This Week in Science

Science  12 Jan 2007:
Vol. 315, Issue 5809, pp. 158
  1. Lunar Differentiation


    Highly siderophile elements (HSE) are concentrated in Earth's core and depleted in its silicate mantle, but little is known about their lunar distribution. Day et al. (p. 217) present Re-Os isotope and HSE abundance data for lunar basalts which indicate that the lunar mantle has chondritic HSE ratios similar to Earth's silicate mantle, but with absolute abundances that are 20 times lower. Thus, the silicate-metal equilibration accompanying core formation must have depleted the HSEs in the silicate mantle of both the Moon and Earth, and continued accretion of meteoritic material replenished their mantles with HSEs. However, this late accretion must have terminated earlier on the moon than Earth, and is likely related to sealing of the lunar mantle by crust formation at or before 4.4 billion years ago.

  2. Supernova Shapes

    Type Ia supernovae are widely used “standard candles” for distance measurements. Wang et al. (p. 212, published online 30 November; see the Perspective by Leonard) have collected spectra in polarized light of 17 such supernovae to investigate the geometry of these explosions. More powerful detonations produced more spherical ejecta, and the outer ejecta layers are more inhomogeneous than inner ones. These findings constrain the physics of burning in the supernovae and tightens the luminosity relations of type Ia supernovae that are used for cosmological measurements.

  3. A Nematic Fermi Liquid


    Previous work on strontium ruthenate has revealed the existence of a quantum critical point where the phase transition is driven by magnetic fields. Borzi et al. (p. 214, published online 23 November; see the Perspective by Fradkin et al.) show that easy and hard directions for current flow can be observed that depend on the direction of the applied magnetic field and that this anisotropy breaks the tetragonal symmetry of the underlying crystal structure. The authors argue that their results are consistent with a recently predicted quantum phase of matter, a nematic Fermi liquid, and may present a test bed to explore other such exotic phases observed in other quantum systems where electronic correlation dominates.

  4. Stability from a Gold Coat

    A major problem for fuel cells in automotive applications is the tendency of the oxygen-reducing platinum cathode to dissolve during the repeated potential cycling required for braking and acceleration. Zhang et al. (p. 220; see the news story by Service) found that nanometerscale gold clusters deposited on carbon-supported platinum particles effectively inhibit dissolution during electrochemical cycling experiments in a perchloric acid electrolyte. Surprisingly, the gold does not significantly inhibit the catalytic O2 reduction, despite the low activity of gold alone in this reaction. X-ray absorption near-edge spectroscopic studies suggest that the presence of gold raises the platinum oxidation potential.

  5. Out of Africa When?

    Some evidence implies that modern humans spread out from Africa some 50,000 years ago and reached central and western Europe about 40,000 years ago. The colonization of northern Europe and Asia has been more difficult to date; northwestern Europe was covered in ice, but the land areas to the east were more open but still frigid (see the Perspective by Goebel). Anikovich et al. (p. 223) now show through a comparison of radiocarbon and luminescence dating and paleomagnetic data that a paleolithic archaeological site on the Don River, Russia (about 400 miles south of Moscow) dates to about 45,000 years ago. Although there are many fossils from this time scattered throughout Europe and Asia, ones from Africa for comparison and to test this hypothesis are scarce.Grine et al. (p. 226) have dated a skull first discovered in 1952 from Hofmeyr, South Africa, to about 36,000 years ago based on luminescence data of attached quartz. The skull displays several features that are more primitive than contemporaneous European skulls but is consistent with the emergence of modern humans from sub-Saharan Africa.

  6. Interference in the Secondary

    The effector molecules in RNA interference (RNAi) are small interfering (si)RNAs. The initial population of “primary” siRNAs, ∼22- nucleotides in length with 5′-monophosphates groups, is generated by the Dicer nuclease. Amplification and “spreading” of the initial trigger population are thought to contribute to strength of the RNAi response in a number of systems and involves an RNA-dependent RNA polymerase (RDRP) (see the Perspective by Baulcombe). To investigate the nature of this secondary response, Pak and Fire (p. 241, published online 23 November) and Sijen et al. (p. 244, published online 7 December) analyzed the course of an experimentally induced RNAi reaction in the nematode worm Caenorhabditis elegans and also examined endogenous small RNAs. They found distinct populations of “secondary” siRNAs that are antisense to the messenger RNA target, that have a di- or triphosphate moiety at their 5′ ends, and that may map both upstream and downstream of the original dsRNA trigger. Primary siRNAs do not appear to act as primers for RdRP, but rather guide RdRP to targeted messages for the de novo synthesis of secondary siRNAs that further boost the RNAi response.

  7. Genome of an Often Disregarded Pathogen

    Trichomonas vaginalis is a common but often neglected sexually transmitted pathogen that colonizes the urogenital tract in men and women. Carlton et al. (p. 207; see the cover) describes its genome, which at 160 megabases is significantly larger than any other parasitic protest known so far, and which provides insight into the parabasilids, which lack mitochodria and peroxisomes and instead bear organelles called hydrogenosomes. The highly repetitive nature of this genome, which expands its genome size and hence cell volume, might provide the parasite with a selective advantage for the phagocytosis of bacteria and host epithelial cells.

  8. Separate Ways

    Two dominant lineages of T cells (αβ and γδ T cells) are highly distinct in function and anatomical location, yet share a common precursor within the thymus. Exactly how one cell fate is decided over another remains unresolved. Melichar et al. (p. 230) present evidence that selection to the γδ T cell branch in the thymus is controlled by the transcriptional factor Sox13, which supports and possibly even initiates γδ T cell development, while opposing differentiation of their (αβ T cell brethren. The authors noted that SOX13 inhibited an important effector of the central T cell developmental signaling pathway mediated by the WNT protein.

  9. Area Versus Isolation in Habitat Reduction


    The worldwide expansion of urban and agricultural land has led to widespread reduction in size and increasing isolation of natural habitat patches. Ferraz et al. (p. 238) examined this phenomenon from a large-scale experimental perspective by quantifying the effects of patch size and patch isolation on the occupancy dynamics of 55 species of forest birds from the central Amazon, Brazil. Patch-size reduction had a consistently strong and negative effect on species occurrence, whereas the effects of isolation were often negative but varied considerably across species. Thus, although isolation is important, many species are absent from small patches simply because of area limitation, regardless of isolation.

  10. One Ubiquitin, Two Ubiquitin, Three Ubiquitin, Four

    The role of protein ubiquitination is well known in promoting regulated protein degradation. Mukhopadhyay and Riezman (p. 201) review what is known about the contribution of protein ubiquitination in other cellular pathways, including intracellular signaling, endocytosis and protein sorting.

  11. Reconstructing Tube Worm Metabolism

    The deep-sea hydrothermal vent tube worm (Riftia pachyptila) plays host to bacterial sulfide-oxidizing endosymbionts. These microbes have not been cultivated, inhabit a remote and nearly inaccessible environment, and form the basis for high degrees of primary productivity at deep-sea hydrothermal vents. Markert et al. (p. 247; see the Perspective by Fisher and Girguis) extend the metabolic reconstruction of the symbionts to reveal mechanisms of dealing with oxidative stress, two carbon fixation pathways, and the sulfide oxidation pathway. In particular, they have been able to infer relative protein stoichiometries, as well as compare symbionts in different physiological niches.

  12. Describing Delicate Interactions

    Quantifying the affinities of interactions of biological networks, particularly transient ones, remains a challenge. Maerkl and Quake (p. 233) describe a high-throughput microfluidic platform that allows the measurement of transient and low-affinity interactions and characterized the DNA binding energy landscapes for four eukaryotic transcription factors. In two cases, the binding specificities were used to predict which genes the transcription factors would bind and likely regulate.

  13. Stealth Genes

    Many bacteria indulge in gene swapping, but the mechanisms that ease and regulate horizontal gene transfer are poorly understood. Doyle et al. (p. 251) have discovered that horizontally transferred plasmids possess a gene that helps the plasmid to enter a new bacterial host with minimal disturbance of its physiology. The stealth gene product strongly resembles the H-NS DNA-binding protein encoded by the Gram-negative bacterial host. H-NS acts as a universal repressor and silences transcription across the genome. When the stealth gene is inoperative, there are widespread changes to the host transcriptome with concomitant impacts on bacterial fitness and virulence. The occurrence of similar genes on other plasmids and on genetic elements, such as pathogenicity islands that are thought to have been transmitted horizontally, points to the widespread use of stealth strategies.

  14. Phycobilin-Containing Picoplankton

    An assemblage of planktonic marine algae called picobiliphytes has been discovered by Not et al. (p. 253) with molecular methods. The authors isolated these organisms' tiny cells from cold temperate coastal marine ecosystems and found that these algae contain pigments known as phycobilins and possess a nucleomorph. The authors speculate that these are phototropic organisms probably involved in endosymbiosis.