This Week in Science

Science  12 Dec 2003:
Vol. 302, Issue 5652, pp. 1857
  1. Gone with the Flow

    The debate about how much liquid water, if any, was present on Mars when the water was flowing and for how long remains unresolved. Malin and Edgett (p. 1931) provide evidence from a meander (below) and alluvial fan, imaged by the Mars Orbiter Camera onboard Mars Global Surveyor, for persistent and recurring water flow over many years in the early history of Mars, when the planet was likely warmer and wetter than today.

  2. Molecules Knocked Cold

    Chemists have long used supersonic gas expansions to create beams of molecules that are “cold” in the sense that the molecules are in low vibrational and rotational states and do not undergo collisions. However, translational motion of the molecules still makes them “hot” in the laboratory frame of reference, and this temperature broadening creates noise that obscures many experimental signals. Elioff et al. (p. 1940) show that single “billiard-like” collisions between the molecules of two molecular beams can produce a population of molecules that essentially come to rest in the lab frame and achieve true temperatures below 1 K. Inelastic collisions between beams of NO molecules and argon atoms produced between 108 and 109 NO molecules in a specific quantum state with speeds no greater than 15 meters per second, which corresponds to a maximum temperature of 0.4 Kelvin.

  3. Hydrogen on Ice

    Hydrogen molecules are formed from H atoms on interstellar dust grains in a process that releases 4.5 electron volts of energy. Whether that energy heats the cloud or is dissipated depends on how much of this energy becomes kinetic energy of the H2 molecule versus released as infrared radiation. Such dust grains can be covered with ice mantles that are expected to be amorphous. Hornekær et al. (p. 1943) show that H2 formation is efficient even at temperatures as low as 8 K and is more efficient for porous films. Energy release also depends on film morphology. For porous films, most of the energy is released into the film, but for dense films, the energy is released into excitation of the internal modes of the H2 molecules.

  4. Saltier Water Sink Deep

    When sea water freezes, the ice that forms contains very little salt and the saltier, denser brine that forms sinks below the ocean surface and helps drive global thermohaline circulation. This mechanism has long been known to operate, but direct observations of the brine rejection process itself have been elusive. Shcherbina et al. (p. 1952) report measurements of brine rejection and the resulting formation of Intermediate Water in the Okhotsk Sea of eastern Russia. They document an uninterrupted interval of brine rejection in a coastal polynya (a largely ice-free area in a field of otherwise ice-covered ocean) that lasted for more than a month during the winter months of 1999 to 2000. These results indicate the importance of this process in winter on shelf regions, and could help explain the key aspects of ocean circulation during the Last Glacial maximum, as well as today.

  5. Comet Tail Gets Sunburned

    Studies of the comas and tails of dust and plasma that comets produce as they approach the Sun can provide insights into their composition and origins. Povich et al. (p. 1949) observed the close solar passage of comet Kudo-Fujikawa using the ultraviolet spectrometer onboard the SOHO spacecraft. They found singly and doubly ionized carbon in the tail, which may be produced by the sublimation and photoionization of atomic carbon from organic-rich mantles on dust grains rather than photodissociation of CO. This production mechanism would support a model of comet formation in which cold interstellar dust grains at the edge of the solar system develop organic refractory coatings during their long exposure to ultraviolet radiation and then aggregate into a cometary nucleus.

  6. Mammal Memoirs

    The origin and diversification of the three main groups of mammals (marsupials, placentals, and monotremes) prior to the Cretaceous-Tertiary boundary have been difficult to discern with certainty because many early mammals are represented only by a few teeth or jaw fragments. Luo et al. (p. 1934; see the Perspective by Cifelli and Davis) now present a nearly complete skeleton of an early metatherian, Sinodelphys, an early ancestor of the marsupials from the Yixian Formation, China, that dates to about 125 million years ago. In contrast, marsupials are mainly extant in the Southern Hemisphere today (the exception being recent migrants to the Northern Hemisphere). The foot of the fossil implies that the animal climbed or lived in trees. This finding, taken together with other fossil evidence from this area, suggests that Asia was a center for diversification of mammals.

  7. Chimp-Human Gene Comparisons

    Human and chimpanzee genomes are thought to be more than 98% identical. What is different between the two, and how have these differences influenced the evolution of each species? Clark et al. (p. 1960; see the news story by Pennisi) compared the sequence of more than 7000 genes from the common chimpanzee with their homologs in humans and mouse to identify genes that are under positive or adaptive evolutionary selection. Organizing these genes into pathways and clusters of genes related by function suggests that changes to particular physiological processes, such as hearing, olfaction, and protein catabolism, have occurred along the human lineage since the split from the common ancestor with chimpanzee.

  8. Tuberculosis and the Proteasome

    The proteasome—a protein complex involved in degrading specific target proteins—is essential in eukaryotes, but a functional role in Archaea and the Actinomycetes family of eubacteria has not been clear. In a global search for pathogen genes involved in resistance to NO-mediated host-cell killing, Darwin et al. (p. 1963; see the Perspective by Pieters and Ploegh) now show that in the Actinomycete, Mycobacterium tuberculosis (Mtb), the proteasome plays an essential role. Both genetic inactivation of proteasome-related accessory proteins and inhibition of the proteolytic core with chemicals markedly sensitized Mtb to NO-dependent killing. Furthermore, Mtb mutated in a proteasome-related gene was markedly less pathogenic in mice.

  9. Bioremediation Bug Genome

    Geobacter sulfurreducens brought to light the important global role of iron reduction in both the carbon and mineral cycles, and later revealed its value for the bioremedial immobilization of uranium. The species may also be useful for biological energy generation. Now Methé et al. (p. 1967) present the genome sequence, which provides key information for harnessing this potential. This is the first genome reported for a delta Proteobacterium, one of the most important and versatile group of anaerobes on the planet.

  10. Elucidating Light Harvesting

    In purple photosynthetic bacteria, light is absorbed by the light harvesting or antennae system and transferred to the reaction center (RC), which produces the proton gradient that drives adenosine triphosphate synthase. Now Roszak et al. (p. 1969) have determined the crystal structure of an intact complex between the purple bacterial RC and its primary peripheral antenna-complex, LH1, at 4.8 angstrom resolution. The RC is surrounded by an oval LH1 complex with an inner and outer ring of helices. One of the 16 helices in the inner ring is out of register with the others, and this orientation prevents closure of the outer ring. This break may be a portal through which the electron acceptor ubiquinone can transfer electrons from the RC to cytochrome b/c1.

  11. Axonal Pathfinding

    In the developing vertebrate nervous system, spinal cord commissural neurons extend axons across the midline region of the embryo and turn anteriorly toward the brain. Lyuksyutova et al. (p. 1984; see the Perspective by Imondi and Thomas) report that the choice to turn toward the brain is controlled by secreted proteins of the Wnt family. Wnt proteins behave as chemoattractants and Wnt4 displays a functional gradient in the floor plate along the anterior-posterior axis. Axons lacking Frizzled3, a Wnt receptor, have defects in pathfinding.

  12. Fateful Dose of Mdm2

    The p53 tumor suppressor pathway is frequently inactivated in human cancer. In response to stress, the cellular levels of p53 protein rise, which can lead either to cell growth arrest or apoptosis. In normal cells, p53 levels are kept low through rapid protein turnover. This process is tightly regulated by the E3 ubiquitin ligase Mdm2, but experiments aimed at dissecting the mechanistic details have yielded conflicting results. Li et al. (p. 1972) now show that the fate of p53 is determined by the level of Mdm2 activity in the cell. Low levels of Mdm2 promote mono-ubiquitination and nuclear export of p53, whereas high levels of Mdm2 promote its poly-ubiquitination and degradation in the nucleus.

  13. Losing Muscleblind Makes for Myotonic Dystrophy

    The neuromuscular disease myotonic dystrophy (DM) is caused by microsatellite repeat expansions in the untranslated regions of two genes. The puzzle has been how repeats that are not translated into protein result in DM pathogenesis. Kanadia et al. (p. 1978) report that mice deficient in muscleblind proteins show many of the features characteristic of DM, such as, myotonia and cataracts. Loss of muscleblind proteins results in abnormal splicing of a number of different mRNAs, including those encoding muscle proteins, such as troponin and the chloride channel. Thus, in DM, muscleblind proteins may be sequestered by the repeat expansions in transcripts of the two DM genes and would thus be unable to carry out their normal RNA splicing function.

  14. Quake Trigger

    In some of the most tectonically active regions, the fault zone is complex and strike-slip faults can intersect thrust faults. Complex fault zones may allow an event on one fault to trigger a smaller or larger event on an adjacent or nearby fault depending on the state of stress of the system. Anderson et al. (p. 1946) have modeled the potential for an earthquake triggering another event on the complex fault zone in the Los Angeles metropolitan area of southern California. Their simulations suggest that an event on the San Jacinto strike-slip fault could trigger a potentially more hazardous event on the Sierra Madre-Cucamonga thrust fault system.

  15. A Root Map

    Gene expression profiling in time and space within a population of cells can be used to generate a molecular movie of development. Birnbaum et al. (p. 1956) describe an analytical approach to provide such a map in the context of the Arabidopsis root tip. Global expression patterns within the organ were mined to infer novel aspects of development, including the discovery of organizing signals that could play important roles in cell fate and differentiation in Arabidopsis.

  16. NO and Hypoxia

    The transcription factor hypoxia-inducible factor (HIF) 1α plays a pivotal role in the cellular response to low oxygen levels; its activation under hypoxic conditions leads to increased transcription of genes implicated in O2 delivery and adaptive responses to O2 deprivation. Hagen et al. (p. 1975) now find that HIF1α is destabilized by nitric oxide (NO) and other inhibitors of mitochondrial respiration by an increase in HIF1α degradation triggered by increased activity of the O2 -dependent enzyme prolyl hydroxylase. This NO-mediated inhibition of mitochondrial O2 consumption may increase O2 availability for nonrespiratory functions (such as prolyl hydroxylases) and cause the cell to fail to register hypoxia. Thus, redistribution of intracellular O2 may be another important consequence of NO action.

  17. An Efficient and Reliable Connection

    Although the functional and anatomical properties of synaptic connections hold essential clues about central nervous system processing, basic synaptic properties, such as the number of functional and anatomically identified release sites per connection and the release probability per site remain unknown at most cortical synapses. Silver et al. (p. 1981) combined in vitro electrophysiology with light and electron microscopy to investigate the quantal parameters of synaptic transmission between layer 4 spiny stellate cells and layer 2/3 pyramidal cells of rat somatosensory cortex. They observed a one-to-one relationship between the number of anatomically identified synaptic contacts and the number of functional release sites. The concentration of glutamate in the synaptic cleft following release was independent of release probability, the probability of release at each identified synaptic contact was extremely high, and the release sites operated independently. The properties are exactly what are required for reliable transmission of spatially distributed, timing-based signals.

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