This Week in Science

Science  03 Aug 2001:
Vol. 293, Issue 5531, pp. 757
  1. Atomic Josephson Junctions

    When two boson condensates are separated by a thin barrier, their phases can couple through wave function overlap and give rise to periodic tunneling. This Josephson Effect, long known in superconductors and more recently in superfluids, has now been demonstrated in a readily tunable Bose-Einstein condensate (BEC). Cataliotti et al. (p. 843) trapped a BEC in an array of optical potential wells so that the barrier height between wells could be controlled. A collective oscillatory motion of the atoms was observed between wells that is a signature of the Josephson effect.

    CREDIT: CATALIOTTI ET AL.
  2. Oxidizing Earth

    The Earth's early environment was relatively poor in oxygen, and observations and modeling indicate that the rise in oxygen levels did not occur until long after oxygen-enriching bacterial photosynthesis had developed. Catling et al. (p. 839; see the Perspective by Kasting) explain the apparent disconnect between oxygenation and the start of photosynthesis by adding methanogenesis to the process. Photosynthesis split water into O2 and H2, and then methanogenesis used H2 to generate CH4. Finally, ultraviolet photolysis converted CH4 and O2 into CO2, O2, and H2. The escape of H2 into space allowed the buildup of O2.

  3. Sticklers About the Time

    High-precision atomic clocks keep time by counting the number of transitions between the hyperfine levels of atomic cesium. Higher frequency standards in the optical range would provide finer time intervals (femtoseconds versus roughly tenths of nanoseconds) that would not only provide greater precision for time metrology but could also help constrain important physical constants, such as the fine structure constant and the Rydberg constant. Building on recent technological advances in generating optical-frequency combs, measuring absolute optical frequencies, and trapping and cooling single ions to extremely low temperatures, Diddams et al. (p. 825) developed an optical clock based on an ultraviolet transition of a single trapped mercury ion. They demonstrate time precision better than that of existing atomic-clock standards.

  4. The Lowdown on Noses

    Nostrils and noses, and more generally the role of smell and extent of air exchange, play a large role in the behavior of many vertebrates. Understanding the role that smell played in dinosaur behavior has been problematic because of the lack of direct fossil evidence for these fleshy structures. Witmer (p. 850; see the cover and the news story by Stokstad) has made comparisons of nostril positions in living animals to conclude that dinosaur noses were not located to the rear of the bony nasal opening and high on the face, as has been generally thought, but low on the face just above the upper jaw. This position now seems to apply generally across reptiles, birds, and mammals.

  5. Ironed Out

    Iron is an essential element in regulating marine phytoplankton growth, and even where iron is present, its bioavailability depends on whether it exists in dissolved form, which is easy for algae to utilize, or as a colloid, which is much less readily used. Wu et al. (p. 847) used a new analytical technique to measure vertical profiles of iron speciation in oligotrophic waters of the North Atlantic and central North Pacific. Much of the iron in these waters (up to 90%) that was previously believed to be dissolved is actually colloidal. The limitation of nitrogen fixation by iron is apparently much more important than had been realized previously.

  6. Death by Deprivation

    Many hematopoietic cells die by apoptosis when deprived of specific cytokines, and this process requires active transcription. Using DNA microarrays to identify specific genes that are transcriptionally activated after cytokine withdrawal, Devireddy et al. (p. 829) discovered an important new player in this cell death pathway. In a study of interleukin-3 (IL-3)-dependent murine FL5.12 pro-B cells, they find that the gene showing maximal transcriptional activation upon IL-3 withdrawal is 24p3. The 24p3 gene encodes a small secreted protein with sequence motifs characteristic of lipocalins, a large family of proteins previously linked to diverse functions ranging from retinol transport to prostaglandin synthesis to modulation of the immune response. Addition of 24p3 to the culture medium induced apoptosis of a wide variety of leukocytes, and apoptotic sensitivity appeared to be determined by the presence or absence of a cell surface receptor for 24p3.

  7. Dicing Developmental RNA

    Small noncoding RNAs, such as the ∼21 nucleotide (nt) let-7 RNA, regulate the developmental timing of the nematode Caenorhabditis elegans. These RNAs are highly conserved in many bilaterally symmetrical organisms, including humans. The similarity in size between the let-7 RNA and those generated during RNA interference (RNAi), in which RNA blocks gene expression, has led to the suggestion that the two may be linked. Hutvágner et al. (p. 834; see the Perspective by Ambros) now show that the enzyme Dicer, which is implicated as the ribonuclease involved in the generation of the small 21- to 23-nt interfering RNAs in RNAi, is also responsible for cleaving the putative ∼70-nt let-7 precursor to generate the let-7 RNA.

  8. A Matter of Timing

    Covalent modification of the amino-terminal tails of the core histones (H2A, H2B, H3, and H4), the proteins that make up the bulk of chromatin in eukaryotes, play an important role in modulating chromatin function. For example, acetylation of histone tails plays a role in transcription, and phosphorylation has been implicated in chromosome condensation. The role of histone methylation has been less well characterized. Wang et al. (p. 853) now show that the enzyme PRMT1 (a protein arginine methyltransferase) methylates the Arg3 residue in H4 tails in vivo. Methylation of Arg3 enhances subsequent acetylation at lysine residues 5, 8, 12, and/or 16 by the histone acetyl-transferase p300, but prior acetylation of these residues inhibits methylation, which suggests that the different modifications are coordinated temporally. Furthermore, H4 methylation facilitates the activation of transcription, providing further evidence for a histone “code.”

  9. News from the Resistance

    The commercialization of transgenic plants that express toxins from Bacillus thuringiensis (Bt) has spurred a number of groups to search for the genes responsible for resistance (see the news story by Stokstad). Gahan et al. (p. 857) combined genetic mapping in a laboratory-generated resistant strain with knowledge of Bt-binding proteins in other species and identified a member of the cadherin superfamily, HevCaLP, as a candidate resistance gene in the tobacco budworm. Griffitts et al. (p. 860) have harnessed the power of genetic analysis in the model organism Caenorhabditis elegans to show that lack of expression of a gene encoding a potential β-1,3-galactosyltransferase in the intestine resulted in resistance to the Cry5B toxin.

  10. A Repeated Theme

    The causative genetic defect of myotonic dystrophy (DM1) was identified in 1992 as an expansion of a CTG repeat in the 3′ untranslated region of the dystrophia myotonica-protein kinase gene (DMPK) on chromosome 19q. Subsequent genetic testing revealed the existence of a clinically similar but genetically distinct form of DM, DM2, linked to chromosome 3q. Liquori et al. (p. 864; see the Perspective by Tapscott and Thornton) now show that DM2 is also caused by expansion of a repeat sequence in a noncoding region of a gene, specifically, a CCTG repeat in intron 1 of the zinc finger protein 9 gene. This discovery provides compelling support for the hypothesis that repeat expansions expressed only at the RNA level can be pathogenic.

  11. Neocortical Building Blocks

    Is there precise connectivity in the highly complex neocortex, or do neurons make random contacts with each other? Kozloski et al. (p. 868) addressed this problem by applying a new method for identifying connected neuronal pairs. They observed extreme specificity among local synaptic circuits in layer V of the primary visual cortex. Cells strongly activated by corticotectal pyramidal neurons fell into a small subset of the many cell types found in this cortical layer, and the relative position of these targeted neurons appeared determined in different animals. The authors suggest that these stereotyped cortical microcircuits are controlled by early events during neocortical development.

    CREDIT: KOZLOSKI ET AL.
  12. The Appearance of Drug Resistance

    STI-571 is a new cancer drug that has shown remarkable efficacy in patients who are in the early stages of chronic myeloid leukemia (CML). The drug inhibits the Abl kinase, which is constitutively activated in CML patients because they carry a characteristic chromosomal translocation that fuses the ABL gene with the unrelated BCR gene. Patients in whom CML has progressed to a stage called blast crisis respond to STI-571 initially, but then develop resistance. Gorre et al. (p. 876; see the 22 June news story by Marx) studied nine patients who had relapsed after STI-571 treatment and found that all nine showed reactivation of the Bcr-Abl signaling pathway. Six patients had acquired the same amino acid substitution in the Abl kinase domain—a change predicted to alter interaction of the kinase with the drug—and the other three showed BCR-ABL gene amplification. Identification of the mechanisms responsible for STI-571 resistance may facilitate the design of next-generation drugs for CML.

  13. Guiding Interneuron Migration

    Projection neurons migrate radially from their place of birth deep in the center of the brain to find their final position in the growing brain. Interneurons, however, while similarly born in the deep portions of the brain, migrate tangentially to find their final homes. Marín et al. (p. 872) now elucidate some of the signals that guide migrating interneurons along their pathway. The secreted signal semaphorins, and their receptors, neuropilins, already known for their ability to guide the course of growing axons in the spinal cord, are instrumental in guiding the path over which these interneurons migrate.

  14. Piecing Together the Clock in Plants

    The circadian clocks translate cyclic changes in molecular function into daily cycles of physiology that anticipate and coordinate with environmental cues. Alabadi et al. (p. 880) now delve into the molecular rhythms of the plant circadian clock and define the molecular partners. A pair of transcriptions factors, LHY and CCA1, begin the morning, and throughout the day gradually turn over the reins to the evening gene, TOC1. The reciprocating balance between these various genes establishes the basis of the circadian cycle in Arabidopsis.

  15. Memory Consolidation and NMDA Receptors

    Reporting on experiments that used a third-generation selective knockout technique, Shimizu et al. (Reports, 10 November 2000, p. 1170) argued that memory consolidation in mice depends on reactivation of N-methyl-D-aspartate (NMDA) receptors in the hippocampal CA1 region after initial learning. Day and Morris, in a comment, find “several reasons to be cautious” about the study's claims, and summarize results of pharmacological experiments in rats that found no effect of “NMDA receptor blockade on memory consolidation.” Those results, in their view, suggest that memory consolidation hinges on “posttraining neural activity in the hippocampus, but not synaptic plasticity,” as suggested by the synaptic reentry reinforcement (SRR) model of Shimizu et al. In a response, Shimizu et al. assert that several “important technical issues lead us to question the accuracy” of the results cited in the comment, and that Day and Morris “may have misunderstood some aspects of our SRR model.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/293/5531/755a.

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