This Week in Science

Science  15 Jun 2001:
Vol. 292, Issue 5524, pp. 1961
  1. Saturation of Ultraviolet Free-Electron Lasers

    Free-electron lasers (FELs) combine the ability to tune the wavelength of emitted radiation with the potential to achieve an x-ray brightness that could be 10 orders of magnitude higher than that of existing synchrotrons. In a FEL, an electron beam is passed through the undulator, a region of modulated magnetic field, and radiation is emitted that interacts with the electron beam. Saturation of this interaction, a key issue in the development of FELs, has so far been limited to millimeter wavelengths. Milton et al. (p. 2037) demonstrate saturation for shorter visible and ultraviolet wavelengths, a critical step on the road to even shorter x-ray wavelengths.

  2. Trailing Blazars

    The high-energy sources of emission creating the extreme brightness of quasars are thought to be supermassive black holes with an accretion disk and two polar jets. About 10% of quasars show high emissions at radio wavelengths, and a fraction of these radio-loud quasars display large outbursts of radiation. The outbursts of these “blazars” have been attributed to relativistic beaming from the jets. A unified scheme suggests that all radio-loud quasars have relativistic jetting—blazars are different only because we are viewing them down the axis of the jet. Ma and Wills (p. 2050; see the news story by Schilling) found indirect evidence of blazars, independent of the viewing geometry, in a survey of radio-loud quasars. They conclude that all radio-loud quasars are produced by similar physical processes.

  3. Linearly Polarized Light from Out-of-Shape Particles

    Spherical semiconductor quantum dots can emit light at optical wavelengths when charge carriers in the dot recombine, and by changing the size of the particle, the emission strength and wavelength can be tuned. This emission is plane polarized, but it would be even more useful in applications such as displays if the emission were linearly polarized. Hu et al. (p. 2060) now show experimentally that elongating CdSe dots even to only an aspect ratio of 2 results in linearly polarized emission.

  4. All Astir

    Some achiral molecules can pack so that they form optically active crystals. Under quiescent conditions, equal populations of left- and right-handed crystals usually form, but in some cases, such as crystallization of sodium chlorate, stirring can cause an entire batch to form crystals all of one handedness (but with a equal chance of the batch being rights or lefts). Ribó et al. (p. 2063; see the Perspective by Feringa) studied the evaporative aggregation of achiral diprotonated porphyrin molecules, which form lyotropic liquid crystals at high concentrations. The aggregates formed are chiral, and the chirality depended on the direction of vortex motion of stirring during evaporation, apparently through the asymmetric influence of vortex motion on the specific binding sites between porphyrins.

  5. From Receptor to Nucleus

    The binding of extracellular ligands to their receptors on the cell membrane can evoke a variety of responses and, in some cases, these responses include a change in gene expression and transcription, which takes place in the nucleus. Santagata et al. (p. 2041; see the Perspective by Cantley) describe a new pathway in which occupancy of a plasma-membrane receptor results in mobilization of a transcription factor to the nucleus. In its inactive state, the tubby protein, which has been connected to adult-onset obesity in mice, binds to the phosphorylated head group of a plasma-membrane lipid. When the G protein-coupled serotonin receptor is activated, phospholipase C hydrolyzes the lipid and releases tubby, which then enters the nucleus, probably via its nuclear localization sequence. An intriguing suggestion is that this result may explain why serotonin-receptor knockout mice display symptoms of adult-onset obesity.

  6. Nanometer-Scale Imaging in a Magnetic Field

    As magnetic devices continue to shrink in size toward to the superparamagnetic limit, there is a need to understand the basic physical behavior of these systems on the nanometer scale in the presence of a magnetic field. Most imaging techniques developed so far provide either high spatial resolution or compatibility with magnetic fields, but not both. Pietzsch et al. (p. 2053) report that spin-polarized scanning tunneling microscopy can be used to image magnetic switching and hysteresis in an array of iron nanowires.

  7. Algae Aquaculture in the Dark

    Diatoms and other sorts of algae, while unwelcome during a summer swim in a pond, are important sources of food and useful phytochemicals. However, for obligate photoautotrophic algae, their success in aquaculture is limited by the amount of light that can be reliably delivered. Zaslavskaia et al. (p. 2073; see the cover and the Perspective by Stephanopoulos and Kelleher) show that transforming a diatom with a gene encoding a human glucose transporter converts them into heterotrophs that can grow without light so long as they are supplied with glucose.

  8. A Busy Gα Subunit

    A typical animal genome contains several genes encoding G protein α subunits, but the Arabidopsis genome appears to encode just one. Two reports show that this one Gα is busy regulating physiological functions in plants (see the Perspective by Ellis and Miles). Wang et al. (p. 2070) found that Gα mediates abscisic acid regulation of stomatal pore size, thus affecting water conservation in the plant. Ullah et al. (p. 2066) show that the same protein is involved in regulation of cell proliferation.

  9. Telomeres Silence in Human Cells

    The reversible silencing of a gene near the end, or telomere, of a chromosome [the telomere position effect (TPE)] depends both on telomere length and the distance between telomere and the affected gene. This effect is well characterized in yeast. By seeding de novo telomere formation in human cells with DNA linked to a luminescent reporter gene, Baur et al. (p. 2075) show that reporters adjacent to the new telomeres produced 10 times less luminescence than those located at internal sites in the chromosomes. As in yeast, increasing the length of the telomeres further increased TPE. Because most human telomeres shorten with age, TPE might provide a mechanism for altering cell expression during the replicative life-span of the cell.

  10. Gene Silencing and CpXpG Methylation

    Methylation of DNA provides an important epigenetic “mark” involved in controlling gene expression and genomic stability and most often results in the silencing of affected genes. The most common mark is found at symmetrical CpG dinucleotides, the methyl group being added by cytosine methyltransferases of the Dnmt1/MET1 class. Although methylation is found at other non-CpG sites—CpXpG, for example—little has been known about the enzymes that make or maintain these marks. Lindroth et al. (p. 2077) have now performed a genetic screen in the plant Arabidopsis to isolate a gene involved in methylation of CpXpG, the cytosine methyltransferase homolog CHROMOMETHYLASE3. Mutations in the gene result in the activation of normally inactive retrotransposon sequences and demonstrate that CpXpG methylation is also involved in gene silencing.

  11. Just-in-Time Delivery

    The transcription of the genes required in the assembly of the bacterial flagellum appears to be carefully timed for maximal efficiency. To understand the organization of genes needed to build functioning flagella without interfering directly with the genetic pathway, Kalir et al. (p. 2080) measured promoter activity by means of a panel of fluorescently labeled reporter plasmids. A precise temporal program of expression was observed which suggested that timing might be regulated by the binding affinity of transcription factors to promoter regions

  12. The Longest Wake

    Islands disrupt prevailing winds to create ocean wakes, but these wakes generally extend no more than a few hundred kilometers. Xie et al. (p. 2057) document an exception to this norm: The wake downwind from the Hawaiian Islands. Satellite observations of this wake reveal that it persists for 3000 kilometers. These long structures are maintained by an interaction between the atmosphere and the ocean that creates a positive feedback involving sea surface temperature, clouds, and wind patterns. This wake is more than just a curiosity—it modifies local atmospheric and oceanic circulation and may influence atmospheric transport of aerosols and trace gases as well as the distribution of plankton and other fishery resources.

  13. Vitamin C and Cancer

    The use of high doses of antioxidants such as vitamin C to protect against cancer has been controversial. Advocates cite epidemiological studies that have revealed an association between high intake of vitamin C and reduced risk of cancer, as well as laboratory studies that demonstrate roles for vitamin C in free-radical scavenging and immune stimulation. Opponents cite the negative results of randomized clinical trials testing antioxidant therapy as well as laboratory evidence that vitamin C might also have a pro-oxidant effect. Lee et al. (p. 2083) now present in vitro data, which show that vitamin C can induce genotoxin formation (agents that damage DNA). If generated in significant amounts, these genotoxins, which are the products of vitamin C-enhanced decomposition of lipid hydroperoxides, could generate cancer-causing mutations.

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