This Week in Science

Science  31 Jan 2003:
Vol. 299, Issue 5607, pp. 621
  1. Charging Against Organelles

    Plastids are vital organelles for the malaria parasite Plasmodium falciparum. Plasmid transit proteins, which mediate protein targeting to these organelles, are potential therapeutic targets, but little has been known about these proteins. Foth et al. (p. 705) found signature charge properties of amino acids near the amino terminus as well as Hsp70 binding sites that enabled them to devise an algorithm to identify target molecules for apicoplasts. Site-directed mutagenesis that altered specific charge characteristics in a model transit peptide severely disrupted organellar targeting in vivo.

  2. Staying in Sync

    In parallel processing, computational tasks are assigned to many computers at once. However, a difficulty is that when several computers are working independently, one processor can get well ahead of the others on its part of the problem. As a result, progress tends to diverge or spread greatly as the number of processors used increases. Korniss et al. (p. 677, see the Perspective by Kirkpatrick) apply recent notions of a small-world network to this problem, and show how just intermittently synchronizing a few processors at random allows the processors to learn the global state of the system and adjust their progress to allow convergence.

  3. Some Hits Are Cooler

    One way to harden a metal surface in order to improve its wear resistance is to form a nitride coating. However, the nitriding process is slow and requires elevated temperatures. Tong et al. (p. 686) found that they could reduce the processing temperature for an iron surface from 500° to 300°C by repeatedly hitting it with stainless steel balls. The mechanical attrition of the iron not only causes it nanocrystallize, but also creates a large number of defects. These defects store sufficient energy to allow the nitriding to proceed, which lowers the barrier to reaction and allows it to proceed at lower temperatures.

  4. Spin Selection Through a Quantum Dot

    The ability to control the spin state of carriers in a semiconductor is one of the key requirements for a viable “spintronic” technology. Although some success has been achieved in injecting spin-polarized electrons into a semiconductor from a ferromagnetic source, the spin state is fixed and can only be changed by reorienting the magnetic field. Folk et al. (p. 679) describe an electron-spin filter, based on the bias-dependent transparency of a quantum dot, that preserves quantum phase coherence, is highly selective, and can have its polarity reversed by simply changing a gate voltage.

  5. Zeroing In on Single Molecules

    The use of conventional fluorescence to detect single molecules normally requires dilute conditions (nanomolar to picomolar conditions). Levene et al. (p. 682, see the cover and the Perspective by Laurence and Weiss) introduce an optical waveguiding method that decreases the observation volume and thus allows the micromolar concentrations more typical of biological conditions to be used. Their zero-mode waveguides consist of pinholes in a metal film over a glass slide that form the wells for the solution. No propagating electromagnetic modes exist in this configuration, so only molecules at the glass surface can adsorb radiation. The activity of immobilized DNA polymerase with fluorescently tagged nucleotides was followed with microsecond resolution.

  6. Ocean of Drought

    A severe drought occurred from 1998 to 2002 across a wide area of the middle latitudes of the Northern Hemisphere, including the United States, the Mediterranean, southern Europe, and Southwest and Central Asia. This drought did not bring unusually warm land temperatures but did cause large deficits in precipitation. Hoerling and Kumar (p. 691, see the news story by Kerr) found that this drought was the result of an unprecedented pattern of tropical sea surface temperature forcing. A component of this oceanic state was consistent with greenhouse gas increases.

  7. X-ray Lightning

    Lightning is initiated by a leader phase, in which channels of ionization are created between the cloud and the ground, and the return stroke phase, in which a large current flows from the ground upward. Large electric fields can be generated in front of the downward propagating leaders, and preliminary observations have indicated that some types of leaders may produce energetic radiation in natural lightning. Dwyer et al. (p. 694, see the Perspective by Krider) report the discovery of intense bursts of x-ray emission from rocket-triggered lightning. The most likely source of these x-rays is the runaway breakdown of air, a process in which electrons are accelerated to relativistic energies by strong electric fields.

  8. X-ing Out Male Expression?

    There is considerable interest in the origin and selection in the sex chromosomes of animals. Because males display one X, one might expect greater selection that is to the male advantage. However, two-thirds of the × chromosomes in a population are contained in females, raising the opposite possibility. Parisi et al. (p. 697, see the Perspective by Schlötterer) now examine global gene expression in the fruit fly, Drosophila, to determine how genes with sex-differential gene expression profiles are distributed among the chromosomes. This analysis shows that more than 30% of the genome is differentially expressed between males and females and most of the sex-biased expression is found in the gonads. In Drosophila males, fewer X-linked genes are expressed in a sex-biased manner, and the few X-linked genes that show male-biased expression are generally not conserved with the mosquito genome. These results suggest that the × chromosome has experienced a “net demasculinization” effect.

  9. Double-Tracked Metabolic Pathways

    Organisms may use two different enzymes to catalyze the same reaction in order to more easily regulate forward and backward fluxes in a metabolic pathway. For example, in Escherichia coli, fumarate reductase (QFR) converts fumarate to succinate under anerobic conditions, and succinate dehydrogenase (SQR) aerobically oxidizes succinate to fumarate. Yankovskaya et al. (p. 700, see the Perspective by Hederstedt) present the 2.6-angstrom structure of SQR. They propose that the presence of O2 switches the pathway because SQR is less likely to yield reactive oxygen species as unwanted by-products of respiration. Genetic disorders in humans that lead to tumors (paraganglioma) and in nematodes that lead to premature aging (mev-1) map to the ubiquinone binding site in SQR.

  10. Japanese Warmth

    Two climate events recorded in ice Greenland cores, the Bölling warming about 14,700 years ago and the Younger Dryas cold period beginning about 12,900 years ago, are seen at other locations but with less precise temporal signatures. Nakagawa et al. (p. 688) present a climate reconstruction for part of the last glacial termination from Japan based on varve-counting and high-precision 14C dating of sediments from Lake Suigetsu. Their results suggest that a Bölling-like warming occurred earlier there than in the North Atlantic region, but that an event like the Younger Dryas began later.

  11. Two Means to a Bad End

    Gastrointestinal stromal tumors (GISTs) have recently attracted much attention because they are exceptionally sensitive to the new cancer drug Gleevec, which inhibits the oncogenic KIT receptor tyrosine kinase that is aberrantly expressed in many of these tumors. Heinrich et al. (p. 708) have now examined GISTs with normal KIT function and find that these tumors harbor activating mutations in a gene that encodes a related tyrosine kinase, platelet-derived growth factor receptor a (PDGFRA). Tumors with mutations in KIT or PDGFRA have similar cytogenetic profiles and display activation of the same downstream signaling pathways. These results reinforce the notion that mutations in distinct genes can activate common cellular pathways that drive cancer development.

  12. Interfering with Amyloids

    The formation of amyloid fibrils is key to the pathology of a variety of diseases including transthyretin amyloidosis, which is responsible for systemic senile amyloidosis, a late-onset cardiac disease. Hammarström et al. (p. 713) have designed small molecules that interfere with the formation of amyloids by transthretin in vitro that have potential for treatment of amyloidosis in patients.

  13. Heterochromatin Open Prison

    Heterochromatin, where genes and other sequences are “locked away,” has been thought to be a hyperstable form of chromatin, but recent results have suggested that many of the proteins that contribute to the architecture of the nucleus are in a very dynamic state. Festenstein et al. (p. 719) and Cheutin et al. (p. 721) used photobleaching to investigate the dynamics of heterochromatin protein 1 (HP1), a principal and very tightly bound component of heterochromatin, within living cells. Contrary to expectations, HP1 in heterochromatin exchanged rapidly with the pool of unbound HP1 in the nucleus. Silenced genes may have the capacity to be rapidly released from heterochromatin and reactivated on receipt of the appropriate cue.

  14. Neurodegeneration of a Different Color

    Animals with prion diseases display a sponge-like brain pathology that is characterized by a progressive loss of neurons and an accumulation of intracytoplasmic vacuoles. He et al. (p. 710) show that mice carrying a null mutation in the mahoganoid coat color gene develop an age-dependent spongiform neuropathology with features strikingly similar to those seen in prion diseases. The mahoganoid gene encoded a RING domain protein with E3 ubiquitin ligase activity in vitro. These results underscore the importance of ubiquitin-mediated protein turnover in neurodegeneration and raise the possibility that the ubiquitination pathway may be disrupted in prion diseases.

  15. Chromatin and the Argonautes

    RNA interference (RNAi), which lies at the heart of a number of RNA-based silencing systems, regulates the formation of specific regions of heterochromatin in the fission yeast Saccharomyces pombe, and proteins of the ARGONAUTE family are known to play a central role in these RNA-based silencing systems. Zilberman et al. (p. 716) now show that the ARGONAUTE4 gene in the higher plant Arabidopsis is critical for the formation of 25 nucleotide small interfering RNAs (intermediates in RNAi), as well as histone methylation and DNA methylation at specific loci.