This Week in Science

Science  25 Oct 1996:
Vol. 274, Issue 5287, pp. 473
  1. An older first appearance of animals

    Animals suddenly first appear in the fossil record at the dawn of the Cambrian, about 565 million years ago. The fossil record has generally been interpreted as evidence of a rapid evolution over a few tens of millions of years. Wray et al. (p. 568 see the Perspective by Vermeij, p. 525) examined this notion by determining rates of molecular sequence divergence among metazoan phyla. The data suggest that animals may have instead arisen about 1 billion years ago and that the radiation was prolonged.

  2. Series switches

    One possible route for the development ofelectronic devices functioning on a molecular length scale is the utilization of photoinduced electron transfer in donor-acceptor (D-A) systems. Debreczenyet al. (p. 584) devised a molecule consisting of covalently bound units in a D-A-A-D sequence and show that photogeneration of ion pairs in the first unit leads to the generation of local electric fields that can be used to control a second photoinduced electron transfer reaction within the same molecule.

  3. Upon reflection


    In theoretical physics, it is sometimes the case that the solution to one problem can be used to solve another by the proper transformation of the system, such as switching the role of electrical fields and charges with their magnetic analogs in electromagnetism (see the Perspective by Girvin, p. 524). Shahar et al. (p. 589) measured the current-voltage characteristics of a fractional quantum Hall effect fluid and its nearby insulating state and found that the results are essentially identical for the two states when current and voltage are interchanged. The existence of this duality symmetry for charge and magnetic flux may lead to new theoretical insights into the quantum Hall effect.

  4. Volcanic hazard

    Mount Vesuvius, responsible for the famous volcanic eruption in 79 A.D. that destroyed Pompeii, has had many less dramatic and more recent eruptions that have occurred as recently as the middle of this century. In an effort to assess its potential hazard to the nearby Naples metropolitan area, Zollo et al. (p. 592) report results of an active seismic study of Mount Vesuvius aimed at understanding the internal geometry of its magma system. The results suggest that a melt zone may be present at depths of about 10 kilometers beneath the volcano.

  5. Fusin and CD4 in HIV-1 entry

    Entry of the human immunodeficiency virus-type 1 (HIV-1) into human cells requires the presence of chemokine core-ceptors such as fusin. Lapham et al. (p. 602; see the news story by Cohen, p. 502) show that when human cells are treated with the HIV-1 envelope glycoprotein gp120, the complex formed between gp120 and CD4 associated with fusin. No similar complex could be isolated from nonhuman cells. The design of molecules that can block this association without interrupting the normal functions of CD4 and chemo-kine receptors may provide another strategy against HIV-1 infection.

  6. DNA data arrays

    An important goal in genome analysis is the rapid determination of variations and mutations in specific sequences for individuals. Chee et al. ( p. 610) have developed a high-density DNA array chip and used it to analyze the human mitochondrial genome (16.6 kilobases). A sequence of length L was probed by hybridization to an array containing 4Lprobes of 15-nucleotide oligomers. Sequences can be read out in minutes by comparing the sample (tagged with one dye color) to the reference sequence (tagged with another color). Rapid analysis of sequence polymorphisms should be possible with this approach.

  7. Inside and off-center

    In chromosomes, DNA is wrapped around complexes of proteins called histones to form structures called nucleosomes that not only condense the DNA but also play a role in gene regulation. Pruss et al. (p. 614; see the news story by Pennisi, p. 503) have analyzed the structure of the nucleosome by attaching photoactivated probes along major groove sites of a DNA sequence. The location of contacts to the globular domain of the histone that links nucleosomes (GH5) suggests that the linker histone is well within the nucleosome core and is located in an asymmetric, off-center position inside the strands of DNA that wrap the nucleosome.

  8. Reducing transmitter release

    Metabotropic glutamate receptors regulate neurotransmitter release in the brain, but the mechanisms by which the receptors inhibit transmitter release have been difficult to ascertain. Takahashi et al. (p. 594) measured electrical activity in individual synapses and demonstrate that activation of the glutamate receptors reduces calcium conductance in the presynaptic cell and thus reduces calcium-triggered neurotransmitter release.

  9. Nuclear recycling

    The paradigm by which proteins are imported to the nucleus from the cytosol is well established-a cytosolic receptor binds to a nuclear localization signal on the nuclear protein after its synthesis in the cytosol and directs the receptor-nuclear protein complex to the nuclear pore for import. Aitchison et al. (p. 624) describe a new nuclear import receptor with a role in returning an unusual class of nuclear proteins to the nucleus-the proteins that help to export messenger RNA from the nucleus and which must be recycled to the nucleus for further rounds of transport.

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