This Week in Science

Science  31 Aug 2007:
Vol. 317, Issue 5842, pp. 1141
  1. Molecular Switching via Hydrogen Hopping


    Large changes in conformation can be expected to change the conductivity of a molecule; in device applications, small changes can help maintain geometries favorable for bonding the molecule to its contacts or allowing it to interact with other switching molecules. Liljeroth et al. (p. 1203) show that the position of the two internal hydrogen atoms on the inner cavity of free-base naphthalocyanine molecules can be switched under cryogenic conditions using the tip of a scanning tunneling microscope (STM). Creation of the new hydrogen tautomer changed the conductance of the molecule. When the molecules were pushed into a chain with the STM tip, a current pulse in an end molecule could induce hydrogen-atom switching in its neighbor.

  2. Aqueous Cascade

    When biosynthetic pathways prove hard to replicate in laboratory model systems, the discrepancy is often attributed to the structural complexity of enzymes. Such was the case for the ladder polyethers, a class of marine toxins associated with red tides. The core of linked tetrahydropyran (THP) cycles appeared most likely to stem from a precursor of multiple epoxides poised for a cascade of consecutive ring-openings, but for more than 20 years, the requisite selectivity for this sequence could not be replicated without adding numerous unnatural substituents needed to direct the reaction. Vilotijevic and Jamison (p. 1189; see the news story by Service; see the cover) show that the problem was the focus on organic solvent media. Neutral water proved an optimal promoter for the reaction and afforded the polycyclic core in good yield and selectivity from an epoxide chain precursor anchored by a single templating THP.

  3. Sustainable Tropical Forest Management?

    The development of conservation strategies in tropical forests requires the assessment of current practices. Oliveira et al. (p. 1233, published online 9 August 2007) used an automated satellite analysis system to detect both forest disturbances and deforestation, down to the level of a few tree falls, caused by natural and anthropogenic processes in the Peruvian Amazon between 1999 and 2005. Although forest disturbances and deforestation have increased recently, three factors have combined to protect the forests: conservation strategies protect against poaching and clearing; the titling of indigenous territories has protected against deforestation and disturbances; and logging concessions have decreased deforestation in the timber harvest areas. Thus, a portfolio of land-use policies can provide broad protection while still allowing for tribal subsistence and income generation.

  4. Solar Heat Waves


    The solar corona is extremely hot gas, extending from the surface of the Sun to millions of kilometers into space. Plasma waves, including incompressible Alfvén waves transmitted along electromagnetic field lines, are thought to be important in heating this gas to temperatures above a million kelvin, but such waves have remained undetected. Tomczyk et al. (p. 1192) have now imaged the Sun and detected the characteristic pattern of Alfvén waves traveling across its surface with a period of about 5 minutes. However, the waves are weaker in strength than predicted, which suggests that other mechanisms are needed to heat the solar corona.

  5. Martian Ferric Oxides

    The geological history of Mars is partly recorded in the various minerals in rocks and soils exposed at its surface. Using infrared spectral data from the Mars Express satellite in orbit around the planet, Bibring et al. (p. 1206, published online 2 August 2007) show that hematite, formed from oxidized iron, is closely associated with layered sulfate deposits across several of the older terranes on Mars. The oxides formed either contemporaneously or subsequent to the sulfates. Finding this association across different regions implies that rising acidic groundwater conditions were pervasive at the time these minerals were formed.

  6. Magnetic Anisotropy of Embedded Atoms

    On a per-atom basis, small molecular magnets and isolated atoms on surfaces can exhibit large anisotropies in their magnetic response with the direction of the applied field at cryogenic temperatures. Hirjibehedin et al. (p. 1199) have used a scanning tunneling microscope to place iron and manganese atoms in a thin layer of copper nitride and measured their magnetic properties at 0.5 kelvin. The large anisotropies observed are explained by density functional calculations, which indicate that these atoms are covalently incorporated into the CuN layer and transfer charge and spin polarization into the surrounding network.

  7. Superconducting in the Middle

    Recent work has revealed that the interface between two oxide insulators, LaAlO3 and SrTiO3, can be metallic. In addition, the conductivity of the interface depends on the thickness of the overlayer. Reyren et al. (p. 1196, published online 2 August 2007) now show that this interface can also be made superconducting, albeit at low temperatures (200 millikelvin), and show that the properties display signatures of a transition expected for a two-dimensional superconductor.

  8. MicroRNAs and Parkinson's Disease

    A variety of nonprotein coding RNA transcripts play roles in development. Kim et al. (p. 1220; see the Perspective by Hébert and De Strooper) now demonstrate a role for microRNAs in the maturation, function, and survival of midbrain dopaminergic neuron cells that are lost in Parkinson's disease. Loss of microRNAs in postmitotic midbrain dopamine neurons leads to a phenotype that resembles Parkinson's disease. The microRNA miR-133b is specifically expressed in human midbrain dopaminergic neurons and is lost in Parkinson's patients. miR-133b functions in a feedback loop with Pitx3, a critical transcriptional regulator of midbrain dopaminergic neurons.

  9. AID Asymmetry

    During somatic hypermutation (SHM), antibody genes that have already generated diversity through somatic rearrangement can diversify further. The enzyme responsible for SHM, activation-induced cytidine deaminase (AID), deaminates cytosines to generate uracils in the DNA strand. This reaction initiates subsequent mutations of adjacent residues through the DNA repair process. Unniraman and Schatz (p. 1227) now show that SHM is an asymmetric process, with only cytosine residues on the nontemplate strand provoking mutations upstream and downstream. AID targets both strands so it cannot be the source of this asymmetry. Instead, the DNA base-repair system was responsible.

  10. Origins and ORCs


    Accurate initiation of DNA replication is essential to life. In eukaryotes and archaea, replication initiation is regulated by adenosine triphosphatases of the origin recognition complex (ORC) superfamily that bind to replication origins and prime the DNA for replisomal assembly. Two studies now describe the structural basis for origin recognition by the archaeal initiation factor Orc1 (see the Perspective by Georgescu and O'Donnell). Gaudier et al. (p. 1213) describe the structure of a single Orc1 subunit in complex with its target origin-binding site, and Dueber et al. (p. 1210) describe the structure of a pair of Orc1 paralogs bound to a second class of origin sequences. Together, the structures provide insight into the stepwise process leading to initiator assembly and activation.

  11. Learning and Recall

    During memory encoding, cell assemblies are thought to be activated and linked together by synaptic plasticity. During subsequent retrieval, it is thought that these assemblies may be reactivated by partial activation and pattern completion. Reijmers et al. (p. 1230) developed mutant mice that allowed active neurons to be tagged differentially during acquisition and retrieval of contextual fear conditioning. In histological sections of the basolateral amygdala the number of neurons that were active during both encoding and retrieval could be counted. Successful memory retrieval was associated with reactivation of neurons that fired during learning.

  12. Intricacies of Cell Contacts

    Cell-cell contacts in multicellular organisms are intricately regulated, and their stability is partly controlled by protein kinases and phosphatases that tune the level of tyrosine phosphorylation. Type IIB receptor protein tyrosine phosphatases (RPTPs) have both adhesive and catalytic properties. Aricescu et al. (p. 1217) determined the crystal structure of the full-length extracellular region of an RPTP, which forms a homophilic trans dimer that is rigid and has dimensions that match the intercellular distance at cadherinmediated junctions. The trans interaction may act as a spacer clamp that localizes phosphatase activity near its target substrates.

  13. Not Lost In Translation

    Protein synthesis by ribosomes normally requires that the messenger RNA (mRNA) template be capped at its 5' end with a modified nucleotide. Starved yeast can undergo a developmental switch to an invasive phenotype, during which most mRNAs are decapped, which prevents their translation by ribosomes. The genes responsible for the switch must be therefore translated in a cap-independent manner. Gilbert et al. (p. 1224) now show that invasive growth-specific genes have internal ribosome entry sites (IRESs) in the unusually long 5' untranslated regions of their mRNAs that allow their translation. Unlike viral IRESs, these cellular IRESs seem to consist of an unstructured sequence of adenosines that recruits the poly-A binding protein, a configuration that can substitute for the 5'cap.