This Week in Science

Science  08 Dec 2006:
Vol. 314, Issue 5805, pp. 1512
  1. Cataclysmic Cannibals


    Cataclysmic variables are binary systems in which a compact white dwarf sucks material from its companion star, which causes their light emission to flicker. Theoretical work has suggested that the donor stars in most fast-spinning cataclysmic variable systems should have lost enough hydrogen to become brown dwarfs, but none have been seen. By accurately timing the eclipses in the short-period cataclysmic variable system SDSS 103533.03+055158.4, Littlefair et al. (p. 1578; see the Perspective by Maxted) show that its donor is a 0.05 solar mass brown dwarf, which was likely cannibalized from a normal main-sequence star. The star's mass is slightly greater than its orbital period would suggest, which implies that brown dwarf radii may be underestimated by current evolutionary models.

  2. Mars Changes in Real Time

    The thin martian atmosphere does little to protect its surface from bombardment by even small objects from space. Malin et al. (p. 1573; see the news stories by Kerr) have found new impact craters that pockmark the surface of Mars through differencing images from Mars Global Surveyor taken 7 years apart. The impact cratering rate they measured is comparable with that seen for the Moon. Also, they spottey measured is comparable with that seen for the Moon. Also, they spotted recent changes in the walls of two craters that they interpret as evidence for recent trickles of liquid water.

  3. Cooling on the Side


    Devices based on quantum systems generally perform better under cryogenic conditions that minimize thermal noise. However, the lowest temperature achievable is typically limited by the cooling system used. Valenzuela et al. (p. 1589; see the Perspective by Chiorescu) introduce a method of lowering the effective temperature of a qubit by using the sideband cooling technique developed for quantum and atom optics. The two-level system under study, a flux qubit, has an ancillary higher level that is used as a passage level from the qubit's thermally excited state toward its ground state. By driving the population to the ground state through the side-band transition, they can cool the qubit to 3 millikelvin, appreciably lower than the several-hundred-millikelvin temperature of its thermal bath.

  4. Helium to Burn

    Stars like the Sun produce 3He as they burn, and when they finally swell to red giants at the end of their lives, the 3He should mix into the convecting outer layers and ultimately be lost in stellar winds. However, very little 3He is seen in interstellar space beyond the predicted amount from Big Bang nucleosynthesis. Eggleton et al. (p. 1580, published online 26 October; see the Perspective by Podsiadlowski and Justham) show by modeling a red giant star in three dimensions that turbulence at the base of the convection zone pushes 3He back down in to the star's engine, where it is burnt further to 4He and H. This turbulence arises from a switch in the mean molecular weight of layers that leads to a Rayleigh-Taylor instability.

  5. Robots, Computers, and DNA

    The use of complex DNA pairing and strand-displacement schemes for computing and robotics is the subject of two reports (see the Perspective by Fontana). Ding and Seeman (p. 1583) have taken a DNA device that normally operates in solution and show that, when mounted on a lattice and placed within a cassette, it retains its functionality. The placement and operation of specific devices at this size scale is a key step in the development of nanorobotics. Seelig et al. (p. 1585) have designed a set of single-stranded DNA molecules that can be used in a modular fashion to build a series of logic circuits such as AND, OR, and NOT operators, as well as an amplifier and a thresh-olding device. The devices work by letting an input DNA strand bind to an exposed or unpaired segment of a gate device, which causes a strand displacement.

  6. Later Than Expected for a Date

    An important australopithecine, StW 573, has been recovered from Sterkfontein cave, South Africa. Originally, only its foot was recovered, but it now appears that most of the skeleton is available. This hominid has been thought to have lived before 3 million years ago (Ma), and earlier work, based on magnetic stratigraphy and cosmogenic dating, put it as old as 4 Ma. Walker et al. (p. 1592) dated the cave deposits holding the fossil with the more accurate U-Pb system. Their ages indicate that the fossil formed only about 2.2 Ma, which implies that the South African australopithecines represent hominids living after the development of tools, rather than before.

  7. From Competition to Cooperation

    Understanding the evolution of cooperation—whether between genes or cells or within animal and human societies—remains one of the fundamental challenges of biology (see the Perspective by Boyd). Nowak (p. 1560) reviews the five main mechanisms of cooperation: kin selection, direct reciprocity, indirect reciprocity, network reciprocity, and group selection. Bowles (p. 1569) contends that the ecological challenges facing humans during the late Pleistocene resulted in intense competition for resources, frequent group extinctions, and intergroup violence. Genetic, climatic, archaeological, ethnographic, and experimental data were used to look at human cooperation in an economics-based, cost-benefit model. Members of a group bearing genes for altruistic behavior pay a tax by limiting their reproductive opportunities in order to benefit from sharing food and information, thereby increasing the average fitness of the group, as well as their interrelatedness. Bands of altruistic humans would then act in concert to gain resources from other groups at a time when humans faced daily challenges to survival.

  8. Toward Biofuels

    Successful biofuels development will require the creation of microbial strains that have high ethanol and glucose tolerance and necessitate the reprogramming of whole segments of metabolism. Alper et al. (p. 1565) changed one member of the global transcription machinery so that the levels of the multitude of genes necessary to achieve ethanol and glucose tolerance could be altered simultaneously. To date, biofuels are produced from monocultures grown on fertile soils. These biofuels are “carbon-positive” because their production and combustion increases atmospheric CO2, although not as much as do fossil fuels. Tilman et al. (p. 1598, see the cover) now find that biofuels produced by polycultures of multiple species can be “carbon negative” and may provide a substantial portion of global energy needs in a sustainable and environmentally beneficial manner without competing with food production for fertile lands.

  9. Switching Neurotransmitter Effects in Development


    The neurotransmitter GABA generally exerts inhibitory effects on neuronal activity during adulthood, but, during early development when circuits are being built, GABA has excitatory effects. Studying chick neurons, Liu et al. (p. 1610) show that the change involves a switch in the direction of the chloride gradient across the cell membrane, which is in turn triggered by changes in nicotinic signaling activity. The change in signaling modality may reflect how neuronal activity and cellular development interact to fine-tune the structure of the brain.

  10. From Oscillations to Patterning

    During early vertebrate development, blocks of mesodermal tissue, somites, are laid down on either side of the notochord in a periodic fashion following rhythmic waves of gene expression in the presomitic mesoderm. The somites subsequently give rise to skeletal muscle, axial skeleton, and part of the dermis. Dequéant et al. (p. 1595, published online 9 November) use a systematic analysis of genes expressed in the presomitic mesoderm over time. Oscillations of the fibroblast growth factor and Notch pathways alternated with components of the Wnt pathway, which suggests that an antagonism between these signaling pathways leads to the generation of phased somites.

  11. A Deadly Duo

    Human immunodeficiency virus (HIV) and malaria are two of the greatest infectious disease concerns that occur together in tropical regions. The interaction between these pathogens during coinfection is poorly understood, but it seems that infection with one predisposes to infection by the other. Abu-Raddad et al. (p. 1603) have examined the human population consequences of HIV and malaria parasite coinfection in a high-risk region of Africa. The authors tested their model on data gathered from Kisumu, Kenya, and found that a synergy operates between the pathogens that explains the propagation of many thousands of HIV infections and almost a million malaria episodes since 1980.

  12. Regulatory RNAs

    Many small regulatory noncoding RNAs act by base-pairing with their target RNAs and interfering with their translation and/or affecting their stability, but not all such RNAs act in this way. The highly conserved prokaryotic 6S RNA interacts with and inhibits the Escherichia coli σ70-containing RNA polymerase (RNAP). 6S RNA contains a single-stranded RNA bulge structure that mimics a gene promoter “open complex” and suggests it might compete with promoters for RNAP. Wassarman and Saecker (p. 1601) show that the bulge does indeed bind in the active site of RNAP in a manner analogous to the open complex, which prevents RNAP binding to legitimate targets. In the presence of nucleotides, the RNAP can synthesize short-product RNAs from the 6S RNA template, which causes the RNAP to dissociate, freeing it to restart transcriptionduring outgrowth from stationary phase and in response to nutrient availability.

  13. Understanding the Master Regulator

    Bacterial pathogens need to be able to endure rapid and extreme environmental shifts and meanwhile ensure appropriate switches in gene expression. The two-component “master regulator” signaling system constituted by the PhoP regulator protein and the PhoQ Mg2+ sensor is an important virulence regulator for pathogenic bacteria. Shin et al. (p. 1607) found that immediately after activation of PhoP and PhoQ, an initial surge in transcription is followed by lower steady-state levels of transcription of target genes. The transcriptional surge was also observed in other two-component systems and may represent a general response that allows organisms to establish a new phenotype immediately in novel conditions.