This Week in Science

Science  04 Nov 2005:
Vol. 310, Issue 5749, pp. 741
  1. Close Up of the Ribosome


    In the last few years, high-resolution structures of the 30S and 50S bacterial ribosome subunits have revealed significant insights into the mechanism of protein synthesis, in particular revealing that the ribosome is a ribozyme—some of the constituent RNAs, rather than proteins, catalyze key reactions. Schuwirth et al. (p. 827; see the Perspective by Moore) describe two structures of the intact Escherichia coli ribosome at 3.5 angstrom resolution. The structures show details of the interaction interface and peptidyl transferase center and reveal molecular motions that are likely to be involved in messenger RNA and transfer RNA translocation.

  2. More Prolific Planet Production

    As the center of a collapsing dust cloud heats up and approaches the conditions for star formation, the dust grains settle into a plane to form a disk around the central core. The particles in this disk eventually combine to form planets and asteroids. Although this picture is consistent with observational data for the brighter dust disks of intermediate mass stars, little is known about planet formation around smaller stars. Apai et al. (p. 834, published online 20 October 2005) present infrared spectroscopic observations of protoplanetary disks around brown dwarfs, objects that are smaller, cooler, and often just short of being stars. The spectra show signs of three key markers for planet formation—dust grain growth, grain crystallization, and grain settling. This kind of planet-forming process may be more widespread than once thought, and thus brown dwarfs should be candidates for future searches for planets outside our solar system.

  3. A Wetter Upper Troposphere

    If increased levels of carbon dioxide (CO2) were the only cause of global warming, we could expect the worldwide average surface temperature to increase by about 1°C during this century. However, climate models project warming roughly three times that much because of feedback effects from water vapor. Up until now, experimental evidence that upper tropospheric water vapor content has actually been increasing has been lacking. Soden et al. (p. 841, published online 6 October 2005; see the Perspective by Cess) have used satellite observations to reveal a distinct radiative signature of increasing upper tropospheric moisture from 1982 to 2004. This moistening is consistent with model reconstructions for the same period.

  4. A Quintuple Bond

    Covalent bonding, or electron sharing between atoms, is the basis of molecular chemistry. In principle, two transition metal atoms can share up to 12 electrons before repulsion starts to push them apart, rather than keeping them together. In practice, however, the quadruple bond of eight shared electrons has been the highest stable interaction in isolated compounds; beyond this number, clusters tend to form instead. Nguyen et al. (p. 844, published online 22 September 2005; see the Perspective by Frenking) have used bulky triphenyl ligands to stabilize a chromium dimer in which one more pair of electrons is shared. Although x-ray crystallography reveals a bent geometry, theory and magnetic measurements support participation of all of the metal d orbitals in the bond.

  5. Intermittent Ionosphere Layer

    Mars's ionosphere, which extends from about 110 to 135 kilometers (km) above the planet, consists of two distinct layers and helps protect the lower atmosphere from removal by the solar wind. The existence of a third lower layer has been predicted, and as discussed by Pätzold et al. (p. 837), has now been detected by radio-wave observations by Mars Express. However, this third layer, which is seen to extend to as low as 65 km above the planet, appears to be intermittent, not permanent as was expected, and is likely formed from the ablation of meteorites.

  6. Decay Discrepancy Reconciled

    The beta decay of 176Lu to 176Hf is an important isotopic system for tracing the geochemical evolution of Earth and other planets, as these elements are fractionated by the formation of continental crust. Application, and particularly comparison to other decay systems requires accurate knowledge of the decay constant (or half-life. However, comparison of the systematics in meteorites and terrestrial rocks have yielded two different values, a discrepancy that has even raised suggestions that other energetic processes might be affecting decay constants. Amelin (p. 839) selected specific samples from meteorites that also have accurate uranium-lead dates and shows that the half-life in meteorites is the same as that in other rocks.

  7. Breakdown to Recovery


    Regulation of immune responses through local catabolic depletion of tryptophan (Trp) was first identified in studies of the maternal T cell response to the fetus. This pathway, which is controlled by the enzyme indoleamine 2, 3-dioxygenase (IDO), has since been identified in a variety of immunological settings. Platten et al. (p. 850) now find that IDO-mediated Trp catabolism also contributes during therapy of a mouse model of multiple sclerosis. By using a form of antigen, termed an altered peptide ligand, T cell responses were prevented from causing inflammation and nervous system pathology, and this effect corresponded with the induction of IDO. Naturally occurring metabolites and a synthetic derivative of the IDO pathway inhibited T cell proliferation and activation of antigen-presenting cells. Remarkably, paralyzed mice recovered after being fed the synthetic derivative.

  8. A Gut Oxidase Reaction

    The mucosal surfaces of the body are constantly exposed to microorganisms and have evolved a variety of protective innate immune mechanisms, including the generation of antimicrobial peptides throughout the animal kingdom. Another hallmark of the innate immune response is the generation of microbicidal reactive oxygen species (ROS) by phagocytes. Ha et al. (p. 847) observe that gut mucosal epithelial cells of Drosophila expressed dual oxidase (dDuox) upon bacterial infection. Flies in which dDuox expression had been silenced were significantly more susceptible to infection, and protection could be restored upon dDuox reexpression. Similar mucosal ROS production mechanisms may be exploited in host defense across different species.

  9. Gliding Motility Factors on the Move

    Little is known about the directional determinants of so-called gliding motility in bacteria. Mignot et al. (p. 855) now show that FrzS, a protein essential for pilus-based gliding motility in Myxococcus xanthus, moves in an oscillatory pattern by disassembling and reassembling clusters at the cell poles as cells reverse their direction of movement. The frequency of the oscillations is controlled by the Frz chemosensory system, which is essential for directed motility. Pole-to-pole migration of FrzS appears to involve directed movement along the length of the cell.

  10. From Post- to Presynaptic Sites


    Postsynaptic Ca2+ signals are somehow transduced into alterations in presynaptic function during enhanced synaptic activity. Yoshihara et al. (p. 858) show that in the Drosophila embryonic neuromuscular junction, miniature endplate potential induction by presynaptic stimulation is blocked by postsynaptic Ca2+ chelation or genetic ablation of synaptotagmin 4 (Syt 4). This blockage can be rescued by postsynaptically targeted “knockin” of Syt 4. Similarly, the reduced amplitude of endplate potentials in Syt 4-deficient mutant can be rescued by postsynaptic rescue of Syt 4. Postsynaptic Syt 4 thus up-regulates transmitter release.

  11. Object Recognition in a Flash

    Humans and other primates have the astonishing ability to recognize and categorize objects within 200 milliseconds. By using a classifier-based decoding approach, Hung et al. (p. 863) characterized the neuronal representation for object recognition in monkey inferotemporal cortex (area IT) and quantitatively examined the underlying neural code. Surprisingly, the activity of small numbers of neurons over very short periods of time was sufficient to support rapid and accurate recognition of object category and identity, which was at the same time invariant to large changes in object position and scale.

  12. Wringing the Neck

    The susceptibility of individual synapses to plasticity induction may be influenced by the ability of signaling molecules to move into and out of the head of the dendritic spine. Thus, regulation of protein movement by the spine neck offers a potentially powerful mechanism to control individual synapses. Bloodgood and Sabatini (p. 866) found that diffusional equilibration across the necks of dendritic spines is directly regulated by activity. By combining two-photon microscopy with two-photon laser photoactivation, protein movement was measured across the necks of a large population of spines. A subclass of spines was effectively isolated from the dendrite. Spine compartments have traditionally been treated as static entities, but it now appears that spine-dendrite coupling is strongly dynamic. Diffusion barriers vary considerably over time in a way that reflects the cell's recent history of spiking.

  13. Differentiating the Male Germ Line

    The TATA-binding protein (TBP) is a conserved transcription factor that binds to the core promoter, and TBP-associated factors, or TAFs, represent one of several classes of coactivators that participate in transcription activation. The presence of varied TAFs allows for plasticity in function for transcribing specific genes. For example, several testis-specific TAFs function in regulating gene expression in Drosophila spermatogenesis. Chen et al. (p. 869) now find that in the male germ line, tissue-specific TAFs regulate gene expression by counteracting the repressive effect of Polycomb protein complexes to allow terminal differentiation. The testis TAFs sequester Polycomb-containing complexes to the nucleolus, which suggests that subnuclear localization functions in regulating transcription.