This Week in Science

Science  20 Sep 2002:
Vol. 297, Issue 5589, pp. 1953
  1. Strength in Smaller Numbers

    At what point do helium atoms in cryogenic clusters show superfluidity effects? Tang et al. (p. 2030; see the cover) obtained high-resolution spectra of the rotational and vibrational motion of OCS molecules in clusters containing two to eight helium atoms. The helium atoms appear to form an equatorial “donut” around the molecule, and the rotational constant falls below the bulk droplet limit for six to eight helium atoms. These results suggest that in smaller clusters, before the onset of superfluidity, the helium atoms in the closest shell are more tightly bound than in bulk clusters, where they can undergo long-path exchange with other helium atoms and thus decrease their binding strength.

  2. Laser Key to Cryptography

    Modern encryption of messages relies on “one-way” functions that are easy to compute in one direction but difficult to deconvolute in the other. Pappu et al. (p. 2026) present a physical one-way function based on the pattern generated by scattered laser light through a disordered medium. The resultant speckle pattern can be processed into a numeric key of fixed length and may be useful as a secure identification and information-encoding technology.

  3. Everyday Wear and Tear

    It is generally assumed that huge floods play a disproportionate role in modifying river courses and eroding bedrock. In a field study on the LiWu River in Taiwan, Hartshorn et al. (p. 2036) found that it is the everyday flows that are mainly responsible for deepening of the bedrock channel in this region of active mountain building. The huge floods act primarily to widen the channel and induce hillslope collapse.

  4. Forest Refuges

    Climatic fluctuations during the Pleistocene caused large-scale vegetation changes in temperate regions. Nevertheless, genetic evidence suggests the existence of refugia where populations of plant species persisted unscathed. A finely detailed pollen analysis of sediments from the Ioannina basin in northwestern Greece by Tzedakis et al. (p. 2044; see the Perspective by Taberlet and Cheddadi) confirm the existence of such refugia in southeastern Europe. Populations of oak, pine, fir, beech, and other tree species persisted through 130,000 years in the Pindus mountains, apparently because local climatic conditions acted as a buffer.

  5. The Ins and Outs of Bacterial Proteins

    In Gram-negative bacteria, secreted and membrane proteins are targeted to a protein conducting pore in the cytoplasmic membrane, the SecYEG translocon. The adenosine triphosphate (ATPase) SecA cycles between soluble and membrane-bound forms and in an ATP-dependent manner extrudes pre-proteins through the translocon. Hunt et al. (p. 2018) determined the crystal structure of soluble SecA with and without Mg-ADP bound at 3.0 and 2.7 angstrom resolution, respectively. Potential pre-protein binding sites are located on a surface that has been implicated in SecYEG binding. The authors propose that nucleotide hydrolysis and release are coupled to a domain-dissociation reaction that gates the binding of SecA to SecYEG.

  6. Running Rings into Polymers

    The synthesis of high-molecular-weight cyclic polymers is often inefficient because the final intramolecular ring closure step demands dilute conditions and is often hampered by chain entanglement. Bielawski et al. (p. 2041; see the Perspective by McLeish) have synthesized cyclic polyethylene molecules with molecular weights up to 200 kilodaltons through a ring-opening metathesis polymerization that tethers the growing chain until it is eliminated as a cyclic species. The cyclic polyethylenes differ from their linear isomers in physical properties such as viscosity.

  7. How Anthrax Murders Macrophages

    Selective destruction of activated macrophages is central to the pathogenicity of Bacillus anthracis. Park et al. (p. 2048) report that lethal factor (LF) produced by the bacterium specifically inactivates the p38 mitogen-activated kinase signaling cascade only in activated mouse macrophages, thus causing cell death. This inhibition prevents the expression of a subset of target genes of the transcription factor nuclear factor B that functions in macrophage survival. This toxic effect of LF on the immune system may play a major role in the virulence of B. anthracis.

  8. One Speck Is All It Takes

    Most metals tarnish quickly in air, but under very low pressure conditions, many metals absorb oxygen but fail to oxidize. Thürmer et al. (p. 2033; see the Perspective by Over and Seitsonen) used scanning tunneling microscopy to show that for lead, impurities are the key to oxidation. Clean lead crystallites, even at elevated temperatures, did not oxidize after exposure to many monolayers of oxygen. By changing temperatures slightly, the shape of the crystallite could be changed, and this process could expose impurities. The unburied impurities rapidly nucleated lead oxide. An analysis of the oxide growth shows how the oxide, once formed, catalyzes its own growth.

  9. The Cost of Being Bigger

    In most mammal species, competition between males for female partners has led to sexual size dimorphism (SSD)—that males are usually larger. Males also tend to die sooner than females, and while male aggression takes its toll, some studies suggest that differences in parasite load between males and females are also to blame. Moore and Wilson (p 2015; see the Perspective by Owens) performed a meta-analysis across many species and found that male-biased parasitism is predictable and depends on the extent of sexual selection, as measured by either mating system or the degree of SSD. Across species, sex differences in parasite loads were associated with sex differences in mortality.

  10. Leaving a Lasting Trace

    The coding, storage, and reactivation of memories at the systems scale (for example, a reaching movement cued by a visual stimulus) has been thought to involve the association of neural firing patterns across the different brain regions that subserve the various functions, such as visual and motor processing. Hoffman and McNaughton (p. 2070) have recorded simultaneously the activity of neural ensembles in four parts of the monkey brain throughout a reaching task. The distributed and correlated activity that occurred during the reaching movement can be detected in three of these regions (in the motor, somatosensory, and parietal cortices), but not in the prefrontal cortex, during a post-task rest period. These findings support the idea that concurrent network activity is the neural representation of a memory.

  11. Haploinsufficiency Comes into Bloom

    A widely accepted model of cancer genetics proposes that tumorigenesis requires inactivation of both alleles of a tumor suppressor gene. However, recent evidence from mouse models suggests that mutation of only one allele (producing “haploinsufficiency,” a state in which the wild-type tumor suppressor is present at half its normal dose) may also affect cancer risk. Goss et al. (p. 2051) now report that mice show an enhanced susceptibility to tumor formation when they carry one mutant and one wild-type copy of Blm, the causative gene for the human cancer predisposition disorder Bloom syndrome. In a companion study of a human population, Gruber et al. (p. 2013) find that Ashkenazi Jews heterozygous for the same BLM mutant allele are more than twice as likely to develop colorectal cancer as control subjects.

  12. Versatile Cytokines

    Type 1 interferons (IFN-α and IFN-β) help defend against viruses through the activation of a variety of protective innate immune pathways. A more controversial mode of action proposed for these cytokines is that they might also regulate IFN-γ, the type 2 interferon responsible for directing the cell-mediated arm of the adaptive immune response. Nguyen et al. (p. 2063) now provide direct evidence that in addition to interleukin-12 (IL-12), IFN-α can activate the IFN-γ-dependent response to viral infection in mice. However, this process required activation of the signal transducer and activator of transcription (STAT)-4, which is also the primary target of IL-12. This finding refocuses the role of type-1 IFNs in controlling adaptive, as well as innate, immune responses to pathogens.

  13. Runaway Mutations

    Germinal centers (GCs) are transient structures within organized lymphoid tissue that are formed by the collaborative response of T and B lymphocytes to foreign antigen. Intense somatic antibody gene mutation (hypermutation) as well as antigen-driven cellular proliferation and death occur in GCs and select for B cells that produce high-affinity antibodies. It has also been assumed that GCs host equivalent events that lead to aberrant B cell responses to self-antigens and autoimmunity. Unexpectedly, William et al. (p. 2066; see the Perspective by Fu and Storb) found that in a background of mice known to possess a strong autoimmune phenotype, somatic hypermutation of genes encoding a self-reactive antibody occurred in regions adjacent to but distinct from GCs. This dissociation of extensive somatic mutation from GCs could reflect a critical element in the progression toward autoimmunity.

  14. Starting Subduction

    Subduction builds mountains, produces earthquakes and volcanism, drives plate tectonics to some degree, and couples plate dynamics to mantle convection, yet we have a poor understanding of how subduction works and how it starts. House et al. (p. 2038) combine thermochronometric dating of uplift with geodynamic models of the Fiordland region of the south island of New Zealand to show that subduction of the Australian plate beneath the Pacific Plate started about 12 million years ago, just offshore of Fiordland. Thus, we are observing the only active initiation of subduction region recognized on Earth, which provides a useful natural laboratory for understanding how subduction works.

  15. Complementarity Makes an miRNA an siRNA

    MicroRNAs (miRNAs), found in scores in animals and now plants, and small intermediate RNAs (siRNAs) are essentially identical in size, ∼22 nucleotides, and are generated by the same enzyme, Dicer. miRNAs lin-41 and let-7 are known to down-regulate the translation of target mRNAs by binding to imperfectly matched binding sites in their 3′-untranslated regions, the mechanism by which all known animal miRNAs are likely to work. siRNAs, which are fully complementary with their target sites, are intermediates in RNA interference, the outcome of which is degradation of the target RNA. Two reports shed light on how this functional distinction can be achieved (see the Perspective by Baulcombe). Hutvágner and Zamore (p. 2056) show that, both in vitro and in vivo, it is purely the degree of complementarity of the let-7 miRNA to its target binding site that determines whether it will function as an miRNA or an siRNA. They also show that the siRNA-protein complex that directs cleavage of the target RNA can function as a multiple turnover enzyme. Llave et al. (p. 2053) show that at least one plant miRNA (from Arabidopsis) has a perfect match with several protein-coding target RNAs. In a similar manner to the siRNA seen in RNAi, and unlike the characterized animal miRNAs, miRNA 39 can direct the cleavage of these targets.

  16. Direct Assistance

    Both B and T lymphocyte responses require support from specialized helper T lymphocytes, which mediate their effects via the cell surface protein CD40. For B cells, this support involves direct interaction with helper T cells expressing CD40-ligand (CD40-L). For CD8+ T cells, such help has been assumed to depend on signals generated via CD40 on third-party antigen-presenting cells (APCs). Bourgeois et al. (p. 2060) developed an in vivo system for priming CD8 responses in which was present on CD8+ T cells, but absent on APCs. As expected, CD4+ cells were required to supply help via CD40-L, but could generate CD8+ T cell memory without the need for CD40 on APC. The role for CD40 on CD8+ T cells, rather than on the APC, indicates a direct form of help paralleling that used in B cell memory formation.