This Week in Science

Science  20 May 2005:
Vol. 308, Issue 5725, pp. 1084
  1. Splashy Surface Electrons


    Many electron transfer processes occur at metal oxide surfaces, and water can play a key role by providing local trap states that open up lower energy pathways for reactions. Onda et al. (p. 1154) studied the (110) surface of titanium oxide at various levels of hydration, both with two-photon photoemission studies and density functional calculations. They find evidence for an excited electronic state on partially hydroxylated surfaces that is 2.4 electron volts above the Fermi level. The calculations indicate that the electrons' environments resemble those of electrons in water clusters, rather than those for electrons on water-covered metal surfaces. This “wet-electron” state relaxes back into the conduction band on time scales less than 15 femtoseconds.

  2. Mapping the Human Transcriptome

    Our understanding of the human genome is continually being improved and we are only now beginning to understand the complexity of the human transcriptome. Cheng et al. (p. 1149, published online 24 March 2005) used high-density oligonucleotide arrays to map the sites of transcription for 30% of the human genome (encoded on 10 chromosomes). The distribution of polyadenylated (polyA+) and nonpolyadenylated (polyA−) RNAs varied within the cell nucleus and cytosol. A much higher percentage of the genome is transcribed either as polyA-, polyA+, or bimorphic (found as polyA- and polyA+) sequences than had been assumed. For example, in the HepG2 cell line, up to 15% of the genome is transcribed. Many of the transcripts identified have not been annotated, and come from the sense and antisense strands or are overlapping. These findings further point out the complexity of the human transcriptome.

  3. Marked Influence on Stem Cells

    Replication-defective retroviral vectors are often used to mark and track stem cell progeny without, it has been assumed, influencing the regulation of the stem cells or conferring any selective advantage or disadvantage. Kustikova et al. (p. 1171) examined the insertion sites present in dominant and long-term repopulating mouse hematopoietic stem cells. They observed a pronounced competitive inequality after insertional deregulation of randomly hit alleles. The genes in question each have recognized roles in the self-renewal, or survival, of hematopoietic stem cells. The findings have implications for clinical gene therapy, and suggest a possible need to revise conclusions generated by gene-marking studies.

  4. Interfering with RNA Interference

    RNA interference (RNAi) is central to a number of natural RNA-based silencing processes and is becoming a common tool used in a wide range of studies in eukaryotes. It is also being explored for its therapeutic potential. Kim et al. (p. 1164, published online 24 March 2005) carried out a genome-wide screen in Caenorhabditis elegans for components of the RNAi pathway using RNAi. Although apparently a “circular” methodology, the screen identified 90 viable and lethal genes involved in RNAi, most of which were not previously linked with the process. Classes of factors include RNA binding and processing factors, chromatin-associated factors, and nuclear import and export factors. The screen also provides insight into the degree of overlap between different RNAi-based silencing pathways.

  5. Catch the Monkey

    Discoveries of new species of mammal are increasingly rare, and discoveries of new primates even more so. Jones et al. (p. 1161; see the news story by Beckman) report the almost simultaneous discovery of two populations of a new species of African monkey in the highlands of southern Tanzania. The new species, named the highland mangabey, is believed to number only a few hundred individuals. Its discovery underscores the importance of the montane woodlands of Tanzania as a conservation focus for primates.

  6. Collagen's Cerebral Side


    Porencephaly is a rare brain disorder that typically is manifested in newborns and that is characterized by degenerative cavities in the cerebral cortex. Gould et al. (p. 1167) characterized mutant mice with phenotypic features reminiscent of human porencephaly. Half of the mutant mice died of cerebral hemorrhage within 1 day of birth, and the surviving pups showed focal disruptions in the vascular basement membrane that was accompanied by porencephaly in a subset of the animals. The causative mutation mapped to the gene encoding procollagen type IV α 1. The mutation led to the inhibition of collagen secretion into the basement membrane. Mutations in the same gene were subsequently identified in two families with inherited forms of porencephaly and cerebral hemorrhage. These results raise the possibility that mutations compromising vascular integrity may increase susceptibility to more common disorders, such as stroke.

  7. Putting Quantum Dots into Cavities

    Cavity quantum-electrodynamics (QED) experiments have been a key tool in understanding and controlling the dynamics of single quantum systems. Although there are advantages, both practical and basic, in carrying out cavity-QED experiments with solid-state emitters, experimental realization has been difficult to achieve. Badolato et al. (p. 1158; see the Perspective by Krauss) present a technique for deterministically coupling an excitation level in a single quantum dot to a single mode of an optical cavity. In their three-step process, they first identify the quantum dot of interest and characterize its excitation spectrum. Next, using the dot itself as a registration marker, they fabricate a two-dimensional photonic crystal cavity that is specially designed with the quantum dot's excitation spectrum in mind, and then place it in a near-optimal position relative to the quantum dot. Finally, they optimize the coupling between the dot and photonic crystal cavity by a series of etch-steps that fine-tune the physical dimensions of the photonic crystal. The observed strong coupling between the quantum dot and the cavity should put the system in the regime for probing cavity-QED in a solid-state environment.

  8. Stress Response, Aging, and Cancer Predisposition


    The activity of FOXO transcription factors is associated with increased life span. Essers et al. (p. 1181; see the Perspective by Bowerman) find that in both Caenorhabditis elegans and mammalian cells, FOXO and β-catenin are associated in a protein complex. In C. elegans, β-catenin promotes the transcriptional activity of FOXO in response to oxidative stress. β-catenin mediates developmental effects of the wingless or Wnt pathway and is implicated in promoting excess cell proliferation in certain cancers. β-catenin's stimulation of FOXO, on the other hand, inhibits progression through the cell cycle. Thus, a critical balance between β-catenin signaling through FOXO or other transcription factors regulated by the Wnt pathway may influence stress responses, aging, and disposition to cancer.

  9. Membrane Engineering

    The intracellular pathogen Salmonella enterica resides in a vacuole from which it translocates effector proteins into the host cell. These bacterial effectors manipulate eukaryotic functions. SifA is a key Salmonella effector protein, and sifA_ mutants are highly attenuated in virulence in mice. Boucrot et al. (p. 1174) now describe how Salmonella uses secreted effectors to negatively regulate the binding of the microtubule-associated kinesin motor onto the bacterial vacuole. SifA targets a host protein, SKIP, that down-regulates the recruitment of kinesin. In this manner, Salmonella controls the kinesin activity associated with its vacuole membrane and, in turn, the dynamics of membrane exchange.

  10. Not Lost in Translation

    The ribosome uses kinetic proofreading and induced-fit mechanisms to ensure the fidelity of the translation reaction. Cochella and Green (p. 1178; see the Perspective by Daviter et al.) analyzed a mutant transfer RNA (tRNA) molecule that promotes mis-incorporation of amino acids. It appears that the tRNA molecule in itself can transmit structural information from the codon:anticodon decoding center to other regions of the ribosome that promote guanosine triphosphate hydrolysis and accommodation of the tRNA in the ribosome acceptor site. Thus, tRNA is more than a passive player in the translation reaction.