This Week in Science

Science  05 Jul 2002:
Vol. 297, Issue 5578, pp. 9
  1. In Brevia

    A technique developed by Park et al. (p. 65) allows refractory oxides, such as zirconium oxide, to be grown as crystalline thin films at low temperatures (∼130°C) and on plastic substrates.

  2. Subsurface Martian Water

    Since February 2002, the Gamma-Ray Spectrometer, Neutron Spectrometer, and High Energy Neutron Detector on board the 2001 Mars Odyssey mission have been used to map the flux of gamma rays and secondary neutrons, which are attributed to hydrogen concentrations in the top meter of the martian surface. Results from just 30 to 60 days of data presented by Feldman et al. (p. 75; see the cover), Mitrofanov et al. (p. 78), and Boynton et al. (p. 81) indicate that, at high latitudes (>60°) near both poles, a putative water ice-rich layer exists about 30 to 60 centimeters below the surface (see the Perspective by Bell). This layer, which is tens of centimeters in thickness, may contain 20 to 35% by weight of water ice, and this ice would fill in most of the estimated porous regolith at this depth. Nearer the equator, the depth of this layer increases to about 100 centimeters, and the amount of possible water ice in the layer decreases. Thus, Mars may have collected and concentrated a significant amount of water just below the surface at both poles. The presence of water is consistent with a wetter past, seasonally modulated water transport, current gully formation by subsurface water-ice erosion, and the potential for a higher concentration of water than expected beneath the martian surface.

  3. RecA Resists for Nanowiring

    One approach to wiring together nanometer-scale electronics relies on using DNA as a template—the ends of a long DNA molecule are designed to hybridize to components bearing complementary strands, and the DNA is then metallized. Keren et al. (p. 72; see the Perspective by Niemeyer) show that the major protein responsible for homologous recombination in Escherichia coli, RecA, can be used to pattern metallized DNA. RecA was polymerized on single-stranded DNA molecules, which were then hydridized to double-stranded DNA substrate molecules. RecA could be used either lithographically to create nonmetallized gaps in the DNA, or to perform recombination reactions that created branched networks.

  4. More from Dmanisi

    Studies at Dmanisi, Georgia, have shown that early Homo migrated out of Africa by 1.75 million years ago. Vekua et al. (p. 85; see news story by Balter and Gibbons) now report a new hominid skull and mandible from this site. Together, these fossils represent the largest collection of individuals from any one site older than about 800,000 years ago. The specimens may even have been from a common group. Regardless, their diversity may provide a view of the variation in morphology of early hominid species. This newly discovered hominid has a relatively small brain, implying that evolution in brain size may not have determined when migration out of Africa occurred.

  5. Getting to the Root of It All

    Despite decades of phylogenetic research, the position of the root of the eukaryotic tree has remained controversial. Stechmann and Cavalier-Smith (p. 89) have used a gene fusion that is almost certainly derived and compared it to the ancestral bacterial condition to show that the root must lie below the divergence point between opisthokonts (animals, fungi, and Choanozoa) and biciliated eukaryotes (which include plants, chromists, and almost all protozoa). This new evidence is congruent with a simple interpretation of the evolution of the eukaryotic cytoskeleton and a basic divergence in mitochondrial structure, and is the most substantial advance to date in clarifying the position of the root and the nature of the first eukaryote.

  6. An On/Off Switch for Oncogenes

    Among the many new strategies being developed for cancer therapy are drugs that inhibit the function of oncogenes that are involved in tumorigenesis. Such drugs might be toxic if used chronically, yet they might be ineffective in the short term because of possible reactivation of the oncogene once treatment is stopped. To investigate the latter possibility, Jain et al. (p. 102; see the Perspective by Weinstein) used a sophisticated mouse genetic model to examine what happens to MYC-induced tumors when the MYC oncogene is briefly inactivated and subsequently reactivated. Surprisingly, they found that transient MYC inactivation leads to permanent loss of the neoplastic phenotype. When MYC was removed, osteogenic sarcoma cells differentiated into bone cells; re-expression of MYC did not restore the cells' tumorigenic potential but, rather, caused the cells to undergo apoptosis. These results suggest that brief inactivation of an oncogene may permanently change the epigenetic context of a tumor cell so that it cannot revert to its original malignant behavior.

  7. Quantum Dot Molecules

    Quantum dots, like molecules, have discrete energy levels that can be tuned by varying their size and composition. Holleitner et al. (p. 70) have controlled the coupling between two quantum dots with a technique based on cotunneling of electrons through each of the dots. They then probe the development of energy spectra analogous to the formation of molecular levels.

  8. Block That Catch?

    Fisheries management is based mainly on population dynamics, but in response to predation, it is conceivable that populations might evolve rapidly, which could lead to diminished yields. Conover and Munch (p. 94) harvested large, small, or randomly sized members of an exploited fish, the Atlantic silverside (Menidia menidia), from captive populations over four generations. Harvesting large individuals initially increased overall catch but quickly led to lower yields, as slow-growing phenotypes began to dominate. The authors suggest that creating no-take reserves or enforcing a maximum size limit may preserve the genetic variability in wild populations.

  9. Second-Sourcing Thymidylate

    It might be assumed that mechanisms for the biosynthesis of essential DNA precursors would be conserved in all organisms. However, Myllykallio et al. (p. 105) show that a large class of proteins that do not have sequence similarity with the classic thymidylate synthase (ThyA) synthesizes thymidylate by a different mechanism. These enzymes (ThyX) are mainly limited to microbial genomes that lack ThyA and are an important drug target as they occur in several human pathogens. In a Perspective, Murzin comments on a rare confluence of events in which the functional characterization is complemented by the recent structural characterization of ThyX by the Joint Center for Structural Genomics.

  10. Compensating for Nonsense

    Mutations that introduce premature termination codons (nonsense) can affect alternative splicing of transcripts by disrupting exonic splicing enhancers. Wang et al. (p. 108) have examined a T cell receptor (TCR) gene that acquires nonsense codons at high frequency during normal T cell development. In contrast to previous explanations, they show that nonsense-associated altered splicing (NAS) is enhancer independent. Instead, it appears that nonsense codons up-regulate alternatively spliced messenger RNA by a translation-like mechanism.

  11. Sticking with It

    Specification of cell lineage requires early decisions that, in some cases, may be reversible. The transcription factor Pax5, is required for entry into the B cell lineage via the repression of non-B cell genes and establishment of the B cell-specific gene profile. After conditionally inactivating the Pax5 gene, Mikkola et al. (p. 110) observed that the developmental program of committed early B cells could be reset. As a result, other myeloid and lymphoid lineages could be derived from these cells in vivo and in vitro. This type of persistent control over early lineage preservation may prove an important general principal in upholding cell fate decisions.

  12. Taking Degradation to Heart

    The ubiquitin pathway of protein degradation has gained prominence as a major regulatory network in mammalian cells. Arginine is commonly conjugated to the amino-terminus of proteins during ubiquitin-dependent degradation, but the physiological functions of arginylation are unknown. Kwon et al. (p. 96) generated mice genetically deficient in one of the Arg-tRNA-protein transferases catalyzing this modification (ATE-1) and found that the embryos die from defects in heart development and angiogenesis. The authors also discover a possible mechanism for the early cardiac defects: Amino-terminal cysteine is oxidized prior to its arginylation by ATE-1, suggesting that the amino-terminal arginylation may function as an oxygen sensor.

  13. Breaking the Mold

    Drosophila protect themselves against bacterial and fungal infection by secreting anti-microbial peptides from the fat body, the insect equivalent of the liver. One mechanism required for initiating this in response to Gram-positive bacteria and fungi is provided by the Toll pathway, which is activated via binding of a proteolytically cleaved form of its ligand Spaetzle. Using mutagenesis screening, Ligoxygakis et al. (p. 114) identified mutants for a serine protease gene named persephone, which suppressed the Toll- dependent antifungal response. Unlike other activators of the Toll pathway, persephone has no inherent microbial recognition domain, which suggests that it depends on an upstream fungal receptor to initiate its cleavage of Spaetzle.

  14. Chain Gangs in Polymers

    When light interacts with a material, heating often occurs, and this causes localized movement away from the hot spot. Conversely, an optical trap can be set up when focused laser light interacts with small particles, pinning them at their current location in solution. Sigel et al. (p. 67) have observed a new optical effect where the laser light causes an increased concentration of polymer chains in a solution, enabling them to temporarily “write” dot and line patterns. Although not fully understood, several criteria have been identified for this writing to occur; the polymers need to have double bonds, chains that are long enough to entangle, and a refractive index higher than the solvent.

  15. Adusting to Ups and Downs

    Photosynthesis is regulated so that chemical output does not exceed the rate of consumption of the cell; excess output, as could occur during high intensity light conditions, can cause photo-oxidative damage. Külheim et al. (p. 91) have studied the adaptive significance of nonphotochemical quenching, which helps to dissipate excess solar input, in Arabidopsis and show that this process is important when adjusting to rapid changes in light intensity, as opposed to steady, high-intensity radiation.

  16. Immune Deficiency in Fish

    The development of reverse genetics approaches has greatly aided the study of gene function in several model organisms, such as Drosophila, Caenorhabditis elegans, and mice. However, methods have been inefficient or lacking for another vertebrate model, zebrafish. Wienholds et al. (p. 99) use a target-selected mutagenesis approach to produce point mutations in a gene whose sequence is known. They examined the role of rag1, a gene required for V(D)J recombination in the assembly of immunoglobulins, and show that mutant fish failed to produce V(D)J rearrangements in lymphocytes. Hence, this work not only provides a large-scale reverse genetics method for the analysis of zebrafish genes, but also demonstrates that zebrafish can be used in studies of the immune system.