This Week in Science

Science  17 Oct 1997:
Vol. 278, Issue 5337, pp. 361
  1. Synthesizing NO signals

    Nitric oxide (NO) participates in physiological functions as diverse as blood flow regulation, neurotransmission, and the immune response. Crane et al. (p. 425; see the news story by Wickelgren, p. 389) present the crystal structure of the oxygenase domain of nitric oxide synthase (NOS), which catalyzes the addition of an OH group (derived from O2) to a guanidino nitrogen of arginine. A comparison of this heme-containing structure to those of the cytochrome P-450 family reveals how the dissimilar fold and heme environment of NOS mediate a similar mechanism. The orientations of the inhibitors imidazole and aminoguanidine provide a template for drug design.

  2. Ancient clues to soil formation

    The chemical reactions that begin the degradation of organic matter in soils and sediments, and thus fix carbon, have been difficult to identify. It has been thought that the classic Maillard reaction was involved, in which sugars and amino acids combine to form compounds with a higher molecular weight (and aromatic by-products that produce the odors of cooked foods). Evershed et al. (p. 432) have now identified a characteristic by-product of the Maillard reaction in preserved plant materials at an archaeological site in Egypt.

  3. Developing DNA computing

    Computing with DNA molecules is massively parallel, and for certain mathematical search problems, the time needed to find an answer grows only linearly in time with problem size, rather than exponentially with a more typical Turing computer. Ouyang et al. (p. 446) incorporated molecular biology methods into a DNA computing algorithm for solving the maximal clique problem (for a given set of vertices and edges, a clique is a set of vertices where each vertex is connected to every other vertex by an edge). Connections in a six-vertex network were represented by DNA sequences, and selective digestion and amplification steps eliminated all but the largest cliques. The authors discuss the need for better algorithms; the amount of DNA needed increases exponentially, and this method would be impractical for networks with more than about 40 vertices.

  4. Triton's tempests

    Voyager 2 showed that the nitrogen-rich atmosphere of Triton, a moon of Neptune, is dynamic, with evidence of seasonal variations in winds and clouds. Elliot et al. (p. 436; see the Perspective by Hubbard, p. 403) observed stellar occultations of Triton's atmosphere from Earth-based telescopes, and in 1995 the Infrared Telescope Facility in Hawaii was situated so that a “central flash,” or partial focusing of starlight by Triton's atmosphere, could be seen. They deduce that the middle atmosphere of Triton is distorted from a sphere, an effect that may be caused by supersonic zonal winds or a distorted internal mass distribution.

  5. Modeling monsoons

    Atmospheric climate models have typically been used with sea-surface temperature prescribed as a boundary condition, but this approach makes investigations of phenomena such as the monsoon rains, for which the atmosphere and oceans are coupled, problematic; simulations, particularly of past climate in equatorial regions, could not always be reconciled with paleoclimate data. Kutzbach and Liu (p. 440) used a combination of an ocean and atmospheric model, forced by orbital variations, to examine the behavior of the climate of the equatorial Atlantic Ocean 6000 years ago. The coupled model agrees with the paleoclimate data and implies that monsoon rains were about 20 percent heavier then.

  6. Cell cycle steps

    Two reports focus on molecular events that accompany the cell cycle. Identifying the substrates of the cyclin-dependent protein kinases (Cdks) has proven difficult. Verma et al. (p. 455) examined molecular control of the transition from the G1 phase of the cell cycle to S phase (when DNA synthesis occurs) in budding yeast. They found that G1-Cdks directly phosphorylate Sic1p, an inhibitor of S phase-Cdks. This phosphorylation triggered ubiquitination and consequent degradation of Sic1p, thus relieving inhibition of the S phase-Cdk. Thus, phosphorylation of Sic1p appears to be a central regulated event that controls the transition from G1 phase to S phase. Visintin et al. (p. 460) enhance our understanding of how the protein degradation machinery acts on the right targets at the right time. Two proteins in budding yeast, Cdc20 and Cdh1, act as substrate-specific activators of the anaphase-promoting complex, which degrades target proteins to allow segregation of chromosomes and exit of the cell from mitosis. It appears that the activity of the proteolytic machinery is finely tuned by proteins like Cdc29 and Cdh1 so that appropriate targets are degraded at the correct time during the cell cycle.

  7. Insulin signaling

    The role of inositol hexakisphosphate (InsP6) in insulin secreting cells has been explored by Larsson et al. (p. 471). InsP6 inhibited activity of protein phosphatases 1 (PP1), 2A (PP2A), and 3 (PP3). The inhibition of the phosphatases can in turn help sustain phosphorylation of voltage-gated L-type calcium channel or an associated protein, which leads to increased open probability of the channel, influx of calcium into the cell, and insulin release. Indeed, application of InsP6 did increase calcium currents through L-type channels in cultured cells. Exposure of the cells to increased concentrations of glucose transiently increased the intracellular concentration of InsP6. Thus, InsP6 may contribute to stimulus-secretion coupling in pancreatic β cells.

  8. Brain sources

    The vertebrate brain arises from a complex pattern of cell proliferation, cell migration, and folding from what begins as a simple tube. Although previous indications had been that the cells of the neocortex, the complex surface layer of higher vertebrate brains, was formed by proliferation of cells within the neocortical primordium, Anderson et al. (p. 474; see the Perspective by Lumsden and Gulisano, p. 402) show that neocortical cells arise from a combination of cells proliferating nearby and cells migrating in from a subcortical zone. The compartments of the mature brain may be less isolated during development.

  9. Deep diamonds

    A few diamonds from Säo Luiz, Brazil, have recently been suggested to have come from Earth's lower mantle, a region where we have not had any known geological samples. McCammon et al. used Mössbauer spectroscopy to analyze the amount of ferric and ferrous iron in several inclusions in these diamonds. The data are consistent with an origin of the diamonds and inclusions in equilibrium in the lower mantle and imply that most of the ferric iron was fractionated into garnet and perovskite.

  10. Protein cooling

    After reactions such as photodissociation of a ligand from the binding site of a protein, the protein is left with excess energy; it subsequently cools through redistribution of the vibrational energy and by dissipating energy into the surrounding water solvent. Mizutani et al. studied the relative contributions of these processes in the cooling of myoglobin after photodissociation of CO with picosecond time resolution. They obtained evidence for two time scales in the heme cooling, which they attribute to a classical diffusion process and a collective motion process.

  11. Supramolecular ordering

    Dendrimers, polymers with a well-defined hierarchy of branched structures, can form the building blocks for supramolecular assemblies. Hudson et al. provide direct structural characterization of these assemblies by electron microscopy, establishing the supramolecular architecture for a series of compounds. The insights gained from such structural studies can be used to tune the structural properties of such assemblies, a prerequisite for rational supramolecular design.

  12. Related cell fates

    Whereas related species retain many developmental similarities, they also display distinct differences. What are the genetic mechanisms behind this variation? Eizinger and Sommer analyzed an evolutionary alteration in the fate of homologous cells among two different nematodes. In Caenorhabditis elegans and Pristionchus pacificus, the fate of 12 precursor cells in the ventral epidermis is to become either vulval cells or nonvulval cells. The nonvulval cells of Caenorhabditis fuse with the epidermis, whereas the nonvulval cells of Pristionchus undergo apoptosis. This difference in cell lineage is controlled by the homeotic gene lin-39, and the cell fates of the two related animals are determined by the differential readout from the lin-39 gene. [See the cover.]

  13. Initiation trees

    One of the hallmarks of nerve cell function is the initiation and firing of an action potential, a wave of membrane depolarization. The classical view, that initiation occurs near the cell body from which the action potential spreads into the axon, has been expanded to incorporate recent observations of active conductances in dendrites that can support action potential propagation backward toward the post-synaptic terminals. Chen et al. argue for a further expansion of this view on the basis of results indicating that action potentials may be initiated within the dendrites and then propagate forward to the cell body. The site of initiation can be regulated by excitatory input to the dendrites and by inhibitory input to the cell body, which suggests that plastic changes within the dendritic tree can occur even in the absence of somatic firing.

  14. Learning in snails

    The mollusk, Aplysia, has been a widely used model system for the study of associative learning. The physical correlate of learning at the synapse has been the focus of much research. Murphy and Glanzman now show at a specific, defined synapse that long-term potentiation is required for associative learning. [See the Perspective by Johnston.]

  15. Becoming glial cells instead of neurons

    During development, cells respond to environmental cues by activating intracellular signaling pathways that result in changes in gene expression and ultimately in differentiation. Bonni et al. explored the signaling mechanisms by which precursor cells of the rat cerebral cortex respond to cytokines and are instructed to undergo differentiation into either neurons or glial cells. Exposure of these cells to ciliary neurotrophic factors triggered the mitogen-activated protein kinase pathway and also the JAK-STAT pathway (JAKs are protein kinases that phosphorylate and activate transcription factors known as STATs). The latter pathway proved to be the critical determinant that promoted differentiation of the precursor cells into glial cells.

  16. Left and right learning

    An asymmetric distribution of cognitive functions in the human brain is well established; for example, language abilities are subserved predominantly by the left hemisphere. By challenging rats to learn a foraging task, LaMendola and Bever provide evidence that spatial learning is asymmetrically distributed in the rat brain. The left hemisphere (linked to the right whiskers) appears to use a maplike representation of space to guide foraging for food, while the right hemisphere (linked to the left whiskers) solves the foraging problem by rote learning.

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