This Week in Science

Science  25 Feb 2005:
Vol. 307, Issue 5713, pp. 1165
  1. Dark Gas in the Milky Way


    Radio-wavelength observations of atomic hydrogen and carbon monoxide (CO) have been used to estimate the abundance and distribution of gas and dust in the Milky Way Galaxy. Some cold molecular gas (dark gas) is hard to trace with radio observations, but cosmic-ray interactions with the gas can make the gas glow sufficiently to be traced. Grenier et al. (p. 1292) combined the radio and gamma-ray observations to show that the cold dark gas forms halos around CO clouds and connects these clouds to diffuse structures dominated by atomic hydrogen.

  2. New Views of Old Mars

    Mars Pathfinder heralded a new generation of exploration of the red planet. Several orbiters and two rovers have successfully followed this lander, and along with telescope observations, returned information on Mars rocks and surface features, topography and gravity— which help infer its internal structure—magnetic field, and atmospheric dynamics and chemistry. Solomon et al. (p. 1214) provide an updated review incorporating these latest results and those from an increasing number of martian meteorites into a history of early Mars, including its formation and the differentiation of its core, mantle, and crust.

  3. A Time for Planets

    Most of the 130 known extrasolar planets were detected by measuring perturbations to the stellar radial velocity caused by the orbiting planet. Some planets were detected by observing decreases of the stellar light when the planet passed in front of its star. Holman and Murray (p. 1288) show that the timing of the transiting planet will vary if there is another planet in the system, so that the presence and mass of a second planet can be estimated from the gravitational interactions between the planets. This method may be able to detect even an Earth-mass planet.

  4. Coupled Qubits Measured Simultaneously

    For a scalable quantum computer to be realized, what will be needed is the ability to read out the entire system of qubits simultaneously, and with high fidelity. However, measurement crosstalk, an undesirable effect where the state measurement of one qubit influences that of others, has presented an experimental barrier to achieving that goal. McDermott et al. (p. 1299, see the Perspective by Mooij) present results on the simultaneous measurement of states of two coupled superconductor phase qubits. For the right timing sequence of the measurement, the influence of crosstalk can be minimized, and the two entangled qubits can be measured with high fidelity. The authors argue that the scheme should be applicable to multi-qubit systems.

  5. Great Tomes of the Past Preserved

    Texts often survived from Antiquity through the Middle Ages by the skin of their teeth, subject to hazards ranging from fire and war to decay and neglect. What were the odds that a manuscript would survive, that an entire work would go extinct, or that the transmission of knowledge itself could be seriously jeopardized? By treating manuscripts as though they were fossils from an extinct population, Cisne (p. 1305; see the Perspective by Gilman and Glaze) shows that explicit, testable estimates of manuscripts' and texts' survival indeed can be found under certain circumstances, and that certain works have had much greater chances of survival than has been guessed from anecdotal evidence. This work suggests a new way of using centuries' worth of exacting scholarship to investigate the survival and dissemination of information.

  6. Restoring the Marshlands of Iraq

    The marshes of southern Iraq were once the largest wetland in the Middle East and home to an indigenous population of tens of thousands of marsh dwellers. They were also a major flyway for migrating birds. Today, less than 10% of the marshes in Iraq remain as fully functioning wetlands because of extensive drainage and upstream agricultural irrigation programs on the Tigris and Euphrates rivers implemented during Saddam Hussein's regime. Richardson et al. (p. 1307) provide an assessment of the ecological status of the Iraqi marshes since the 2003 war. Nearly 20% of the original 15,000- square-kilometer marsh area was reflooded by March 2004. Reflooding has partly restored some of the former marsh areas. However, high salinity and toxicity may persist in reflooded marshes unless flow-through of fresh water is maintained by careful hydraulic design. It seems that the marshes can be restored as long as sound ecological restoration principles are followed.

  7. Great Medieval Earthquake

    Trenching along the Main Frontal Thrust fault of the Himalayan mountains has revealed evidence for one great earthquake with a moment magnitude of about 8.8 and an offset of about 17 meters over a length of about 240 kilometers. Lavé et al. (p. 1302) dated the offset in the trench at about 1100 A.D., and they suggest that no additional great earthquakes have occurred since then. Such a great event could account for 25 to 50% of the shortening across the mountains and would recur in 1800 to 3000 years.

  8. βAPPtists and Tauists Unite


    In Alzheimer's disease (AD) pathological lesions of various types are seen in the brains of patients and in mouse models of the disease as the disease progresses. Stokin et al. (p. 1282) provide evidence that axonal transport deficits are likely to be an early characteristic of AD, contributing to progression of disease phenotypes, and perhaps being an early initiating event. The data also provide a useful unifying theme bringing amyloid precursor protein processing and tauopathy, two processes thought to be mechanistically distinct in neurological disease progression, together into a single disease pathway for AD.

  9. Variety and Fitness in Natural Bacterial Populations

    Microbiologists have long used clonal isolates as model systems to study processes promoted by bacterial populations. However, little is known about how functionally representative such clones may be of populations in their native habitats. Thompson et al. (p. 1311) show that vast genotypic diversity exists within a natural population of coexisting bacteria bearing nearly identical 16S ribosomal RNA genes (the biomarker most frequently used to identify bacterial strains). Individual clones are present in the environment at such low concentrations that, for all practical purposes, no two are alike. These findings suggest that any individual clone is relatively unimportant in overall population function and that much of the variation in the genomes does not lead to fitness differences among the members of the population.

  10. The Bad and the Ugly?


    The fungus Cryptococcus neoformans is an opportunistic human pathogen that has become more prevalent, in part because of the increased incidence of immunocompromised patients. Loftus et al. (p. 1321, published online 13 January 2005) have sequenced the genome of two inbred strains, JEC21 and B-3501A, which differ in virulence, and have compared this sequence information with other fungal genomes. The C. neoformans genome shows evidence of alternative splicing and antisense transcripts, suggesting widespread genetic regulatory mechanisms, and is transposon-rich. The functional distribution of many of C. neoformans genes mirrors that of Saccharomyces cerevisiae.

  11. Sword and Shield in Bacterial Pathogenesis

    The serotype diversity that characterizes Shigella (and other bacterial pathogens of mammals) could have evolved under the selective pressure of the innate immune response of the host or as an escape mechanism to the adaptive response. West et al. (p. 1313; see the Perspective by Normark et al.) now show that Shigella has acquired mechanisms to shorten the O-antigen of lipopolysaccharide (LPS) it expresses at the cell surface without decreasing its mass for the purpose of better exposing its major weapon, the type III secretory system. It does this without affecting the capacity of LPS to resist the innate immune responses of the host. Bacteriophage-mediated glucosylation of the LPS O-antigen leads to a shift from a linear to helical conformation, shortening the LPS without altering its quantity. This process provides a strong selective advantage to Shigella for maintaining lysogenic bacteriophages in its genome.

  12. Mistic and Membranes

    Structure determination of membrane proteins remains a challenge because of difficulties in expressing sufficient quantities of protein and in obtaining ordered crystals for analysis by x-ray crystallography. Roosild et al. (p. 1317) have taken steps forward in two directions. First, they developed techniques that allowed them to determine the structure of a four-helix bundle integral membrane protein from Bacillus subtilis by nuclear magnetic resonance spectroscopy. Second, they show that this protein, which they name Mistic, can be used to assist in recombinant expression of other membrane proteins because it can insert autonomously into the cell membrane.

  13. Neuroligin and Inhibitory Synapse Formation

    Achieving an appropriate balance between excitatory and inhibitory synapses as the central nervous system is wired up during development is critical for building smoothly flowing information pathways. Neuroligins are postsynaptic adhesion molecules that also play a role in synapse formation. Chih et al. (p. 1324, published online 27 January 2005; see the Perspective by Hussain and Sheng) have analyzed knockdown mutants of several neuroligin isoforms and found that various neuroligins have overlapping but not identical functions. Disruption of neuroligin function leads to a loss of excitatory synapses and results in a functional imbalance of excitatory and inhibitory transmission in the rodent hippocampus.

  14. Heat Capacity of Fermi Gases

    The ability to tune the interaction strength between atoms in strongly interacting Fermi gases provides an experimental system to study many-body physics. One example is the crossover from a Bose-Einstein condensate (BEC) regime, in which the atoms are strongly coupled into pairs, to the weak regime that mimics Bardeen-Cooper-Scheiffer (BCS) coupling of electrons in superconducting metals. As in superconductivity and superfluidity, heat capacity measurements have provided important information on phase transition and characterization of the excitations. Kinast et al. (p. 1296, published online 27 January 2005) have now made similar measurements on an optically trapped cloud of Fermi atoms and found a clear transition at the predicted BCS-BEC crossover regime. Such measurements should prove an important probe of the fundamental thermodynamics of these gases.

  15. Reconstructing an Enzyme's Past

    Rigorous tests of hypotheses about adaptation must identify those mutations responsible for changes in function and show that they are selectively advantageous under particular conditions. Zhu et al. (p. 1279, published online 13 January 2005) reconstruct the selective basis of an adaptive event that occurred at the dawn of the eukaryotes billions of years ago, focusing on two variants of isocitrate dehydrogenase (IDH) that have different cofactor preferences, either nicotinamide adenine dinucleotide, NAD, or its phosphate, NADP. Genetic engineering techniques were used to create an ancestral version of an enzyme. Subsequent selection experiments showed why the cofactor specificity of the enzyme changed. NADP is linked to use of acetate as a substrate, and the appearance of the derived NADP-IDH from the ancestral NAD-IDH reflects the type of substrate used to generate adenosine triphosphate and reducing power. By eliminating other sources of reduced NADPH, selection acts against the NAD-dependent phenotype. Thus, growth on acetate favored the evolution of an NADPH isocitrate dehydrogenase.

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