This Week in Science

Science  16 Dec 2005:
Vol. 310, Issue 5755, pp. 1737
  1. Comparing Correlations


    Quantum mechanics offers an exact solution to the forces binding an electron to a proton in the hydrogen atom. However, adding just one more proton and electron to the system presents an intractable complication arising from the correlated motion of the electrons. Vanroose et al. (p. 1787) have improved the approximate solution by numerical computation. They analyze the trajectories of both electrons upon double ionization of the hydrogen molecule by a single photon, specifically focusing on the influence of changing the internuclear separation. The result is distinct from the path taken on double ionization of the helium atom, These findings indicate that significant correlation effects stem from a molecular geometry (an electron pair shared between two protons), as opposed to an atomic geometry (an electron pair symmetrically surrounding two protons).

  2. Begin with a Backflip

    The initiation of diffusion of molecules on surfaces is mainly thought of in terms of translational motion. Backus et al. (p. 1790, published online 10 November; see the Perspective by Ueba and Wolf) followed the diffusion of CO molecules on a stepped Pt surface with ultrafast vibrational spectroscopy by using changes in CO stretching frequencies to distinguish different adsorption sites. Photoexcitation of the CO with a laser pulse revealed very fast motion (a time constant of only 500 femtoseconds) that was associated with CO rotation rather than translation. Density functional theory calculations show that the excitation of frustrated rotational motion of the CO molecule is needed for the molecule to hop to an adjoining adsorption site.

  3. The Reptile-Dinosaur-Bird Conundrum

    Examination of the histology of fossil bones has shown that most dinosaurs, like birds and mammals today, attained their adult size at about the same age after a period of rapid growth, independently of environmental factors. In contrast, many reptiles adjust their growth in response to temperature and other factors, and may attain adult size at rather different ages. By examining a large collection of fossils from central Europe, Sander and Klein (p. 1800; see the news story by Gramling) now show that the most common Triassic dinosaur, the large prosauropod Plateosaurus engelhardti grew more like turtles, snakes, and alligators, unlike later dinosaurs, whose growth response resembles that of birds and mammals.

  4. Small Reactors


    Positron emission tomography (PET) achieves local sensitivity in medical imaging of organs by detecting the emissive decay of isotopically unstable molecular probes. This instability also requires the rapid and efficient synthesis of probe compounds. Lee et al. (p. 1793) have built a computer-controlled device, roughly the size of a penny, for optimizing the speed and cost of such preparations. The micrometer-scale valves and channels achieve rapid mixing and solvent exchange, and efficient heat transfer, as demonstrated in the multistep synthesis of 18F-radiolabeled 2-deoxy-2-fluoro-D-glucose, the most widely used PET probe.

  5. The Genetics of Skin Pigmentation

    Little is known about the specific genes that contribute to the variations in human skin color. An exciting clue has now emerged from an unlikely source, a tiny aquarium fish. Working with a mutant line of zebrafish called golden, whose stripes are paler than those in wild-type fish, Lamason et al. (p. 1782; see the cover and the news story by Balter) found that the altered pigmentation was caused by a mutation in the slc24A5 gene, which encodes a protein potentially involved in cation exchange. The gene is highly conserved in vertebrates, and expression of the human gene in the golden zebrafish restored wild-type pigmentation. European populations carry a slightly different version of the slc24A5 gene than do African and East Asian populations. A genetic polymorphism that changes one amino acid in the coding region of the gene correlates with skin pigmentation levels, which suggests that slc24A5 may contribute to skin color in humans.

  6. Hiding in the Long Grass

    Since the seminal work of MacArthur and Wilson on the theory of island biogeography, studies on this topic have focused mainly on the relation of species richness with island parameters such as area, distance, and habitat variability. The population biology of individual species in the island context has received much less attention. Schoener et al. (p. 1807; see the Perspective by Thorpe) report results from an experiment using Anolis lizards and an introduced lizard predator on small islands in the Bahamas archipelago. In the absence of the predator, there was a highly regular (decreasing) correlation of lizard survival to a key habitat variable (vegetation height). In the presence of the predator, the situation was nearly reversed, such that prey survival was highest in the tallest vegetation.

  7. Modification of Electrical Synapses

    The brain has two main types of synapses, chemical and electrical. Electrical synapses represent a major form of communication between interneurons in the mammalian nervous system. They play an important role in synchronization of activity in local cell populations because their speed and reliability allows signals to spread across whole networks at a time scale that is sufficient to preserve precise timing of signals between distant neurons. In spite of these potentially vital functions, electrical synapses have generally been regarded as stereotypic and nonflexible. However, Landisman and Connors (p. 1809) found that transmission across electrical synapses can undergo long-term modifications just like chemical synapses. The modulation depends on activation of metabotropic glutamate receptors, which presumably trigger intracellular signal cascades modulating the connexins that constitute the electrical synapses.

  8. Snails on the Rampage


    There has been unprecedented and massive die-off of southeastern United States salt marshes during the past 5 years, with potentially serious consequences for coastal protection and integrity. Silliman et al. (p. 1803) surveyed more than 1200 kilometers of coastline and found high-density fronts of plant-grazing snails (~1500 individuals per square meter) mowing down marsh plants at 11 of 12 die-off sites. Die-off was initiated by drought-induced stress. Snail fronts developed at the edges of the die-off zones, and then spread across remaining healthy areas. These interactions between climatic and trophic factors may lead to further degradation or even collapse of these ecologically and economically important systems.

  9. Inhibiting Brain Repair

    Neuronal axons in the mammalian central and peripheral nervous system are generally ensheathed in myelin that is generated by nonneuronal cells. In response to injury in the peripheral nervous system, new axons can sprout from unmyelinated gaps called the Nodes of Ranvier, but this response rarely occurs in the central nervous system (CNS). Huang et al. (p. 1813, published online 17 November) have identified a precursor oligodendrocyte cell type whose processes envelope nodes in the CNS and inhibit axon sprouting. The processes express a glycoprotein previously thought exclusive to compact myelin. Mice lacking the glycoprotein exhibited abnormal node formation and nodal axon sprouting. Overcoming the inhibitory nature of these cells may be clinically important in recovery from injury.

  10. MicroRNA Management of the Genome

    MicroRNAs (miRNAs), small, ~22-nucleotide noncoding RNAs that have been found in most of the plants and animals so far studied, generally regulate gene expression by suppressing the activity of messenger RNAs (mRNA) bearing complementary target sequences. These targets, or “seeds,” are apparently only seven to eight nucleotides long, and so, all things being equal, should occur randomly throughout the genome with relatively high frequency. Farh et al. (p. 1817, published online 24 November) now show that all things are not equal: Expression of regulated seed-bearing mRNAs correlates with the presence of the appropriate miRNA. However, nonregulated mRNAs present at high levels in miRNA-expressing tissues have a paucity of complementary seed matches in their sequence. Thus, miRNAs are influencing the expression, the evolution, or both of the majority of mRNAs.

  11. Just-in-Time Competency

    Many bacteria can take up exogenous DNA, an ability known as natural competence. The causative agent of cholera, Vibrio cholerae, is not known to have this property, but somehow it has clearly acquired virulence attributes, including cholera toxin, from some other source. V. cholerae does possess the genes used by other bacteria to assemble the necessary machinery for DNA uptake, for example, type IV pili. Meibom et al. (p. 1824; see the Perspective by Bartlett and Azam) now show that a chitin (which can be found in the exoskeleton of crabs, a natural host for the bacteria) triggers V. cholerae to produce pili, and to release and exchange functional DNA. This competency remained unnoticed in a pathogen that has been studied for 60 years, which suggests that other noncompetent bacteria may become so under the appropriate growth conditions.

  12. Looking at Glasses at Length

    At the glass transition, the viscosity of a liquid increases dramatically, but static snapshots of the extent of ordering in these amorphous systems (their static correlation lengths) reveal little difference. However, numerical simulations and indirect experimental evidence indicates that as a liquid cools, regions of correlated movements of particles become larger, impeding the overall fluid flow, so that the length scales upon which the material is dynamically heterogeneous appear to be increasing. Berthier et al. (p. 1797) derive a lower bound on the four-point, time-dependent susceptibility χ4(t), a general measure of dynamic correlation lengths, and analyze data from two experimental systems—supercooled glycerol and colloidal hard spheres—as well as simulated glass, a binary Lennard-Jones mixture. All three systems display peaks in their appropriate dynamic susceptibility whose height increases for slower time scales, which indicate the presence of a growing dynamic length scale as the liquids cool.

  13. Bridging the Damaged DNA Gap

    The main replicative DNA polymerase has evolved to recognize DNA with high fidelity, but this capability also makes it very poor at dealing with damaged DNA, where it very often stalls at the point of damage. A series of “damage-specific” DNA polymerases that can handle distorted or abasic templates are recruited to the stalled polymerase partly through the ubiquitinylation of proliferating cell nuclear antigen (PCNA)—but how this occurs has been a mystery. Bienko et al. (p. 1821) now show that all the Y-family damage-specific translesion synthesis (TLS) polymerases contain two previously undetected types of ubiquitin binding domain. Colocalization of two TLS polymerases with PCNA in replication factories depends on these ubiquitin binding domains, as do the ability to interact with ubiquitinylated-PCNA and the ability to facilitate DNA repair.

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