This Week in Science

Science  01 Sep 2006:
Vol. 313, Issue 5791, pp. 1197
  1. Corridor Confirmation

    CREDIT: ELLEN DAMSCHEN AND THE U.S. FOREST SERVICE

    Although “corridors” connecting patches of habitat are proposed to be beneficial in terms of preserving biodiversity, this theory has never been tested experimentally at large scales. Using replicated experimental 50-hectare landscapes consisting of open patches in longleaf pine forest connected by similarly open corridors, Damschen et al. (p. 1284) show that corridors increase the species richness of herbaceous plants. These findings confirm the validity of corridors as a tool for conservation and landscape managers.

  2. Steering Retinal

    Because of the wave-particle duality inherent in quantum mechanics, different states along the pathway of a molecular rearrangement can interfere with each other like vibrations on a string. The phases and amplitudes of spectral components in light pulses that initiate photochemical reactions can now be created that can steer small molecules along distinct reaction trajectories by inducing constructive or destructive wave interference among states. Prokhorenko et al. (p. 1257; see the Perspective by Chergui) show that this approach can modulate the efficiency of retinal isomerization in the protein bacteriorhodopsin (a rearrangement closely related to the vision response) by as much as 20% in either direction. The extent of modulation is remarkable in light of the many degrees of freedom in the protein environment that might be expected to randomize the wave phases.

  3. P2 Gently Generated

    Although elemental nitrogen and oxygen are most stable as diatomic molecules, their heavier congeners, such as phosphorus and sulfur, are inhibited from multiple bonding by core electron repulsion, and so tend to exist as polyatomic clusters instead. Piro et al. (p. 1276) have prepared a niobium precursor that releases P2 at 65°C, and thereby facilitates exploration of the solution-phase chemistry of this unusual molecule, which is otherwise only accessible through decomposition of the P4 cluster above 1000°C. The authors show that P2 can be trapped by successive Diels-Alder coupling to two cyclohexadiene molecules, which is consistent with the presence of a reactive triple bond.

  4. Snapshots of Working Catalysts

    Vanadium phosphates (VPOs) are used industrially to catalyze the partial oxidation of n-butane to maleic anhydride, which is then used as a starting material for products such as resins and lubricants. However, the reaction proceeds at elevated temperatures (in excess of 400°C), and VPO phases stable under those conditions will transform to other phases at ambient conditions, so an understanding of this catalyst demands that it be studied near its working conditions. Conte et al. (p. 1270) have used powder x-ray diffraction, as well as laser Raman spectroscopy and electron paramagnetic resonance spectroscopy, to determine the transformation of VPO phases as a function of temperature and with various reactants and products present over the catalyst. They conclude that the presence of the reactants rapidly converts ω-VOPO4 to δ-VOPO4, but that the initial formation of the ω phase may create the V5+ sites associated with increased catalytic activity.

  5. Tiny Fluorous Flasks

    CREDIT: SATO ET AL.

    Fluorocarbons have been increasingly applied as media for chemical reactions and separations because their solubilizing properties are distinct from those of both water and traditional organic solvents. Sato et al. (p. 1273; see the Perspective by Gladysz) have created a nanometer-scale fluorous environment within a polar organic solvent. Arrow-shaped ligands with perfluoroalkyl tails self-assemble with palladium ions in dimethyl sulfoxide to form a shell in which the fluorinated chains are directed inward toward the center. By varying the lengths of these chains, the shell size could be tuned to encapsulate a liquid-like, disordered phase of ∼2 to ∼6 perfluorooctane molecules, which were characterized spectroscopically and crystallographically.

  6. Round the Bend

    The Hawaiian Islands chain of volcanoes sits within a long line of seamounts stretching 6000 km across the Pacific. The Hawaiian volcanoes had been considered to be produced by the relative motion of the Pacific plate over a southward drifting locus of melting in the mantle. About 3500 kilometers west of Kilauea, there is a sharp bend in the chain. Sharp and Clague (p. 1281; see the Perspective by Stock) inferred a time line for the formation of the Hawaiian-Emperor seamount chain by measuring 40Ar/39Ar ages for eight volcanoes. They give an average age for the bend of about 50 ± 1 Ma, older than previous estimates. The ages, increasing to the north, imply that rates of migration have varied considerably. These results imply the plate motion must have changed at this time, which coincides with the development of subduction zones around the Pacific plate boundary.

  7. Direct Delivery

    The life cycle of the malaria parasite in its mammalian host begins with a liver-specific stage, in which sporozoites delivered by the mosquito invade hepatocytes, where they develop into merozoites that invade red blood cells. Merozoites must enter the bloodstream, although precisely how they move from hepatocyte to the lumen of the liver sinusoid has remained a matter of speculation. In a study of a rodent form of the parasite, Sturm et al. (p. 1287; see the Perspective by Cowman and Kappe) reveal that as the merozoites induce death of the hepatocyte, they simultaneously hold in check the normal cues that would signal phagocytosis of the dying cell. This alteration allows membrane-bound extensions of the infected cells, which the authors term merosomes, to bud off and shuttle the merozoites directly into the bloodstream. Thus, the parasites modify the host response to dying infected cells to ensure better survival and replication.

  8. A Tale of Two Spirals

    A facile route to crystal growth is for atoms to attach to a surface at a screw dislocation. Hannon et al. (p. 1266; see the Perspective by Voorhees) studied atomic growth of two silicon surfaces, the (111) and (001) faces, at 1100°C with low-energy electron microscopy. On the (111) surface, growth proceeded smoothly with a spiral pattern, in accord with the classic model. However, on the (001) surface, growth occurred along a spiral with an S-shaped undulating profile, and the step edges rotated with an almost ratchet-like motion. The origin of this difference is attributed to the nonuniform strain field created by the two possible surface terminations of the (001) surface, and the growth profiles were analyzed in detail with a continuum step model.

  9. Out of the Shadows

    CREDIT: ALEX MOLNAR/AGRICULTURE AND AGRI-FOOD CANADA

    Phytophthora species are oömycetes and belong to the kingdom Stramenopila, which is evolutionarily distant from plants, animals, and fungi. Importantly, nonphotosynthetic stramenopiles, including the oömycetes, are believed to have lost their plastids at some point in evolution. The two Phytophthora genome sequences presented by Tyler et al. (p. 1261) provide compelling evidence that their ancestor indeed harbored a photosynthetic endosymbiont. The genomes also show a striking diversification of infection-associated genes, which consists of about 350 genes in each genome and reflects intense coevolutionary processes occurring between these parasitic species and their hosts.

  10. Genetic Measures of Human Evolutionary Proximity

    Gene sequences that show a pronounced human lineage-specific increase in copy number and that also encode multiple copies of a domain of unknown function (DUF1220) have been identified by Popesco et al. (p. 1304). These domains show significant hyperamplification in the human lineage and generally increase in copy number as a function of a primate species' evolutionary proximity to humans. Antibody studies indicated that DUF1220 sequences are abundantly expressed in structures of the neocortex and in particular subsets of neurons. These sequences might be important to cognitive pathways and synaptic function.

  11. An Ounce of Prevention

    Members of groups subject to stereotyping are more likely to behave in a fashion that conforms to the stereotype when the stereotype is made salient; for instance, women score lower than men on tests when the tests are identified as math as opposed to problem-solving. Cohen et al. (p. 1307; see the Perspective by Wilson) report the results of two field studies in which a brief, value-affirmation intervention at the beginning of the school year appeared to buffer the effects of a stereotype threat on 7th-grade African Americans such that they maintained their achievement levels (as did European American students) throughout the remainder of the school year, in comparison to African American students in the control condition.

  12. Brain Regions and Social Organization

    There is general agreement that humans can represent the mental states of others (theory of mind), and the current consensus appears to be that we are unique in this respect. Nevertheless, other animals have been shown to possess some aspects of social intellect, but precisely who knows what is unclear. Rudebeck et al. (p. 1310) have carried out a lesion study in monkeys to examine the differential contributions of two neighboring cortical areas, the anterior cingulate and the orbitofrontal region. They find that the gyrus of the anterior cingulate is needed in order to orient toward a specifically social stimulus, such as the face of another monkey, in contrast to other potent stimuli, such as a moving snake, which are processing in the orbitofrontal cortex.

  13. Holding the Heart Together

    To elucidate molecular mechanisms in heart development, Yi et al. (p. 1301, published online 20 July) have used a genetic screen for Drosophila mutants. Five mutants were observed in which the cardioblast layer of the heart does not adhere to the adjacent layer of pericardial cells. This phenotype is labeled as “broken hearted.” Gene cloning shows that this phenotype arises from mutations in genes that encode components of the geranylgeranyl synthesis pathway downstream of HMG-CoA reductase. Statins inhibit this pathway, and treatment with statins during development yielded a similar phenotype. Hence, isoprenoid biosynthesis functions in heart development, with defects in geranylgeranylation of the G protein Gγ1 causing severe defects in heart formation because of abnormal adhesion of cardioblasts to pericardial cells. Because of the conservation of mechanisms in heart formation, these findings may illuminate mammalian heart development and congenital heart disease and also the effects of statins on heart function.

  14. Both Ways Now

    Glycosyltransferases, which are known to attach carbohydrates to biological molecules, have been thought of as unidirectional catalysts. Now Zhang et al. (p. 1291) show that glycosyltransferases involved in the biosynthesis of the anticancer compound calicheamicin and the antibiotic vancomycin can also remove nucleotide sugars from these natural products. This reversibility of glycosyltransferase reactions is likely to be general and suggests a new direction for altering the critical sugar residues attached to complex natural products in order to address problems such as drug resistance.

  15. Planemo Pair

    Brown dwarfs are extremely dim stars that are too small to light up by burning hydrogen. Some have low masses that make them quite similar to gas giant planets like Jupiter. These planetary mass objects—or “planemos”—are free-floating, and it is not known how they form. Do they condense from fragmenting gas clouds, like stars, or have they been kicked out from planetary systems? One clue comes from the discovery by Jayawardhana and Ivanov (p. 1279) of a planemo pair. The two companions form a wide binary system that is held together only lightly by gravity and so favors formation from a gas cloud in situ rather than by more dynamic processes. The spectra and colors of the planemos are similar and also suggest they were formed together.

  16. Righty Tighty, Lefty Loosey

    One of the primary mechanisms that microorganisms (and tumor cells) utilize to fight off the effects of drugs is to pump them out of the cell. One of the largest groups of bacterial membrane proteins involved in multidrug efflux is the resistance nodulation division (RND) family; these proteins pump out drugs by relying on a transmembrane gradient of protons. Seeger et al. (p. 1295) describe two new crystal forms of the trimeric RND protein AcrB. Unlike earlier AcrB structures in which the three monomers were modeled as symmetric, these crystals offer a view at three distinct monomer conformations—called loose, tight, and open. The internal, hydrophobic cavities within these conformations are suggestive of a pumping mechanism in which each monomer cycles through these states, driven by a flux of protons, and takes in drug molecules from the membrane and pushes them out into the extracellular space.

  17. This End Up

    In early embryo development, polarity is established as early as the one-cell stage, when sperm entry triggers the asymmetric distribution of proteins and generation of an anteroposterior axis. Jenkins et al. (p. 1298, published online 27 July) now report the identity of a sperm-donated factor that affects protein polarity and the actomyosin cytoskeleton within the early nematode embryo. At fertilization, the sperm contributes CYK-4 protein to the egg. Proteins that participate with CYK-4 for polarity initiation include rho-1/RhoA, cyk-4/GTPase-activating protein, and ect-2/guanine nucleotide-exchange factor. ECT-2 and RHO-1 proteins promote, but CYK-4 inhibits, activated myosin light-chain and actomyosin contractility. Because CYK-4 is expressed in sperm of other species, it will be of interest to know if this polarity mechanism is conserved among other species that display actomyosin contraction.

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