This Week in Science

Science  24 Feb 2006:
Vol. 311, Issue 5764, pp. 1068
  1. Dissecting Stratospheric Temperature Trends


    Since 1980, the lower stratosphere has cooled significantly. This cooling trend has been ascribed to the influence of anthropogenic effects—mainly stratospheric ozone depletion and the buildup of greenhouse gases. However, this process occurred in two major steps. Ramaswamy et al. (p. 1138) investigated the temporal structure of the trend using simulations with a climate model, in order to delineate the roles of natural and anthropogenic forcings. Although the overall downward trend in temperature is the result of anthropogenic factors, natural forcing by changes in solar irradiance and volcanic aerosols have superimposed on the gradual longer term decrease the shorter time-scale structure recorded in the observations. Thus, while anthropogenic factors are responsible for the 25-year-long stratospheric cooling trend, the steps were caused by natural forcing.

  2. Early Aquatic Mammal

    Mesozoic mammals have been thought to have been small, nocturnal, and confined to a few niches on land until the demise of the dinosaurs 65 million years ago. Most are recorded by isolated jaw fragments or teeth. Ji et al. (p. 1123; see the cover and the Perspective by Martin) now describe a Jurassic mammal from China that breaks this mold. The fossil is well preserved, and impressions of fur can be seen on its body and scales on a broad tail (similar to a beaver overall). The animal was fairly large, approaching not quite half a meter in length, and the shape of its limbs suggest that it was adapted for swimming and burrowing. The combination of both primitive and derived features in this early mammal, and the demonstration that mammals had occupied aquatic habitats by this time, expands the evolutionary innovations of early mammals.

  3. Segregating Old and New Chromatids


    During chromosome replication, paired chromatids ultimately separate during cell division to become individual chromosomes in daughter cells. Although one might expect segregation of chromatids (with old versus newly synthesized strands) to daughter cells to be random, some studies have suggested that nonrandom segregation can occur. Armakolas and Klar (p. 1146) looked for evidence of chromosome-specific nonrandom strand segregation in various cell types. After mitotic recombination, mouse chromosome 7 shows random segregation in cardiomyocytes, pancreatic, and mesoderm cells, whereas nonrandom segregation is seen in embryonic stem cells, endoderm cells, and neuroectoderm cells. These segregation patterns may be important for developmental decisions and have implications for imprinting and inheritance.

  4. Wind Up

    Recent evidence suggests that short-duration gamma-ray bursts are produced by fast mergers of compact objects, such as double-neutron stars and neutron-star, black-hole binaries. However, lingering x-ray emissions seen hundreds of seconds after some bright gamma-ray bursts are still a problem for this model because simulations predict the merger should happen in seconds. Dai et al. (p. 1127) suggest that the merger process proceeds less catastrophically, producing a differentially rotating millisecond pulsar rather than a final black hole. Because the pulsar's layers spin at different rates, its magnetic fields become wound up and release energy sporadically through reconnection-driven explosive x-ray flares.

  5. Red and Blue

    Globular star clusters in elliptical galaxies come in two colors, red or blue. Many astronomers have assumed the colors reflected age differences, such that blue clusters formed more recently than red ones, and implying two epochs of globular cluster formation during the growth history of elliptical galaxies. Yoon et al. (p. 1129, published online 19 January; see the Perspective by Freeman), however, show that a single coeval population of globular clusters can exhibit color bimodality due to a nonlinear relationship between color and metallicity in stars. Galactic spectral models that include treatment of horizontal branch stars can reproduce the color distributions even with stars of similar age, removing the need for multiple populations of globular clusters.

  6. Finding the Path for Quantum Computing

    Quantum computers hold great promises for solving difficult problems otherwise intractable on classical computers. However, actually finding algorithms, or the quantum circuitry on which the algorithms can be implemented, is challenging because the number of components in the quantum circuits should grow only polynomially with the complexity of the problem you want to solve. While manipulation of a single qubit can be thought of as the rotation of a unit vector in a sphere, a quantum computer will typically have n interacting qubits, giving rise to a 2n-dimensional space, Thus Nielsen et al. (p. 1133; see the Perspective by Oppenheim) recast the problem of finding an efficient quantum algorithm in terms determining the shortest path between two points in a certain curved, or Riemannian, geometry. The mathematical tools of Riemannian geometry can then be used to provide an understanding of quantum computation and a possible route to determine efficient quantum algorithms.

  7. Bacteria Have Social Lives Too

    Quorum sensing provides a mechanism for bacteria to monitor one another's presence and to modulate gene expression in response to changes in population density. Camilli and Bassler (p. 1113) review how the synchronous response of bacterial populations to small molecule autoinducers that is involved in quorum sensing confers social behavior to bacteria. Autoinducers are packaged in a variety of ways and have varying half-lives, depending on their roles. Autoinducer signals are integrated within each cell by second-messenger systems, probably by cdiGMP signaling.

  8. Positive and Negative Transcription Regulators

    The Drosophila Polycomb group (PcG) and Trithorax group (trxG) of epigenetic regulators maintain, respectively, either repressed or active chromosomal transcriptional states. They act via the same dual-function chromosomal elements to exert their effects. Transcription through these elements switches them from silent Polycomb response elements (PREs) to active Trithorax response elements (TREs). Sanchez-Elsner et al. (p. 1118) show that noncoding RNAs generated by PRE/TRE transcription in the ultrabithorax (Ubx) locus function to recruit the histone methyltransferase Ash1, an activator of Ubx expression. Ash1 interacts specifically with the chromatin-associated TRE noncoding RNAs. Although TRE noncoding RNAs are retained at Ubx TREs, possibly through RNA-DNA interactions, they can also act in trans to recruit Ash1 to their counterpart TREs and activate Ubx transcription.

  9. Timely Demise and Immune Control


    Apoptosis, or programmed cell death, is a fundamental means by which the immune system regulates itself. Autoimmunity develops when components of the cell death machinery, such as the cell surface receptor Fas and its ligand, are mutated or absent. Generally, this change is considered to be due to direct defects in lymphocytes, leading to their aberrant activation and proliferation. However, Chen et al. (p. 1160; see the news story by Marx) challenge this assumption by revealing that correctly regulated cell death of another central immune cell—the dendritic cell (DC)—is also required to maintain immune control. To prevent apoptosis, a transgene encoding a caspase inhibitor was targeted to DCs in mice, resulting in the accumulation of these cells; both in their resting state, as well as in situations of antigen-priming. As a consequence, T cells in these animals became chronically activated and dysregulated, leading to telltale signs of autoimmunity.

  10. DNA Damage-Transcription Links

    Damage to DNA in cells (like that produced by some anticancer drugs) is sensed by the cell and causes cellular responses that determine whether a cell lives or dies. Wu et al. (p. 1141; see the Perspective by Bartek and Jiri) provide a new link by which this signal can be conveyed from the nucleus to the cytoplasm. The protein kinase ataxia telangiectasia mutated (ATM) is activated in response to DNA damage and directly phosphorylates NEMO, one of the proteins in the IκB kinase (IKK) complex that regulates the activity of the transcription factor NF-κB. NF-κB in turn mediates signals that promote cell survival. After DNA damage, ATM was exported from the nucleus and then interacted in the cytoplasm with another protein in the IKK complex, ELKS. Activated IKK then caused activation of NF-κB-dependent transcription.

  11. Searching for a Damaged Needle in a DNA Haystack

    How does a DNA repair enzyme find a deleterious base lesion within a huge excess of normal base pairs? Banerjee et al. (p. 1153) show that a bacterial DNA glycosylase can examine an intact DNA helix, and does not need to extrude damaged base pairs. Instead, a conserved phenylalanine residue inserts into the helical stack and causes buckling at the intercalation site. The probe residue senses a deformed base within the intact helix and allows for base extrusion events only at damaged sites.

  12. Long-Term Planktonic Coupling

    Plankton bacteria primarily subsist on phytoplankton production, and this is clearly seen in successions following phytoplankton blooms. In the ocean, phytoplankton and bacterioplankton live short lives (days) and interact over short distances (micrometers). In general, it has proved difficult to document such trophic coupling in terms of a correlation between chlorophyll contents of the water and bacterial abundance. Li et al. (p. 1157) analyzed serial observations of chlorophyll and bacterioplankton at several coastal and open ocean stations during the past decade and find that these microbial groups do change in tandem, increasing or decreasing together over many years.

  13. Silencing the Supernumerary X

    In mammals, females have two X chromosomes and males have only one. Potentially lethal X-chromosome gene imbalance is prevented by dosage compensation, where one of the two X chromosomes in female cells is inactivated (Xi) while the other remains active (Xa). Silencing of the “supernumerary X” chromosome in females is regulated by the X-inactivation center (Xic) on the X chromosome. The mutually exclusive nature of the Xa/Xi choice suggests that there must be some form of communication between the two X chromosomes. Xu et al. (p. 1149, published online 19 January; see the Perspective by Carrel) now show that in mouse cells undergoing dosage compensation and X inactivation, such communication takes the form of a transient physical interaction between the Xic of the two X chromosomes. Placing additional copies of the Xic on autosomes induces ectopic X-autosome interactions which interfere with the normal process of X-chromosome inactivation.

  14. Ionospheric Response

    The particles and magnetic fields that make up the solar wind buffet planetary ionospheres. Mendillo et al. (p. 1135) have recorded the instantaneous change in the ionosphere of Mars by catching a solar x-ray flare as it hit both Earth's ionosphere and then Mars's. The flare, which was spotted in data taken in 2001, was caught by radio instruments on the Mars Global Surveyor spacecraft in orbit around Mars and by terrestrial ionosonde stations and x-ray satellites. This tandem measurement allowed a direct comparison of the response of the planets' ionospheres to the perturbing event.