This Week in Science

Science  19 Nov 2004:
Vol. 306, Issue 5700, pp. 1255
  1. Nanotube Yarns and Forests


    Spinning fibers to make stronger yarns and ropes is an old technology. By looking at the fundamentals of this process and scaling them down to fibers with nanometer-sized diameters, Zhang et al. (p. 1358) have developed a technique to spin carbon nanotube yarns from mats of fibers. The twisted yarns can be infiltrated with a polymer to improve their strength. Unlike larger diameter materials, a knot can be made in the ropes without a loss of strength. Many methods have been developed for making single-walled carbon nanotubes, but there are still significant limitations to making the tubes in large quantities and free from impurities or residual catalysts. Hata et al. (p. 1362; see the news story by Service) modified the standard chemical vapor deposition synthesis by adding a small amount of water, which removes the residual carbon from the catalyst particles and keeps them chemically active for longer periods. The nanotube forests are easily removed from the bed of catalyst particles, which continue to be chemically active.

  2. Toward Optical Metamaterials

    Metamaterials that are engineered to exhibit negative indices of refraction can provide a number of advantages in optics, such as the fabrication of a “perfect” lens, and much effort is being directed to push the frequencies at which negative indices can be achieved into the optical regime. Using nanofabrication techniques to shrink the dimensions of gold nanostructures making up the metamaterial, Linden et al. (p. 1351) show that the magnetic response can be raised to 100 terahertz. On the theoretical side, Pendry (p. 1353) introduces an alternate route to the design of metamaterials exhibiting negative refraction that may prove easier to prepare than the present structures, which are based on tuning the electric and magnetic response. The proposed structure relies on chirality and consists of a series of helically folded metallic foils. Designers should be able to work with the polarization of either the magnetic or the electric field, rather than both.

  3. Improving Optical Clocks

    The development of frequency-stabilized sources of laser radiation, together with the associated coupling of frequency cycles in the optical regime, offer the potential to exceed the accuracy set by atomic standards that operate in the lower frequency microwave regime. Margolis et al. (p. 1355) have developed an optical frequency standard based on measuring the transition frequency of a trapped strontium ion. The transition frequency is determined to nearly 1 Hertz in 1015 and represents a fractional uncertainty within a factor of three of the primary cesium atomic-clock standards.

  4. The Spin on Martian Argon


    Argon concentration in the martian atmosphere can be used to trace the planet's rotational dynamics and seasonal patterns. Measurements from the Gamma-Ray Spectrometer on the Mars Odyssey spacecraft by Sprague et al. (p. 1364, published online 7 October 2004; see the Perspective by Forget) suggest that non-condensible argon is enhanced at mid-latitudes during the summer and decreases at more polar latitudes in early autumn, even though carbon dioxide is condensing out of the atmosphere onto the polar cap in the southern hemisphere. The data suggest that meridional mixing occurs, which is contrary to the idea that separate vortices of material, particularly at the poles, are driven by planetary rotation.

  5. Before the Divide with the Great Apes

    The group that includes humans and great apes is thought to have diverged from other apes (such as gibbons) in the Middle Miocene, about 10 to 15 million years ago. Few relatively complete fossils are available from this time; all are thought to be related to later great apes from Eurasia. Moyà-Solà et al. (p. 1339; see the news story by Culotta) have recovered a remarkably preserved fossil of a new ape species from Spain dating to about 13 million years ago. The cranium, which is nearly complete and undistorted, the thorax, and bones including the wrist show a mix of both primitive, derived, and very modern features. The skeleton also shows that the distinctive posture of great apes had evolved by this time. The fossil may be close to the last common ancestor of the great apes and humans.

  6. Understanding Mimi

    Mimivirus is an extremely large DNA virus that grows in amoebae. Raoult et al. (p. 1344, published online 14 October 2004) have sequenced and analyzed the genome of the Mimivirus, which is 1.2 megabases long—more than three times larger than any other viral genome previously sequenced. Among its 1200 open reading frames are genes not previously thought to be part of the classical definition of a viral repertoire, including genes with homology to transfer RNAs (tRNAs), translation initiation factors, polysaccharide synthesis enzymes, tRNA synthetases, and enzymes involved in nucleic acid metabolism. Mimivirus appears to represent a new family of nucleocytoplasmic large DNA viruses that emerged early in evolution.

  7. Identifying the Chosen Strand

    Small interfering (si)RNAs provide the sequence information that allows the RNA-induced silencing complex (RISC) to destroy target messenger RNAs. siRNAs generated by the enzyme Dicer are double-stranded (ds), but the “guide” RNA used by RISC needs to be a single stand. The stability of the base pairs at the 5' ends of both of the siRNA strands plays an important role in distinguishing between them. Tomari et al. (p. 1377) now provide insight into how this choice is made. The RISC loading complex, which consists of Dicer itself together with the dsRNA-binding protein, R2D2, can detect and differentiate between the siRNA 5' end stabilities, with R2D2 binding to end with the most double-stranded character. As the siRNA is unwound, the guide strand would then be transferred from R2D2 to RISC, while the other strand would be destroyed.

  8. Now You See It, Now You Don't


    Existing fluorescent protein highlighting techniques are irreversible and preclude repeated monitoring of the same protein to study its temporal regulation. Within cells, protein movement is regulated by many different factors and may be altered by changes in the cellular state. Measurements of protein dynamics are affected by the geometry of both the cells and the highlighted regions, and any changes in movement should ideally be assessed using data from a single cell. Ando et al. (p. 1370) describe the engineering and application of a fluorescent protein, Dronpa, which can be reversibly highlighted to study spatiotemporal protein dynamics in living cells. The authors directly visualized the influx and efflux of a key regulator of intracellular signaling, mitogen-activated protein kinase, into and out of the nucleus.

  9. Virus Exploits a Serotonin Receptor

    JC virus (JCV) is a common human polyomavirus responsible for the fatal demyelinating disease, progressive multifocal leukoencephalopathy (PML), in immunocompromised individuals—about 5% of AIDS patients develop this currently untreatable fatal disease. Typical and atypical antipsychotic drugs inhibit JCV infection of glial cells. Elphick et al. (p. 1380) now find that the cellular receptor for JCV on glial cells is a serotonin receptor. The findings contribute to the understanding of the pathogenesis of PML in AIDS patients and suggest that therapy based on existing serotonin receptor inhibitors may be feasible.

  10. Please Release Me

    During times of food deprivation or increased energy demand, mammals begin to use the intracellular triglycerides stored in fat tissue as a primary energy source. Mobilization of these stored lipids requires activation of enzymes that degrade them so that free fatty acids, the molecules that supply energy to most tissues, are released into the blood. Zimmermann et al. (p. 1383) identify a new enzyme, adipose triglyceride lipase (ATGL), that is expressed at high levels in mammalian adipose tissue and catalyzes the initial step in triglyceride degradation. Because abnormalities in lipid metabolism are often associated with obesity and type 2 diabetes, ATGL could represent an important new drug target for these conditions.

  11. The Nuclear Pore, Up Close and Personal

    Cryoelectron tomography of intact cells or organelles has been developed to study molecular structures in their native environments, unaffected by isolation and purification procedures which may entail the loss of components. Beck et al. (p. 1387, published online 28 October 2004) studied intact nuclei from Dictyostelium discoideum by cryoelectron tomography with a focus on the structure of the nuclear pore complexes. The images detail the components of the pore and reveal putative transport substrates.

  12. Regulating Oscillatory Calcium Signals

    Variation in the intensity and frequency of intracellular calcium signals impact numerous calcium-dependent cellular responses, but the underlying mechanisms that regulate oscillatory calcium signaling have not been fully resolved. Launay et al. (p. 1374) report that generation and maintenance of the calcium oscillations that control the production of the cytokine interleukin-2 in stimulated T cells involve a calcium-activated nonselective cation channel called TRPM4. In response to a rise in intracellular calcium, TRPM4 is activated and contributes to depolarization of the membrane potential, which suppresses further calcium influx. Subsequent repolarization closes TRPM4 channels and reestablishes conditions for further calcium influx.

  13. Assessing the Rate of Gene Duplication Events

    Gene duplication is thought to play a significant role in the evolution genomic novelty, yet the rate of gene duplication is still unclear. Previous estimates of the rate of origin of new gene duplicates using the molecular clock model are very high—about one event per billion years. Gao and Innan (p. 1367) compare the genomes of six closely related yeast species and Saccharomyces cerevisiae to calculate the rate of gene duplication without reliance on the molecular clock model and taking into account gene conversion, or the concerted evolution of duplicate genes (which would confound clock based estimates). The new estimate for the rate of gene duplication is two orders of magnitude lower, about 0.01 to 0.06 duplications per billion years.

  14. Shedding Light on a Light Meter

    Microbes generally find it difficult to seek shelter from the Sun and must be able to sense and adjust to varying illumination conditions to protect themselves from light-induced damage. Sensory rhodopsins are a subset of the large family of archaeal and eubacterial rhodopsins. Unlike their ion-pumping brethren, they use an integral chromophore, a retinal derivative, to sense the wavelength of incident light and communicate this information through protein-protein interactions. Vogeley et al. (p. 1390, published online 30 September 2004) describe the crystal structure of a cyanobacterial (Anabaena) sensory rhodopsin which has the unusual property of existing in two photoactive stable states; most rhodopsins are stable in a dark-adapted state and cycle through a sequence of conformational changes upon illumination. These orange- and blue-sensitive states would enable Anabaena to assess the quality (color composition) of ambient light and to adjust its repertoire of light-harvesting complexes.