This Week in Science

Science  22 May 2009:
Vol. 324, Issue 5930, pp. 983
  1. Viral Battery


    In developing materials for batteries, there is a trade-off between charge capacity, conductivity, and chemical stability. Nanostructured materials improve the conductivity for some resistive materials, but fabricating stable materials at nanometer-length scales is difficult. Harnessing their knowledge of viruses as toolkits for materials fabrication, Lee et al. (p. 1051; published online 2 April) modified two genes in the filamentous bacteriophage M13 to produce a virus with an affinity for nucleating amorphous iron phosphate along its length and for attaching carbon nanotubes at one of the ends. In nanostructured form, the amorphous iron phosphate produced a useful cathode material, while the carbon nanotubes formed a percolating network that significantly enhanced conductivity.

  2. Fuel Economy for Growing Cells

    Sophisticated 21st-century analyses of the signaling pathways that control cell growth have led researchers back to the seminal work of Otto Warburg, who discovered in the 1920s that tumor cells generate their energy in an unusual way—by switching from mitochondrial respiration to glycolysis. The advantage conferred by this metabolic switch is puzzling because mitochondrial respiration is a more efficient way to produce ATP. Vander Heiden et al. (p. 1029) review arguments that rapidly growing cells have critical metabolic requirements that extend beyond ATP and that a better understanding of these requirements may shed new light on the “Warburg effect” and ultimately lead to new therapies for cancer.

  3. African Origins

    The modern human originated in Africa and subsequently spread across the globe. However, the genetic relationships among the diverse populations on the African continent have been unclear. Tishkoff et al. (p. 1035; see the cover, published online 30 April) provide a detailed genetic analysis of most major groups of African populations. The findings suggest that Africans represent 14 ancestral populations. Populations tend to be of mixed ancestry which documents historical migrations. The data mainly support but sometimes challenge proposed relationships between groups of self-identified ethnicity previously hypothesized on the basis of linguistic studies. The authors also examined populations of African Americans and individuals of mixed ancestry from Cape Town, documenting the variation and origins of admixture within these groups.

  4. Future Fuels

    In the scramble to develop more environmentally friendly transportation methods, a wide range of questions must be answered, including how automobiles may best be powered. Biomass might be an important source for that power, either to generate electricity for electric vehicles, or to make ethanol for combustion vehicles. Campbell et al. (p. 1055; published online 7 May; see the Policy Forum by Ohlrogge et al.) examined each approach's demands on land-use, using a lifecycle assessment model, for a range of energy feedstocks, conversion methods, and vehicle types. In nearly all cases, bioelectricity would have a significantly higher net transportation efficiency per area of land used than bioethanol, owing to the greater efficiency of electric engines. Furthermore, bioelectricity would result in a smaller CO2 burden. Thus, bioelectricity could represent an important bridge to more advanced transportation technologies with less environmental impact.

  5. “Lake” Victoria?

    After having explored the Eagle and Endurance craters, which are separated by only 800 meters, the Mars Exploration Rover Opportunity spent 2 years at Victoria, a much larger impact crater located 6 kilometers south across Meridiani Planum. Sedimentary rocks previously analyzed at Eagle and Endurance point to local environmental conditions that included abundant liquid water in the ancient past. Now, an analysis of rocks in the walls of Victoria by Squyres et al. (p. 1058) reveals that the aqueous alteration processes that operated at Eagle and Endurance also acted at Victoria. In addition, sedimentary layering in the crater walls preserves evidence of ancient windblown dunes.

  6. Getting Below the Surface


    Two-dimensional electron systems have proved to be a productive area for the study of low-dimensional transport. Despite decades of intense effort, the actual dispersion of the electronic excitations—how their energy behaves as a function of momentum as the they move through their two-dimensional space—have eluded experiments. This elusiveness is due to the samples being thin and buried. Kukushkin et al. (p. 1044; published online 30 April; see the Perspective by Simon) now introduce a technique based on launching surface acoustic waves through the sample. The excitations “ride” these waves, with their energy and momentum mapped out using pulses of laser light. The technique should also be applicable to other hard-to-get-at electronic systems.

  7. Death Mirrors Life

    The accuracy with which fossil assemblages represent the actual composition of ancient ecological communities is very often uncertain. One way to assess the degree of fidelity between living and dead assemblages is to patiently compare modern living communities and the records they leave behind them in real time. Western and Behrensmeyer (p. 1061) recorded the composition of vertebrate herbivore communities in the Amboseli reserve in Kenya over four decades and compared this to the assemblages of bones left behind as the animals died. The results suggest a high level of fidelity of life-to-death assemblages, both in the composition and the relative abundance of species. Thus, older fossil assemblages might indeed give a reliable picture of past and extinct communities.

  8. A Jump on Catalyst Kinetics

    A common method for interrogating fast reaction kinetics is to create an abrupt initiation point, and, for reactions catalyzed by metals on oxide supports, one option is to induce a rapid temperature jump, provided that the part of the catalyst undergoing heating can be probed locally. Thibault-Starzyk et al. (p. 1048) used femtosecond laser pulses to heat catalysts for nitrogen oxide removal and followed the reaction with Fourier-transform infrared spectroscopy. These catalysts are designed to remove NO in fuel-efficient lean-burn engines through reaction with CO, rather than with hydrocarbons. A cyanide reaction intermediate was identified that moves from the silver nanoparticle to the alumina surface.

  9. Viruses and Mobile Phones

    While traditional cellphones have been relatively immune to viruses, smartphones (like Palm, Blackberry, iPhone, and many Nokia brands) that can share programs and data with each other could potentially be vulnerable to virus epidemics. Why then haven't we experienced any major mobile virus outbreak so far? Wang et al. (p. 1071; published online 2 April; see the Perspective by Havlin) believe that the answer is related to the spreading patterns possible in the current mobile phone systems. They studied the anonymized billing record of a mobile phone company, representing the calling patterns and the coordinates of the closest mobile phone tower each time a group of 6.2 million mobile phone subscribers used their phone. While a Bluetooth virus could reach all susceptible users given sufficient time, its spread, limited by human mobility, was relatively slow. In contrast, MMS viruses would have an explosive spreading pattern, but could only reach the group of individuals that know each other and carry the same phone. Thus, no major virus outbreak is likely until one operating system encompasses a larger fraction of the total smartphone market.

  10. Chromatin Modifier Modulates Gene Expression


    Modification of chromatin structure is usually thought of as a global, relatively nonspecific way of modulating gene expression. However, Wellen et al. (p. 1076; see the Perspective by Rathmell and Newgard) demonstrate that such regulation helps link growth factor–stimulated increases in metabolism to appropriate changes in gene expression. Adenosine triphosphate (ATP)–citrate lyase (ACL), which converts citrate to acetyl–coenzyme A (CoA) in the mitochondria of mammalian cells during metabolism of glucose, was also found to be present in the nucleus, where it might regulate activity of histone acetyl transferases (HATs) by controlling the availability of acetyl-CoA. Indeed, depletion of ACL from cultured human colon carcinoma cells specifically decreased histone acetylation in the nucleus, but appeared not to affect the overall amount of acetylation of proteins in the cells. Loss of ACL in cultured mouse 3T3-L1 cells diminished the increase in histone acetylation normally associated with hormone-stimulated differentiation of these cells and inhibited the increase in expression of specific genes, such as that encoding the Glut4 glucose transporter. Thus, ACL may help cells link metabolic activity to changes in gene expression.

  11. Down But Not Out

    The K-complex, a defining characteristic of slow wave sleep, is the largest spontaneously occurring component of the healthy human electroencephalogram (EEG) but little is known about its physiological characteristics in the human cortex. Cash et al. (p. 1084) investigated the intracortical origin of K-complexes in humans undergoing surgery for epileptic seizures. In simultaneous subdural EEG and intracortical multisite microelectrode recordings, K complexes represented cortical downstates reflecting a decrease in neural firing. These down-states are a fundamental mode of cortical operation that have been well studied in animals and may contribute to sleep preservation and memory consolidation.

  12. ABA Receptor Rumbled?

    The plant hormone abscisic acid (ABA) is critical for normal development and for mediating plant responses to stressful environmental conditions. Now, two papers present analyses of candidate ABA receptors (see the news story by Pennisi). Ma et al. (p. 1064; published online 30 April) and Park et al. (p. 1068, published online 30 April) used independent strategies to search for proteins that physically interact with ABI family phosphatase components of the ABA response signaling pathway. Both groups identified different members of the same family of proteins, which appear to interact with ABI proteins to form a heterocomplex that can act as the ABA receptor. The variety of both families suggests that the ABA receptor may not be one entity, but rather a class of closely related complexes, which may explain previous difficulties in establishing its identity.

  13. Rewarding Bursts of Dopamine

    Dopaminergic neurons are thought to be involved in the cognitive and hedonic underpinnings of motivated behaviors. However, it is still unclear whether dopaminergic neuron activation is sufficient to elicit reward-related behavior and which type of neuronal activity pattern serves this purpose. Tsai et al. (p. 1080; published online 23 April) directly compared tonic versus phasic firing of dopaminergic cells in the ventral tegmental area, and the effects on both behavior and dopamine release. Using a transgenic system and virus injection in mice, they targeted the dopaminergic cells with rhodopsin. Light stimulation was then used to drive dopaminergic cells either with a tonic low level of pulses or bursts of high-frequency pulses, with the number of pulses being equal across conditions. Only the high-frequency phasic firing induced a conditioned place preference and dopamine release.

  14. Nuclear Import/Export Receptor

    Nuclear transport receptors constantly shuttle cargo between the nucleus and the cytoplasm through nuclear pore complexes. In the nucleus, RanGTP promotes the dissociation of cargo from importins, which import cargo into the nucleus (where RanGTP is guanosine 5′ triphosphate–bound Ran). Conversely, nuclear RanGTP promotes cargo-binding to exportins, which export cargo from the nucleus. Cargo is released from exportins in the cytoplasm upon hydrolysis of RanGTP. Cytoplasmically assembled RNA splicing components enter the nucleus together with an import adapter snurportin 1 (SPN1), but how then does the import adapter release its cargo and exit the nucleus to collect further cargo? The nuclear exportin CRM1 exports a broad range of substrates—including SPN1, ribosomes, and many regulatory proteins. Monecke et al. (p. 1087) describe the crystal structure of CRM1 bound to SPN1 and RanGTP. The structure shows that SPN1 cannot simultaneously bind its import cargo and the exportin CRM1, ensuring that only cargo-free SPN1 is returned to the cytoplasm. There are no direct contacts between Ran and SPN1 in the ternary complex, suggesting that RanGTP promotes cargo-binding through long-range conformational changes in CRM1.