This Week in Science

Science  02 Nov 2001:
Vol. 294, Issue 5544, pp. 953
  1. Cometary Origins

    Comets are thought to have formed through the freezing out and accretion of volatiles from the solar nebula at the edge of our solar system, where the temperatures range between 30 to 80 K. Kawakita et al. (p. 1089) collected high-resolution spectra from the Subaru telescope of comet C/1999 S4 (LINEAR) to determine the ortho-to-para ratio of NH2. By assuming that the NH2 is derived from the photodissociation of ammonia by the solar radiation, they derived a spin temperature of the ammonia of about 38 K. If this ammonia is primordial, then the spin temperature indicates that the comet formed between 8 and 15 astronomical units in the solar nebula.

  2. When Order Breaks Down

    Phase transitions reflect changes in ordering of a system, and insights can often be gained from studies of how the phase transition changes in response to deliberate disordering. Bellini et al. (p. 1074) examined the effect of disordering a type of liquid-crystal phase transition, the melting of a smectic phase, in which molecules are oriented and packed in layers, into a nematic phase, where the molecules are still oriented but have no distinct layers. They introduced the liquid crystal into an aerogel network, whose highly irregular surface perturbs the ordering on the nanometer scale. By comparing the experimental results to two theoretical descriptions, they show how long-range periodicity is destroyed. Some local ordering still persists and exhibits universality behavior similar to that of bulk phase transitions.

  3. Keeping an Eye on the Pulse

    As the development of photonic devices advances, so too will the need to monitor the transient behavior of the optical pulses as they propagate through the device structure. However, peeking inside a photonic structure is far from trivial, particularly when the pulses are on the order of several tens of femtoseconds. Balistreri et al. (p. 1080) introduce a noninvasive technique based on an optical scanning tunneling microscope that can be used to “visualize” the pulse as it propagates through an optical waveguide with temporal and spatial resolution.

    CREDIT: BALISTRERI ET AL.
  4. Making Connections

    A simple method for connecting pairs of electrodes immersed in water with microwires has been developed by Hermanson et al. (p. 1082). They added colloidal gold particles to the solution and turned on an alternating current (ac) between a pair of planar electrodes. The colloidal particles attached to one electrode and grew toward the other to form a bridging wire. The process can be enhanced by placing conductive islands between the two electrodes, causing the ac field to become asymmetric, and the thickness of the wires can be controlled by the particle size, concentration, the electric field and current, and the electrolyte concentration. The addition of latex particles to the solution tended to coat the wire to form a partial insulating layer.

  5. A Matter of Life and Death

    Shell beds containing the fossilized remains of molluscs are abundant and have been central to the interpretation of the history of life on Earth. Do these “death assemblages” provide more than a qualitative record of the presence and absence of species at particular times? Kidwell (p. 1091) performed a meta-analysis of many quantitative studies on the correspondence between dead and live molluscs in marine sediments. She shows that these sediments provide a reliable record of the relative abundance of molluscan species in their original living communities, thus indicating that death assemblages can reveal important information about paleoecology.

  6. Sea Changes

    Previous work that has suggested that the chemistry of the oceans, based on major ions such as Na+, K+, Ca+, Mg2+, and Cl, has been constant during the Phanerozoic (the past 540 million years), but this is inconsistent with observed changes in the mineralogy of marine limestones and evaporites. Lowenstein et al. (p. 1086; see the Perspective by Goldstein) measured the major ions trapped in fluid inclusions, which provide samples of ancient seawaters, in halite grains from marine formations throughout the Phanerozoic. Seawater chemistry has varied over time, and the variations are consistent with changes in sea-floor spreading rates, volcanism, and sea level.

  7. Mammoth Developments

    The fossil record of the woolly mammoth in Eurasia is one of the most extensive for any large mammal for the past 2 million years. Lister and Sher (p. 1094) have completed a detailed analysis of patterns of change in skull morphology and dentition spanning the mammoth's entire geographical and temporal range. They build a picture of gradual evolutionary change that resolves many of the contested questions in mammoth evolution and provides an unprecedented documentation of evolutionary tempo and mode in a large vertebrate.

  8. Map Factor

    Different regions of the mammalian cortex are dedicated to different neurological functions, but the molecular mechanisms by which the functional map is defined have been obscure. Fukuchi-Shimogori and Grove (p. 1071; see the Perspective by Rakic) have manipulated the expression of the fibroblast growth factor family member FGF8 in the developing forebrain. The effects of too much—or too little—FGF8 indicate that an intrinsic source of FGF8 in the anterior cortical primordium is responsible for determining the general structure of a neocortical map.

  9. Grabbing onto a Branch(point)

    During RNA processing in eucaryotic nuclei, intervening sequences (introns) are removed. The bond between the last nucleotide of the exon and the first nucleotide of the intron is cleaved by using an intronic adenosine residue as the attacking nucleophile. This adenosine is referred to as the branch point A because this reaction yields a branched structure known as a lariat. From the solution structure of the complex between splicing factor 1 (SF1) and the branch point sequence (BPS), Liu et al. (p. 1098) find that the branch point A is buried within the protein. They suggest that the subsequent formation of an RNA double helix between the BPS and U2 snRNA bypasses this sequestered adenosine, and the dissociation of SF1 would then leave this unpaired nucleotide exposed.

    CREDIT: LIU ET AL.
  10. Coordinating Energy Consumption

    The mammalian target of the immunosuppressant (and anticancer agent) rapamycin (mTOR) controls ribosome biogenesis in response to nutrients and is now shown to be a sensor of the energy status of the cell. Given that protein synthesis consumes a lot of energy, Dennis et al. (p. 1102) found that treatments that decreased the cellular concentration of ATP also decreased mTOR activity. In fact, mTOR exhibits a Michaelis constant for ATP similar to the concentration of ATP in a mammalian cell. Thus, mTOR seems to function as a key node for regulation of ribosome biogenesis, replete with distinct mechanisms to coordinate ribosome function with availability of ATP or amino acids.

  11. HIV-Induced Nuclear Blebbing

    The HIV protein Vpr is known to halt cell proliferation. De Noronha et al. (p. 1105; see the Perspective by Segura-Totten and Wilson) observed the formation of blebs or herniations in the cell's nuclear envelope. These herniations would occasionally rupture, leading to the mixing of nuclear and cytosolic contents, including cell cycle regulators. It will be important to determine how these events contribute to the inhibition of cellular proliferation.

  12. Connect to the Light

    Light perception in plants is mediated by photoreceptors called phytochromes for the red/far-red portions of the spectrum. Sweere et al. (p. 1108) show that the first component in the signal transduction pathway for phytochrome B in Arabidopsis is the response regulator ARR4. Phytochrome B regulates the expression of ARR4, which in turn interacts with phytochrome B to further light signaling. Hormone-regulated expression of ARR4 and the effect of phosphorylation on its function suggest how the light signaling pathway may interact with other signaling pathways.

  13. Fusion Machinery Components

    The SNARE proteins are parts of the cellular fusion machinery and are thought to act as specificity markers on intracellular membrane-bounded compartments (see the Perspective by Scales et al.). Schoch et al. (p. 1117) generated knockout mice that lack the SNARE synaptobrevin 2 and found that it was not absolutely required. Wang et al. (p. 1111) looked at the role of two isoforms of another protein with a role in fusion, synaptotagmin. Whereas overexpression of synaptotagmin I prolonged the interval from fusion pore opening to dilation during exocytosis of dense-core, noradrenaline-containing granules, synaptotagmin IV reduced this interval. Both of these proteins restricted the transfer of noradrenaline through the open pores, suggesting that synaptotagmins interact directly with the fusion pore.

  14. The Long and the Short of Memory

    Although small, the fruit fly Drosophila can learn and remember, and the sophisticated genetics of this fly has made it possible to probe several aspects of short- and long-term memory. Pascual and Préat (p. 1115) find that the alpha-lobes-absent mutant is missing either the two vertical lobes or two of the three median lobes of the mushroom body. Long-term memory requires the two vertical lobes, but not the three median lobes, while short-term memory requires only one of the three median lobes.

  15. Anomalies in Ozone Isotopomer Formation

    Gao and Marcus (Research Articles, 13 July, p. 259) presented theoretical arguments to explain why both mass-independent and mass-dependent oxygen isotope fractionation can be observed for ozone synthesis under different conditions. In a comment, Janssen acknowledges that the theory “successfully explains most of the experimental data,” but takes issue with the study's conclusion that both isotope effects “are in a sense symmetry-driven.” Although that description is “clearly appropriate” for the nonstatistical effect cited by Gao and Marcus to explain the mass-independent fractionation, according to Janssen, “the same term should be avoided” to describe the zero-point energy fractionation (ZPEF) driving the large unconventional mass-dependent effects, because such fractionation “is connected only accidentally, not causally, with molecular symmetry.” In a response, Marcus explains the subtler sense under which ZPEF can also be termed symmetry-driven. The full text of these comments can be seen at

    www.sciencemag.org/cgi/content/full/294/5544/951a

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