Editors' Choice

Science  13 Aug 2004:
Vol. 305, Issue 5686, pp. 919

    Breaking Away

    The streptomycetes group of soil bacteria is familiar as the source of medically important antibiotics, such as streptomycin and other small macrocyclic molecules collectively known as polyketides for their mode of biosynthesis. During the life cycle of S. coelicolor, multinuclear aerial structures called hyphae extend from the substratum into the air; subsequently, each nucleus is walled off to form a spore. In order to push the hyphae through the air/water interface, the bacterium secretes SapB, a spore-associated peptide that acts as a surfactant and reduces surface tension.

    Kodani et al. connect bacterial morphogenesis to antibiotic biosynthesis by showing that SapB is encoded by the ramS gene and that its structure is akin to those of antimicrobial oligopeptides known as lantibiotics, named for the posttranslational processing that generates intramolecular thioether (lanthionine) crosslinks. These posttranslational modifications are effected in part by the protein encoded by another gene of the ram (rapid aerial mycelium) operon, ramC, which exhibits sequence similarity to known lantibiotic cyclases. — GJC

    Proc. Natl. Acad. Sci. U.S.A. 101, 11448 (2004).


    An Ice-Free Arctic?

    Coupled atmosphere-ice-ocean climate models predict that greenhouse global warming in the Arctic should be greater than the global mean. Two questions spring to mind. First, how much of the changing atmosphere-ice-ocean conditions in the Arctic is a consequence of natural climate processes, and how much is due to external factors such as anthropogenic greenhouse gas forcing? Second, what fraction of the Arctic sea-ice cover will disappear in this century due to anthropogenic factors?

    Johannessen et al. have compiled observational data of surface air temperature and sea ice for the quarter century between 1978 and 2003, and compared them with the output of the ECHAM4 and HadCM3 models. They conclude that warming in the early 20th century was due to natural internal climate system variability but that a substantial part of the recent changes is a response to anthropogenic forcing. The area of Arctic sea ice has decreased by 7.4% or 8 × 105 km2 in the past 25 years, with the minimum summer ice coverage occurring in September 2002. Their simulations predict that the Arctic will be almost free of sea ice during the summers toward the end of the this century (for another climate prediction for the late 21st century, see Meehl and Tebaldi, this issue, p. 994). — HJS

    Tellus A 56, 328 (2004).


    Tougher and Stiffer

    Composites are typically designed so that the addition of a small amount of secondary material, in the form of particles or fibers, enhances specific properties of the primary matrix material. However, when the size of the added particles approaches the nanometer scale, they can alter the crystal structure or bulk morphology, and hence can stabilize metastable or otherwise inaccessible phases.

    Shah et al. have examined composites of clay particles (montmorillonite) dispersed into poly(vinylidene fluoride) (PVDF), a commercially important polymer with five known crystalline forms. The α form is most common, but it is the all-trans β form that exhibits the largest piezo- and pyroelectric responses and thus is of the greatest technological interest. Mixing in a surface-modified montmorillonite clay made it energetically favorable for the polymer chains to intercalate between the layers of the clay particles and promoted a transition from ordered α crystallites to disordered, fiber-like β crystallites in the PVDF matrix. In mechanical tests, composites made with this modified clay showed an increase in both toughness and stiffness, whereas in most cases an increase in one property comes at the expense of the other. — MSL

    Adv. Mater. 16, 1173 (2004).


    A Nuclear Bouncer

    In eukaryotic cells, the nucleus contains a defined set of proteins, which are, of course, manufactured in the cytosol. This compartmentalization is enforced by the double membrane of the nuclear envelope, which is punctuated by nuclear pore complexes. Extensive trafficking of proteins and RNAs between the cytoplasm and the nucleus via the nuclear pores is essential for regulating gene expression. Mingot et al. now characterize the role of the protein RanBP16/exportin 7, which mediates the nuclear export of a variety of protein substrates, such as p50RhoGap and 14-3-3s. Exportin 7 binds to folded motifs that contain basic residues, unlike the short leucine-rich linear stretches of amino acids that are recognized by one of the other well-known nuclear exporters, CRM1. Exportin 7 escorts multiple cytosolic proteins from the nucleus back into the cytoplasm, and thus may function to exclude numerous proteins that otherwise would interfere with gene expression if allowed to gather in the nucleus. — SMH

    EMBO J. 10.1038/sj.emboj.7600338 (2004).


    Laser-Produced Radioactive Isotopes

    Tagging pharmaceuticals with short-lived positron-emitting isotopes makes it possible to use the imaging technique positron emission tomography (PET) to follow the fate of these compounds non-invasively; for instance, in measuring neurotransmitter receptor distributions in the brain. Although essential from a biosafety point of view, the short lifetimes of these radioactive isotopes present logistical limitations on the locations of the scanners—requiring them to be in proximity to an isotope-producing nuclear facility or synchrotron—which may not always be where the patients are. Using the powerful VULCAN laser, Ledingham et al. present a proof-of-principle demonstration in which radioactive isotopes of carbon and fluorine are produced in sufficient abundance during the interaction between petawatt laser pulses and a solid target such as gold, aluminum, or mylar foils. As high-power lasers continue to shrink in size, the prospects for radioactive isotopes being produced in-house will improve. — ISO

    J. Phys. D App. Phys. 37, 2341 (2004).


    Aggression and Survival

    The visible burrow system, comprising an open field, tunnels, and chambers, provides rats with a seminaturalistic environment. Rats housed in such a structure explore the field, sleep in the chambers, and compete for access to food and water via two narrow ramps. These behaviors and, in particular, the associated social interactions result in the establishment of a dominance hierarchy after only 3 days of habitation.

    Kozorovitskiy and Gould document the extent of neurogenesis in the dentate gyrus of the hippocampal formation as a function of social position. They find that subordinate and dominant rats generate the same number of new neurons, but that more of these cells survive in the more aggressive individuals. The difference in number does not appear to be due directly to stress (which might be expected to dampen neurogenesis), because corticosterone levels did not differ; it is possible, however, that behavioral history may influence the glucocorticoid sensitivity of neuronal proliferation, as suggested by Mirescu et al. in a related study. — GJC

    J. Neurosci. 24, 6755 (2004); Nature Neurosci. 7, 841 (2004).

  7. STKE

    Dying Cells Promote Proliferation

    Huh et al. used the developing Drosophila wing to determine whether activation of the apoptotic pathway in a cell would stimulate proliferation of adjacent cells. The impetus for this experiment was the observation that even if more than half of the cells in the wing disc are lost, the surviving disc cells will proliferate and replace the missing cells, producing a normal-sized wing. In cells of the posterior wing compartment, the cell death pathway was activated by expression of the head involution defective (Hid) protein, while at the same time actual death was prevented by expression of the baculovirus p35 protein that inhibits effector caspase activity (but not activation). When Hid and p35 were both present, the number of cells in the posterior wing compartment increased, and when a dominant-negative form of the caspase Dronc was introduced along with p35 and Hid, this proliferative response was blocked. Thus, dying cells may transmit a signal through a Dronc-dependent mechanism that stimulates their neighbors to replenish the population. — NG

    Curr. Biol. 14, 1262 (2004).