This Week in Science

Science  21 Dec 2001:
Vol. 294, Issue 5551, pp. 2425
  1. In Brevia

    A previously unknown species of large squid, up to 7 meters in length and with distinctive morphology, has been sited by Vecchione et al. (p. 2505) in four deep-ocean basins.

    CREDIT: VECCHIONE ET AL.

    Analysis of the tiny marine organism Oikopleura dioica by Seo et al. (p. 2506) reveals the smallest genome found so far for a chordate, part of the lineage that gave rise to the vertebrates and ultimately humans.

  2. Reasons to Transfer

    The Marcus cross relation allows the rate of electron transfer between different species to be systematized in terms of equilibrium constants (ground-state energetics) and self-exchange rates. Hydrogen atom (H·) transfers (an electron plus a proton) are radical reactions that exhibit wide variations in rates for similar driving forces. Roth et al. (p. 2524) now show that the rates of a wide variety of H· transfers obey a Marcus cross relation, and compounds that show slow electron or proton transfer often undergo slow H· transfer.

  3. Our Fair Share?

    A study by Vitousek et al., 15 years ago, based largely on extrapolations from limited field-based studies, estimated that humans co-opt about 42% of the global terrestrial production. Rojstaczer et al. (p. 2549; see the Perspective by Field) now provide an update based on recent, mainly satellite-based surveys and an error analysis. They reach a similar conclusion through this different methodology, but caution that the uncertainties are still very large for several key parameters, namely, the productivity of agricultural land and the biomass of secondary forests.

  4. Black Holes and Growth Spurts

    The excessive luminosity of active galactic nuclei (AGNs) has been attributed to a period when black holes at their core were accreting the bulk of their mass. Page et al. (p. 2516) studied the x-ray flux of AGNs where their luminosity peaks (those with redshifts from about 1 to 3). They assign the emission to dust from starburst activity, which reflects star formation, in the bulges of the galaxies. Starburst activity and the period of major growth of the black holes appears to be coeval, which suggests that the source materials for rapid star formation and black hole growth should be similar. Thus, galaxy morphologies in the bulge and the center developed at the same time in a relatively hyperactive universe.

  5. Exciting Landings on Surfaces

    When a molecule adsorbs on a surface, bond formation releases energy. Generally it has been assumed that for low-energy bonds (less than half an electron volt), this energy is dissipated as heat (vibrations in the surface, or phonons), and that electronic processes are unimportant. Gergen et al. (p. 2521; see the Perspective by Auerbach) now show that adsorption of water, nitric oxide (NO), nitrous oxide (NO2), and even xenon onto a polycrystalline solver film all produce electron-hole pairs that can observed with a low-barrier Schottky diode. This finding not only revises the conventional model of chemisorption but also suggests new possibilities for sensors.

  6. Exotic Behavior with Sheets

    Low-transition-temperature (Tc) superconductors, in which the superconductivity is described by an electron-phonon coupling mechanism, generally display isotropic pairing of the electrons in momentum space—they are said to exhibit s-wave symmetry. However, like some of their high-Tc cuprate cousins, a few low-Tc superconductors have revealed a more complex pairing symmetry. One mechanism put forward to explain this apparent discrepancy has been additive contribution of several Fermi surface sheets. Yokoya et al. (p. 2518) used an ultrahigh-resolution angle-resolved photoemission technique that could resolve different Fermi-surface sheets in such a superconductor (2H-NbSe2) and confirmed that the underlying mechanism is s-wave pairing.

  7. A New Twist on Aromaticity

    Thiamine diphosphate, a cofactor in many enzymatic reactions, contains a thiazole moiety that can yield a nucleophilic carbanion at the C-2 position. Chabrière et al. (p. 2559; see the Perspective by Frey) offer crystallographic evidence that the free-radical intermediate in the degradative reaction catalyzed by pyruvate:ferredoxin oxidoreductase contains a partially localized electron after decarboxylation of the pyruvate has occurred but before oxidation is complete. Twisting of the thiazole ring during catalysis greatly reduces its aromaticity and prevents complete delocalization of the lone electron.

  8. A Syringe in Action

    Projections from bacteria have been predicted to play a role in the export of specific proteins and DNAs during conjugation and pathogenesis. However, a direct demonstration of such a role has been lacking. Jin and He (p. 2556) have now directly visualized the extrusion of a protein from the Hrp pilus of Pseudomonas syringae. The immunogold labeling protocol used will help to provide unambiguous answers in other systems where similar questions remain open.

    CREDIT: JIN AND HE
  9. Walk, Don't Walk

    Signals from a circadian clock in a region of the mammalian brain called the suprachiasmatic nucleus (SCN) must somehow be translated into the rhythmic behavior of processes such as locomotion and rest. Kramer et al. (p. 2511; see the news story by Barinaga) show that neurons in the SCN secrete transforming growth factor-α (TGF-α) that acts on cognate receptors in the hypothalamus to inhibit locomotion in hamsters. Expression of TGF-α is correlated with cycles of locomotion and rest. Hence, secretion of factors that act locally in the brain is one mechanism by which the clock exerts its cyclic effects.

  10. Transcription and Cofactor Methylation

    In order to access DNA that is packaged into nucleosomes, histone-modifying factors come into action to allow subsequent DNA transcription. Xu et al. (p. 2507; see the Perspective by Nishioka and Reinberg) now show that the histone methyltransferase CARM1 also has a nonchromatin substrate and acts in two separate but coordinated ways to influence gene expression. With in vitro chromatin templates, CARM1 acts positively to stimulate nuclear receptor transcription. However, the factor can block transcription by methylating the coactivator CBP at an amino acid that interferes with the action of CREB. Thus, CARM1 potentiates transcriptional activation by retinoid receptors whereas it can block cAMP (adenosine-3′,5′-monophosphate) signaling and allow for cross-talk between the two signaling pathways.

    CREDIT: XU ET AL.
  11. The End-Joining Justifies the Screen

    A genome-wide screen in yeast for mutants defective in nonhomologous end-joining (NHEJ), a form of genetic recombination critical in immunoglobulin rearrangement, double-strand break repair, and yeast mating type-switching, has been performed by Ooi et al. (p. 2552). Thousands of these mutants were analyzed at one time with a functional assay in which cells were transformed by linear or circularized plasmids containing a selectable marker. One of the genes identified as being involved in NHEJ, NEJ1, interacts with the amino-terminal end of a homolog of a human protein that protects cells against cancer.

  12. From Stem to Sperm

    A pair of studies sheds light on the molecular mechanism and the spatial organization of a signaling pathway that regulates stem cell fate in the testis of fruit flies (see the Perspective by Wasserman and DiNardo). A group of somatic cells located at the testis apex (the hub) is surrounded by stem cells that can generate sperm. When needed by the fly, a stem cell can divide asymmetrically to yield a daughter stem cell and a cell that will subsequently differentiate into a mature sperm. Tulina and Matunis (p. 2546) and Kiger et al. (p. 2542) now show that the hub sends regulatory signals from the JAK-STAT signaling pathway in order to maintain these stem cells in an undifferentiated state. When this pathway is defective, cells begin to go through spermatogenesis. A better understanding of how stem cells are maintained and directed down different paths of differentiation should prove useful in medical applications.

  13. Genetic Clue to Prostate Cancer

    Prostate cancer affects one in eight men and causes 32,000 deaths each year in the United States alone. About half of these cancers show allelic loss at chromosome 10p15. Narla et al. (p. 2563) find that one of the genes mapping to this region, KLF6, encoding a Kruppel-like putative transcription factor, is mutated at high frequency in sporadic prostate tumors. Transient expression of wild-type KLF6, but not KLF6 containing tumor-associated mutations, slowed the growth of prostate cancer cells in culture, an effect mediated through the cell-cycle regulatory protein p21/WAF1. These features of KLF6 suggest that it functions as a tumor suppressor gene in prostate cancer.

  14. Useful Information in the Neighborhood

    How is information distributed over populations of neurons? Cortical neurons with selectivity for similar stimulus features can be found close to each other. Reich et al. (p. 2566; see the Perspective by Richmond) simultaneously recorded from single neurons in primary visual cortex and found that the responses of nearby neurons are only correlated to a relatively small degree. Simple averaging strategies, such as population histograms, imaging, or field potentials, are suboptimal and vastly underestimate the information processing that is occurring, even at such an early stage in sensory processing.

  15. Warping of Tree-Ring Dates

    The accuracy of 14C dating of tree rings depends on how well a key assumption holds—that the average carbon isotopic composition of atmospheric CO2 incorporated by all mid-latitude trees is the same during any given year. Two reports re-examine this assumption (see the Perspective by Reimer). Using one of the infrequent instances when tree ring radiocarbon equality did not exist, Manning et al. (p. 2532) revised the chronology of the eastern Mediterranean region for the later-third to early-first millennia B.C. based on data from Anatolian juniper trees. A latitudinal gradient in tree ring 14C/12C that existed for a brief window of time at around 800 B.C. produced a 22-year shift to older ages from a time line previously determined with tree ring data from Germany and Ireland. This small but significant difference has important implications for the history of the eastern Mediterranean and the Aegaen. Kromer et al. (p. 2529) identified small but important regional differences in contemporaneous tree ring radiocarbon ages at times of reduced solar activity and related global cooling. During certain periods when there were rapid increases in the concentration of atmospheric 14CO2, trees from Germany and Anatolia recorded different values of 14C/12C and thus apparently different ages. These differences could be the result of offsets in the timing of growing seasons at the different locations or the physiological effects of regional changes in warmth and water availability.

  16. Divide and Conquer

    The properties of the methyl cation bear on questions in fields as diverse as combustion, chemical kinetics, and astrochemistry—yet this small species has proven difficult to characterize spectroscopically. Liu et al. (p. 2527) now assign rotationally resolved vibrational spectra for CH3+, obtained by an analog of photoelectron spectroscopy in which the negatively charged particle that is ejected is not an electron, but instead is a Cl ion, in this case produced by photodissociation of CH3Cl. This approach could also be applied to other carbocations and transient ionic species.

  17. How to Imprint a Genome

    In mammals, the expression of some genes depends not on the genotype of the embryo, but on which parent they come from, a phenomenon known as “genomic imprinting.” Imprinted genes are involved in prenatal growth, in development, and in behavior, and have also been implicated in human disease. Imprinting often involves the de novo methylation of the DNA of affected genes, yet little is known about how these imprints are established during gametogenesis. Bourc'his et al. (p. 2536) have now identified Dnmt3L as one of the components of the maternal gametic methylation machinery. Dnmt3L is required for the establishment of maternal methylation imprints—the maintenance of imprinting is not affected. Curiously, Dnmt3L does not look like a DNA methyltransferase enzyme and may function as a regulator of methylation at imprinted loci.

  18. Centromere Heterochromatin Ensures Segregation

    During cell division, the newly replicated chromosomes pair up, each with their own duplicate, via cohesin proteins, so that when arranged in the middle of the about-to-divide cell, one of each pair can be sent to the two daughters. Bernard et al. (p. 2539) found that the heterochromatic nature of the centromeres—often visible as a constriction toward the middle of condensed chromosomes—plays a vital role in holding the sister chromosomes together prior to the final separation of the daughter cells. Specifically, the heterochromatin protein Swi6 allows association of cohesins with the centromere and thus the correct segregation of the chromosomes.

  19. In Silico Mapping of Mouse Quantitative Trait Loci

    Grupe et al. (Reports, 8 June 2001, p. 1915) described a novel computational algorithm for prediction of chromosomal locations of quantitative trait loci (QTLs) in mice. In a comment, Chesler et al. suggest that the algorithm does not work when an important strain, CAST/Ei, is omitted, and express concerns that, though the method “does not appear to be a viable alternative to current QTL methodology,” it could jeapordize funding for, and attention to, conventional QTL studies. Separately, Darvasi comments that the new method “is not of practical use for most relevant cases of QTL mapping” because of the large numbers of strains required for the correct calculations. Usuka et al., in response, identify a potential error in the implementation of their method by Chesler et al., and state that their method “should markedly increase interest in and productivity from mouse genetic research,” including conventional QTL studies. Further, they argue that Darvasi's calculations are not applicable to their computational method and that “every new source of QTL information is valuable.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/294/5551/2423a

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