This Week in Science

Science  09 Jan 2009:
Vol. 323, Issue 5911, pp. 183
  1. Turn of the Screw

    CREDIT: JOANNA AIZENBERG/HARVARD UNIVERSITY

    To make chiral superstructures, one usually needs to start with either chiral starting materials or a driving force, like stirring in one direction, that promotes a specific handedness to the helicity. Pokroy et al. (p. 237) show an exception to this with the agglomeration of soft polymeric pillars. The pillars are covered with droplets of an organic liquid, and, as the liquid evaporates, surface tension draws the pillars together. The process can be controlled to bias the helicity over large areas, and the drying process can be used to capture particles within a web of pillars.

  2. Nonlinear Quantum Fluids

    The quantum-level description of many-body electronic effects has generally been simplified to include only a linear degree of interaction between the particles. Recent measurements, however, reveal that this linearization is an oversimplification and fails to describe the complex interparticle effects that may arise at low temperatures. Imambekov and Glazman (p. 228, published online 27 November; see the Perspective by Cheianov) introduce a nonlinear theoretical description of correlation effects in a one-dimensional quantum system. The nonlinear description may represent a universal description of many-body phenomena in condensed-matter quantum systems.

  3. Follow the Moving Polymer Chains

    When materials deform, changes occur within them all the way down to the atomic and molecular level. When rubbery polymers begin to be pulled apart, the classic model put forward by Eyring predicts that the molecular chains will become more mobile. Lee et al. (p. 231, published online 27 November; see the Perspective by Weitz) have measured chain mobilities in a rubbery polymer, poly(methyl methacrylate), using an optical photobleaching method. They find that while the Eyring model works at low stress, once the material starts to flow, mobility values jump up and the distribution of values narrows. Mobility decreases when even greater stresses are applied that induce strain hardening. These results provide a guide for developing more general theoretical treatments of plastic flow.

  4. What We Will Reap

    CREDIT: BATTISTI ET AL.

    Global warming is making growing seasons longer, but higher growing season temperatures, and the episodes of extreme heat that are expected to accompany them, could cause serious damage to agriculture worldwide. Battisti and Naylor (p. 240; see the news story by Holden) use the outputs of 23 climate models to predict how temperatures will change by the end of the 21st century, and historical examples of how heat waves have affected crop production, to weave a cautionary tale. Their results suggest that heat stress on crops could cause profound negative impacts on global agriculture and food security in the absence of adequate adaptation.

  5. Evolution of Darwin's Thinking

    The principal sources of Darwin's originality in the formulation of his theory of evolution by natural selection are reviewed by Bowler (p. 223), who focuses on Darwin's recognition that the evolution of life must be represented as an irregularly branching tree. The author contrasts this idea with other models available at the time and identifies the source of Darwin's idea in his work on biogeography. The role of artificial selection in Darwin's thinking and his appeal to the metaphor of the “struggle for existence” are also analyzed.

  6. Chromatin and Stemness

    Ubiquitination of one of the histones, H2B, has consequences for modifications such as methylation of other histones. These interactions cascade down to control the general activity of the genes bound up with these histones. Buszczak et al. (p. 248, published online 27 November; see the Perspective by Smith and Shilatifard) now show that in Drosophila a ubiquitin protease, scrawny, helps keep genes silent. Scrawny's functions seem particularly important for stem cells in the germline, in epithelia, and in the intestine. In various types of stem cells, the balance between stem and differentiated fates might be tipped by a common chromatin modification route.

  7. Retinal Protection

    Mechanisms controlling apoptosis or cell death in photoreceptor cells are of importance because loss of these cells can result in blindness. Chen and Cepko (p. 256) report that histone deacetylase (HDAC) 4 functions in mouse retina to support survival of retinal neurons. Depletion of HDAC4 increased cell death. Overexpression of HDAC4, on the other hand, supported retinal cell survival. HDACs produce some of their effects by modifying histones associated with DNA, which alters transcription. However, in this case, HDAC4 was predominantly found in the cytoplasm, and mutation of HDAC4 to promote its exclusion from the nucleus did not prevent its protective effects on the retinal cells. Cytoplasmic HDAC4 may act by interacting with the transcription factor hypoxia-inducible factor 1β (HIF1β); deacetylation of HIF1β promotes its stability and accumulation in the nucleus.

  8. Tassel Tussle

    Sex expression in the maize tassel (which only has male flowers) results from the halting of female organ development, but the genetic regulation of this process is unknown. Acosta et al. (p. 262) identify a major gene involved in male floral specificity by mapping and identifying the function of the Tasselseed1 gene. Surprisingly, the gene is not a standard transcription factor but rather appears to be plastid localized and involved in the jasmonic acid pathway. The ts1 mutants have reduced jasmonate level, and its flowers contain both pistils and stamens, instead of male tassels. Exogenous jasmonate can rescue the mutant phenotype. Thus, jasmonate (or its metabolites) is required to suppress female development in male flowers in maize.

  9. Hole in Two

    CREDIT: FRONZES ET AL.

    Type IV secretion systems (T4SS) are used by many Gram-negative bacteria to inject toxin or effector molecules into host cells or transfer plasmids harboring antibiotic resistance genes between bacteria. Fronzes et al. (p. 266) report a cryoelectron microscopy structure of the core complex of a T4SS. The double-walled structure spans the bacterial inner and outer membranes and is open on the cytoplasmic side and constricted on the extracellular side. These details may provide insights into how secretion might be regulated.

  10. Paradoxical Producers

    It is still not clear why certain cooperative behaviors persist when the benefit to nonparticipants appears to outweigh the advantage to the cooperators. In exploring a synthetic microbial system, Chuang et al. (p. 272) contend that although cooperators might be disadvantaged locally because of structural heterogeneities in the population, across the entire population, cooperation has a selective advantage. This study shows how entangled the parameters of natural populations can be and why statistical outcomes might be ambiguous or misleading.

  11. Black and White Reaction?

    Researchers have demonstrated that humans are stupendously poor at forecasting how they will feel in emotionally challenging situations. Usually and happily for our innate dispositions, our estimate of how bad we expect to feel is much worse than the actual feelings we experience. Kawakami et al. (p. 276; see the Perspective by Smith and Mackie) have applied this insight to the question of why prejudicial attitudes and behavior (assessed in the context of white-black relations) persist even though publicly expressed reactions toward prejudicial actions are overwhelmingly censorious. In a series of studies, they show that witnesses of a racist remark made by a white subject toward a black subject consistently predict that they would feel worse than they actually do and that they would shun the white subject (in a partnership task) more than they actually do.

  12. Fixing Nitrogen

    Nitrate is an essential macronutrient for marine algae, the availability of which could help explain glacial-interglacial changes in the concentration of atmospheric CO2 through its effects on marine photosynthetic productivity. The largest source of nitrate in the oceans comes from nitrogen fixation by bacteria. Ren et al. (p. 244, published online 18 December) report measurements of the nitrogen isotopic composition of planktonic foraminifera from Caribbean marine sediments, which indicate that there was less nitrogen fixation in the Atlantic during the last Ice Age than afterward. These data support a long-suspected, but poorly documented, feedback between denitrification and nitrogen fixation across the global ocean.

  13. Minimizing Metallic Carbon Nanotubes

    Single-walled carbon nanotubes (SWNTs) are synthesized as a mixture of two forms, metallic and semiconducting, and much effort has been devoted to separating them so that their properties in given applications can be optimized. For example, the semiconducting SWNTs can be useful as gate materials for thin-film transistor (TFT) applications, but the presence of metallic SWNTs can lead to unacceptably high currents in the off state of the devices. Kanungo et al. (p. 234) show that cyclo-addition of fluorinated olefins to the sidewalls of the as-synthesized SWNTs decreases the conductivity of the metallic SWNTs without appreciable degradation of the conductivity properties of the semiconducting SWNTs. Separation of two forms is not necessary, and the process creates a dense ink that can be spin-coated onto substrates in a reproducible manner. They observe on-off ratios for TFTs in excess of 105 with mobilities of 100 centimeters squared per volt-second.

  14. CD30-ARNT Interaction

    CD30 is a member of the tumor necrosis factor receptor family associated with certain cancers of lymphoid cells. In a screen for proteins that interact with CD30, Wright and Duckett (p. 251) identified ARNT, the aryl hydrocarbon receptor nuclear translocator (also called hypoxia-inducible factor 1α), a transcription factor better known for its roles in mediating responses to the toxin dioxin or cellular responses to hypoxia. In a human cell line, ARNT was associated with internalized CD30. CD30 regulates gene expression in part through the transcription factor NF-κB, and depletion of ARNT-enhanced NF-κB-dependent transcription altered the association of NF-κB subunits with target promoters. Thus, signals impinging on ARNT may indirectly alter signaling by CD30 by modulating the function of the shared ARNT signaling component.

  15. Expanding the Code

    A central tenet of the genetic code is that there is a one-to-one correspondence of triplet codon with specific amino acid. Codons in a genome can be recoded to specify other amino acids but, it has been thought, not both amino acids, unambiguously and at the same time. Turanov et al. (p. 259) show that insertion of two amino acids at one codon can indeed occur unambiguously in different places in the same protein. In the ciliate Euplotes crassus, cysteine (Cys) is encoded by the codons UGU, UGC, and UGA. UGA can also code for selenocysteine (Sec) in the presence of a Sec insertion sequence (SECIS) element. In the E. crassus protein thioredoxin reductase 1, which has seven in-frame UGA codons as well as an SECIS element in its 3′ untranslated region, the first six UGA codons code for Cys and the last for Sec. Thus, UGA codes unambiguously for two different amino acids, which suggests that the genetic code can be extended by recoding subsets of codons.

  16. Intoxicating AMPylation

    Vibrio parahaemolyticus kills infected host cells within hours using three parallel mechanisms, including autophagy, cell rounding, and cell lysis. Yarbrough et al. (p. 269, published online 27 November) now describe a molecular mechanism used by this pathogen to induce cell rounding. The effector, VopS, disrupts signaling by the Rho family of guanosine triphosphatases (GTPases) by modifying a threonine residue in the GTPases with phosphoadenosine (AMP). This modification, AMPylation, prevents the GTPases from interacting with downstream effectors, which are required to mediate actin assembly.

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