This Week in Science

Science  22 Mar 1996:
Vol. 271, Issue 5256, pp. 1645
  1. Young but not necessarily tolerant

    Neonatal animals show much less of an immune response when challenged by antigens than do adults. Three reports indicate that neonatal T cells are not more easily tolerized by antigens than are adult T cells but generate responses that may have been mistaken for tolerance (see the news story by Pennisi, p. 1665). Ridge et al. (p. 1723; see cover) show that challenge of newborn T cells with nonself spleen cells also exposes the T cells to inappropriate antigen-presenting cells (APCs) that lead to a tolerizing response; exposure to professional APCs, however, leads to their activation. Sarzotti et al. (p. 1726) show that for a viral antigen, low doses lead to a cytotoxic T lymphocyte response but that high doses lead to a nonprotective type 2 cytokine response. Forsthuber et al. (p. 1728) show that the response to a foreign protein in mice (hen egg lysozyme) is not tolerization but a T helper 2 response.

  2. In a shape of flux

    Aromatic organic cations play an important role in organic chemistry, but the shape of the benzene cation, C6H6+, has been unresolved despite numerous theoretical and experimental studies. Electronic degeneracy of the cation should lead to Jahn-Teller distortions that would normally lower the symmetry of the ground state. Lindner et al. (p. 1698) studied the rotational and vibrational structure of C6H6+ in zero-kinetic-energy threshold photoionization experiments. They conclude that C6H6+ is fluxional—its distorted isomers easily interconvert through pseudorotation, and the molecule essentially retains D6h symmetry.

  3. Keeps meandering along

    The meander patterns of streams and rivers, from a small stream to a large river such as the Mississippi, appear to be similar. By using simulations of a freely meandering river and comparison with natural rivers, Stolum (p. 1710) suggests that planforms of meandering rivers may oscillate between two states. In one, the meander pattern is ordered and sinuosity is low, in the other it is chaotic. Both can occur on the same river. Clusters of migrating cutoffs induce transitions between these two states.

  4. Singles and doubles

    Recent advances in molecular detection and manipulation have led to the development of single-molecule spectroscopic and microscopic techniques that can provide detailed information about local chemical environments. In these techniques, absorption of a single photon leads to a transition from the ground to the excited state. Plakhotnik et al. (p. 1703) now show that simultaneous two-photon absorption on the single-molecule level is possible. This nonlinear optical interaction should open the way to nonlinear single-molecule scanning microscopy.

  5. Epilepsy gene

    Mutations in a gene on chromosome 21 have been linked to progressive myoclonus epilepsy (Unverricht-Lundborg type, or EPM1). Pennacchio et al. (p. 1731; see the news story by O'Brien, p. 1672) found mutations in the gene encoding cystatin B, a protease, in cells from affected patients. These occurred at a 3' splice site and a stop codon site and were associated with decreased levels of messenger RNA expression.

  6. Tumor immunotherapy

    T cell activation depends on the outcome of competing stimulatory and inhibitory signals, and Leach et al. (p. 1734; see the Perspective by Pardoll, p. 1691) have used such signaling to develop a new approach to enhancing the immune response to tumors. They blocked the inhibitory signals with an antibody to the appropriate T cell-surface receptor, named CTLA-4. Mice with established tumors, which were fatal if left untreated, mounted strong antitumor responses when treated with the antibody to CTLA-4. These responses were sufficient to clear tumors and to induce immunity to rechallenge.

  7. Mending your RNAs

    Most pre-messenger RNA introns begin with the dinucleotide GU and end with the dinucleotide AG. Recently, a minor class of introns beginning with the dinucleotide AU and ending with AC has been recognized. Because splicing of introns requires pairing between the intron and small nuclear RNAs (snRNAs), it was proposed that splicing of the minor intron class would involve different snRNAs than does splicing of the major class. Hall and Padgett (p. 1716; see the Perspective by Mount, p. 1690) have now found that the U12 snRNA, of previously unknown function, is required to splice the minor class.

  8. Resetting the clock

    In Drosophila, the products of the period (per) and timeless (tim) genes form the core of the circadian clock. Two reports show how light interacts with this system to reset the clock, a process necessary for keeping organisms in tune with their environment (see the news story by Barinaga, p. 1671). Myers et al. (p. 1736) and Lee et al. (p. 1740) found that light causes a rapid degradation of the TIM protein, which disrupts the PER-TIM complex and advances or delays the circadian clock.

  9. Centrosome surplus

    Centrosomes are major microtubule-organizing centers in eukaryotic cells and play an important role in mitotic fidelity by ensuring balanced chromosome segregation. Centrosomes normally duplicate once per cell cycle. Fukasawa et al. (p. 1744) show that cells deficient in the tumor suppressor protein p53 accumulate multiple copies of functional centrosomes, which results in unequal segregation of the chromosomes. This finding suggests that p53 may help maintain genomic stability through regulation of centrosome duplication.

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