This Week in Science

Science  10 Sep 2004:
Vol. 305, Issue 5690, pp. 1527
  1. Dendrimeric Diblock Copolymers


    Diblock copolymers can phase-separate into a rich array of morphologies, and dendrimer polymers allow many different functionalities to be placed onto highly branched compact molecules. Cho et al. (p. 1598) combined these two architectures into a single molecule and examined the phase behavior of a dendron grafted onto a long linear chain segment. The molecules show the same spherical, cylindrical, and lamellar structures seen in normal diblock copolymers, but also an unusual continuous cubic structure. The mechanical and charge transport properties of the polymers could be correlated with the observed phases.

  2. Composition of Jupiter's Atmosphere

    When the Cassini spacecraft flew by Jupiter on its way to Saturn, the Composite Infrared Spectrometer (CIRS) took measurements of the jovian upper atmosphere. Kunde et al. (p. 1582, published online 19 August 2004) found enhancements of some hydrocarbons in the aurorae associated with temperature and magnetic field effects. Carbon dioxide and hydrogen cyanide added to the stratosphere by the impact of comet Shoemaker-Levy 9 have not been transported or diffused very much, possibly because polar vortices are inhibiting the diffusion of these species to higher latitudes.

  3. Macrocyclic Libraries via DNA

    DNA recognition has been exploited in several ways to synthesize libraries of macro- cycles. Gartner et al. (p. 1601, published online 19 August 2004) linked single-stranded DNA to the peptide-like building blocks of macrocycles. DNA recognition of complementary strands brought the ring components into proximity so that ring- closure reactions could be performed. Specific macrocycles can then be selected for their protein affinity or enzymatic inhibition, and then identified by amplifying their DNA tags. A library of 65 such compounds was constructed.

  4. Suddenly Turbulent

    Despite having been studied for more than 100 years, the transition from laminar to turbulent flow in pipes is not understood. For other flow geometries, the source of the initial instabilities can be identified, but theory predicts that pipe flow should remain laminar for all flow rates. Recent numerical calculations suggested that traveling waves may be the reason the flow becomes turbulent. Hof et al.'s (p. 1594; see the Perspective by Busse) hypothesis is now confirmed through experimental observations.

  5. Slippery But Still Wet

    The hydrophobic effect (the poor solvation of nonpolar parts of molecules) is thought to play a key role in protein folding. Large nonpolar side chains would create a layer largely depleted of water when hydrophobic domains are brought together. However, this situation is based mainly on a consideration of van der Waals interactions between solutes and water. Zhou et al. (p. 1605) have performed molecular dynamics simulations of the BphC enzyme, a two-domain protein that collapses into a globular structure in which complementary hydrophobic faces align. Only a weak water depletion, with a water density about 10 to 15% lower than the bulk, was formed between the hydrophobic domains. The authors find that when electrostatic effects are artificially removed in their simulations, the dewetting transition reappears and the collapse transition occurs at a much faster rate.

  6. Phytoplankton Feel the Heat

    The marine pelagic ecosystem is the largest one on Earth, yet little is known how global warming might affect it. Phytoplankton make up the base of the marine food web and support the rest of the larger organisms in the oceans. Richardson and Schoeman (p. 1609; see the news story by Stokstad) studied the impact of climate change on the abundance of marine planktonic food web over large space and time scales in the Northeast Atlantic. Their analysis of more than 100,000 samples over 45 years shows that climate warming has increased in the abundance of phytoplankton in cooler regions and a decrease in warmer ones.

  7. Ensuring Adequate Gas Supplies

    In an uncertain world, survival may depend on leaving nothing to chance. In biochemical terms, the way to place a spontaneously occurring process under control is to make an enzyme that catalyzes the reaction. Biological membranes are inherently permeable to gases, such as oxygen, yet Khademi et al. (p. 1587; see the cover and the Perspective by Knepper and Agre) now describe a bacterial protein that functions as an ammonia channel. The crystal structure of AmtB reveals a vestibule where the water-soluble species NH4+ is deprotonated and a hydrophobic conduit enables NH3 to cross the membrane. The human analog of AmtB is the well-known rhesus or Rh factor.

  8. Take That Copper


    Methanotrophic bacteria oxidize methane, and copper plays a central role in the metabolism of these organisms. However, their copper trafficking mechanism is not well defined. Kim et al. (p. 1612) have identified and determined the structure of methanobactin, a copper-sequestering small molecule from the methanotroph Methylosinus trichosporium OB3b. Structural similarities to ironsiderophores suggest that this molecule may function as a copper-siderophore by binding copper extracellularly and mediating its transport into cells.

  9. Forming Hearts sans Fusion


    In the early developing vertebrate heart, bilateral cardiac mesoderm migrates to the ventral midline and then fuses to form the primitive heart tube. Subsequently, looping morphogenesis and chamber specification are observed. It has been generally thought that the fusion event must occur in order for these latter events to take place. However, Li et al. (p. 1619) now show that looping and septation can occur in the absence of heart fusion as seen with Foxp4 mutant embryos, which display two complete hearts without fusion. The early bilateral precardiac mesoderm is preprogrammed to differentiate multiple cell types and to complete the complex morphological steps required for formation of the mature four-chambered heart.

  10. Bacterial Persistence and Antibiotic Resistance

    The inherent persistence of bacterial populations after exposure to antibiotics or other stress is well known but little understood. Such persistence is distinct from acquired antibiotic resistance and, on regrowth, such bacteria are still antibiotic sensitive (see the Perspective by Levin). Balaban et al. (p. 1622, published online 12 August 2004;) investigated the growth dynamics of various mutant and wild-type Escherichia coli using a microfluidic device to track individual organisms. At least three different phenotypes were revealed. Those with a normal growth rate were killed. Type I persisters exited stationary phase very slowly—hours rather than minutes after nutrients were restored. Type II persisters arose by a spontaneous switch from the normal growth rate to grow consistently more slowly, regardless of growth conditions, and, rarely, could switch back to the normal growth rate. Many pathogens have become resistant to the β-lactam antibiotics, like penicillin, by a variety of mechanisms, including mutation of penicillin-binding protein genes, destruction of the antibiotic by β-lactamases, or by inhibition of uptake by the bacterial cells. Miller et al. (p. 1629, published online 12 August 2004; see the Perspective by Levin) describe another mechanism for avoiding the lethal effects of antibiotics. Damage to penicillin binding protein 3 activates the DpiBA two-component signal transduction cascade and eventually triggers the SOS DNA repair response. When SOS kicks in, cell division pauses, and the bacteria escape lethal damage, at least from short-term antibiotic exposure, because synthesis of new cell walls shuts down.

  11. Tracking Iron Sources of Pathogenic Bacteria

    In geochemistry, different isotopes are classically used to track the source of an element. Skaar et al. (p. 1626; see the Perspective by Rouault) have devised a technique for use in living systems that combines stable isotope labeling with computational genome analysis. They could distinguish whether iron was taken from heme or from transferrin by the pathogenic bacterium Staphylococcus aureus, and discovered a previously unrecognized heme uptake system. Mutations in this system attenuate pathogenicity in model infections in the worm, Caenorhabditis elegans, and in the mouse. Drugs that target this system could prove useful in treating human infections.

  12. Falling Together

    Although coevolution has led to the tight interdependence of many species, little is known of the frequency with which the demise of one species causes the demise of another. Koh et al. (p. 1632) present a probabilistic model, scaled with empirical data, to estimate the number of such coextinction events across a wide range of coevolved systems. From this analysis, they derive a quantitative estimation of the possible cascading effects of species loss of endangered taxa. This work has implications for the understanding of historical extinctions and coevolution, as well as the conservation of biodiversity.

  13. Cdc42: You Are Being Watched

    One of the goals of imaging research is to observe protein activity within individual living cells. Nalbant et al. (p. 1615) describe the production of a biosensor for the small guanosine triphosphate-binding protein, Cdc42, which can be used to study endogenous protein activity with high sensitivity. Cdc42 is activated at precise times and places to control cell protrusions. The sensitivity provided by the biosensor enabled high-resolution kinetic studies, which showed that Cdc42 activation kinetics is an important determinant of cell extension and retraction rates.

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