This Week in Science

Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1865
  1. Multidrug Transporter Caught in the Act


    Multidrug transporters are integral membrane proteins found in bacteria, which can expel a wide range of drugs and thereby complicate the treatment of a variety of bacterial infections. One such protein, EmrE is a proton-dependent transporter that confers resistance to positively charged hydrophobic antibiotics, including tetracycline. Pornillos et al. (p. 1950) now report the structure of EmrE in complex with a translocation substrate, tetraphenylphosphonium, at 3.7 angstrom resolution. Two EmrE polypeptides form an asymmetric, antiparallel dimer with substrate bound at the dimerization interface. The structure suggests a mechanism in which an asymmetric translocation pathway confers unidirectional transport.

  2. The Zircon's Tale

    Earth's oldest preserved continental crust dates to about 4 billion years ago, much after Earth's formation (4.55 billion years ago); a major question has been how much continental crust had formed previously and been recycled back into the mantle. Some early rocks in Australia contain relic crystals of zircon, recycled from earlier rocks. Zircon harbors uranium, and these have been dated to up to 4.4 billion years ago. Harrison et al. (p. 1947, see Perspective by Amelin) have analyzed lutetium and hafnium isotopes in a large number of these early zircons. This isotopic system provides information on the differentiation of major silicate reservoirs on the Earth. The data imply that significant continental crust must have formed on Earth early on, perhaps by nearly 4.5 billion years ago.

  3. Seeing the Forest for the Trees

    Tree plantations are a potentially valuable tool for slowing the increase of carbon dioxide concentrations in the atmosphere, but they also can affect the water and soil resources on which they depend. Jackson et al. (p. 1944) analyze these often-neglected effects, using a combination of field research, regional economic and climate modeling, and more than 600 already-published observations, to show that afforestation can dramatically reduce water availability, as well as salinize and acidify the surrounding soil. They find that tree plantations caused nearby streams to dry up in more than one-tenth of the cases studied, and that stream flow was reduced by half, on average. These findings should help illuminate the costs of carbon sequestration by afforestation, rather than only their benefits.

  4. Mars, Above and Below


    The Mars Express satellite carries an instrument called MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding), which has been imaging Mars with radar waves. The radar waves penetrate the surface, including the kilometer-thick polar ice caps, to reveal subsurface features. As described by Picardi et al. (p. 1925, published online 30 November), the data reveal the base of icy deposits near the martian north pole, showing that the crust there is rigid, and a buried circular crater, 250 km in diameter, in the Chryse Planitia lowlands. The radar echoes also reveal information about the martian ionosphere. Gurnett et al. (p. 1929, published online 30 November) show that reflections occur where there are sharp changes or gradients in electron density, and with characteristic frequency signatures. In many scans of the ionosphere, Gurnett et al. record a range of echo types, including oblique signals in regions where the relic magnetic field preserved in Mars' crust is strong.

  5. Controlled Conversion

    In the absence of a magnetic field, the two nuclear spin states of an isolated hydrogen atom are completely equivalent. However, in molecules with more than one hydrogen atom, the spins interact with one another, and the total energy changes slightly with their relative orientations. In low-pressure conditions, such as interstellar space, interconversion of such isomers is poorly understood. Sun et al. (p. 1938; see the Perspective by Hougen and Oka) have used the differential absorption of infrared light by the four nuclear spin isomers of ethylene (C2H4) to produce a nonequilibrium population, depleted in one isomer. By monitoring the evolution of this gaseous sample, they find that the isomers of similar inversion symmetry can interconvert efficiently, but do not transform to isomers of opposite symmetry.

  6. MicroRNAs and the Aging Worm

    MicroRNAs are present in diverse organisms, including humans, and control processes such as cell division and cell death. Boehm and Slack (p. 1954) now extend that repertoire of functions to include aging. In the nematode Caenorhabditis elegans, lin-4, a microRNA that is a key regulator of the stage-specific timing of cell division patterns during the larval stage, also influences the life span and the pace of aging in the adult. The microRNA and its target, lin-14, act in insulin/insulin-like growth factor-1 signaling pathway to influence life span and the pace of aging. Loss of lin-4 shortens worm life span. A common mechanism thus serves to control the timing of two processes—development and aging.

  7. Maintaining the Amino Acid Supply Chain

    The efficiency and fidelity of protein synthesis is a key factor in cellular survival under a variety of growth conditions. Now Vabulas and Hartl (p. 1960) show that, under conditions of acute restriction in amino acid supply, continued protein biogenesis in mammalian cells is maintained by proteasomal degradation of preexisting proteins. Amino acid deficiency leads to severe depletion of the intracellular amino acid pool within minutes of proteasome inhibition and, concomitantly, protein translation is impaired. Both nascent and newly synthesized polypeptides remain protected from proteasomal degradation. At most, only a few percent of polypeptides are immediately degraded upon synthesis, indicating that, in contrast to previous estimates, protein biogenesis is a highly efficient process.

  8. Prospects for Limb Regeneration


    Salamanders are able to regenerate a lost limb, a feature of ongoing development sadly lost to humans. Brockes and Kumar (p. 1919) review what is known about amphibian limb regeneration and speculate on how these observations could inform application of stem cell and regenerative medicine to mammalian cases. Zebrafish as well can regenerate their fins. Regeneration occurs through initial formation of a clump of undifferentiated cells, the blastema, which through growth and differentiation elaborates a replacement fin. Whitehead et al. (p. 1957; see the Perspective by Antebi) have now identified one of the signaling factors critical to formation of the blastema. In zebrafish the dob (devoid of blastema) mutation affects a gene that encodes signaling factor Fgf20, which seems to be used specifically for regeneration rather than for normal embryonic development.

  9. Observing the Formation and Recollection of Memories

    Recent advances in analyzing the large data sets collected during functional brain imaging studies have revealed patterns of neuronal activity that can be associated reliably with the recall of remembered stimuli. After seeing pictures or listening to sounds, subjects are able, when prompted, to retrieve or reactivate their memories of these items, and brain scans taken during the retrieval period are similar to those taken when the same items were studied directly. Polyn et al. (p. 1963) now show that reactivation of such stored representations occurs prior to a verbal report of recollection in a free recall paradigm, where subjects were not prompted to remember specific items, but were reporting which of these items “resurfaced” in their memory and when. These results provide support for the theoretical framework of shifting brain states in dynamic cognition.

  10. Challenging Immune Diversity Dogma

    The adaptive immune system has been thought to be confined to the realm of jawed vertebrates, where somatic mechanisms of genetic variation have evolved to generate immune receptors in great diversity that are clonally dispersed among its lymphocytes. However, recently jawless fish have been shown to be able to generate diversity among immune-like receptors, and indeed some invertebrates produce diverse immunoglobulin-like molecules. Extending their original discovery of variable lymphocyte receptors (VLRs) in the sea lamprey, Alder et al. (p. 1970) now provide information on the form, function, and potential extent of somatic genetic diversity in this system. Leucine-rich repeats (LRRs) are randomly selected from a large bank of LRR modules by a sequential mechanism of rearrangement so that an estimated diversity of VLRs rivaling that of immune receptors in mammals is possible. Furthermore serial immunization of lampreys was found to elicit the responses expected in a developing adaptive immune response to an antigen.

  11. Rapid Radiation of Animals

    Despite many years of effort, the relationships within and between major groups of metazoa remain uncertain and controversial. Using substantial quantities of sequence data from several key animal taxa, Rokas et al. (p. 1933; see the Perspective by Jermiin et al.) find a contrast between the history of the metazoan and fungal kingdoms—two groups that originated at a similar time in life's history. In particular, for animals, the lack of resolution of ancient clades is a signature of closely spaced series of clade-generating events. This explicit molecular support for the rapid radiation of animals is in agreement with previous inferences from the fossil record.

  12. Swapping O for N

    The uranyl ion, O=U=O2+, despite its abundance, is a rare example of multiple bonding between a light atom and an actinide metal. A fundamental question in the formation of such complexes is the specific role of partially occupied f-orbitals, or electron distributions unique to the lanthanide and actinide elements. Hayton et al. (p. 1941) have prepared two analogs of uranyl, in which the multiply bonded oxygens are replaced by either alkyl or aryl nitrogen groups. These complexes result from an efficient iodine oxidation of uranium metal in the presence of an amine and are shown by x-ray crystallography to resemble uranyl in their trans N=U=N coordination geometries. Density functional theory calculations shed light on the orbitals involved in the bonding, and the comparative electronic structures of the nitrogen and oxygen compounds.

  13. Linking NO Production and Prostaglandin Synthesis in Inflammation

    Principal inflammatory responses in mammals include the induction of nitric oxide and prostaglandin synthesis in cells of the immune system. Kim et al. (p. 1966) report that the enzymes that produce these molecules, nitric oxide synthase and cyclooxygenase-2, respectively, interact in macrophage cells. Nitric oxide synthase delivers NO to cyclooxygenase-2. This modification of cyclooxygenase-2 by nitric oxide, known as S-nitrosylation, activates the enzyme. This synergism between the two systems may help in the design of potential novel anti-inflammatory drugs.