Editors' Choice

Science  30 Jul 2004:
Vol. 305, Issue 5684, pp. 575

    Acquiring a Tan

    Forty years ago, a neuropeptide called bursicon was shown to trigger the hardening and darkening of the new exoskeleton that is exposed when an insect molts. Now, Dewey et al. are finally able to describe the gene that encodes bursicon. Using a partial amino acid sequence of bursicon that had been purified from cockroaches, the authors searched the Drosophila melanogaster genome and located a candidate gene, for which homologs could be identified in the honeybee and mosquito genomes. The gene encodes a 15-kD protein that appears structurally similar to cystine knot proteins, a group of proteins that contain intramolecular disulfide linkages and includes molecules involved in vertebrate development, such as transforming growth factor-β and platelet-derived growth factor. Bursicon was detected in Drosophila thoracic and abdominal neurons, and mutant flies not only were defective for sclerotization but also exhibited wing expansion abnormalities, pointing to an additional yet-to be-described function of the hormone during development. — LDC

    Curr. Biol . 14, 1208 (2004).


    Chromosome Structure

    RNA interference (RNAi) has been linked with heterochromatin formation, gene silencing, and chromosome segregation in fission yeast. Transcripts from the outer centromere repeats of the yeast chromosomes are chopped up by the RNAi machinery, forming small interfering (si)RNAs, which are required for the formation of heterochromatin over the outer repeats. In RNAi mutants, heterochromatin fails to form over the repeats, perturbing the binding of cohesin to the centromeres. This affects sister chromatid cohesion in mitosis and hence prevents the proper segregation of the chromosomes during cell division.

    Does RNAi have a similar role in vertebrates? Fukagawa et al. have addressed this question by making a conditional knockout of Dicer, the enzyme that cleaves double-stranded RNAs to generate siRNAs, in a chicken cell line that also contains a single copy of human chromosome 21. Cells lacking Dicer, and therefore RNAi, showed transcription of both strands of centromeric repeat DNA, in a manner somewhat reminiscent of fission yeast, as well as the possible presence of repeat-derived small RNAs in wild-type cells. Furthermore, in Dicer-deficient cells, binding of HP1 proteins, central components of heterochromatin, was perturbed at the centromeres. Finally, these cells showed defective sister chromatid cohesion of human chromosome 21, as well as of the chicken chromosomes, and substantial mis-segregation of the human chromosome, suggesting that RNAi may indeed play a role in heterochromatin formation at centromeres in vertebrates. — GR

    Nature Cell Biol. 10.1038/ncb1155 (2004).


    Sushi to Go

    The only ancient corpse to have been recovered from the ice of a North American glacier has provided a glimpse into human habits and diets of 500 to 600 years ago. Dickson et al. have pored over the gut contents of Kwäday Dän Ts'ìnchí, whose body was preserved with its intestinal tract intact in a glacier in the St. Elias Mountains in British Columbia. Although Kwäday died some 80 km inland, his gut contents suggested a largely coastal diet of intertidal saltmarsh plant material, a marine crustacean, chum salmon, and eggs of fish tapeworm. Stable isotope analysis of muscle and bone confirmed that his diet was largely of marine origin. The sequence of gut contents, as well as the plant remains on Kwäday's clothes, reveal that he journeyed inland in the days before his death, carrying with him a supply of seafood. The purpose of the journey, and how he met his untimely death on the glacier, are likely to remain a mystery. — AMS

    Holocene 14, 481 (2004).


    Golden Glow

    The performance of organic display materials in a variety of applications can benefit from the inclusion of luminescent compounds that have long emission lifetimes at visible wavelengths, and metal complexes containing small clusters of gold atoms can display these optical properties. Wang et al. now report on a set of isostructural complexes containing a tetrahedral core of three gold atoms and one silver atom; these complexes allow the relation of structure to emission to be explored in more detail.

    In these compounds, the tripod of gold atoms is capped on one side by the silver atom and on the other side by an oxygen, sulfur, or selenium atom. The three gold-silver edges are bridged by diphenylphosphine-2-pyridine ligands. X-ray crystallography reveals that dimers form in the solid state through an aurophilic bridge between the exposed gold-gold edges. The emission is blue for the oxygen capping ligand, yellow or green for sulfur (which was also prepared as the diphenylphosphine-4-methylpyridine complex), and red for selenium. The authors assign the emission, based on the relative energy of the lone pair orbitals of these capping ligands, to a ligand-to-metal-metal charge-transfer process. — PDS

    J. Am. Chem. Soc.10.1021/ja048091d (2004).


    The End of Molecular Biology?

    During the past five decades, beginning with the structure of DNA and ending with the sequence of the human genome, understanding biology has signified, for many, understanding the structures and functions of molecules. Woese looks back at the past half-century, and even further, in order to look ahead to a revisionist biology that strives to understand the evolutionary emergence of complexity. In some quarters, of course, the notion that multivalent and nonlinear interactions guide how organisms function and evolve has already begun to convert molecular prospectors into systems analysts. Copious quantities of genomic data are revitalizing old debates about the division, if there is a single one, between eucaryote and prokaryote, and about how thoroughly horizontal gene transfer might have muddied what we see around us today, even in as basic a process as translation of the genetic code. Might there have been, in a pre-Darwinian era, thriving cellular communities with a bustling marketplace where molecules and genes were freely traded, and, if so, how did bacteria, archaea, and everything else arise? These are some of Woese's questions for the new biologists. — GJC

    Microbiol. Mol. Biol. Rev. 68, 173 (2004).


    Binding Be Brightly

    The production of beryllium (Be) materials, such as alloys and components in neutron moderators, requires constant environmental monitoring because Be is highly toxic. Detection is currently achieved with large instruments, either by atomic emission or mass spectrometry, and hence portable analytics and biological studies depend on the development of selective fluorescent chelating ligands.

    Unlike Mg or Ca, Be exists in aqueous solution as polynuclear complexes, such as Be3(OH)3. Keizer et al. explored using two bridging ligands to bind this polynuclear motif, specifically 2-hydroxyisophthalic acid and 2,3-dihydroxybenzoic acid (DHPA). The addition of Be at a pH of 7 causes a sharp increase in the intensity of fluorescence emission for DHPA, even in a background that contains aluminum and iron, both of which can interfere strongly with indicators that bind Be as a mononuclear species. A 10 μM solution of DHBA can be used to detect 50 nM Be (4.5 pg per ml). — PDS

    J. Am. Chem. Soc.10.1021/ja047637t (2004).


    Buckle Up

    Thin films made from soft materials such as polymers are increasingly finding use as coatings, filters, and lithographic resists. Although nanoindentation can be used to measure the mechanical properties of thin films of hard materials, such as metals or ceramics, it is far less accurate for soft viscous materials, and it tends to tell you more about the underlying substrate than the film itself.

    Stafford et al. have developed a technique for the rapid and quantitative measure of the elastic modulus of a thin stiff film by placing it on a thick softer substrate and then gently applying stress. This causes the film to buckle, with the resulting periodicity and amplitude dependent on the material properties of both the film and the substrate. The buckling pattern is measured as a diffraction pattern from a scattered laser beam. The authors validated their method using polystyrene films on a polydimethylsiloxane substrate, which included films with varying thickness across their length. They also showed that the method could be applied to more technically challenging porous organosilicate films, which are difficult to analyze but of considerable interest as low-k dielectric materials. — MSL

    Nature Mater. 10.1038/nmat1175 (2004).