This Week in Science

Science  16 Apr 1999:
Vol. 284, Issue 5413, pp. 397
  1. A Core Full of Jelly

    The solid inner core jiggles around within the liquid outer core, and the period of oscillation of some of the modes have been measured with superconducting gravimeters. Smylie (p. 461) estimated the viscosity just above the inner core boundary by assuming that the Coriolis acceleration balances the viscous forces within the boundary layer and that the derived periods of oscillation should match the gravimeter measurements. The estimated viscosity for the boundary layer, 1.22 × 1011 Pascal seconds, is also consistent with a two-phase fluid model of the boundary layer, in which iron-rich liquid and solid iron particles rain down to the inner core.

  2. Global Warming Feedbacks

    Increasing concentrations of greenhouse gases, particularly carbon dioxide (CO2), are threatening to increase global surface temperatures. The combination of higher CO2 and higher temperatures may create positive feedback loops between the atmosphere, the ocean, and the terrestrial biosphere, but our understanding of many of the underlying mechanisms remains limited. Joos et al. (p. 464) use a simple model representing oceans, the atmosphere, and the biosphere (both marine and terrestrial). They apply a wide range of scenarios for global CO2 emissions and atmospheric CO2 concentrations that have been published by the Intergovernmental Panel for Climate Change and then analyze the role of the ocean circulation and ocean biosphere in oceanic carbon uptake. At high atmospheric CO2 concentrations, North Atlantic circulation collapse leads to a significant reduction in carbon uptake because of reduced mixing, whereas at lower CO2 values, the key factor that reduces uptake is sea surface warming.

  3. Mini Magnets

    The demands for greater computer memory will likely push the size of magnetic storage bits ever smaller; two reports focus on the motion and orientation of nanoscale magnetic domains. A simple method for following the motion of a submicrometer-sized magnetic domain is presented by Ono et al. (p. 468). The resistivity of a sandwich of two ferromagnetic layers separated by a nonmagnetic metal layer depends on the relative magnetization directions. As a small domain propagates through one ferromagnetic layer, it switches the magnetization direction and decreases the resistivity on a microsecond time scale. How small can a magnetic domain be and hold a magnetic orientation? Below the superparamagnetic limit (∼10 nanometers for cobalt, for example), thermal fluctuations alone will cause switching of the magnetization direction. Majetich and Jin (p. 470) used the Foucault method of Lorentz microscopy to image such direction changes in nanoparticles of SmCo5, magnetite (Fe3O4), and carbon-coated iron-cobalt alloys. Such studies allow the effects of surface roughness and modification on the dynamics of magnetization to be measured.

  4. Tumor Suppression and Protein Degradation

    Mutations in the von Hippel-Lindau (VHL) tumor suppressor are associated with cancer of the kidney and the central nervous system. Stebbins et al. (p. 455) describe the crystal structure of the VHL protein in complex with ElonginB and ElonginC, two components that together with ElonginA are involved in transcriptional regulation. The VHL protein contains two structural domains, α and β. The former appears to contain a structural motif similar to one known as SOCS (suppressor of cytokine signaling) and binds to ElonginC, and many of the known mutations map to this interface. The β domain also contains a surface hotspot at which many of the other known mutations cluster, which suggests that a functionally important and as yet unidentified partner binds at this site. On the basis of sequence and structural similarities to other components, the authors propose that the VHL protein might also connect to pathways involving protein degradation.

  5. Setting CO Straight

    Early studies of carbon monoxide (CO) toxicity found that binding of CO to heme proteins, such as myoglobin (Mb), was much weaker than to bare porphyrin complexes, and crystallographic studies offered an explanation—CO adopted a “bent” geometry in the heme proteins caused by steric effects, whereas in bare complexes it could adopt a linear conformation (perpendicular to the porphyrin) and form a stronger bond. However, recent spectroscopic studies have questioned the bent heme geometry. Kachalova et al. (p. 473) now report a 1.15 angstrom resolution study at room temperature of deoxy and CO-ligated Mb, which shows that CO indeed adopts a linear geometry in the MbCO complex; concerted protein motions needed to overcome steric inhibition during binding cause the reduced affinity.

  6. Pulling Two Pathways Together

    Two growth factors, LIF and BMP2, function together during differentiation of astrocytes. Each growth factor stimulates its own receptor and signaling pathway leading to distinct transcription factors, yet together they support differentiation of neural progenitor cells. Nakashima et al. (p. 479; see the Perspective by Janknecht and Hunter) now show that the molecular basis of this cross talk lies in formation of an unexpected complex. The transcription factors from the two pathways, STAT3 and Smad1, bind to opposite ends of the transcriptional coactivator p300 thereby coordinating these two signals.

  7. Kill the Messenger?

    Gene expression can be controlled at many different levels. Laroia et al. (p. 499) examine the mechanism used by cytokines and proto-oncogenes that limits the usage of their messenger RNA (mRNA) template by degrading them rapidly (within 5 to 30 minutes, compared to hours for other mRNAs). In the mRNA degradation pathway, the protein AUF1 normally binds to a sequence element in the 3′ untranslated region, and this structure is subsequently degraded. However, under the stress of higher temperatures, the heat shock protein prevents AUF1 from recruiting the mRNA to the ubiquitin-proteasome degradation pathway. Hence, a link is made between heat shock, rapid mRNA decay, and the ubiquitin-proteasome pathway.

  8. Natural Drug Synthesis

    In addition to the well-known machines for making polymeric biomolecules such as proteins and nucleic acids, there are also complex enzymatic machines that synthesize a variety of smaller compounds that are based on a repeating unit. Because two of the classes of these compounds contain widely used and useful antibiotics, such as penicillin and erythromycin, learning how to manipulate and recombine the constituent synthetic modules would offer a pathway to improving existing antibiotics or to new ones. Gokhale et al. (p. 482) explore the substrate specificity of the gramicidin synthetase by loading chemically produced aminoacyl-coenzyme A esters onto the free sulfhydryls of the donor and acceptor modules. The donor site can tolerate many different amino acid side chains, whereas the more restrictive acceptor site constrains initiation of the peptide chain to the correct amino acid. Belshaw et al. (p. 486) focus on the intermodule connectors within the 6-deoxyerythronolide synthase and find that these are critical for the appropriate progression of intermediates as seven 3-carbon units are polymerized into the cyclic core of erythromycin.

  9. Revamping Vancomycin

    There is a pressing public health need to develop new antibiotics against bacterial infections. The glycopeptide vancomycin is one of the last efficacious drugs for the treatment of Gram-positive infections, and it acts by inhibiting the cross-linking of amino acids in the formation of the bacterial cell wall. Ge et al. (p. 507; see the Perspective by Walsh) show that alterations in the carbohydrate moieties render vancomycin analogs much more active, even against bacterial strains that have developed resistance to vancomycin or methicillin. Their data suggest that the carbohydrate derivatives inhibit a different step in bacterial cell wall synthesis, possibly a transglycosylation reaction, and thus point toward new compounds that may overcome the problem of vancomycin resistance.

  10. Open Wide

    The ability to create small-diameter vascular grafts would have medical implications for treating atherosclerotic vascular disease. Although it is possible to manufacture such vessels from synthetic materials, they have only shown low levels of blood flow. Niklason et al. (p. 489; see the news story by Ferber) have generated blood vessels in vitro from bovine or porcine smooth muscle cells that were grown under conditions in which growth medium is pumped through the vessels (providing the type of physical stress they would normally experience during development) on a biodegradable polymer matrix. The pulsatile conditions resulted in vessels that were stronger than those grown without stress, that remained unobstructed longer when transplanted into miniature swine, that had collagen levels which were closer to those of normal vessels, and that showed appropriate contractile responses to prostaglandin.

  11. Big Bacteria

    Bacteria in ocean sediments have been recognized as the important intermediaries in the global cycling of nitrogen and sulfur. Schulz et al. (p. 493; see the cover and the news story by Wuethrich) now describe a new species of sulfur bacterium in Namibian sediments that not only is important in the cycling of sulfur and nitrogen but also is 100 times larger than other known bacteria. The huge cell volume and internal vacuole may allow them to store these elements for long periods of time when environmental supplies are limited.

  12. Eyeing the Clock

    Light can reset the phase of the circadian clock, which is located in the hypothalamus of the mammalian brain and controls the daily periodicity of functions such as sleep, activity, and metabolism. The eyes are necessary for this light-induced resetting, but the identity of the photoreceptor is not known, although transgenic mice that lack rods still show photoentrainment, which excludes rods as the photoreceptor. Now, Freedman et al. (p. 502) and Lucas et al. (p. 505) construct a line of mice lacking cones and a line lacking both rods and cones (see the news story by Barinaga). Each of these lines is still able to follow a circadian rhythm, as assessed by monitoring the daily activity of the mice and by acute inhibition of melatonin levels. Thus, there must be another type of photoreceptor that relays the light signal to the circadian clock.

  13. Structures from Powders

    Many materials do not readily form single crystals, and thus there have been many efforts to enhance the structural information that can be obtained from powdered samples. Wessels et al. (p. 477) show that by investigating textured polycrystalline samples, that is, those with a preferential orientation of crystallites, with synchrotron radiation, the previously unsolved complex structures of a zeolite and a molecular sieve can be determined. The method involves a full quantitative texture analysis of a single sample, so even compounds that form powders with only a single texture can be tackled.

  14. Red Light Signals

    Plant phytochromes are responsible for sensing the red and far-red portions of the light spectrum, and specific phytochromes mediate the developmental responses to specific portions of the spectrum. Hoecker et al.(p. 496) now identify a downstream signaling component that seems to process the signals from a particular phytochrome. The SPA1 protein, identified in Arabidopsis, alters morphogenesis specifically in response to light activation of phytochrome A. Other signaling components are shared among more types of phytochromes.

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