This Week in Science

Science  29 Sep 2000:
Vol. 289, Issue 5488, pp. 2237
  1. Flashy Jets in a Radio Galaxy

    Supermassive black holes may be detected by the relativistic jets created by the accretion process. Gómez et al. (p. 2317) observed the jets associated with the radio galaxy 3C120 for 16 months with the high-resolution Very Large Baseline Interferometer in order to better understand how these jets are created and how the local environment may alter their structure. They found knot structures in the jet that could not be readily explained by opacity or Faraday effects, but instead require the rotation of the magnetic field in the core of the galaxy. Other changes in the knots, such as flaring, accretion, and disappearance, can be related to denser molecular clouds in the surrounding medium.

  2. Separating the Almost Equal

    The rare earth elements exhibit similar chemical reactivity, which has made the separation of individual elements a difficult process that includes solvent extraction or ion exchange steps. The expense of the resulting pure rare earths limits some of their potential applications in electronic, magnetic, and optical materials. Uda et al. (p. 2326, see the Perspective by Fray) introduce a separation technique that takes advantage of the different redox potentials of rare earth elements for selective reduction and the differing vapor pressure of their chloride compounds for distillation. For the separation of samarium and neodymium, the separation factor was improved by nearly two orders of magnitude compared with conventional processes. Further improvements may result by replacing the chlorides with iodides.

  3. How Low Is Low?

    The New Madrid Seismic Zone is an intraplate region in the central United States that produced three large earthquakes between 1811 and 1812. Recent geodetic measurements suggested that strain accumulation rates are now low—but this finding may not mean that the probability of another large-magnitude event in the next few hundred years is also low. Kenner and Segall (p. 2329) modeled this intraplate region by incorporating a weak layer of lower crust that transfers stress to the overlying crust. Their calculated interseismic strain rates are consistent with past seismicity and geologic structure but are also so small that they may not be detected by geodetic measurements. They propose some long-term measurements that may be able to determine such small strain rates.

  4. Eyeing Movements

    When we watch a movie, we assume that we can easily watch and interpret all of the complex scenes and movements shown on the screen. However, many different levels of analysis must be performed in the visual system of our brain before a complex motion is perceived. An important step in the analysis of image movement is already performed in the eye. More than 35 years ago, Barlow and Levick characterized direction-selective neurons in the retina, ganglion cells that fire strongly only when an image moves in their preferred direction. Taylor et al. (p. 2347, see the news story by Barinaga) have now analyzed the cellular and synaptic mechanisms for the computation of direction selectivity in these cells. Their electrophysiological study shows that the critical asymmetric inhibition is caused by inhibitory cells impinging directly onto the dendrites of ganglion cells. Thus, processing in neuronal dendrites is a fundamental part of direction selectivity.

  5. Observing Quantum Conduction Channels

    Electron flow through a restricted geometry on the nanometer scale leads to the now familiar observation of quantized conductance in which electrons flow through an integer number of channels, each of which has a fixed conductance described in terms of fundamental constants. A scanning probe microscopy study of conduction through a quantum point contact by Topinka et al. (p. 2323) now shows directly that the number of conduction channels increases as the size of the contact is increased. Their images also reveal the coherent transport of the electrons through the contact and the angular distribution of the conduction channels, in agreement with theoretical predictions.

  6. Frozen Fuel

    The largest known reservoir of hydrocarbons in Earth is not coal or oil, but the methane contained in submarine gas hydrates. Methane hydrates occur in the sediments of continental slopes as a thin layer roughly 200 to 500 meters below the sea floor where they are stable, but little has been known about how they actually form. Fehn et al. (p. 2332) used accelerator mass spectrometry measurements of cosmogenic iodine-129 to date the pore waters associated with gas hydrates from the western North Atlantic. The iodine in these pore waters dates to about 55 million years ago, far older than the surrounding sediments, and is likely produced from older and deeper organic sources. They suggest that these organic species are also the source of the methane, which migrated up through 1 to 3 kilometers of sediment before freezing in its present location.

  7. Cachexia and NF-kappa B

    Patients with chronic diseases such as cancer and AIDS often develop cachexia, a life-threatening disorder characterized by extensive weight loss and degeneration of skeletal muscle. The cytokine tumor necrosis factor (TNF) is a critical mediator of the muscle wasting in cachexia, but little else is known about its molecular pathogenesis. Two reports discuss the role that the transcription factor NF-kB, a key component in the inflammatory response induced by TNF, plays in this disease. Guttridge et al. (p. 2363, see the Perspective by Tisdale) show that NF-kB in muscle cells suppresses the activity of MyoD, a transcription factor needed for muscle repair, by reducing the levels of MyoD messenger RNA. Lee et al. (p. 2350) describe the phenotype of knockout mice lacking the A20 protein, an inhibitor of signaling through NF-kB. The mice show severe inflammation and cachexia and die prematurely. The results suggest an inhibitory physiological role for A20 early in the signaling pathway leading from the TNF receptor to the activation of NF-kB. Paradoxically, A20 also appears to protect cells from programmed cell death.

  8. Separating the Sexes

    The evolution of separate sexes in plants (where hermaphroditism is the norm) has been widely studied. Gender dimorphism is a widespread evolutionary trend in plants and occurs in nearly half of the angiosperm families. Miller and Venable (p. 2335) propose a new pathway for the evolution of dimorphism, in which polyploidy is the factor that triggers gender specialization. Their main evidence comes from a single genus, but supporting data that come from 12 genera in seven different families indicate that polyploidy has broken down self-incompatibility, which would set the stage for gender dimorphism to evolve through an outcrossing mechanism.

  9. An Open or Shut Case

    Calcium waves, or oscillations, have been observed in various plant and animal cells. Allen et al. (p. 2338) have now made a connection in plants between cytoplasmic calcium oscillations and stomatal closure. Guard cells of mutant Arabidopsis plants showed calcium oscillations in response to some, but not all, of the physiological triggers and were unable to effect stomatal closure. When calcium oscillations were imposed by external treatments, however, stomata could close.

  10. Follow the Leader

    The daily rhythms of our bodies, and those of other mammals, are thought to be coordinated by a “master” clock located in the hypothalamus of the brain, which in turn controls peripheral “slave” clocks in the other organs. Balsalobre et al. (p. 2344) identify glucocorticoid hormones as one of the signals that the master clock may use to coordinate the slaves. A synthetic glucocorticoid, dexamethasone, can transiently alter the phase of the rhythms in liver, kidney, and heart at any time of day or night. However, the master clock in the brain, which contains no glucocorticoid receptor and so is not affected by dexamethasone treatment, responds to entraining signals only at certain times in its cycle.

  11. Going After the Tag

    Bacteria possess a specialized machinery for degrading proteins whose synthesis has stalled for some reason, such as a lack of the appropriately charged aminoacyl transfer RNAs. They add an 11-residue peptide tag known as ssrA that marks newly synthesized but incomplete proteins for unfolding and degradation by the energy-dependent protease ClpXP. Levchenko et al. (p. 2354) describe a ribosome-associated protein, SspB, that stimulates degradation of proteins bearing the ssrA tag. Because SspB production is stimulated by starvation—which is likely also to increase the amount of ribosome stalling—the system could provide a means of efficiently recycling amino acids for synthesis of essential proteins during nutrient stress.

  12. A Do-It-Yourself Approach

    The DNA of all eukaryotic genes is buried in chromatin, the major protein components of which are the histones, around which the DNA is wrapped. A critical step in the activation of a gene is the “freeing up” of the chromatin so that the general transcription factors have access to the DNA. Although much is known about the posttranslational modifications of histones that add acetyl and methyl groups to the proteins, a less well understood modification that is correlated with the activation of gene expression is the addition of ubiquitin to the histones. Pham and Sauer (p. 2357, see the Perspective by Mizzen and Allis) set out to identify the protein in cells responsible for this modification. It turns out that the culprit is none other than one of the components of the general transcription machinery itself, TAFII250, a central subunit of transcription factor TFIID. A mutation in the portion of TAFII250 thought to be responsible for adding ubiquitin to histones affected transcription and reduced the level of ubiquitination in the fruit fly Drosophila.

  13. Going Backward

    When animals are cloned by using cellular nuclei from somatic cells, rather than the germ cells that are normally responsible for propagation, the nucleus must be reprogrammed from a differentiated state in the reverse direction to a totipotent ground state. This process will lead to changes in the structure and the complement of regulatory proteins found within the nucleus. Kikyo et al. (p. 2360) have now analyzed an adenosine triphosphatase, ISWI, that contributes to the remodeling of chromatin in Xenopus cells. This enzyme helps release the TATA binding protein, a critical general transcription component, from the nuclear matrix.

  14. Competing Magnetism in Low Dimensions

    Strongly correlated electron systems in reduced dimensions provide an important test-bed for studying the dynamics of quantum phase transitions; that is, phase transitions where the competition between ground states is caused by zero-temperature quantum fluctuations. Eom et al. (p. 2320) introduce results on time-dependent evolution of spin-ordering in a two-dimensional electron gas for quantum Hall states that are normally ferromagnetic but that can become antiferromagnetic under applied pressure. After perturbing this natural ferromagnetic state, they can watch how the mixed magnetic phases evolve and interact.

  15. Faster Overall

    The sequence of mitochondrial genomes not only reveals important information useful for several purposes, including phylogenic studies, forensic investigations, and studies of human genetic disorders, but also can be used to assess overall mutation rates. Denver et al. (p. 2342) passaged the progeny of the nematode, Caenorhabditis elegans, for 214 generations and sequenced the complete mitochondrial DNAs from 74 progeny lines. They found 26 mutations, some of which affected coding regions, and several of which occurred in mutational hotspots. Because the worms are self-fertile, mutations accumulate independently of natural selection, and the authors could obtain a direct estimate of the total rate of mutation (14.3 per site per millions of years) that is two orders of magnitude greater than other indirect predictions.

  16. Older and Wiser

    Swedish society keeps very good records of its births and deaths. The high precision of this information from 1860 to the present allowed Wilmoth et al. (p. 2366) to determine statistically the cause of the increase in maximum age at death seen during the last century in most developed countries. Most of the increase in the prevalence of older citizens is from reductions in the death rates of people over age 70, with the rate of this reduction accelerating since 1969. Although such good data are not available for any other country, these conclusions likely apply to most developed nations.

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