This Week in Science

Science  04 Dec 1998:
Vol. 282, Issue 5395, pp. 1781
  1. Recording Tropical and Temperate Climates

    Climate records from tropical glaciers in the Andes allow researchers to assess whether the events recorded in polar glaciers were truly global in their extent and timing. Thompson et al. (p. 1858) present a high-resolution carbon-14 paleoclimate record from a Bolivian glacier that extends back to the Last Glacial Stage (LGS, about 20,000 years ago). The record adds to the growing body of evidence that the LGS was global in extent and shows an abrupt cold reversal during deglaciation at about the same time as the Younger Dryas cooling in Greenland records. In contrast to polar locations, cooler global temperatures were associated with greater precipitation and reduced evaporation. Most of our detailed records of climate during the last several glaciations have come from marine sediments or coral; few records are available from continental interiors, and radiocarbon dating extends to only about 45,000 years ago. Dorale et al. (p. 1871) now provide a record of climate changes in central North America from 75,000 to 25,000 years ago from stalagmites dated by thorium-230 in Crevice Cave, Missouri, which is just south of the farthest extent of ice during the last glaciation. The climate records are replicated in four stalagmites, indicating that their formation was not biased by local processes. Stable carbon and oxygen isotope data show that a marked episode of cooling began about 55,000 years ago, and forest replaced prairie at the site.

  2. Tuning the Ear

    Sound waves reaching the ears must be transformed into nerve impulses, but comparisons of the response properties of auditory nerve fibers and the basilar membrane of the inner ear have been indirect, as data from different subjects had to be used. Narayan et al. (p. 1882) present recordings from the basilar membrane and auditory nerve fibers in the same cochlea. In chinchillas, cochlear frequency selectivity can be explained by the vibrations of the basilar membrane. This observation is a strong argument against the hypothesis of a second filter in the mammalian cochlea.

  3. Altered Asteroids

    Chondrites, primitive meteorites composed of rounded mineral aggregates called chondrules and refractory grains, are thought to be the first components to accrete in the solar nebula. However, the mineral phases in a chondrite may have been altered by high temperatures, high pressures, and infiltration of fluids even on the relatively small parent bodies that later formed. Ion microprobe analysis of fayalite grains in the Mokoia carbonaceous chondrite by Hutcheon et al. (p. 1865) revealed a radiogenic chromium-53 excess derived from the decay of manganese-53. They infer that the fayalite crystallized after the solar nebula had dissipated during later alteration processes.

  4. Watching Single Enzyme Molecules in Action

    Studies of single molecules allow processes that would normally be hidden in studies of large ensembles of molecules to be revealed. Lu et al. (p. 1877) present evidence for a “memory effect” in the activity of the enzyme cholesterol oxidase. The rate of reaction could be determined with confocal microscopy because the enzyme's cofactor, flavin adenine dinucleotide, is fluorescent only in its oxidized form. The distribution of times for fluorescence to “switch on” varied considerably between single enzyme molecules, and even for a single enzyme molecule, the distribution of times deviates from exponential behavior in ways that can be attributed to slow fluctuations in the conformation of the molecule.

  5. Magnetite in Bacteria

    Magnetotactic bacteria contain chains of small crystals of magnetite (or other minerals) several nanometers across. Dunin-Borkowski et al. (p. 1868) have now used electron holography to image the magnetic microstructure of individual crystals and crystal chains in situ in bacteria. In this approach, they compare an electron wave passing through the sample with a reference wave to create a holographic image. The magnetization directions of the smallest crystals were controlled by the larger crystals in the chains. This approach, which shows how these crystals may interact within the bacteria, might be extended to magnetite crystals in higher organisms.

  6. Clarifying Consciousness

    In an exploration of the basis of consciousness, Tononi and Edelman (p. 1846) identify key properties of the conscious experience and then survey known neural phenomenon in search of a match. They find that consciousness is integrated (cannot be subdivided) and differentiated (a conscious experience is one of a huge number possible). An ever-changing subset of neural systems in the occipital, temporal, and frontal parts of the brain are also integrated and differentiated, which leads the authors to propose the “dynamic core” hypothesis for the mechanism of human consciousness.

  7. Ironing Out an Enzyme

    The chemical dexterity with which microorganisms carry out simple reactions is exemplified by hydrogenases, which catalyze the conversion of two protons and two electrons into molecular hydrogen. Peters et al. (p. 1853; see the Perspective by Adams and Steifel) present the crystal structure at 1.8 angstroms of a hydrogenase from the anaerobic bacterium Clostridium. This enzyme, a medium-sized protein of less than 600 amino acids, contains a wealth of iron atoms (20 per molecule) that are arranged in five distinct clusters: Six in the active site H cluster (one [4Fe-4S] and one binuclear Fe-S) and fourteen in the four accessory iron-sulfur clusters (three [4Fe-4S] and one [2Fe-2S]). Likely pathways for how the electrons and protons travel inward to meet at the H cluster are visible.

  8. Tracking Birds Isotopically

    Studies of the population dynamics of migratory birds are hampered by the great distances between breeding and wintering habitats and the different arrival times of males and females. Marra et al. (p. 1884; see the cover and the news story by Wuethrich) use habitat-specific isotope signatures to link American redstarts (Setophaga ruticilla) in their tropical wintering grounds and temperate breeding areas. Arrival times at the breeding grounds and reproductive success are heavily influenced by competition for resources during the preceding winter: The more successful birds had carbon-13 isotope values which indicated that their insect prey were associated with richer forested habitats.

  9. Mediating Stress

    Members of the mitogen-activated protein kinase (MAPK) family are activated by cascades of protein kinases that begin with the MAPK kinase kinases (MAPKKKs). The MAPKKK called MEKK1 can lead to activation of two different sorts of MAPKs known as JNK and ERK, but it has been unclear what its physiological targets are in vivo. Yujiri et al. (p. 1911) produced mice that do not express the MEKK1 protein and showed that coupling of MEKK1 to specific MAPKs was highly dependent on the type of external stress. For example, activation of JNK in response to cold stress or serum factors was lost, but not the responses to ultraviolet radiation or heat shock. In some cases, MEKK1 contributed to activation of ERK, but other stimuli that activated MEKK1 resulted only in activation of JNK.

  10. Meeting the Current Challenge

    The M-current is a potassium conductance crucial for the control of excitability and thus the response properties to synaptic inputs of many neurons throughout the nervous system. Wang et al. (p. 1890; see the news story by Barinaga) now show that the biophysical characteristics, the antagonist pharmacology, and the pattern of messenger RNA expression of two members of the KCNQ potassium channel gene family (KCNQ2 and KCNQ3) are identical to the M-channel behavior and distribution in vivo. They conclude that the ion channels underlying the M-current are made of heteromers of these two subunits.

  11. Continuing Repression

    In Drosophila embryonic development, the homeotic genes play pivotal roles such as specifying whether an appendage will be an antenna or a leg. Segmentation gene products perform the initial spatial restriction of homeotic (or HOX) gene expression, and this repression is later continued by Polycomb group (PcG) gene products. Kehle et al. (p. 1897) have now identified a link between these regulatory components. The protein dMi-2 interacts with the segmentation gene product Hunchback (Hb). In addition, genetic analyses demonstrate that dMi-2 functions in both the Hb- and PcG-mediated repression of HOX genes. Because the vertebrate homolog of dMi-2 is a component of a histone deacestylase complex, repression of HOX genes may involve histone deacetylation.

  12. Tau Isoforms and Neurodegeneration

    Mutations in the gene encoding the microtubule-associated protein tau are associated with several neurodegenerative diseases. In frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), filamentous aggregates containing tau proteins are found in the brains of patients and are similar to aggregates seen in patients with Alzheimer's disease. Hong et al. (p. 1914) show that different mutations seen in patients with FTDP-17 have different effects on splicing of tau exons (and thus on the isoform composition of tau in the brain) and on the ability of tau to bind to or promote assembly of microtubules.

  13. Oxygen-16 Enriched Droplets

    The refractory calcium-aluminum-rich inclusions (CAIs) found in primitive meteorites are thought to be the first particles that crystallized and accreted in the solar nebula. Yurimoto et al. (p. 1874) measured the oxygen isotopic composition of minerals in a CAI from the Allende carbonaceous chondrite with a secondary ion mass spectrometer. Several melilite grains were enriched in oxygen-16 relative to previously measured melilite grains in carbonaceous chondrites and other minerals in this CAI. They suggest that the variation in oxygen-16 abundance between mineral phases within one CAI is consistent with CAIs starting as oxygen-16 enriched molten droplets in the solar nebula and crystallizing in the presence of an oxygen-16-poor nebular gas.

  14. Given the Signal to Go

    As connections get wired together during development of the brain, neurons in the cortical layer extend axons in search of distant targets. The first stage of navigation is that the axons grow away from the cortical surface. Polleux et al. (p. 1904) investigated the signals that direct these early steps. The key signaling molecule is a member of the semaphorin family that triggers responses in developing neurons through the neuropilin-1 receptor.

  15. Genetic Influences to AIDS Progression

    Genetic variation in alleles for CCR5 and CCR2 chemokine receptor genes has been associated with protection against HIV infection and delayed progression to AIDS. Martin et al. (p. 1907) have demonstrated, in an analysis of five cohorts of patients, that variation in the promoter region upstream of the CCR5 gene is associated with more rapid progression to AIDS, especially in the first 4 to 6 years after infection.

  16. Controlling and Halting Cell Division

    Proper regulation of the cell division cycle requires that mitosis is not begun until DNA synthesis is completed in S phase. Michael and Newport 1886 provide evidence for a biochemical mechanism by which such entry into mitosis is controlled. After DNA replication is complete, the protein kinase Wee1 is degraded in a manner dependent on the ubiquitin-conjugating enzyme Cdc34. Entry into mitosis is controlled by the protein kinase Cdc2, and Wee1 phosphorylates and inactivates Cdc2. If DNA replication was blocked, degradation of Wee1 was inhibited. Thus, controlled proteolysis of Wee1 apparently provides a means for coupling mitosis to the completion of the preceding S phase. DNA damage causes cells to halt the cell division cycle through a checkpoint mechanism that depends on the ATM (ataxia telangiectasia mutated) protein. Matsuoka et al. (p. 1893) report that ATM-dependent arrest in the G2 phase of the cell cycle occurs through phosphorylation and activation of the Chk2 protein kinase, a homolog of the Rad53 protein, which has a similar function in the yeast Saccharomyces cerevisiae. Chk2 then phosphorylates the protein phosphatase Cdc25C. Such phosphorylation apparently leads to inhibition of Cdc25C activity, and thus prevents dephosphorylation and activation of the cyclin-dependent kinase Cdc2.

  17. Unhindered by Packaging

    Transcription of class II genes has been reconstituted in vitro by the addition of RNA polymerase II and the general transcription factors. These factors are sufficient for transcription of a naked DNA template but are not enough for DNA that is packaged with histone. LeRoy et al. (p. 1900) have now purified a factor, RSF (remodeling and spacing factor), that functions in transcribing DNA templates that are assembled into chromatin. The combination of general transcription factors, FACT (facilitates chromatin transcription), and RSF define the minimal requirements for activated transcription through in vitro-assembled chromatin templates. [See the Perspective by John and Workman.]

  18. Interpreting Late Precambrian Microfossils

    C.-W. Li et al. studied 580-million-year-old microfossils in Early Vendian Doushantuo phosphate deposits in central Guizhou (South China) and identified the remains of sponges and metazoan embryos (Reports, 6 Feb., p. 879). Y. Zhang et al. comment that “the putative parenchymella larvae of sponges … are actually acanthomorphic acritarchs … comparable to remains of cysts of planktic eukaryotic algae.” They provide figures that show how “taphonomic changes” and other degradation of the fossils could lead to misinterpretation. In response, Li et al. state that they “are now able to reconstruct the sequential embryogenesis of the Wengan sponge,” and they provide figures showing that “collapsed acritarchs” illustrated in the comment are more likely “filamentous bacteria” that grew on organic remains. They call for “further research and evaluation” of Wengan specimens. The full text of these comments can be seen at

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