This Week in Science

Science  10 Nov 1995:
Vol. 270, Issue 5238, pp. 893
  1. Play at the plates

    Reconstruction of plate motions from the details of magnetic anomalies formed at spreading ridges is critical for interpreting the causes of many geologic events and features. A key area for reconstructing the motions of many plates is in the South Pacific Ocean, where spreading has been occurring between the Antarctic and Pacific plates. Cande et al. (p. 947) analyze magnetic anomalies in this poorly surveyed region to reconstruct the spreading history of these plates during the past 65 million years. A major change in the relative motion between the Pacific and Antarctic plates occurred about 6 million years ago.

  2. Tune-up and alignment

    Organic molecules that show strong second-order nonlinear responses have many potential uses in optics and electronics. However, this response depends not just on molecular properties but requires a specific molecular arrangement in the material. Kauranen et al.(p. 966) were able to force organic molecules into the desired arrangement by attaching them to a helical backbone. This supramolecular approach increases the nonlinear response of the material and could be used to optimize the second-order nonlinear response of other compounds.

  3. Plastic oscillator

    Polymer transistors and logic circuits are desirable for low-cost mass-produced electronic applications, but their performance needs to be comparable to that of silicon devices to make practical devices. Brown et al. (p. 972) have developed polymer field-effect transistors that exhibit voltage amplification—a prerequisite for constructing more complex circuits. Fabrication was made possible by the use of solution processing of conjugated polymers. A five-stage ring oscillator was built to show that the polymeric transistors could drive subsequent gates.

  4. PCB processing

    Polychlorinated biphenyls (PCBs), once released into the environment, can be long-lived pollutants. Some microbes can degrade these stable aromatic compounds by adding two hydroxyl groups and cleaving the modified aromatic ring, thus forming more biodegradable products. Han et al. (p. 976) present the crystal structure of a metalloenzyme that catalyzes the cleavage reaction, enabling them to define the coordination of the modified PCB by the iron atom and possible flexibility in handling chlorinated substrates.

  5. Quality, not quantity

    Precise cell division patterns may play a role in the determination of cell fates during development. Such divisions may be needed for proper distribution of determinants necessary to ensure the correct cell fate. De Nooij and Hariharan (p. 983; see the news story by Roush, p. 916) examined the requirement for the second mitotic wave in the establishment of proper fates in the Drosophila eye. After blocking the second mitotic wave with the expression of a human cyclin-dependent kinase inhibitor p21, each cell type was still specified even though the proper number of precursor cells was not present. Thus cell fate can be uncoupled from the normal pattern of cell division in the eye.

  6. Lymphocyte homeostasis

    The immune response relies on selection and massive expansion of a small number of lymphocytes of appropriate antigenic specificity. Subsequent down-modulation of the response prevents the build-up of large numbers of activated lymphocytes. Unlike activation, down-modulation has been refractory to molecular analysis. Now Waterhouse et al. (p. 985; see the Perspective by Allison, p. 932) suggest that the T cell surface molecule CTLA-4 plays a critical role in negative regulation. Mice lacking CTLA-4 suffer a severe lymphoproliferative disorder and die within weeks of birth; T cells from these animals proliferate spontaneously. Adding further interest, CTLA-4 shares sequence homology with CD28, a surface receptor known to be involved in the stimulatory phase.

  7. Benign HIV-1 strain

    Not all strains of HIV-1 necessarily produce immune deficiency. Deacon et al. (p. 988; see the news story by Cohen, p. 917) have sequenced HIV-1 from a blood donor and a group of six recipients who have not shown HIV-disease symptoms despite being infected for 10 to 14 years. Deletions were found in the nef gene and the U3 region of the long-terminal repeat. Because the lack of disease progression appears to depend on the virus instead of the host immune system, these results suggest a possible use of such HIV strains in live vaccines.

  8. Shifting origins of replication

    In certain viral genomes, the origins of DNA replication are defined by a specific DNA sequence. In higher organisms, however, the nature of a DNA replication origin has resisted definition. Hyrien et al. (p. 994), in studying the ribosomal RNA genes of the frog, Xenopus, show that initiation of nuclear DNA replication may be modulated by the developmental state of the cell. Early in development, when the embryo is undergoing a period of rapid cell division with little transcriptional activity, DNA replication in the ribosomal RNA gene cluster initiates without regard to a specific DNA sequence. However, after the mid-blastula transition, replication initiation becomes confined to the intergenic ribosomal DNA spacers.

  9. Low-impact insect flight

    Animal navigation requires not only the ability to move but also the ability to detect and evade objects and predators. How, for example, does an insect avoid collisions with approaching objects that vary in size and velocity? Hatsopoulos et al. (p. 1000) present an analysis of computation performed by a visual neuron of the locust that multiplies angular acceleration by an inverse exponential function of angular size. This function increases with approach but reaches a maximum prior to impact, providing enough time for escape.

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