This Week in Science

Science  23 Apr 1999:
Vol. 284, Issue 5414, pp. 553
  1. Megaflood Basalt

    Flood basalts are large outpourings of magma (up to millions of cubic kilometers) that occur in a relatively short period of time (few million years), such as the Deccan flood basalt, which formed near the end-Cretaceous extinction, and the Siberian flood basalt, which formed during the end-Permian extinction. Marzoli et al. (p. 616; see the Perspective by Olsen) now suggest that perhaps the largest flood basalt province formed 200 million years ago during the major extinction at the Triassic-Jurassic boundary. They dated extensive dikes and other basaltic rocks in North Central Brazil to 200 million years ago and correlated these rocks with other basalts with similar ages spanning the coastal regions of North and South America, Africa, and Europe. Altogether, the volume of the basaltic magma may have been 3 million cubic kilometers.

  2. Barely Solvating an Electron

    Water not only solvates ions but can even solvate single electrons, as occurs when aqueous iodide ions eject their electrons into the solvent during ultraviolet irradiation. Lehr et al. (p. 635) have used femtosecond-resolved photoelectron spectroscopy to determine how small a water cluster is needed to support a solvated electron. Clusters of four water molecules around an iodide ion showed a simple decay after photoexcitation, but clusters with five or six water molecules appeared to rearrange their structure and stabilize the free electron as bulk water does.

  3. Emergence of Homo

    The time from 2 to 3 million years ago marks several fundamental developments in human evolution: The first use of tools, expansion of diet to include carnivory, changes in habitat and posture, an increase of brain size, and, finally, the emergence of Homo. Fossils from this critical time have been scarce, however, so it has been difficult to reconstruct much of this history and the phylogeny with confidence. Asfaw et al. (p. 629) now describe a new species of Australopithecus from Ethiopia, based on the remains of a partial cranium, that may be an ancestor of early Homo and descended from A. afarensis. Other remains include a mandible and femur and other postcranial bones (likely from several individuals). de Heinzelin et al. (p. 625; see the cover) show that these fossils date to 2.5 million years ago. They also describe scarred and crushed bones, which suggest that early hominids were acquiring meat and marrow with primitive tools (see the news story by Culotta).

  4. Use Glue Sparingly

    Silica sols can be used to produce microporous materials (aerogels), and a simple route to creating new porous materials would be to use the silica to “glue” other small particles, such as metal colloids or polymer particles, together. However, as often happens in building a model airplane, too much glue is incorporated, and the silica usually coats the other particles completely. Morris et al. (p. 622) show that drying composite gels of silica sols and metal, oxide, carbon, or polymer particles under supercritical conditions creates nanocomposites that retain the properties of both components. For example, incorporating carbon blacks creates electrically conducting (and optically absorbing) aerogel networks.

  5. Hanging On to the Host

    Latency-associated nuclear antigen (LANA) is a protein encoded by the Kaposi's sarcoma-associated herpesvirus (KSHV) that is commonly expressed in KSHV-infected tumors. Ballestas et al. (p. 641) provide evidence that LANA mediates persistence of the viral DNA as an episome (an autonomously replicating element) in infected cells by tethering it to the host chromosomes. Blocking this process might be a new way of preventing or treating these tumors.

  6. Losing RNA in a Hairpin Turn

    Transcription termination is one of the processes used to regulate gene expression, and this process has been simulated in vitro by Yarnell and Roberts (p. 611; see the Perspective by Landick) for an intrinsic Escherichia coli terminator. The two termination elements, a U-encoding element and a hairpin structure, promote polymerase pausing and extract RNA from the transcription complex, respectively. In this context, an antiterminator counteracts termination by inhibiting hairpin formation and, as a result, allows transcription readthrough. A model for intrinsic termination and antitermination is proposed in which the terminator hairpin extracts the nascent RNA from RNA polymerase as the polymerase molecule translocates forward in the absence of RNA synthesis.

  7. T Cell Kinases

    Multiple kinases are required to convey signals from lymphocyte antigen receptors. The Tec kinase family member BTK is critical for B lymphocyte antigen receptor signaling. T lymphocytes contain three Tec kinases, but whether they play a role in T cell development and function has been unclear. Schaeffer et al. (p. 638) report that removing one Tec kinase does not render the T cell system impotent, but mice genetically deficient in Tec family members Rlk and Itk had disrupted signaling through phospholipase C, which led to a lack of cytokine production, proliferation, and a defective immune response to Toxoplasma gondii. Thus, Tec kinases are critical for the development of effective T cell function and immunity.

  8. No NO Means Yes

    How many “safeties” must a system have to prevent inadvertent activation? The cell death pathway initiated by the binding of the cell surface receptor Fas to its ligand has multiple levels of controls built in. The heart of the effector pathway is the caspase protease cascade in which each intermediate protease activates the next protease through cleavage. Mannick et al. (p. 651) found that the zymogen form of caspase-3 is nitrosylated on its catalytic site cysteine and that denitrosylation correlates with caspase activation.

  9. Too Much of a Good Thing

    Although plants thrive on light, too much light can give the plant a biochemical equivalent of sunburn. When there are not enough of the photosystem molecules responsible for transforming photons into biochemical redox power, the excess energy from light may be expressed as reactive oxygen intermediates that can damage nearby molecules. Karpinski et al. (p. 654; see the Perspective by Foyer and Noctor) now show the existence of a component that can move systemically through the plant and signal the need to prepare for more incoming photons, even to leaves still in the shade.

  10. Tumor Suppression by Ubiquitination?

    The von Hippel-Lindau (VHL) tumor suppressor gene is mutated in most kidney cancers and in VHL disease, a familial cancer syndrome that predisposes individuals to a variety of tumors. The VHL protein exists in a complex with proteins that participate in transcriptional regulation, ubiquitin-mediated protein degradation, and cytokine signaling. Kamura et al. (p. 657) and Skowyra et al. (p. 662) identify a new protein in the VHL complex, called Rbx1, and show that its homolog in budding yeast is a component of a ubiquitin ligase that is required for ubiquitination of multiple substrates, including the G1 cyclin Cln1. Rbx1 appears to both recruit substrates to the ligase and stimulate its intrinsic ubiquitination activity. The VHL complex may regulate the degradation of multiple proteins, which would explain the remarkable pleiotropy of VHL mutations (see the Perspective by Tyers and Willems).

  11. Integral Kinase

    During retroviral infection, a complementary DNA copy of the viral genome becomes integrated into the host cell DNA. This process requires the DNA nicking and joining activities of the virally coded integrase protein. Daniel et al. (p. 644) show that retroviral integration also requires a cellular DNA repair protein, DNA-dependent protein kinase (DNA-PK). This finding suggests that the initial events in integration are detected as DNA damage by the host cell and that completion of the process involves the DNA-PK-mediated repair pathway.

  12. Preventing Zinc-Related Neuronal Death

    Tissue plasminogen activator (tPA) has recently been introduced as a new treatment for stroke. In addition to its well-established action of dissolving clots, Kim et al. (p. 647) show that it may also have further neuroprotective effects. tPA reduces zinc-induced neuronal death in vivo and in vitro. The mechanism of action is not due to tPA binding of zinc but rather to an antioxidant effect. Depending on the route of administration, this action can counterbalance another excitotoxic side effect of tPA that is mediated by plasmin.

  13. Genetic Component of Homosexuality Revisited

    Several years ago, Science published genetic analyses of families which suggested that a region of Xq28 was associated with homosexuality. In a study of 52 families, Rice et al. (p. 665) report that their data for four markers does not support such a genetic linkage. It will be necessary to accumulate even larger data sets in order to understand the underlying bases of this complex behavior (see the news story by Wickelgren).

  14. Coregulators in Vivo

    Proper organismal development requires that the regulated expression of numerous genes be orchestrated both temporally and spatially. Activators and repressors increase or decrease regulated gene expression, respectively. Additional factors called coregulators are now known to form complexes with activators and repressors to effect their function. Until recently, most of the research on coregulators has been performed through in vitro biochemical means or with cell culture techniques. Mannervik et al. (p. 606) review recent functional studies performed in vivo with coactivators and corepressors. This work provides us with a better understanding of the many and varied regulatory mechanisms used by coregulators within the organism.

  15. Not so fast

    New Madrid, Missouri, was struck by several large earthquakes in 1811 and 1812. These have been enigmatic because they occurred far from plate boundaries. Paleoseismic data and geodetic data have implied that these earthquakes might recur every several thousand years or so. Newman et al. (p. 619) now present a longer geodetic study that implies that little if any motion is occurring across the seismic zone and thus that the recurrence interval may be very long.

  16. T Cell Turnover in SIV Infection

    H. Mohri et al. (Reports, 20 Feb. 1998, p. 1223) studied the turnover of CD4 and CD8 T lymphocytes in normal rhesus macaques and in those infected with simian immunodeficiency virus (SIV). Their analysis showed a “large difference between death and proliferation rates for each of the lymphocyte populations,” requiring a “large source … of new cells to maintain the dynamic equilibrium,” thus they raised the question, “Why … is CD4 T cell depletion observed and not CD8 T cell depletion?”

    Z. Grossman et al. disagree with this interpretation, noting that “the death of immune activated cells serves to control their earlier accelerated proliferation and need not affect the equilibrium condition of the population as a whole” and “has little bearing on the issue of depletion.” They offer a model “of normal, ongoing immune activation” that could account for the data without invoking “‘killing’ of … lymphocytes in infected individuals, in access of proliferation.” I. M. Rouzine and J. M. Coffin propose an alternative explanation to that in the report, one based on dilution of the label (that was used to mark the turnover of T cells) according to a model of T cell replenishment.

    In response, A. S. Perelson et al. provide new data collected “19 weeks after the stop of labeling.” They state, “We agree with both comments that immune stimulation in SIV infection may be elevated and may be the cause of the increased proliferation of all of the various lymphoid populations that we measured. However, because as many as 30 to 40% of T cells are labeled after 3 weeks and no antigen was intentionally given to stimulate a response, we believe that the majority of cells we monitored are representative of the general T cell population, although a minority may be antigen-activated.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/284/5414/555a

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