This Week in Science

Science  04 Apr 1997:
Vol. 276, Issue 5309, pp. 9
  1. Structural clues to cis and trans integrase function

    Site-specific recombinase enzymes catalyze the exchange of DNA strands. One puzzle that this family of enzymes has presented is that, during the formation of the complex of two DNA strands and four recombinase monomers, some integrases act on the phosphate nearest them (action in cis), whereas others act on the phosphate across from them (action in trans). The crystal structure of the catalytic core of λ -integrase reported by Kwon et al. (p. 126) reveals a conservative fold and shows that the attacking nucleophile, tyrosine-342, is located on a flexible loop about 20 angstroms across from a basic groove that contains the other catalytic residues. In a Perspective, Jayaram (p. 49) discusses modeling results which show how different loop lengths could explain why different integrases adopt cis or trans modes of attack on the phosphates.

  2. Abnormal matter

    How old are the oldest stars? Visser (p. 88) uses the energy conditions derived from classical general relativity, the currently accepted value for the Hubble parameter (the rate at which the expansion velocity changes with distance in the universe), and observations of old stars to attempt to answer this question. He concludes that no possible combination of normal matter is compatible with the range of accepted values of the Hubble parameter. Either large amounts of abnormal matter or fundamental changes in the theory that describes the early structure of the universe may be required.

  3. Hot, wavy exospheres

    The Voyager missions determined that the exospheres of the giant outer planets were hotter than could be explained by any solar mechanism, and the thermal profile of the upper atmosphere derived from Galileo probe data confirmed the presence of a hot jovian exosphere. After further refinements to the probe data, Seiff et al. (p. 102) show that wavelike oscillations are persistent in the jovian thermal profile above the tropopause. Two reports provide explanations for the waves and high temperatures in the jovian exosphere that could also be applied to the other giant planets, for which there are fewer direct observations. Waite et al. (p. 104) suggest the equatorial x-ray emissions observed by the ROSAT satellite are caused by heavy ion precipitation, which would also provide a heating mechanism. Young et al. (p. 108) suggest that the observed probe oscillations are consistent with gravity waves and that the viscous dissipation of these waves would produce excess heat.

  4. Aerosols and smog

    Organic molecules from gasoline vapor have an important influence on the formation of organic aerosols, an essential factor in urban smog but, to date, detailed information on the aerosol-forming potential of organic mixtures has been limited. Odum et al. (p. 96; see the news story by Kaiser, p. 33) irradiated smog chambers containing different mixtures with sunlight and analyzed their aerosol-forming potential. The sum of the contributions from the aromatic fraction of the gasoline vapor can account for the aerosol-forming potential. These results may facilitate modeling and help in understanding the factors contributing to urban smog.

  5. Deep ocean mixing

    The amount and pattern of mixing in the deep ocean affects recycling of chemicals and nutrients and heat flow, but data detailing these processes have been sparse. Polzin et al. (p. 93) report results of a large survey to examine deep mixing in the Brazil Basin of the South Atlantic Ocean. Mixing is greatly enhanced near bathymetric features, such as the Mid-Atlantic Ridge, but lower above the smooth abyssal plains.

  6. Alkane metathesis

    Metathesis reactions, which involve the exchange of chemical groups between reactants, are well known for alkenes and alkynes. Vidal et al. (p. 99) show that metathesis reactions can also occur for the much less reactive alkanes. They used transition metal hydride catalysts to transform linear and branched alkanes into lower and higher alkanes under mild conditions. Such reactions may be used to transform the relatively abundant light alkanes into higher molecular weight hydrocarbons.

  7. Hot on the TRAIL

    Some cytokines, such as tumor necrosis factor (TNF), promote apoptosis or programmed cell death. TRAIL, a member of the TNF family that causes death of transformed cell lines but seems to be expressed without toxicity in many normal human tissues, has attracted attention for possible use in cancer therapy. Pan et al. (p. 111) now report the identification of the receptor for TRAIL. Although it shares some similarities with other members of the TNF receptor family, it appears to promote cell death through a distinct mechanism.

  8. Biological scales

    Several scaling laws are known that are widely applicable in biology; for example, rates of energy use, cellular metabolism, heartbeat, embryonic growth, population growth, and life-span all exhibit power-law scaling with body mass. However, the physical origin of these relations has been poorly understood. West et al. (p. 122; see the news story by Williams, p. 34) have developed descriptions of biological processes that may provide a general model for allometric scaling laws. They apply it specifically to vertebrate cardiovascular systems, but it is anticipated to have relevance to a wide range of other systems. The model is based on a fractal network of branching tubes and makes the central assumption that energy use is minimized

  9. Preserved minerals

    The common form of SiO2 in the crust is quartz but at very high pressures it transforms into coesite. Small inclusions of coesite have been found in metamorphic rocks now exposed at the surface and indicate that these rocks were once buried to depths of 100 kilometers or more (see the Perspective by Liou et al., p. 48). Darling et al. (p. 91) now describe the occurrence of cristobolite, a low-pressure high-temperature form of SiO2, in otherwise high-pressure metamorphic rocks. This mineral may have formed from trapped melt and its preservation seems to require negligible influx of water.

  10. Regulated up and down

    Insect metamorphosis, regulated by the hormone ecdysone, depends on a series of developmental events that follow a carefully regulated progression through time. White et al. (p. 114) analyzed the molecular events that form the basis of this regulation. The hormone-induced orphan nuclear receptors and the genes they regulate form an intricate mesh of interactions in which the receptor is sometimes a positive regulator, sometimes a negative regulator. The end result is orderly and a finely tuned expression of a sequential cascade of genes.

  11. Pheromone pathways

    In response to mating pheromones, yeast cells undergo major structural reorganization of the cytoskeleton. Signals from the pheromone receptors are transmitted through members of the Rho family of small guanosine triphosphatases (GTPases), including Cdc42p. However, the proteins regulated by Cdc42p have been unknown. Evangelista et al. (p. 118) identified one possible target for Cdc42p, a protein called Bni1p. Bni1p was found to interact with Cdc42p, the key structural protein actin, and two other actin-associated proteins. Such protein complexes, which are located at the tips of mating projections, may help link the signaling pathway activated by pheromones to changes in the actin cytoskeleton.

  12. Transmembrane NMR structure

    Nuclear magnetic resonance (NMR) methods have been used by MacKenzie et al. (p. 131) to determine the structure of the transmembrane domain of the protein glycophorin A. This 40-residue peptide was solubilized in aqueous detergent micelles. The monomers form α helices despite the large number of amino acids with a low propensity for helix formation, and the interface between the monomers is stabilized not by hydrogen bonding but by van der Waals interactions.

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