This Week in Science

Science  22 Aug 2008:
Vol. 321, Issue 5892, pp. 1016
  1. Black Hole Rising

    CREDIT: BONNELL AND RICE

    Our Galaxy contains a massive black hole, which was discovered by carefully observing the elliptical motions of stars around it. But this raises the questions of how did these stars get there, and why are they mostly massive stars? The black hole should have disrupted and consumed a molecular cloud from which such stars generally form, and capture of material from farther out seems unlikely. Bonnell and Rice (p. 1060; see the Perspective by Armitage) present a series of simulations that show that, although a molecular cloud is greatly disrupted by a black hole, sufficient gas remains to form a disrupted disk that can nucleate stars, and, under appropriate conditions, most of these become massive stars. These results thus explain the observations, confirm that the stars are indeed revealing a black hole, and provide an explanation for this type of star formation.

  2. Tibetan Uprising

    The Tibetan plateau, the highest extensive region on Earth, formed broadly in response to the collision of India with Asia. But the cause and timing of the uplift of the plateau have been difficult to resolve. Royden et al. (p. 1054; see the News story by Kerr) review the geologic history of the collision, focusing on Tibet, and propose that the history of the plateau is closely tied to tectonics in the Pacific. Before about 20 million years ago, Pacific and Indonesian trenches were migrating away from the Himalayan collision zone, which allowed some accommodation of the excess crustal mass produced by the collision. Once trench migration ceased or slowed, extensive uplift of Tibet commenced.

  3. Circulating More

    CREDIT: PAULUIS ET AL.

    Earth's atmosphere receives more solar energy at low latitudes than at high latitudes, resulting in more heating near the equator than at the poles. Because such a thermal gradient cannot be maintained by the fluid atmosphere, poleward transport by atmospheric circulation redistributes heat to correct the imbalance. The conventional picture is that air rises predominantly in the tropics, within a structure known as a Hadley Cell, and begins its movement toward the poles at high altitudes. That view, however, is predicated on the representation of entropy that ignores the water content of the air. By considering atmospheric moisture, Pauluis et al. (p. 1075) show that considerably more air rises in the mid-latitudes than previously realized. The resulting mass transport by atmospheric circulation is nearly twice the canonical value, with as much as half of the air ascending into the upper troposphere doing so in the mid-latitudes.

  4. Cs2 Unspun

    Rotational and vibrational degrees of freedom have hampered full cooling of molecules required for the formation of Bose Einstein condensates (BECs), which can now be achieved regularly with atomic samples. Danzl et al. (p. 1062, published online 10 July) show that by using a coherent Raman pumping scheme, discrete packets of energy can be removed from loosely bound cesium dimers in an atomic BEC to create tightly bound, translationally cold Cs2 molecules in specific rotational-vibrational levels of the electronic ground state. The experiment effectively removes all rotational excitation, but leaves in vibrational energy that could possibly be dissipated in a second pumping cycle. Key to the technique is preservation of phase coherence between two pumping lasers tuned to substantially different frequencies, achieved by referencing to a frequency comb.

  5. Fine Wire Finesse

    An electron-current can drive atomic diffusion, a process known as electromigration. In very small copper wires, this can lead to the formation of voids or breaks in a wire, which presents a significant problem in modern electronics. Using in situ electron microscopy, Chen et al. (p. 1066) find that the electromigration process can be influenced by the presence of specific micro-structural features. Atomic diffusion is significantly reduced where a twinned grain meets a grain boundary. At this junction, new steps on the crystal need to form for migration to occur, which adds an incubation or nucleation time to the process. It is thus possible that the lifetime of fine copper wires can be improved by engineering crystal grains in copper wires.

  6. Jam the Signal

    Many bacterial pathogens sense that they are in a potential host because they can detect adrenergic molecules. These organisms share a sensor kinase that picks up the signal and relays it to virulence gene loci, thus kicking in the responses needed to ensure bacterial establishment. This pathway makes a good target for broad spectrum antibiotic development because virulence inhibition should not present a strong selective pressure for resistance. Rasko et al. (p. 1078) have had some success with this approach using a non-toxic small molecule inhibitor of the sensor kinase QseC, which blocks signaling in a sensitive and specific manner. In animal models, the inhibitor was somewhat effective against gastrointestinal infections with enterohemorrhagic Escherichia coli and Salmonella typhimurium. Encouragingly, the lead molecule was more effective against the systemic pathogen Francisella tularensis: A single oral dose protected 80% of infected mice from death.

  7. Fixed But Flexible

    To what extent is stochastic noise responsible for phenotypic variability in biological systems? Feinerman et al. (p. 1081) combine computer modeling with measurements of expression levels of multiple proteins required for signaling in populations of T cells, along with the response of the same cells to antigenic stimulation. Together, these revealed how stochastic variation in expression of different components of antigen-induced signaling pathway had distinct effects on the response potential of T cells. For a clone of T cells responding to a given antigen, this could help by optimizing flexibility within the responding cell population, while maintaining the overall uniformity required for reliable antigen discrimination.

  8. The Making of a Centromere

    Centromeres play a critical role during cell division in ensuring that eukaryotic chromosomes are evenly distributed between daughter cells. How and where centromeres form on chromosomes remains something of a mystery, because during evolution new centromeres seem to have arisen independently of the underlying DNA sequence. Ishii et al. (p. 1088) developed a system for generating new centromeres at will in fission yeast, by snipping out the existing centromere from one of the three chromosomes. Survivors of this generally fatal event were found either to form a new centromere at one of the subtelomeric regions of the eviscerated chromosome, or to fuse the aberrant chromosome with another chromosome. The formation of new centromeres required components of the RNA interference machinery.

  9. Sensing Alarm

    CREDIT: BRECHBÜHL ET AL.

    In 1973, Hans Grueneberg observed the presence of a structure at the tip of the rodent nose that, he thought, belonged to the Nervus terminalis. Recently, using transgenic techniques, several groups reported the rediscovery of this structure. They named this structure the Grueneberg ganglion in memory of the original work. However, the function of these cells remains a matter of controversy. Despite the lack of typical olfactory neuronal features, the ganglion was suggested to have some olfactory function, based on the expression of olfactory marker protein and on its neural connections to the olfactory bulb of the brain. Brechbühl et al. (p. 1092) have now identified a function for the Grueneberg ganglion cells. A combination of anatomical, surgical, and behavioral techniques were used to suggest that the Grueneberg ganglion is involved in alarm pheromone detection.

  10. To Divide or Not to Divide

    Cells can stop dividing and remain in a nonproliferative state (like most differentiated cells in an organism), but, under some conditions, the cells can reenter the cell division cycle and begin to proliferate again. Sang et al. (p. 1095) provide evidence that a transcriptional repressor known as Hairy and Enhancer of Split1 (HES1) has an important role in determining whether or not a cell can resume cell division. When proliferation of human fibroblast cells in culture was inhibited, reduction of HES1 activity prevented cells from reinitiating cell division once the block to proliferation was removed. Inappropriate reentry into the cell cycle is a property of tumor cells, and treatments that inhibited Hes1 function in skeletal muscle tumor cells also inhibited proliferation and promoted differentiation.

  11. Deciding the Undecided

    How do our unconscious biases affect rational decision-making, and can the decisions we make affect these biases? Galdi et al. (p. 1100; see the Perspective by Wilson and Bar-Anan) find in a longitudinal study of voting intentions in a population of voters that, as expected, the conscious beliefs of already-decided voters predict the choices that they made when quizzed again at a later date, and that the automatic associations of undecided voters likewise predicted their choices later on. Surprisingly, however, the conscious beliefs of the decided voters, which, at the first question period, showed no correlation with their automatic or implicit preferences, did, in fact, predict their implicit preferences during the second session, suggesting that the interaction between unconscious and conscious cognition is a two-way street.

  12. A Stable Si(0) Dimer

    Many isolatable molecular transition metal compounds, such as iron pentacarbonyl, are stabilized by neutral ligands without being oxidized. This motif occurs much more rarely among the lighter electropositive elements, which tend to give up electrons during coordination to H, N, O, or halide substituents. Wang et al. (p. 1069; see the Perspective by Dyker and Bertrand) show that by using bulky neutral N-heterocyclic carbene ligands (L), they can synthesize a stable Si2 complex in which both silicon centers retain their valence electrons, persisting in the zero oxidation state. The compound's Z-shaped L-Si-Si-L structure was uncovered by x-ray crystallography and its bonding analyzed by density functional theory.

  13. Cobalt Catalyst for Oxygen Evolution

    The widespread implementation of many important energy technologies, including fuel cells and solar energy harvesting, is hampered by the “oxygen electrode problem.” Platinum electrodes can overcome this issue and oxidize water to produce O2 rapidly at low excess driving potential, but platinum's scarcity and cost have spurred a search for alternatives. Kanan and Nocera (p. 1072, published online 31 July) report the in situ formation of an efficient water oxidation catalyst when Co2+ is reduced over an indium tin oxide electrode in the presence of a potassium phosphate buffer. A precipitate formed that has Co: P:K ratios in the range from 2:1:1 to 3:1:1 and is active for water oxidation at neutral pH. An analysis of the pH dependence of the current generation indicated that HPO42− acts as the proton acceptor.

  14. Cell Cycle Regulation by Adenovirus

    Adenovirus small e1a drives human cells into S-phase by displacing the Retinoblastoma (RB) proteins from E2F transcription factors to derepress cell cycle genes, and binding the p300/CBP histone acetyltransferases. It was unclear how the interaction with p300/CBP promotes cell cycling. Horwitz et al. (p. 1084) and Ferrari et al. (p. 1086) now show that the e1a-p300/CBP interaction causes a global reduction in total histone H3 lysine 18 acetylation (H3K18ac). Upon infection, e1a binds to most human promoter regions in a temporally ordered manner, causing relocalization of the RB proteins to repress antiviral responses and differentiation, and p300/CBP and H3K18ac to activate cell cycle genes. These data reveal a defined process of epigenetic reprogramming that leads to cellular transformation and may have relevance to human cancers.

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