This Week in Science

Science  01 Dec 2006:
Vol. 314, Issue 5804, pp. 1349
  1. Fast Flickering

    CREDIT: AHARONIAN ET AL.

    Among the very few known extragalactic emitters of very high energy tera-electron volt (TeV) γ rays are blazars, which are galaxies with relativistic particle jets that point toward Earth. It has been suggested that the TeV γ rays originate in those jets. By monitoring the nearby radio galaxy M87, whose twin jets are oriented in the plane of the sky rather than pointed at us, Aharonian et al. (p. 1424, published online 26 October; see the Perspective by Fabian) show that γ rays in active galaxies are actually produced near the central black hole. M87 is bright in γ rays up to 10 TeV, and its brightness varies daily. Such fast variations imply the source of the γ rays lies near the Schwarzschild radius of the supermassive black hole that lies at the heart of the M87 galaxy. Although this behavior may fit some leptonic models for γ-ray production, an alternative mechanism of proton curvature radiation near to the black hole is proposed.

  2. Phytoplankton Clouds

    Phytoplankton produce compounds that can become aerosols, which suggests that biological productivity might exert an important control on cloudiness over the ocean if these aerosols act as cloud condensation nuclei. Meskhidze and Nenes (p. 1419, published online 2 November) combine satellite observations of surface ocean chlorophyll a content and cloud cover to show that biological productivity can have a significant effect on shallow marine clouds. Cloud droplet number concentrations over a phytoplankton bloom in the Southern Ocean doubled, and cloud effective radius was reduced by 30%, which led to a large change in the short-wave radiative flux at the top of the atmosphere.

  3. In Tune for a Second

    High-resolution spectroscopy generally requires a trade-off between the size of the ensemble being probed and the coherence of that sample during the course of the measurement, so that increasing the sample size to raise signal strength often broadens the signal of interest. Boyd et al. (p. 1430) have used an optical trap to inhibit the random motion of strontium atoms in order to maintain coherence of the photoexcited sample for ∼1 second. By careful frequency stabilization of the probe laser, an absorption line at ∼1014 hertz could be measured with a corresponding width of ∼1 hertz. The attained ratio of frequency to linewidth, or quality factor, exceeds previous values by an order of magnitude. Such capabilities facilitate high-precision unit standardization and enhanced measures of fundamental physical constants.

  4. Monsoon Violence

    Most climate models have predicted that extreme rainfall events will become more common as air temperature rises, but observational evidence of this trend has been hard to find. Goswami et al. (p. 1442) used a daily rainfall data set for central India to show that there was an increase in the frequency and intensity of heavy rain events, and a decrease in the frequency of light to moderate rain events, for the monsoon seasons from 1951 to 2000. The mean rainfall did not show a significant trend because the increasing contribution from heavy events was offset by a decreasing one from light ones. These findings suggest that severe rain events over India will become more common if global warming continues as expected.

  5. Controlled Coupling of Qubits

    Performing logical operations on quantum computers will require the coupling and decoupling of qubits so that individual qubits can be prepared in a given quantum state, allowed to interact, and be read out once the final state is achieved. Hime et al. (p. 1427) demonstrate on-and-off control on a pair of superconducting-flux qubits coupled through their mutual inductance. With both qubits also coupled to a nearby superconducting quantum interference device (SQUID), their mutual inductance and the extent of the coupling strength could be controlled by varying the working parameters of the SQUID.

  6. Lining Up at the Front

    CREDIT: GOSWAMI ET AL.

    Self-assembly of molecules can create nanoscale features on flat surfaces, but the maximum extent of a single domain is usually on the order of tens of micrometers. Van Hameren et al. (p. 1433) show that disk-like molecules, in which three porphyrin groups bearing long alkyl groups assemble around a central core, form very long aligned chains over areas of several square millimeters through a dewetting process. On mica, single-column stacks form lines parallel to the evaporation front of smaller droplets, whereas for larger droplets, longer evaporation times cause larger lines of aggregates to grow normal to the evaporation front. Patterns formed on rougher glass surfaces were less regular but could still be used to align liquid crystal molecules.

  7. More Boys Preferred

    Biases in sex ratio at birth have led to the suggestion that females may manipulate the sex of their offspring. Gomendio et al. (p. 1445) now show that males may also influence offspring sex ratio. In red deer, more fertile males tend to produce proportionally more sons who are likely to inherit high fertility rates. Sperm collected during the rut from males living in natural populations was used for artificial insemination to minimize known female effects on sex ratio. Such male contributions to biases in offspring sex ratio suggest an evolutionary scenario in which conflicts of interest between males and females in relation to the sex of their offspring may play an important role.

  8. Turing Patterning in the Mouse Hairs

    More than 50 years ago, Alan Turing provided a theoretical explanation of biological pattern formation through a hypothesis of reaction-diffusion, whereby patterns, such as that for hair follicles or feather distribution, can form as a result of positive and negative feedback regulation of an inhibitor and activator. Turing models have since been used to account for patterns in many chemical systems, but have not been successful in explaining biological pattering in developmental model systems such as the fly. Sick et al. (p. 1447, published online 2 November; see the Perspective by Maini et al.) have now examined hair follicle arrangements in mice that arise through the WNT activator protein and its inhibitor DKK and show through computation modeling that reaction-diffusion can account for the patterning observed.

  9. Not Lost in Translation

    CREDIT: PFINGSTEN ET AL.

    The canonical mechanism for initiation of protein synthesis in eukaryotes involves a nucleotide cap on messenger RNA (mRNA) that is recognized by an initiation protein factor. However, a variety of pathogenic viruses and cellular mRNAs bypass the canonical mechanism by using structured RNA sequences, called internal ribosomal entry sites (IRESs), to initiate translation. Pfingsten et al. (p. 1450) have determined the structure of the ribosome-binding domain of an IRES at 3.1 angstrom resolution. The RNA prefolds to create a specific ribosome-binding structure. By docking the structure onto cryoelectron microscopic reconstructions of an IRES-ribosome complex, contacts were identified that drive binding and induce conformational change in the ribosome.

  10. Of Genes and Gut Reactions

    Inflammatory bowel diseases (IBDs) such as Crohn's disease and ulcerative colitis are thought to be caused by an inappropriate immune response to commensal intestinal bacteria. There is strong evidence that these disorders have a genetic component; for example, individuals carrying specific sequence variants of the NOD2/CARD15 gene are at increased risk. Now, in a genome-wide association study, Duerr et al. (p. 1461, published online 26 October; see the Perspective by Cardon) find that a rare sequence variant of the gene encoding the receptor for interleukin-23 (IL23R) significantly lowers an individual's risk of developing IBDs. Interleukin-23 is a cytokine that has attracted increasing attention because of its role in a wide range of chronic inflammatory diseases in mouse models, including IBDs, multiple sclerosis, and arthritis.

  11. Progesterone and Breast Cancer

    Mutations in the breast cancer susceptibility gene BRCA1 greatly increase a woman's risk of developing breast and ovarian cancers. Why do these mutations predominantly affect hormone-responsive tissues when the mutant gene is widely expressed throughout the body? Poole et al. (p. 1467; see the news story by Marx) suggest that this tissue specificity is caused in part by BRCA1-mediated effects on signaling by the hormone progesterone. Mammary epithelial cells (MECs) of Brca1/p53-deficient mice accumulated high levels of progesterone receptors, probably through defective degradation by the proteasome, and developed aberrant proliferation of the MECs. Treatment with the progesterone antagonist mifepristone (RU 486) prevented or delayed mammary tumor development in the mice.

  12. Singles on CD

    Single-molecule spectroscopy has recently highlighted many cases in which conformational averaging obscures precise details of reaction and photoresponse pathways. Hassey et al. (p. 1437, published online 2 November) extend this resolving technique to circular dichroism (CD), a long-standing method of characterizing chiral mixtures that often yields a response only loosely understood at the molecular level. They probed two diastereomeric helicenes that rested on a polyolefin substrate and were restricted from interconverting by a bound camphor group that blocked internal rotation. The aggregate signals in each case contained substantial minority contributions oppositely polarized from the prevailing sense. Thus, an ensemble measurement of this system would imply a deceptively weak intrinsic molecular response. The authors attribute the broad range of signal responses to distinct conformations of the helicenes relative to the surface.

  13. Primitive Spheres

    Round globules of organic material pepper some primitive meteorites. Many are hollow spheres with thin carbon-rich shells just micrometers in size. Nakamura-Messenger et al. (p. 1439) have measured isotope ratios of 15N/14N and D/H for these carbon nanospheres in the Tagish Lake meteorite and show that the ratios are much higher in the spheres than in the rest of the matrix. Similar hotspots were seen before in meteorite organic material but not localized to particular grains. The isotope ratios indicate that the spheres were formed at very low temperatures, around 10 to 20 kelvin, that are characteristic of cold pre-solar molecular clouds.

  14. Turn On the PIPs

    Many cellular proteins are inhibited when phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] is hydrolyzed by phospholipase C, and reactivated when a kinase restores the PtdIns(4,5)P2 (see the Perspective by McLaughlin, published online 9 November). Whether depletion of PtdIns(4,5)P2 is the direct signal closing KCNQ potassium channels, an important ion channel in the nervous system, upon receptor activation is still controversial. Suh et al. (p. 1454, published online 21 September) now show that a loss of PtdIns(4,5)P2 indeed suffices to close the channel. Furthermore, KCNQ channel opening was stimulated by synthesis of extra PtdIns(4,5)P2 but only minimally affected by production of extra phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3]. Small GTPases (guanosine triphosphatases) have many roles in cellular regulation and often transiently associate with the plasma membrane. Heo et al. (p. 1458, published online 9 November) monitored the cellular localization of fluorescently tagged small GTPases and found that the majority of the enzymes that associated with the membrane contained a cluster of positively charged amino acids. Suspecting that these regions might bind negatively charged lipids, the authors showed that PtdIns(4,5)P2 and PtdIns(3,4,5)P3 were required for targeting the GTPases to the plasma membrane. The lipids may act as signaling hubs that control multiple aspects of cellular control systems.

  15. Making Connections

    Metagenomic analyses of complex communities are dominated by genome “shrapnel”; unless the microbial community is dominated by one or a few species, it is hard to reconstruct the community by computation. Ottesen et al. (p. 1464) have been developing microfluidic devices to perform a variant of digital polymerase chain reaction that samples individual bacteria in complex environmental consortia. They tested their device on the bacterial flora of the termite gut to identify bacterial contributors to homoacetogenesis by tracing pairs of small-subunit ribosomal RNA genes and the key metabolic gene, thereby mapping taxonomic position with function.

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