This Week in Science

Science  24 Jan 2003:
Vol. 299, Issue 5606, pp. 469
  1. Point-and-Shoot Fluorescence

    Molecular fluorescence is normally induced by the absorption of an ultraviolet photon that promotes an electron to an excited state, which then decays and emits a photon. Qiu et al. (p. 542) show that characteristic fluorescence can be detected from molecules adsorbed on surfaces when probed with the scanning tunneling microscope. The tunneling electron not only induces fluorescence, but the emission observed can vary depending on which part of the molecule is being probed with the tip.

  2. Controlling Interest

    The advent of ultrafast-pulse lasers led to hope that bond-selective chemistry would be just around the corner. However, energy deposited in one bond usually leaks into the rest of the molecule before the desired chemistry can take place. Recently, iterative genetic algorithms have been used to create much more complex pulse shapes over a broad range of frequencies that can in fact select different reaction pathways. Daniel et al. (p. 536; see the Perspective by Rabitz) explored why the pulse used one of these reactions, in which the ionization of the compound CpMn(CO)3 (where Cp is the cyclopentadienyl) can occur with or without loss of a CO group, selects a particular outcome. They combined ab initio calculations and femtosecond pump-probe spectroscopy to identify the underlying excitation and ionization steps.

  3. A High Water Mark

    The assignment of the emission and absorption spectrum of water is important for several fields, including the understanding of the absorption of sunlight by water in the atmosphere and the fate of water in extreme environments, such as at the surface of the Sun. The accuracy of theoretical assignments of rotational- vibrational spectra has unfortunately lagged behind those of experiments, in part because bending motions cause extensive rehybridization. Polyansky et al. (p. 539) took into account a wide range of effects not usually included in such calculations and raised the accuracy of such calculation by an order of magnitude. This study suggests that calculations with experimental accuracy may be within reach.

  4. Tilted Core

    The structure of the inner core can be inferred from the observed properties of seismic body waves that traverse the core, or from normal-mode oscillations (the “ringing” of the Earth). Unfortunately, the results of two methods have not agreed. Beghein and Trampert (p. 552; see the Perspective by Sambridge) remedy the discrepancy by using a neighborhood algorithm to sample the entire model space of possible inner core structure for normal-mode oscillations. They speculate that the anisotropy they see in their most robust model arises from tilted hexagonal close-packed iron crystals in the outer half of the inner core and a different iron phase in the center of the core.

  5. One for the Many, Two for the Slow

    Most plants and animals are diploid, with two copies of each chromosome, but some eukaryotes are haploid and carry just one copy. Both schemes have relative advantages—diploid organisms should be better protected against deleterious mutations, and haploid organisms should acquire beneficial mutations more readily. Zeyl et al. (p. 555; see the Perspective by Grieg and Travisano) experimentally tested these ideas by propagating both haploid and diploid yeast populations for 2000 generations and measuring their rates of adaptation to the culture conditions. Haploid populations adapted significantly at large population sizes, but this advantage was eliminated in much smaller population sizes. Mating in diploids, which was not allowed in these experiments, would likely lead to further selection advantages in small populations.

  6. Coherent Coda

    The coda is the late part of a seismic signal that is relatively heterogeneous because it represents the scattering of sound waves off of small-scale structures in the lithosphere. Campillo and Paul (p. 547) identified a coherent signal of surface waves by correlating the codas from 101 earthquakes to a few specific stations. The noisy pattern at the end of a seismic signal can yield important information about the structure of the lithosphere that cannot be derived from other seismic phases.

  7. Changing Where You Send the Bill

    Migratory neural crest cells are one of the sources of developmental information that are integrated into the shaping of the head and face. Schneider and Helms (p. 565; see the Perspective by Trainor) used cellular chimeras of duck and quail to produce “qucks” and “duails” to analyze the molecular information exchange that directs beak formation in these birds. The transplanted neural crest cells form the right type of structures in the right locations as required by the host, but the specific form of those structures is directed by the source of the neural crest cells. The transplanted cross-species neural crest cells also directed development of host craniofacial structures with which they interact. Thus, the final shape of neural crest derivatives in the face is a triple interplay of reciprocating information streams.

  8. Bug Breathing

    Openings in the exoskeleton of insects connect to a vast network of tubules, the respiratory tracheal system, and it has been thought that insects rely on passive diffusion and physical activity to move gases through this system. Westneat et al. (p. 558; see the cover) used an x-ray source to visualize tracheal respiration and report a previously unobserved tracheal compression and expansion cycle in inactive insects similar to that of vertebrate lungs.

  9. Distributing Chromosomes in Bacteria

    In eukaryotic cell division, an elaborate spindle structure partitions chromosomes to daughter cells. In bacteria, no such apparatus has been found. Ben-Yehuda et al. (p. 532) describe a protein termed RacA that plays a role in chromosomal segregation during spore formation in Bacillus subtilis. RacA appears to anchor chromosomes at the ends of the cell via the chromosomal origin of replication and to help promote the distribution of chromosomes to the spore and the mother cell. In nonsporulating cells, increased levels of RacA protein cause chromosomes to shift from the center of the cell to the poles.

  10. How PARP Produces Puffs

    Polytene chromosomes in the salivary glands of the fruit fly Drosophila undergo “puffing” at specific regions that often correspond to increased gene expression. Tulin and Spradling (p. 560; see the Perspective by Pirotta) now show that ribosylation and poly(ADP)-ribose polymerase (PARP) mediate chromosome puffing. The authors propose that PARP dissociates chromatin proteins at induced chromosomal loci, which then allows increases in transcriptional activity.

  11. It's not just the Nitrogen

    Policies for coastal ecosystem management and restoration have been driven by the widely accepted paradigm that these ecosystems are nitrogen limited. Sundareshwar et al. (p. 563) show that unlike plants, soil bacteria were phosphorus limited. Phosphorus enrichment also depressed the potential rates of N2O production, denitrification, and heterotrophic nitrogen fixation. Hence, phosphorus levels will affect ecosystem-level inputs and outputs of nitrogen. These findings have major implications for the global carbon cycle in that primary producers and decomposers in other ecosystems may also be differentially limited by nutrients.

  12. Live Lean, Live Long

    Restricting food generally increases the life-span of mammals, and this effect may be related to the decrease in caloric intake or to the accompanying reduction in fat mass. Studying genetically modified mice that eat normally but cannot properly store energy in fat, Blüher et al. (p. 572) show that leanness alone is associated with a modest (18%) increase in mean life-span, possibly through effects on insulin signaling.

  13. Imaging the Formation of Memories

    Functional brain imaging of the activation of the hippocampus during the process of encoding and remembering new items has been technically challenging because of the small and compact architecture of this region of the brain. Zeineh et al. (p. 577) have developed an advanced imaging analysis that enabled them to “flatten” the hippocampal region and visualize neuronal activation more finely during a memory task. Activity in the cornu ammonis fields 2 and 3 and the dentate gyrus correlated with the number of new items and with the periods of encoding. Activity declined as fewer new items remained to be memorized and was not correlated with periods of retrieving these memories. Conversely, the subiculum displayed activity that correlated with periods of retrieval and that declined as fewer new items needed to be recalled.

  14. Hearing and Listening in the Brain

    The main cortical areas of the primate auditory system have been long known, but the full extent of its projections has been more difficult to map out. Poremba et al. (p. 568) isolated one hemisphere from all auditory inputs and then looked at where auditory input caused relatively increased activity in the other hemisphere. The results show that the auditory system is not only large like the visual system, but also overlaps with the visual system in specific areas.

  15. Shockingly Fine Features

    Chondrules are rounded aggregates of minerals within chondritic meteorites (chondrites) and are thought to be the first solids that quenched out of shocked regions of the solar nebula. Carbonaceous chondrites are further distinguished within this meteorite class by their higher abundances of hydrated minerals and carbon. Indeed, many carbonaceous chondrites are rimmed by a concentration of fine-grained hydrated minerals whose formation has been attributed to liquid water reactions on the parent body. Ciesla et al. (p. 549) present a model in which the fine-grained rims formed by liquid water reactions produced by shock waves moving through icy regions of the nebula. Thus, chondrule and fine-grained rim formation were nearly simultaneous.

  16. Localized Traffic Only

    Transforming growth factor-β (TGF-β) helps regulate cell growth, differentiation, and cell fate specification. Receptors at the cell surface transduce TGF-β signals to intracellular proteins called Smads, which are then deployed to the nucleus to effect transcription. Tang et al. (p. 574) show that a major cytoskeletal protein of the β-spectrin family regulates this signaling pathway by controlling Smad localization. In the absence of spectrin, mislocalized Smads do not have access to TGF-β receptors and hence, signals are not propagated. The cytoskeletal protein appears to function as an adaptor that assembles components of the receptor-Smad signal transduction pathway.

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