This Week in Science

Science  17 Aug 2001:
Vol. 293, Issue 5533, pp. 1217
  1. Toward a Light-Wave Generator

    Mixing light waves of different frequencies is a routine technique for synthesizing waveforms of arbitrary shape and spectral composition. For applications in coherent quantum control, ultrafast optical pulses over a range of wavelengths may be required. Shelton et al. (p. 1286; see the Perspective by Brown et al.), building on recent advances involving phase-locking of femtosecond optical pulses, generation of a comb of optical frequencies, and precision measurement of optical frequencies, now show that two waveforms can be coherently stitched together from separate femtosecond lasers to form a coherent pulse train.

  2. Carbon Nanotubes Close the Loop

    A method for forming large rings from single-walled carbon nanotubes (SWNT) is described by Sano et al. (p. 1299). They solubilized SWNTs through a cutting technique and then lightly etched them to generate a distribution of tubes that are oxygenated at both ends. Nanotubes of intermediate length could form closed loops that, after a subsequent chemical reaction and annealing, were stable when treated with ring-opening reactants or subjected to high temperatures.

  3. Langmuir-Blodgett Films Come to Order

    The formation of Langmuir-Blodgett films, in which ordered monolayers of amphiphilic molecules at a surface are transferred to solid support, is one of the oldest nanoassembly methods. However, as films build up, defects introduced by reordering transitions limit the usefulness of these films. Takamoto et al. (p. 1292) report that by working at higher solution pH (8.5 instead of the typical 7), films of cadmium arachidate that are harder to draw actually form nearly defect-free films. The high-pH films, which on the aqueous phase form a “pseudo-herringbone” structure, convert to a hexagonal structure after deposition without the further reorganization that disrupts film layering, as is the case with pH-7 films.

  4. COmbustion

    Both aerobic and anaerobic microbes possess an enzyme, carbon monoxide dehydrogenase (CODH), that converts CO into CO2 and that is important for forward and reverse reactions that cycle carbon globally between CO2 and acetate. In some cases, the CO2 can be used as a carbon source through incorporation into acetyl-coenzyme A. Dobbek et al. (p. 1281; see the cover and the Perspective by Thauer) describe the crystal structure at 1.6-angstrom resolution of CODH from Carboxydothermus hydrogenoformans. This enzyme utilizes an unusual [Ni-4Fe-5S] cluster in which CO binds to the Ni, thus rendering it susceptible to attack from a hydroxyl moiety bound to a neighboring Fe.

  5. The Queen Gets Her Way

    Sex ratio studies in ants provide one of the better tests of parent-offspring conflict and kin-selection theories. Theory predicts that workers prefer a more female-biased sex ratio than do queens, and the prevailing view is that workers win this conflict. Passera et al. (p. 1308; see the news story by Pennisi), however, show that sex ratio can sometimes be under queen control. Queens of the fire ant Solenopsis invicta can force workers to produce males by limiting the number of female eggs. This finding provides an explanation for sex investment ratios departing from the worker optimum in this and many other ant species.

  6. Sensing with Nanoscale Surfaces

    The use of nanoscale surfaces provides a new twist for designing sensors with semiconductors and with liquid crystals. The adsorption of charged molecules or ions on the surfaces of semiconductors can lead to changes in their surface conductivity that can be exploited in chemical sensing. Cui et al. (p. 1289) increased the sensitivity of this effect by using semiconducting nanotubes instead of planar surfaces. Functionalized boron-doped silicon nanotubes were used to detect changes in pH and the concentrations of an antibody and of Ca2+ in real time. Competitive binding to molecular receptors on a surface has been used to create a colorometric sensor based on liquid crystal ordering with part per billion sensitivity to organic molecules. Shah and Abbott (p. 1296) patterned glass surfaces that have nanometer-scale corrugation with self-assembled monolayers of alkyl thiols bearing molecular receptors (for example, carboxylic acid groups for targeting amines). This layer was covered with a film of a liquid crystal that binds weakly to the molecular receptor. This weak-induced ordering of the liquid crystal, which can be observed with polarized light, is disrupted by the much stronger binding of the desired analyte. Such systems could be used as wearable sensors for monitoring exposure limits.

  7. A Lead into Arctic Circulation

    Knowledge of the variability of water circulation in the Arctic Ocean during the past century is relatively limited, as is the understanding of how many anthropogenic pollutants are transported there. Using lead as a tracer, Gobeil et al. (p. 1301; see the Perspective by Mysak) paint a picture of Arctic boundary currents that clearly identifies their connection with the Atlantic Ocean and shows that the ocean, not the atmosphere, dominates the transport of anthropogenic lead to Arctic waters. The distribution of contaminant lead and its isotopic composition allows the authors to reconstruct Arctic Ocean current stability during the past 50 years.

  8. Tailing Tuna

    Sustainable management of the Atlantic bluefin tuna, one of the most valuable commercial fish, has been hampered by inadequate knowledge of the bluefin's migratory movements, spawning grounds, and population mixing. To record these movements, Block et al. (p. 1310; see the Perspective by Magnuson et al.) used electronic archival data tags programmed to remain on fish for up to 1 year before floating to the surface so that the accumulated data can be downloaded via satellite. Tuna resident in the West Atlantic return to breeding grounds in the Gulf of Mexico and in the Mediterranean, and suggest that more rigorous protection will be necessary in these spawning grounds if the bluefin is to be conserved.

  9. Myosin-V Regulation

    Myosin-V is a microtubule-based motor responsible for promoting organelle movements around the cell, particularly the movement of melanophores around melanocytes. During mitosis, the activity of myosin-V is down-regulated, probably by phosphorylation, as the microtubule machinery is co-opted into spindle assembly and membrane traffic shuts down. Karcher et al. (p. 1317; see the Perspective by Cheney and Rodriguez) describe the precise phosphorylation site that is present in the organelle-binding carboxyl terminal of the protein. The authors suggest that such phosphorylation to abrogate cargo binding may be used more generally.

  10. The Tie that Binds

    During cell division, sister chromatids are held together by complex of proteins called cohesin. In yeast, proteolysis of the cohesin subunit known as Scc1p is thought to be the event that actually releases the chromatids, which then migrate to the daughter cells in anaphase. However, in vertebrate cells, most of the cohesin dissociates from the chromosomes before metaphase. Hauf et al. (p. 1320) now show that the small amount of cohesin that remains on chromosomes and is lost at the start of anaphase must be cleaved to allow separation of the sister chromatids. They made mutants of the human cohesin subunit SCC1 that could not be cleaved. In human cell lines that express physiological amounts of the modified SCC1, chromatid separation and subsequent cytokinesis were disrupted. The authors note that defects in this mechanism could contribute to genomic instability in tumor cells.

  11. Current Trends in Calcium Currents

    Identification and characterization of new calcium channels that may function to control calcium-dependent signaling continues at a rapid pace. Sano et al. (p. 1327; see the Perspective by Levitan and Cibulsky) describe the human LTRP2 channel, a member of the transient receptor potential (TRP) family that is abundantly expressed in the blood. The channel contains a motif in its cytoplasmic tail that is associated with nucleotide hydrolase activity. In cells transfected with LTRP2, conductance of the channel was directly activated by adenosine diphosphate ribose (ADPR), or β-nicotinamide adenine dinucleotide (β-NAD). In immunocyte cell lines, they observed a similar inward calcium current when cells were exposed to ADPR or β-NAD. In a Perspective, Levitan and Cibulsky summarize other recent work that has characterized a different TRP-family member and in which different regulatory properties and ADPR pyrophosphatase activity are ascribed to LTRP2.

  12. A Write-Only Memory

    From our subjective perspective, the process of memory appears seamless and easy, until trauma or the aging process interferes. McGuire et al. (p. 1330; see the Perspective by Waddell and Quinn) have developed a controlled version of such interference in Drosophila by conditionally expressing the gene shibire, which encodes a dynamin guanosine triphosphatase, in the mushroom bodies of the fly's brain. This protein plays an essential role in synaptic vesicle recycling. By using changes in temperature to turn off expression of this gene, mushroom-body function could be turned on and off during different phases of an odor-recognition task. Mushroom bodies are necessary for memory retrieval but not for acquisition or consolidation. Thus, the fly can learn and store the information, but cannot later remember it.

  13. Sea Floor Rain Gauge

    The Cariaco Basin is a small anoxic marine basin on the northern shelf of Venezuela that has proven to be an unusually informative source of information about paleoclimate in the tropics. During the summer, large amounts of rain fall in regions that drain directly into the Basin or the nearby Orinoco River, transporting terrigenous materials into the Basin and surrounding shelf where they accumulate as sediments. During the winter, rainfall and local river runoff is diminished, and upwelling along the coast increases biological production in the surface ocean. This seasonal rhythm has produced a continuous, mostly laminated sediment sequence of alternating light-colored plankton-rich layers and darker-colored terrigenous grain-rich layers during the last 14,500 years. Haug et al. (p. 1304) analyzed these sediments and present a record of tropical Atlantic precipitation variations that suggests that changes in the position of the South American Intertropical Convergence Zone, in phase with El Niño-Southern Oscillation variability in the Pacific, are responsible.

  14. Out of Control

    Introduced biological control agents (predators, parasites, and pathogens) may control insect pests (introduced or native) but have also harmed non-target species. Henneman and Memmott (p. 1314; see the news story by Stokstad) show that biological control agents have heavily infiltrated a remote Hawaiian community of endemic moth species. They find that 83% of the parasitoid wasps reared from lepidopteran larvae swept quantitatively from transects in the Alaka'i Swamp area of Kauai, Hawaii were introduced biological control agents, while 14% were accidental introductions and only 3% were indigenous species.

  15. Vascular Pathways to Tumor Suppression

    Peutz-Jeghers syndrome is an inherited disorder characterized by the development of gastrointestinal polyps and increased risk of cancer. The causative mutations lie in the LKB1 gene, which encodes a serine/threonine kinase. To gain insight into the function of LKB1, Ylikorkala et al. (p. 1323) created mice deficient in the kinase. The absence of LKB1 caused embryonic lethality, which was associated with severe defects in the development of the vasculature and aberrant upregulation of vascular endothelial growth factor (VEGF). These results suggest that LKB1's role as a tumor suppressor may in part be due to its ability to prevent growth of the blood vessels that are needed for tumor progression.

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