This Week in Science

Science  25 Apr 1997:
Vol. 276, Issue 5312, pp. 505
  1. Telomerase goes retro

    Telomerase is a ribonucleoprotein enzyme that replicates the ends of eukaryotic chromosomes or telomeres. Although the RNA components and several telomerase-associated proteins have been isolated, the catalytic protein subunit has remained elusive. Lingner et al. (p. 561; see the news story by Barinaga, p. 528) have characterized a 123-kilodalton telomerase protein from the ciliated protozoan Euplotes and shown that it contains signature motifs of reverse transcriptases (RTs), enzymes that replicate retroviral genomes and transposable DNA elements through an RNA intermediate. The yeast homolog of p123 was identified as Est2p, a protein known to be required for telomere maintenance. Mutagenesis of the RT motifs in Est2p led to telomere shortening and senescence in yeast, implying that these motifs are critical to telomere elongation and likely constitute the enzyme active site.

  2. Landscapes under ice

    The relative effects of tectonism and erosion on producing or limiting high elevation and relief have been uncertain. As one test of the importance of these processes, Brozovic¥ et al. (p. 571) examined the distribution of elevations and hillside slopes in the Himalayas. Hillside slopes reach a minimum near where glaciation has occurred, and elevations evidently do not correlate in detail with tectonic denudation rates. These and other observations suggest that glaciation limits elevation there.

  3. Tectonics under ice

    The sea floor around Antarctica is key for reconstructing past plate motions because it connects the Pacific and Atlantic plates, but critical regions of the sea floor are covered with ice. McAdoo and Laxon (p. 556) obtained detailed satellite gravity data of the ice-covered regions; the data reveal lineations and spreading patterns that describe past plate motions. The data imply that Antarctica comprised two separate plates before about 61million years ago.

  4. Autoantigen identified

    Primary Sjögren's syndrome is a disease in which the immune system attacks and destroys moisture-producing glands, such as tear and salivary glands. The precise target of the attack has not been identified. Now, using a mouse model of Sjögren's syndrome, Haneji et al. (p. 604) have come up with a candidate: α-fodrin, a component of the cytoskeleton. They describe T cell and antibody responses to α-fodrin and show that intravenous injection of a recombinant form of the protein protects against development of the disease in mice. Also, patients with primary Sjögren's syndrome show antibody and T cell responses to α-fodrin, while patients with other autoimmune diseases do not.

  5. Connecting sight and sound

    Sensory processing is generally thought of as a one-way pathway in which incoming signals travel to higher processing and integration centers of the brain. It has been suggested that imagining a visual scene causes top-down activation of the primary visual cortex. The primary visual cortex has also been thought to be active during reading of Braille. Calvert et al. (p. 593) found that the primary auditory cortex is active not only when listening to speech, as expected, but also when lip-reading silent speech, which suggests that the communication between different modalities can occur quite early in sensory processing and perception.

  6. Sucking up

    The early evolution of plants is thought to have had a marked effect on Earth's atmosphere, notably by decreasing atmospheric CO2 levels. Retallack (p. 583; see Perspective by Berner, p. 544) describes evidence from a fossil soil in Antarctica that well-drained forests had developed by the Middle Devonian, about 380 million years ago. This evolution coincides with predictions and other measurements of a dramatic drop in CO2 levels.

  7. Stopping cystitis

    There is a high incidence of cystitis resulting from Escherichia coli infection in women 18 to 40 years old (resulting in millions of hospital visits per year). Langermann et al.(p. 607; see the news story by Service, p. 533) found that an adhesin (FimH) at the end of a structure called a pilus on E. coli can be used to immunize mice and prevent bacterial colonization of the bladder in an animal model of the infection.

  8. Imaging fractions of a charge

    Incorporation of a single electron transistor into a sharp glass scanning probe tip has allowed the detection of extremely small changes in electrical potential. Yoo et al. (p. 579) measured the current that tunnels through a small metal island (100 nanometers across); the external field induces oscillations in this current, and by monitoring the periodicity they can detect just 1 percent of an electron charge. They used this device to map electric fields at the surface of a silicon-doped GaAs/AlxGa1-xAs heterostructure.

  9. Reliable transmissions

    The synapses that mediate the signal from nerve to muscle are highly reliable, in part because of the high density of receptors on the muscle for acetylcholine, the neurotransmitter that carries the signal across the synapse. Sandrock et al.(p. 599) now show that a neuregulin, an extracellular ligand for receptor tyrosine kinases on the muscle, is necessary to maintain the acetylcholine receptors. Knockout mice deficient for only the subtype of neuregulin that contains an immunoglobulin-like domain are myasthenic (their muscles cannot sustain rapid, high-frequency stimulation) and the density of their acetylcholine receptors is reduced.

  10. Crystal engineering

    A layered molecular material has been made that can be modified to incorporate a wide variety of guest molecules. Russell et al. found that guanidinium cations and alkane- or arene-substituted monosulfonate anions form two-dimensional hydrogen-bonded networks. Hydrogen bonds between the layers pucker the two-dimensional sheets and act as flexible hinges. The alkane and arene groups on the anions act as pillars inside the interlayer space and help control the type of guest molecule that can be accommodated by the void spaces. [See the cover and the Perspective by Zimmerman.]

  11. Neuron regeneration in culture

    Injuries to the spinal cord are particularly serious because damaged neurons are not normally replaced. Kehl et al. have found that postnatal rat spinal cord indeed contains cells with the capacity to proliferate and differentiate in vitro into functional neurons. The results suggest new possibilities for therapeutic approaches to spinal cord injuries.

  12. Trypanosome chromosomes

    Trypanosomes are of interest not only because they cause disease but also because as primitive eukaryotes, they display unusual biological features. For example, they carry two quite different sets of chromosomes: A relatively standard set of large chromosomes plus a cohort of 100 or so minichromosomes. Ersfeld and Gull studied chromosomal segregation in dividing trypanosomes and found that the two populations use different mechanisms. The large chromosomes are segregated by kinetochore microtubules whereas the minichromosomes are dependent on the central mitotic spindle. The minichromosomes do seem to segregate faithfully, suggesting an important biological function. This finding links back to the health threat: The minichromosomes carry copies of genes that allow the parasite to periodically vary its surface coat protein and thereby to evade the host immune response.

  13. Magma times

    Arc volcanic rocks are generated above regions where hydrated oceanic crust is returned to the mantle in subduction zones. Fluids and other elements from the subducted crust are thus thought to be involved in generating the magmas, and the distribution of key elements and isotopes can be used to track the processes. Products of the decay of uranium are a key for tracing the evolution because they can also be used to infer the rates involved. Hawkesworth et al. overview the uranium series and other geochemical data from several ocean arcs. The data imply that fluids are transferred from the subducted material through the overlying mantle in about 30,000 to 120,000 years.

  14. De-gassed mantle

    Rare gases from the mantle provide key information on Earth's evolution because most are lost to the atmosphere, unlike many other elements, which are extensively recycled. But actually determining the gas composition of the mantle has been difficult. Burnard et al. describe measurements of rare gases in overpressured vesicles in a key sample of mid-ocean ridge basalt known as the popping rock. They show that there has been atmospheric contamination of argon and helium even in this rock, and that key isotopic ratios measured represent end-member values for the mantle. The composition of heavy rare gases resemble solar abundance patterns.

  15. Regulating immunity

    Abnormalities in the gene encoding BCL-6 are among the most common genetic changes in non-Hodgkin's lymphomas. To investigate the function of BCL-6, Dent et al. produced mice with a homozygous disruption of the BCL-6 gene: These animals were characterized by a series of immunological defects, including uncontrolled inflammation and an inability to produce immunoglobulin G (IgG) antibodies in response to immunization. The former was due to elevated levels of the pro-inflammatory cytokines associated with the type 2 T helper cells, such as interleukins-4, -5, and -6; the latter was due to a lack of germinal centers, the structures within which IgG antibody production is initiated. How might disruption of BCL-6 cause these phenomena? Dent et al. showed that BCL-6 inhibits interleukin-4 signaling through the transcription factor Stat6, which controls the development of type 2 T helper cells. As the STAT transcription factors mediate signaling by multiple cytokines, BCL-6 may be a pivotal factor in regulating both T and B cell responses.

  16. Function of a Blimp

    What factors interact to control whether an activated B cell undergoes proliferation or differentiates into an antibody-secreting plasma cell? One molecule at the heart of the differentiation program is Blimp-1, a zinc-finger protein. Another protein, c-Myc, is present in dividing cells and blocks terminal differentiation in a number of cells types; plasma cell tumors (plasmacytomas) often have a dysregulated c-myc gene. Now Lin et al. have discovered a key link between these two factors: Blimp-1 is identical to plasmacytoma repressor factor, an uncloned factor that represses c-myc expression. Thus, direct repression of c-myc is one of the functions of Blimp-1, a “master regulator” of B cell differentiation.

  17. Continuous chemical evolution

    Evolutionary principles can now be observed in a test tube using a population of molecules. Wright and Joyce constructed a system in which a series of RNA molecules were amplified and selected on the basis of their ability to catalyze a reaction. Approximately 300 successive populations were obtained in 52 hours without intervention. Mutations that occurred in the RNA molecules and were naturally selected improved the catalytic rate substantially. [See Perspective by Ellington.]

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