COVER A growing number of U.S. corporations are reacting to reports of poor student performance on national and international tests by contributing time and money to efforts aimed at improving math and science education in U.S. schools. To find out what they are doing and how well it’s working, see page 1030 .
Photo illustration: Kelly Krause/Science (images: Jupiter Images; Getty)
This Week in Science
Highlights of Research in This Issue
Volume 319, Number 5866, Issue of 22 February 2008
©2008 by The American Association for the Advancement of Science.
Carboxysome Building Blocks
The carboxysome is a bacterial microcompartment, roughly 100 nanometers in diameter that sequesters enzymes involved in carbon fixation. The proteinaceous outer shell is icosohedral, a geometry that typically involves a combination of hexameric and pentameric building blocks such as those found in certain viral capsids. Four abundant shell proteins from two known types of carboxysomes are known to form hexamers. Now Tanaka et al. (p. 1083) report the structure of two carboxysome proteins that form pentamers with the appropriate size and shape to be inserted into hexagonally packed layers so that they form vertices in an icosohedral shell. Model-building gave two plausible ways to pack the hexamers and pentamers to build a complete carboxysome shell.
CREDIT: T. YEATES
Ductility in Ceramic Composites
Nature has found routes to take weak or brittle starting materials and make stronger or tougher composites, such as incorporating biopolymers and calcium carbonate to form nacre. Although scientists have been able to use better starting materials, they have not been able to achieve the same level of improvement or sophistication. Bonderer et al. (p. 1069; see the Perspective by Ortiz and Boyce) fabricate a composite from sub-micrometer-thick alumina embedded in a polymer matrix (chitosan) that has a high tensile strength. By controlling the thickness of the ceramic films, they can determine the mode of fracture and ensure that the composites remain ductile.
Photon Turnstile
High-quality optical microresonators can influence or delay the propagation of the light in coupled optical fibers. Dayan et al. (p. 1062) now show that coupling of an atom to the evanescent field of the resonator can have a more dramatic effect on the propagating light in the fiber. When the atom is coupled, the input stream of photons is regularized into an output of single photons. Such an optical turnstile should prove useful as a light source for quantum information processing.
Whither Weather Out West
Recently, more winter rain and less snow has fallen in the western United States than previously, and more snow has melted earlier in the year. These changes have increased river flows in the spring and decreased them in the summer. Barnett et al. (p. 1080, published online 31 January) conducted a multivariate climate change detection and attribution study of the western United States from 1950 to 1999 and found that up to 60% of the trends of river flow, winter air temperature, and snow pack reflect anthropogenic climate change despite high variability throughout the records. These results suggest that a crisis in water supply for the western United States, which relies heavily on a late spring snow melt for agriculture, may be looming.
A Little Nudge
Scanning probe microscopes have been used to manipulate individual atoms and create structures at the atomic scale, but despite their widespread use, it has been difficult to determine the forces needed to move each atom. Ternes et al. (p. 1066; see the Perspective by Custance and Morita) report on simultaneous measurements of the vertical and lateral forces exerted on an adsorbate, either Co atoms or CO molecules, during lateral manipulation using an atomic force microscope (AFM). They simultaneously detected the tunneling current between the oscillating AFM tip and the sample surface, which recorded variations of the tunneling gap conductance with the force measurements.
CREDIT: TERNES ET AL.
Social Quakes
Earthquakes constantly color Earth’s plate boundaries, but how and to what extent are any two quakes connected, let alone a series of quakes? Clear aftershocks cover only a small number of quakes and primarily series that are successively weaker. Overall, the question is partly statistical, and partly mechanistic in determining how one quake affects stress on another fault. Marsan et al. (p. 1076) develop a model of triggering based on two simple assumptions without the need for fitting a large number of parameters for each fault. Their model, applied to southern California quakes, implies that small earthquakes have a disproportionate effect on triggering and that cascading events are common.
Tau and Microtubule Motors
The microtubule-associated protein tau is implicated in neurodegenerative diseases, including Alzheimer’s disease, but neither its normal nor pathological role are well understood. Microtubules in cells are densely coated with microtubule-associated proteins, including tau, and Dixit et al. (p. 1086)) wanted to learn how motor proteins navigate on coated microtubules. The authors incorporated several isoforms and truncations of tau in a cell free assay to build a complex system mimicking the cell environment. Encounters between individual motor proteins and tau on microtubules were observed directly, which revealed dramatic differences between the effects of tau on the motility of two microtubule-based motors, dynein versus kinesin. It appears likely that tau can spatially regulate the balance of microtubule-dependent axonal transport in neurons.
A Viral Culprit in Cancer?
The discovery that a virus contributes to the pathogenesis of a human cancer provides new options for prevention and treatment, as illustrated by the development of the HPV (human papillomavirus) vaccine for the prevention of cervical cancer. Feng et al. (p. 1096, published online 17 January; see Perspective by Viscidi and Shah) provide tantalizing evidence that a previously uncharacterized human polyomavirus may be a contributing factor in Merkel cell carcinoma (MCC), a rare but highly aggressive form of skin cancer. The authors initially detected viral DNA sequences in MCC samples by digital transcriptome subtraction, a methodology that reveals rare foreign transcripts, and then sequenced the full genome of the virus, which they termed Merkel cell polyomavirus (MCV). Although MCV sequences were present in a small percentage of control tissues, they were strongly associated with MCC and were integrated within the tumor genome in a pattern which suggests that MCV infection preceded clonal expansion of the tumor cells.
CREDIT: FENG ET AL.
Beyond Out of Africa
Analysis of DNA sequence variation has increased our understanding genetic diversity in humans, revealing, for example, that humans migrated out of Africa and dispersed across the globe, displacing other hominids in the process. In order to gain a deeper understanding, Li et al. (p. 1100) studied more than half a million single nucleotide polymorphisms from the Human Genome Diversity Panel, representing 51 populations from around the world. The broad sweep of the analysis uncovers both fine-scale population structure, for example, distinguishing Orcadian, French, and Northern Italian populations from Bergamo and Tuscan groups, and supports the “serial founder model,” in which non-African populations form a sequential chain of colonies as they radiated out from Africa.
Tagging + Recruitment = Learning
Stabilizating long-term memories requires the expression of new gene products in the nucleus to generate physical changes or “tags” at a small subset of synapses on the dendrites of neuron. Synthesis of AMPA receptors is increased during learning is suggestive of a possible role in this process. Matsuo et al. (p. 1104) developed transgenic mice to monitor the trafficking and turnover of newly synthesized AMPA receptors in a fear-conditioning paradigm. By coupling expression of fluorescent receptors to neuronal activity, they specifically examined the pool of newly synthesized receptor. New receptors were delivered to spines several hours after behavioral training, which suggests that at the time of learning, changes occur in some spines that allow the capture of newly synthesized AMPA receptors at later time points.
Spike Coding in the Retina
The retina’s task is to transduce visual images into neural signals, to process these signals, and to transmit the result through the optic nerve to the brain. The general notion, as in other sensory systems, is that a time-varying visual stimulus will produce time-varying firing rates among retinal ganglion cells, and these rates are what drives the processing in subsequent visual stations. It is unclear, however, whether this type of sensory encoding could support the rapid signal detection and image processing of which the visual system is capable. Gollisch and Meister (p. 1108) show, instead, that a single spike per ganglion cell is sufficient to accurately communicate a new visual image. The meaning of the spike is conveyed by its precise time of occurrence, relative to spikes from other ganglion cells. By several criteria, this message is more powerful and more robust than that conveyed by the firing rate. Most importantly, it transmits image information in the shortest possible time.
Predicting Human Behavior
How well can neuronal networks model human interactive decision-making? Recently, the important role of regret in human decision-making has been demonstrated. Marchiori and Warglien (p. 1111; see the Perspective by Cohen) have now modified neuronal network feedback in 21 interactive games to take the role of regret into account. Introducing regret into the feedback dramatically improved the efficacy of neuronal networks and allowed more precise prediction of human behavior than conventional economic learning theories.
Exploiting Nonlinearity in Molecular Junctions
The use of molecules as switching elements in electronics stems not just from their small size but from the possibility that a molecular junction could exhibit nonlinear responses that enable more compact realizations for logic operations. Galperin et al. (p. 1056)) overview the theory of how electronic transport in molecular junctions moves beyond the tunneling regime. At higher voltages, up to about 0.3 volts, molecular vibrations scatter electrons and provide spectroscopic signatures. At higher biases, strong coupling allows the electron to reside for longer times on the molecule and leads to strong polarization and charging, as well as nonlinear responses such as Coulomb blockades, negative differential resistance, and dynamical switching.
Tracking Interfacial Mixing
High-resolution electron microscopy can now identify columns of atoms in a crystalline material and even identify individual vacancies or the structure of the defects at a grain boundary. Using a fifth-order-corrected scanning tunneling transmission electron microscope, Muller et al. (p. 1073) determined chemical species and bonding in a La0.7Sr0.3MnO3/SrTiO3 multilayer. They observed asymmetries in intermixing at the interfaces and an uptake of La into the titanium oxide layers.
Telomere Tag
Telomeres–the ends of linear mammalian chromosomes–are regulated in length and protected from recognition by DNA damage repair systems by the shelterin complex, which includes the proteins telomeric repeat binding factors 1 and 2 (TRF1 and TRF2). These two proteins recruit other members of the shelterin complex and associated telomere factors, which can negatively regulate telomere length. Chen et al. (p. 1092, published online 17 January) show that TRF1 and TRF2, which are structurally quite similar, interact with such factors in distinct ways. TIN2 binds TRF1 through its N-terminal TRF homology (TRFH) domain and to TRF2 through a C-terminal domain whereas Apollo binds TRF2 via its TRFH domain in the same manner that TRF1 binds TIN2, with two loops in the TRFH domain determining these distinct binding specificities. The binding regions of TIN2 and Apollo share a pentameric sequence that may help identify other telomere accessory proteins.
Regulating the Regulon
In the presence of galactose, the yeast Saccharomyces cerevisiae transports the sugar into its cytoplasm, where the galactose binds the Gal3 protein, which in turn sequesters the Gal80 repressor away from the transcriptional activator Gal4, resulting in the induction of galactose metabolizing enzymes. The induction is so rapid that it has been suggested another unknown factor must somehow be involved in this “galactose regulon.” Kumar et al. (p. 1090) have identified this factor as nicotinamide adenine dinucleotide (NAD) and its phosphate (NADP). In the Gal4:Gal80 complex, NAD is sandwiched between the two proteins. NADP, on the other hand, inhibits the interaction between Gal4 and Gal80. Mutations that have the potential to affect the NAD-binding site in Gal80 result in even faster induction, which suggests that Gal80 is sensing the balance between NAD and NADP and thus the metabolic state of the cell.
Editors’ Choice
Volume 319, Number 5866, Issue of 22 February 2008
©2008 by The American Association for the Advancement of Science.
EVOLUTION: Under Strict Control
In many species of animals, males and females are strikingly (and visibly) different. Three evolutionary genetic pathways can produce such sexual dimorphism. From the outset, the expression of the alleles for dimorphic traits could be limited to one sex. Or the alleles could be expressed initially in both sexes but subsequently promoted only in one sex or repressed only in the other. Darwin believed that the bright colors and ornaments of males were male-limited from their first appearance, whereas Fisher held that they had been lost from females through the accumulation of suppressors. Coyne et al. weigh these alternatives by examining female hybrids between bird species [such as Costa’s (left) and black-chinned (right) hummingbirds] whose males show distinct ornamental traits (for example, plumage patterns and the presence or absence of elongated feathers). Unless the suppressors are completely dominant, these females should show some expression of the male traits. In none of the 13 hybrid cases (from four orders and eight families) that met their search criteria did the females express a male-limited trait. This suggests that male ornaments were sex-limited at their first appearance (perhaps by hormonal or physiological differences between the sexes) or are now controlled by completely dominant modifier alleles or regulatory elements. — SJSEvolution 62, 214 (2008).
PLANETARY SCIENCE: Telling Time on Mars
Our primary information on the surface age and geologic history of Mars comes from crater densities on different parts of the planet and from dating of Martian meteorites (though their source region on Mars is not known with certainty). Crater ages imply that most of the crust is over 2 billion years old. In contrast, dating of one distinct group of Martian meteorites, by a variety of isotopic systems, has generally indicated young ages, from a few hundred thousand to about 1 billion years. These numbers imply that Mars had enough recent magmatic activity to be sampled by a significant fraction of the available Martian meteorites. A problem, however, is that all of these rocks show evidence of alteration and impact events (required for but not limited to their excavation from Mars), as well as hints of the very early geochemical history of Mars, so interpreting their ages is not generally straightforward. Bouvier et al. present additional analyses of two of the purportedly young meteorites using several isotopic systems and evaluate the ages of all of the Martian meteorites. In general, the Sm-Nd and Lu-Hf ages derived from minerals were young, but the Pb-Pb whole-rock ages were much older (~4 billion years). In contrast to some other recent analyses, the authors interpret this difference, and ultimately the other young ages, as indicating later alteration that has re-initialized mineral-based isochrons. — BHEarth Planet. Sci. Lett. 266, 105 (2008).
APPLIED PHYSICS: Plasmons on a Wire
The miniaturization of optical elements such as waveguides is limited by the wavelength of light, typically from a few hundred nanometers up to the micrometer scale. By comparison, electrical circuits can be patterned down to a few tens of nanometers. A promising route for integrating these two scales is direct conversion of an optical signal into an electrical one via surface plasmons created at the surface of metal dielectric interfaces. Schmidt et al. use a photonic crystal fiber as the optical carrier and show that they can fill in the air holes that run the length of the fiber with gold or silver, introduced in molten form, to create an array of metal nanowires embedded within the silica matrix. Characterization of the hybrid structure shows that plasmon resonances are excited in the nanowires, thereby demonstrating the direct integration of optics and nanoelectronics. — ISOPhys. Rev. B 77, 33417 (2008).
PLANT SCIENCE: Resisting Resistance
Bacillus thuringiensis (Bt) toxins are bacterial proteins that kill some insects and are widely used in crops for pest control. Plants expressing the Bt toxins Cry1Ac and Cry1Ab make up the majority of insect-resistant transgenic corn and cotton, respectively. A major concern has been that the pests will develop resistance; refuges, where non-transgenic plants grow next to transgenic plants, have been hypothesized to delay the evolution of resistance because resistant individuals will mate with susceptible individuals from the refuge and hence not confer resistance (if inheritance is recessive) to their offspring.Tabashnik et al. have analyzed field studies from around the world to determine the effectiveness of the refuge strategy. They found that resistance has evolved in populations of the corn earworm at some U.S. locations, but not elsewhere nor in five other pests. Furthermore they conclude that the refuge strategy has been successful in slowing the evolution of resistance and that expressing multiple Bt toxins in single plants may further slow the appearance of resistance. — LMZ
Nat. Biotechnol. 26, 199 (2008).
CHEMISTRY: Air to the Rescue
Chemists generally take pains to ensure the purity of their reagents, lest a highly reactive contaminant doom the anticipated outcome. Once in a while, though, an adventitious impurity proves more help than hindrance. The reported synthesis of quinine by Woodward and Doering in 1944 was a landmark achievement in the history of organic chemistry, but in the ensuing years, controversy swelled over the report’s reliance on a final reduction step published 26 years earlier by Rabe and Kindler. Was the chemistry in this last step really viable? Had Woodward taken his synthesis all the way to fruition, or actually stopped at the penultimate step? In a meticulous effort to reproduce the Rabe study, Smith and Williams now find that modern concerns probably arose on account of excessively pure materials. They show that the reduction proceeds most efficiently when the aluminum powder called for in the published procedure has been partially oxidized by exposure to air, a reasonably likely circumstance in 1918. — JSYAngew. Chem. Int. Ed. 47, 1736 (2008).
IMMUNOLOGY: Exceptional Acceptance
In 1960, Medawar and Burnet were awarded the Nobel Prize for Physiology or Medicine for showing that immunological tolerance to foreign grafts can be acquired. What this and many studies in animal models have shown since is that it is possible to “re-educate” the immune system of the recipient to the foreign antigens carried by the donor cells and grafts. The induction of full immune tolerance to transplanted organs has also been pursued in the clinic because this would dispense with the need for extensive immunosuppressive regimes currently used to prevent organ rejection.Scandling et al. and Kawai et al. report cases of human kidney transplantation in which hematopoietic cells from the donor were co-transplanted with the organ, followed by distinct protocols of conditioning involving depletion of host cells. In the first study, this resulted in ongoing mixed chimerism; meaning that there were comparable proportions of donor and recipient hematopoietic cells circulating in the body. In the second study, the extent of chimerism was considerably less and was also transient. Nevertheless, in both clinical settings it was possible to discontinue immunosuppressive drugs without significant adverse responses to the grafted organs; furthermore, cellular evidence of immunological tolerance was detected. In a third study, Alexander et al. describe a female liver transplant patient in which a spontaneous switch occurred in the blood group and–because the donor was male–the sex of the circulating leukocytes. Impressively, the removal of immunosuppressive drugs, which was undertaken to resolve hemolytic anemia in the patient, resulted in complete chimerism (meaning that all hematopoietic cells were of donor origin) and long-term acceptance of the grafted liver. These three studies reveal that immunological acceptance of transplanted organs is a realistic goal worthy of further exploration. — SJS
N. Engl. J. Med. 358, 362; 353; 369 (2008).
PHYSIOLOGY: We’re Never Alone
Each of us carries a community of microorganisms in our intestinal tract that is necessary for optimal health but varies in composition from person to person. To learn which gut microbes most influence human physiology, Li et al. surveyed these symbionts in seven members of a four-generation Chinese family. By sequencing rRNA genes, they assembled a phylogenetic picture of the resident microbes-mainly Bacteroidetes and Firmicutes spp. The Chinese microbiomes showed individual and male/female differences in their composition, although all differed markedly from those seen in Americans. A parallel analysis of urine samples from the same family allowed the authors to correlate variations in resident microbes with variations in excreted metabolites, which served as a proxy for the individuals’ metabolic state. They found several microbial species that appeared to significantly affect their hosts: for instance, Faecalibacterium prausnitzii was associated with the presence of dimethylamine, suggested to be an indicator of metabolic syndrome and diabetes. This functional metagenomics approach can be extended by combining full genomic sequencing of the microbes with measurements of metabolic or clinical features of interest. Ultimately, the hope is to identify which gut inhabitants are associated with disease states and which with human well-being. — KKProc. Natl. Acad. Sci. U.S.A. 105, 2117 (2008).
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