This Week in Science

Science  22 Sep 2006:
Vol. 313, Issue 5794, pp. 1700
  1. Ribozymes Lost and Found


    It has been suggested that self-cleaving RNAs and other ribozymes represented a step—the RNA world—in the origin of life (see the Perspective by Been). Now Klein and Ferré-D'Amaré (p. 1752) report crystal structures of the glmS ribozyme, which regulates the synthesis of glucosamine-6-phosphate (GlcN6P), a key metabolic precursor of the bacterial cell wall. The structures cover the precleavage state, both unbound and bound to the competitive inhibitor glucose-6-phosphate, and the postcleavage state. Unlike other riboswitches, where metabolite binding regulates activity by inducing a conformational change, in GlmS the ribozyme conformation is similar in all three states. GlcN6P binds to a preformed site and is precisely positioned to serve as a coenzyme. Few self-cleaving ribozymes have been detected in mammals, leading to speculation that they have been lost over evolution. Salehi-Ashtiani et al. (p. 1788) identified a self-cleaving ribozyme in the human genome that shares biochemical and structural properties with hepatitis delta virus ribozymes.

  2. Spawned from Vesta


    Meteorites offer glimpses of the earliest stages of planetary formation. Stony-iron meteorites come in two main classes, pallasites and mesosiderites, and it was thought they may have had similar origins. Greenwood et al. (p. 1763, published online 24 August 2006; see the Perspective by Clayton) have found that their oxygen isotope properties differ, suggesting they come from distinct places. The characteristics of mesosiderites suggest they came from the third largest asteroid, Vesta, the target of the NASA Dawn Mission. Pallasites are made of mixed core-mantle material from a disrupted asteroid, indicating that extensive asteroid deformation was an integral part of planetary accretion in the early Solar System.

  3. All in One Shot

    Ultrafast laser studies have relied on one light pulse to initiate a chemical reaction and a second one to probe the outcome. Dynamics are measured by continuously repeating this process while successively lengthening the time between the two pulses. Because this approach requires many laser shots, easily depleted samples such as ordered crystals often decompose before sufficient data can be acquired. Poulin and Nelson (p. 1756, published online 31 August; see the Perspective by Apkarian) developed a scheme in which a delay gradient across a single femtosecond probe pulse allows chemical events spanning a 10-picosecond period to be tracked with subpicosecond resolution. They measured the photodissociation and recombination dynamics of crystalline I3 and find that geminate recombination rates depend sensitively on the crystal structure.

  4. A Small Advantage

    Capacitors work by storing charge in conductive foils separated by dry nonconducting layers. Supercapacitors also store charge in a static form, but resemble batteries in that they use porous conductors and an electrolyte to store and conduct the charges. It has been assumed that larger pores should lead to better performance because they increase the mobility of the anions and cations. However, Chmiola et al. (p. 1760) now show an anomalous increase in capacitance for pores smaller than 1 nanometer that may allow development of supercapacitors with higher energy densities.

  5. Earthquakes Unzipped

    Earthquake rupture has long been thought to occur by propagation of a crack, but more recent observations and theory seem to indicate a “pulse-like” or “self-healing” mode of rupture propagation. In a series of model experiments, Lykotrafitis et al. (p. 1765; see the Perspective by Marone and Richardson) use a combination of dynamic photoelasticity and laser interferometry techniques to watch various rupture modes propagating along frictionally held, incoherent, interfaces and address the question of what controls slip at a point on a fault during an earthquake in realistic settings. The results show that self-healing pulses are typical and that crack-like or pulse-like modes, or both, can pertain depending on conditions.

  6. All Mixed Down

    Turbulence near the surface of the ocean helps transport nutrients to deeper regions and exchange gases with the atmosphere. Most assessments of turbulent mixing have focused on physical drivers, such as wind. Kunze et al. (p. 1768; see the news story by Kerr) report that the dusk ascent of abundant krill (a type of pelagic crustacean) from their daytime depth of 100 m to the surface generates significant turbulence, up to four orders of magnitude greater than that observed at other times, in a coastal inlet. If the effect is widespread, surface mixing could have a significant biological origin.

  7. Icing Up

    Cirrus clouds reflect shortwave radiation from the Sun and absorb reflected longwave radiation. The magnitude of these effects depends on the properties of their constituent ice particles and how they form and grow. Abbatt et al. (p. 1770, published online 31 August) describe that ice can form via heterogeneous nucleation on solid ammonium sulfate aerosols. Prevailing theories have assumed that ammonium sulfate aerosol nucleate ice from the liquid state through a homogeneous process. These findings raise the question of how anthropogenic ammonia emissions, which now exceed natural ones, might impact the formation of upper tropospheric ice clouds.

  8. Climate and Genetic Change

    Some organisms undergo genetic change when they are exposed to higher than normal temperatures. However, whether recent global warming might already be driving such changes has been uncertain. Balanyá et al. (p. 1773) compiled data on chromosomal polymorphisms covering periods of 13 to 46 years for 26 populations of the fruit fly Drosophila subobscura on three continents. Weather records for the same periods and locations suggest that recent climate warming is associated with genetic change in 22 of the populations, favoring genotypes characteristic of low latitudes.

  9. Social Experience and the Need to Sleep

    Sleep is widely observed in the animal kingdom and yet we still don't know why it is beneficial. Studying Drosophila, Ganguly-Fitzgerald et al. (p. 1775) developed a strategy for elucidating the mechanisms underlying the need to sleep. They observed that a rich social experience, versus an impoverished one, increased the duration of sleep, which in turn was promoted by processes that underlie learning and memory, such as dopamine and cyclic adenosine monophosphate signaling pathways. Mutations in 17 genes were found to disrupt experience-dependent changes in sleep.

  10. Infectious Amyloid


    β-amyloid plays a key role in Alzheimer's disease. There also exist marked pathological similarities between Alzheimer's disease and so-called prion diseases like Mad cow disease. Meyer-Luehmann et al. (p. 1781) now show that cerebral β-amyloid-amyloidosis can be induced by the injection of exogenous, β-amyloid-rich brain extract, and that cerebral amyloid induction is dependent on intrinsic properties of the injected β-amyloid agent and the host that receives the injection. The results suggest the occurrence of polymorphic β-amyloid species with varying biological activities, reminiscent of prion strains. The findings underscore the commonalities among diseases of protein aggregation and assembly.

  11. Tumors Send for Help

    Solid tumors require a blood supply for their growth, and they recruit surrounding host endothelial cells to build new blood vessels. The extent to which tumors enlist the help of the endothelial progenitor cells (EPCs) that circulate in the blood has been controversial. Studying mouse models, Shaked et al. (p. 1785) show that treatment of tumors with drugs called vascular disrupting agents (VDAs) leads to a sudden and dramatic mobilization of EPCs to the tumor rim. When EPC mobilization was prevented, the tumors were more responsive to the therapy. Thus, under certain circumstances, the contribution of EPCs to tumor angiogenesis is indeed substantial.

  12. Protein Pathways in Epilepsy

    One cause of epilepsy is mutations in proteins that function in the brain. Fukata et al. (p. 1792; see the Perspective by Snyder) identified the partners of a complex of proteins located at rat brain synapses. Of the various components, one (LGI1) seems to function as a ligand, one (ADAM22) as a receptor, and one (PSD-95) as a scaffolding anchor. LGI1 controls the strength of excitatory synapses. Both the ligand and the receptor of this complex are implicated by genetics and mutations as being causative for certain types of epilepsy.

  13. Beyond Self--Non-Self for MHC

    Proteins of the major histocompatibility complex class 1 (MHC1), which are important in identifying self and non-self tissue for the immune system, are also found in the brain. Syken et al. (p. 1795) show that a receptor, PirB, to which the MHC1 proteins bind, is also found in neurons of the brain. In mice carrying a mutant PirB lacking its signal transduction capabilities, the overall structure of the brain remained normal. However, these mice showed greater than normal plasticity in the visual cortex. Thus, intercellular signaling through PirB seems to be critical for keeping visual plasticity within limits.