This Week in Science

Science  10 Apr 1998:
Vol. 280, Issue 5361, pp. 173
  1. Change in momentum

    In the BCS (Bardeen-Cooper-Schrieffer) theory of superconductivity, the formation of electron pairs lowers the energy of the system and open up a gap energy that is typically only a few millielectron volts (twice the thermal energy kTc at the superconducting transition temperature Tc). Because typical energies of electrons near the top of the conduction band (Fermi energies) are a few electron volts, the mixing of electrons with similar energies is further constrained by a momentum requirement-an electron with momentum k mixes strongly only with electrons of momentum k or -k (moving parallel or antiparallel) because of the low gap energy. Shen et al. (p. 259) obtained angle-resolved photoemission spectra from optimally doped samples of the high-Tc superconductor Bi2Sr2CaCu2O8+δ and found changes in energy at certain momentum of up to 300 millielectron volts, or 40 times kTc, which occurred with a substantial transfer in momentum [on the order of (0.45π, 0)]. This value is very near that required by the theory of Emery and Kivelson that connects microscopic antiphase domains or spin and charge ordering with superconductivity.

  2. Intradot interaction

    Semiconductor quantum dots, such as islands of one semiconductor embedded in another, confine charge carriers in three dimensions. This confinement resembles that of electrons in atoms and results in a discrete energy spectrum. Landin et al. (p. 262; see the commentary by Gammon, p. 225) have studied optical emissions from single indium arsenide dots in gallium arsenide and show that the electrons and holes in the quantum dots interact with each other through Coulomb interactions to produce fine structure in the spectra. Such interactions between charge carriers in quantum dots have been predicted theoretically.

  3. When a gel breaks

    The failure of materials through cracking is a familiar phenomenon. Crystalline materials generally break instantaneously once a well-defined specific force is applied. Bonn et al. (p. 265) show that in polymer gels, delayed fracture can occur, with delays of up to 15 minutes for the materials and the applied forces they studied. They find that the behavior can be modeled when the connectivity of the gel network is taken into account, allowing calculation of the activation energy of crack formation.

  4. Hot on the trail of catalysts

    Many chemical reactions are exothermic, and thus one way to screen catalysts is to measure the relative amount of heat liberated. Taylor and Morken (p. 267) show that thermal imaging can be used to screen active catalysts for reactions run in solution. They demonstrate the method for an acylation reaction; both known catalysts and those created from a library (approximately 3000 different potential catalysts) were supported on resin beads and imaged with an infrared camera (chloroform was used in the solvent so that the beads just float, thus reducing solvent interference with transmission). They show that the most active catalysts are the ones most strongly selected with this assay.

  5. Soft support

    Although materials such as silica have long been used as supports for catalysts, their inherent polarity can make them less than ideal for many highly reactive transition metal catalysts. Roscoe et al. (p. 270) report that noninteracting polymer supports can be used with metallocene polymerization catalysts. Reaction occurs inside catalyst-loaded polystyrene beads in the 50- to 100-micrometer size range to form polyolefin beads roughly 1 millimeter in diameter.

  6. Being rubbed the wrong way

    Atomic force microscopy (AFM) allows the determination of the forces between sample surface and tip when the tip is scanned across the sample. However, the molecular organization of the sample cannot yet be directly determined by this technique and requires the use of additional experimental techniques. Liley et al. (p. 273) have combined electron diffraction and Brewster angle microscopy with AFM to study the friction anisotropy of a lipid monolayer on a mica surface and show that a small molecular tilt results in a measurable friction anisotropy. This anisotropy is counterintuitive: The friction force is smaller when scanning against the “cat fur” direction than stroking with it.

  7. Getting the message across

    In messenger RNA translation, transfer RNAs (tRNAs) with attached amino acids dock into the P (peptidyl) and A (aminoacyl) sites of the ribosome for peptide bond formation. Previous work has identified the 23S ribosomal RNA (rRNA) as the ribosomal component that participates in peptidyl transferase function at the P site. Green et al. (p. 286) now show that a different domain of the 23S rRNA participates at the A site. Cross-linking analysis showed that in order for the A site to form, the P site has to be bound by tRNA, indicating a cooperative interaction between the P and A sites of the ribosome.

  8. Biofilm formation

    Certain bacteria can come together and differentiate to form a complex, multicellular structure called a biofilm. Davies et al. (p. 295; see the commentary by Kolter and Losick, p. 226) have shown that Pseudomonas aeruginosa uses a diffusible, density-dependent signal that is a product of lasI to induce differentiation of the biofilm. Biofilm formation by P. aeruginosa is a medical problem when it occurs in catheters or in the lungs of patients with cystic fibrosis. A mutation that disrupts the cell-to-cell signal made the biofilm sensitive to the detergent SDS. Agents that inhibit this signal could be helpful in preventing biofilms.

  9. Stopping molecular motors

    The molecular motor kinesin is responsible for many aspects of intracellular motility, including the movement of chromosomes along the mitotic spindle. The motor protein moves along intracellular tracks, the microtubules carrying its cargo through the cell. Studies of kinesin function have been hampered, however, because of the lack of specific agents to block its motor activity. So far only nucleotide analogs are known to act as potent inhibitors of motor function, but they lack specificity, which limits their usefulness in complex assay systems. Sakowicz et al. (p. 292) have now discovered a specific kinesin inhibitor in extracts from a marine sponge. The inhibitor appears to act by mimicking the microtubule and blocking motor-microtubule interactions.

  10. Kingdoms united in splicing

    Splicing of transfer RNA precursors (pre-tRNAs) is essential for the production of mature tRNA. In Eucarya and Archaea, this process requires an endonuclease that recognizes the splice sites and releases the intron. Li et al. (p. 279) determined the crystal structure of the tRNA splicing endonuclease from the archaeabacterium Methanococcus jannaschii. Although the eucaryal and archaeal endonucleases are known to recognize their RNA substrates by very different means, the new structural data indicate that the two groups of enzymes share a common cleavage mechanism resembling that of ribonuclease A. Accompanying results from Fabbri et al. (p. 284), who examined endonuclease cleavage of artificial pre-tRNA substrates, support the evolutionary relatedness of the eucaryl and archaeal enzymes.

  11. Locating intestinal T cells

    The T cells of the intestines are thought to develop outside of the thymus, unlike other T cells. Saito et al. (p. 275; see the news story by Williams, p. 198) have found a new primary lymphoid organ in the mouse small intestine, the recently identified “cryptopatches,” and show that this is where the local precursors for intestinal T cells reside, not in the Peyer's patches or the mesenteric lymph nodes.

  12. Stopping transcription

    Increasing evidence has shown that the transcription apparatus is intimately linked with the messenger RNA (mRNA) processing machinery. Although mRNA polyadenylation signals are necessary for transcription termination, the molecular components of the mRNA cleavage-polyadenylation complexes involved in transcription termination remain unknown. By using temperature-sensitive mutants of polypeptides in the mRNA processing complexes, Birse et al. (p. 298) show that in yeast, polypeptides that function in endonucleolytic cleavage of nascent transcripts, but not polyadenylation, are required for efficient polymerase II termination.

  13. Flight control

    The halteres of dipterous flies evolved from hind wings and are believed to provide direct input to the forewing motor circuitry. Although the halteres do not function aerodynamically, their integrity and input is required for stable flight. In an integrated anatomical and physiological study, Chan et al. (p. 289; see the news story by Pennisi, p. 201) demonstrate that visual input directly influences motor control of the halteres and thereby indirectly alters the sensory output of the halteres and how that output regulates the flight musculature.

  14. Hairpin reverse

    The RAG proteins are the core enzymes responsible for the DNA rearrangements that comprise the making of an active antigen receptor gene. RAGs can cleave the DNA at recombination signal sequences to make blunt or “hairpin” ends. Melek et al. (p. 301) now show that the RAGs can also reverse the hairpin reaction with a mechanism reminiscent of the “dis-integration” reaction of retroviral integrases, to rejoin the DNA. This information explains how certain DNA by-products of reaction are formed and may provide a basis for understanding the organization of the antigen receptor loci.

  15. Linkage Disequilibrium Mapping and Parkinson's Disease

    M. H. Polymeropoulos et al. (Reports, 27 June 1997, p. 2045) identified a mutation “in the α-synuclein gene, which codes for a presynaptic protein thought to be involved in neuronal plasticity, in an Italian kindred and in three unrelated families of Greek origin” with inherited Parkinson's disease. Other investigators, however, have not found evidence of this mutation in their Parkinson's disease study populations (Letters: The French Parkinson's Disease Genetics Study Group, 20 Feb. 1998, p. 1116; T. Lynch et al., 14 Nov., p. 1212; and 5 Dec. 1997, p. 1696; Technical Comments: W. K. Scott et al., 18 July 1997, p. 387 and T. Gasser et al., p. 388).

    B. Rannala and M. Slatkin show that “the recently developed theory of LD [linkage disequilibrium] mapping can be used to quantitatively assess” whether the α-synuclein mutation is indeed causative or if it is “a neutral variant in linkage disequilibrium with some other, causative, mutation.” Rannala and Slatkin apply a method that “accounts for several demographic factors that may influence levels of disequilibrium, including population growth, genealogical associations, and sampling effects.” They conclude from their analysis that, if the mutation identified by Polymeropoulos et al. is not the causative one, then it is probably very close to it on the genome.

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