This Week in Science

Science  22 Jun 2001:
Vol. 292, Issue 5525, pp. 2213
  1. Stomatal Matters

    Global temperatures and the concentration of atmospheric CO2 are remarkably well correlated over the past 400,000 years, and there is reason to expect that they also were correlated further back in time. However, proxy measurements of CO2 for some warm intervals during the past 60 million years are grossly inconsistent, ranging from 300 parts per million (ppm) to more than 3000 ppm. Royer et al. (p. 2310) address this disagreement by estimating the concentration of atmospheric CO2 with measurements of the stomatal density (a function of ambient CO2 levels) of fossil leaves from warm intervals during the Miocene and the Paleocene/Eocene boundary. They find that CO2 concentrations during these periods remained relatively low, mostly between 300 and 450 ppm, and that the amount of warming expected from the corresponding CO2 forcing is much less than that which has been estimated from a variety of paleotemperature proxies. These results imply that factors other than CO2 are needed to explain these intervals of global warmth.

  2. Lining Up Liquid Crystals

    For most liquid crystal (LC) devices, the substrate for the device needs to be treated to induce a particular alignment of the LC at the surface. The standard procedure is to rub the surface, producing an alignment parallel to the rubbing direction. Stöhr et al. (p. 2299) use near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to correlate the orientation bond order at the surface with various treatment methods. They find that any method that induces order at a carbonaceous surface can be used to generate a substrate, and that ion-beam irradiation of diamond-like carbon films can be used to replace rubbing and thus form the basis for the next generation of LC devices.

  3. Acoustic Oscillations in Accord

    A standard model of the evolution of the universe assumes that an initial inflationary phase was followed by a Hot Big Bang. This process created a universe full of ionized plasma during its first 100,000 years, and the competing interactions between photon pressure and gravitational pressure in higher density clumps created acoustic oscillations in the plasma. These acoustic oscillations left features in the cosmic microwave background (CMB), which represents a radio-wavelength remnant of the early universe, that were recently measured. Miller et al. (p. 2302) have analyzed the acoustic oscillations imprinted on the matter-density distribution of clusters of galaxies and individual galaxies formed in the recent universe and found that the oscillations are consistent with the CMB results. Thus, oscillations recorded in young galaxies match with oscillations recorded in the very old plasma and provide further support for the Inflationary Hot Big Bang model of formation of the universe.

  4. African Rains

    The tropics are thought to be an important source of global climate variability, as well as an area whose response to climate change must be understood better in order to anticipate possible impacts of global warming (see the Perspective by Gasse). However, high resolution records of continental temperature and precipitation of this area are relatively scarce. Barker et al. (p. 2307) report results from oxygen isotopic analyses of silica from diatoms that lived in alpine lakes on Mt. Kenya and conclude that the changes in the amounts of rainfall and evaporation, rather than variations in temperature, were responsible for the important isotopic shifts that occurred there during the Holocene.

  5. Precise and Reliable Brain Trains

    The precision of action potential timing has become a central issue for the understanding of interneuron function in the cortex. Using paired recordings in brain slices, Galarreta and Hestrin (p. 2295; see the news story by Helmuth) studied the reliability of precise spike transmission between pyramidal cells and fast-spiking interneurons, a subset of cortical inhibitory cells. Neuronal firing at this connection occurred during a limited time window that lasted for about 1 millisecond and corresponded closely to the rising phase of the excitatory postynaptic event. This precise transmission may constitute an essential mechanism that enables the synchronous firing of fast-spiking GABAergic networks.

  6. A Bit Too Quiet in Old Star Clusters

    Globular clusters are spheroidal structures found in the halos of galaxies that formed early in the universe (about 12 to 14 billion years ago) before galactic disks formed. Grindlay et al. (p. 2290; see the 18 May news story by Watson) studied the stellar population in 47Tuc, a globular cluster in the Milky Way Galaxy using the Chandra X-ray Observatory. They found that about 50 to 55% of the x-ray sources are millisecond pulsars (MSPs), 30% are accreting white dwarfs, 15% are binaries in flare outburst, and a few are quiescent, low-mass x-ray binaries. MSPs may be spun up by accretion onto the neutron star in quiescent x-ray binaries, but the number of quiescent binaries in 47Tuc is too small to explain the number of MSPs, so alternative modes of formation may be needed to explain the population of MSPs in 47Tuc. These and other findings for 47Tuc suggest the need for revisions to models of early stellar evolution.

  7. Redox Control of Growth

    Prokaryotes sense change in environmental oxygen through a two-component signal transduction system and adjust their metabolism accordingly. Surprisingly, oxygen is not the molecular signal that regulates this process, which involves a membrane-associated sensor kinase and a response regulator that modulates gene expression. Georgellis et al. (p. 2314) have determined that the redox state of membrane-bound quinones regulates the ArcB sensor kinase of the bacterium Escherichia coli; quinones are carriers in electron transport chains that are linked to metabolic reactions.

  8. Whither CO2?

    Calculation of the amount of carbon dioxide fixed by photosynthesis is crucial to predicting the course of future climate change (see the Perspective by Wofsy). The size of the U.S. terrestrial carbon sink has been a subject of much debate. Pacala et al. (p. 2316) provide a comprehensive analysis comparing estimates derived from forest inventories with those from atmospheric models of carbon dioxide concentration. Although the estimates of the sink vary by a factor of nearly two (from 0.3 to 0.6 petagrams of carbon per year), the range of estimates from both methods is similar. Fang et al. (p. 2320) report changes in the storage of living forest biomass carbon over the past half century in China, using data from the continuous national forest inventory. Their results suggest that reforestation in China may be contributing to global terrestrial carbon sinks.

  9. Collateral Damage

    Potentially dangerous drug-drug interactions can occur when xenobiotics, such as the St. John's wort ingredient hyperforin, activate the nuclear pregnane X receptor (PXR) because this receptor is a transcription factor that stimulates expression of cytochrome P450, an enzyme that metabolizes many small molecules, such as the immunosuppressive drug cyclosporin. Watkins et al. (p. 2329) have determined the crystal structures of the ligand-binding domain of the human PXR alone and in complex with the cholesterol-lowering drug SR12813. Although the PXR is known to bind a variety of compounds, several polar residues within a mostly hydrophobic cavity were shown to specify a directed promiscuity and to mediate binding of SR12813 in three distinct orientations; targeted mutagenesis of these residues served to graft sensitivity to SR12813 onto the formerly inert mouse PXR.

  10. Make or Break

    Large bacterial viruses (phage) break out of their host cells by making a hole in the host's cell membrane, which allows the release of a virus-produced enzyme that degrades the cell wall (see the Perspective by Hatfull). The small, single-stranded counterparts of these phages have a different strategy for attacking the cell wall and produce proteins that inhibit various steps in bacterial cell-wall synthesis. Bernhardt et al. (p. 2326) show that the protein A2, from the group III ssRNA phage Qb, blocks the first enzyme in cell wall synthesis, and, as a result, 20 minutes later the cell walls of the growing bacteria begin to disintegrate.

  11. Executive Recruiters

    Transcription activators can recruit the histone acetyltransferases (HAT) complexes SAGA and NuA4 to stimulate transcription from chromatin-assembled DNA templates, but the mechanism for targeting these complexes to promoters is unclear. Brown et al. (p. 2333) now demonstrate that acidic activators interact with Tra1p, a subunit that is found in both of these complexes; hence, Tra1p is likely to be an essential component involved in the recruitment of HAT to promoters for the modification of histones.

  12. Ionic Mechanism of Hearing

    The outer hair cells of the mammalian cochlea are specialized cells and function early on in the hearing process. Recently, prestin, the motor protein responsible for the electromotility of outer hair cells, has been identified and cloned. Oliver et al. (p. 2340) found that the unusual voltage sensitivity of this protein is not intrinsic but depends on the presence of intracellular anions, such as bicarbonate and chloride, which behave like extrinsic voltage sensors. In binding to the motor protein, they trigger the structural rearrangements in the molecule that underlie outer hair cell electromotility.

  13. Conformation Versus Charge Transfer

    The changes in conductance seen in molecular switches often are attributed to changes in charging, or redox state, of functional groups such as nitro and amino groups. Donhauser et al. (p. 2303) have looked at the structure and conductivity of several phenylene ethynylene oligomers in two different settings by scanning tunneling microscopy, either packed together rigidly on the surface in islands (both with defects and in well-ordered arrays) or diluted individually into alkane thiol monolayers. Molecules in sites that allow more conformational change (defects or in alkanes) showed more spontaneous switching between high and low conductance states than did molecules in well-ordered, tightly packed environments. They suggest that electric field-induced switching may be driven by movement of charged groups.

  14. Reading History from Sequence

    The hepatitis C virus (HCV) is a leading cause of liver disease worldwide. Pybus et al. (p. 2323) estimate the epidemiological variables that determine the rate of spread of HCV, which permit quantitative predictions about its future epidemic behavior and risk to public health. The results indicate significant differences in behavior among subtypes, due to differences in transmission route rather than the genetics of the virus. The epidemiological model permits the estimation of the basic reproductive number of an infectious disease from gene sequence data, providing an important conceptual bridge between the disciplines of mathematical epidemiology and population genetics.

  15. Plasticity Near and Far

    The linkage between cellular activities at the synapse, such as long-term potentiation (LTP), and the changes in neuronal connectivity that reflect higher levels of cortical plasticity during development has been debated. Di Cristo et al. (p. 2337) show that modulators of extracellular signal-regulated protein kinase (ERK) function affect both cellular LTP and experience-dependent plasticity. The ERK signal transduction cascade couples the plasticity shown in the visual cortex during the critical period for establishment of ocular dominance to the phenomenon of activity-dependent changes in synaptic strength as seen in vitro at individual synapses.