapeutic agents because of low potency, instability, and inefficient delivery to cells. Walensky et al. (p. 1466;) now show that these problems could be overcome when a BH3 domain that promotes apoptosis was held in its native α-helical form by a chemical modification they call a hydrocarbon staple. The modified peptide showed increased binding affinity for its target, was relatively protease resistant, and could cross cell membranes. Preliminary studies in animals even showed that the modified peptides could decrease growth of transplanted tumors in mice. The activity of caspases, the cysteine proteases that mediate cell death by apoptosis, is held in check by the inhibitor of apoptosis proteins (IAPs). The protein known as Smac promotes apoptosis by binding to IAPs and relieving inhibition of caspases. Li et al. (p. 1471) show that the effect of the Smac peptide can be potently mimicked by a small membrane-permeable molecule. Studies with the compound revealed that the well-known requirement for inhibition of protein synthesis to allow apoptotic effects of tumor necrosis factor α (TNFα) likely reflects decreased IAP- mediated inhibition of caspases. The new compound sensitized cancer cells in culture to TNFα-induced cell death.

  9. A Disarming Approach to Predation


    Predation has often been considered to be an important force in driving evolution. Several periods in Earth's history seem to record rapid evolution of both predators and prey; one of these is the Mid-Paleozoic Marine Revolution, about 440 to 360 million years ago. To test whether increased predation might be recorded in the fossil record directly, and whether it might have driven this marine revolution, Baumiller and Gahn (p. 1453; see the news story by Stokstad) examined the damage to arms of crinoids. Crinoids often sacrifice or shed one or more of their arms to attackers, then regenerate them. The distribution of crinoids with damaged arms jumped abruptly during this revolution, supporting the predation hypothesis.

  10. A Swell Way to Grow

    Early self-replicating systems that acquired an encapsulating membrane would presumably have gained vital protection from the environment, but acquisition of a membrane would have also required that the membrane grow and divide in synchrony with the replicator. RNA is a candidate early replicator, and Chen et al. (p. 1474) have looked at the link between RNA-based replicators and fatty acid-based vesicles. Encapsulated RNA exerts osmotic pressure on the membrane. These swollen, hypertonic vesicles grow by scavenging membrane from isotonic vesicles with low osmotic pressure. Thus, vesicles containing more effective RNA replicators (or, indeed, any replicator that exerts osmotic pressure) would have grown and outcompeted less-effective vesicle-encapsulated replicators.

  11. Tiny Star Grains

    High-resolution isotopic and microstructural analyses of two presolar grains from the Tieschitz meteorite indicate that corundum and amorphous alumina form from asymptotic giant branch stars. These grains condensed in the circumstellar disk around the stars and survived without much further processing in the interstellar medium and during the formation of the solar system. Thus, as Stroud et al. (p. 1455) conclude, these grains confirm observations of corundum and amorphous alumina and provide chemical details to refine models of stellar and planetary system formation.