This Week in Science

Science  19 Oct 2012:
Vol. 338, Issue 6105, pp. 301
  1. Black Hole Close-Up


    M87 is a giant elliptical galaxy about 55 million light-years away. Accretion of matter onto its central massive black hole is thought to power its relativistic jet. To probe structures on scales similar to that of the black hole's event horizon, Doeleman et al. (p. 355, published online 27 September) observed the relativistic jet in M87 at a wavelength of 1.3 mm using the Event Horizon Telescope, a special purpose, very-long-baseline interferometry array consisting of four radio telescopes located in Arizona, California, and Hawaii. The analysis suggests that the accretion disk that powers the jet orbits in the same direction as the spin of the black hole.

  2. All Change

    Research on early warning signals for critical transitions in complex systems such as ecosystems, climate, and global finance systems recently has been gathering pace. At the same time, studies on complex networks are starting to reveal which architecture may cause systems to be vulnerable to systemic collapse. Scheffer et al. (p. 344) review how previously isolated lines of work can be connected, conclude that many critical transitions (such as escape from the poverty trap) can have positive outcomes, and highlight how the new approaches to sensing fragility can help to detect both risks and opportunities for desired change.

  3. Transcription Around the Clock

    The biological clock that controls daily rhythms in mammalian physiology and behavior is thought to be regulated in large part by transcriptional events (see the Perspective by Doherty and Kay). Koike et al. (p. 349; published online 30 August) produced a comprehensive analysis of these transcriptional events across the entire mouse liver genome over a 24-hour period. Only ∼22% of cycling messenger RNA transcripts were driven by de novo transcription, suggesting that posttranscriptional events also play an important regulatory role in the mammalian clock. Biological timing in organisms can also respond to rhythmic cues from the environment. Morf et al. (p. 379, published online 23 August) explored how one such cue, cycles in ambient temperature, influence circadian timing in mammalian cells. Cold-inducible RNA–binding protein (CIRP) accumulates when body temperature is low. A systematic search for binding partners of CIRP identified RNA encoding core components of the circadian clock. Loss of CIRP decreased the amplitude of circadian gene expression and cells lacking CIRP adapted more quickly to temperature cycles.

  4. Beyond Quantum Dots


    Semiconducting colloidal nanoparticles—quantum dots—are of interest for their unusual properties. One current challenge is the controlled assembly of colloidal particles into larger structures, such as two-dimensional lattices on a substrate, or three-dimensional superparticles. Wang et al. (p. 358) present a two-step self-assembly of CdSe/CdS semiconductor nanorods to form mesoscopic colloidal superparticles. The particles show well-defined super-crystalline domains with dimensions ranging from hundreds of nanometers to several microns, and with the particle morphology controlled by the number of constituent rods. Films of the needle-shaped superparticles were able to act as polarizing light-emitting diodes.

  5. A Twist of Light

    The angular momentum of photons can be used to encode and transmit information. Cai et al. (p. 363) developed a method for generating and emitting controllable orbital angular momentum states of light from a reconfigurable and scalable silicon photonic chip. Using micro-ring resonators embedded with angular gratings allowed the imprinting of optical angular momentum on the light propagating in the whispering gallery modes of the resonator. The method may enable large-scale integration of optical vortex emitters on complementary metal-oxide–semiconductor-compatible silicon chips.

  6. Too-Hot Times

    Climate warming has been invoked as a factor contributing to widespread extinction events, acting as a trigger or amplifier for more proximal causes, such as marine anoxia. Sun et al. (p. 366; see the Perspective by Bottjer) present evidence that exceptionally high temperatures themselves may have caused some extinctions during the end-Permian. A rapid temperature rise coincided with a general absence of ichthyofauna in equatorial regions, as well as an absence of many species of marine mammals and calcareous algae, consistent with thermal influences on the marine low latitudes. Sea surface temperatures approached 40°C, which suggests that land temperatures likely fluctuated to even higher values that suppressed terrestrial equatorial plant and animal abundance during most of the Early Triassic.

  7. Dating Carbon

    Radiocarbon dating is the best way to determine the age of samples that contain carbon and that are younger than ∼50,000 years, the limit of precision for the method. There are several factors that complicate such age determinations, however, some of the most important of which include variability of the 14C production in the atmosphere (which affects organic samples whose radiocarbon inventories are derived from atmospheric CO2), surface ocean reservoir effects (which affect marine samples that acquire their radiocarbon signatures from seawater), and variable dead carbon fraction effects (which affect speleothems that derive their carbon from groundwaters). Bronk Ramsey et al. (p. 370; see the Perspective by Reimer) avoid the need to make such assumptions, reporting the 14C results of sediments from Lake Suigetsu, Japan. Analysis of terrestrial plant macrofossils in annually layered datable sediments yielded a direct record of atmospheric radiocarbon for the entire measurable interval up to 52.8 thousand years ago.

  8. African Origins


    Humans originated in Africa and then spread across the globe. The high genetic diversity found in sub-Saharan Africa is consistent with this view, but the relationships among and within African populations have been less well explored. Schlebusch et al. (p. 374, published online 20 September) genotyped 220 individuals from 11 populations representing groups from Southern Africa to determine their relationships and history. The data suggest that modern-day human populations arose from a complex blend of mixing between groups and genetic stratification.

  9. Made and Modified

    The polytheonamides are 48-residue toxins derived from marine sponges that include 18 D-amino acids, as well as many other unusual amino acid modifications. Given the complexity, one might guess that these peptides are the product of nonribosomal, peptide synthetase (NRPS). However, Freeman et al. (p. 387, published online 13 September now show that polytheonamides are produced by a bacterial symbiont using a ribosomal pathway. Six candidate enzymes for the 48 posttranslational modifications were identified and three were functionally validated. Such ribosomal systems could be useful in bioengineering.

  10. Cleave and Leave

    Plants produce ethylene gas, which acts as a hormone and is essential for the ripening of fruit, the resistance of plants to pathogens, the adaptation of plants to stress conditions, and stem cell maintenance. Although many components of the ethylene gas signaling pathway have been well studied, little is known about how the ethylene receptors located in the endoplasmic reticulum (ER) membrane can transmit the signal to the nucleus. Studying Arabidopsis, Qiao et al. (p. 390, published online 30 August) found that perception of ethylene gas in the ER promotes signal transduction via cleavage and rapid ER-nucleus translocation of the cytosolic portion of the transmembrane ETHYLENE INSENSITIVE2 protein, which activates ethylene-dependent gene expression and other ethylene response phenotypes in plants.

  11. A Fine Balance

    Intellectual and neurological disabilities can arise from diverse developmental aberrations. Novarino et al. (p. 394, published online 6 September; see the Perspective by Beaudet) have now determined the genetic basis for one such disorder for a small group of patients. Exome sequencing led to identification of mutations in a kinase BCKDK (Branched Chain Ketoacid Dehydrogenase Kinase) that regulates metabolism of branched-chain amino acids such as valine, leucine, and isoleucine. Mice with homozygous mutations in the BCKDK gene showed developmental and neurological abnormalities resembling those in certain mouse autism models. Analysis of transport mechanisms responsible for carrying amino acids across the blood-brain barrier revealed competition between the branched-chain amino acids and large neutral amino acids. Nutritional supplementation with extra branched-chain amino acids in the diet of mice carrying homozygous mutations in the BCKDK gene normalized their phenotype.

  12. Making a Riboswitch

    During RNA synthesis by RNA polymerase (RNAP), nascent RNA will start to fold. This cotranscriptional folding can affect the final conformation and the function of the full-length RNA. Using single-molecule spectroscopy and optical tweezers, Frieda and Block (p. 397) followed the cotranscriptional folding of the pbuE riboswitch from Bacillus subtilis directly in real time, observing the formation of the adenine-binding aptamer structure or the alternative transcription terminator hairpin structure. The structures formed within seconds of their sequences clearing the RNAP footprint, confirming that the pbuE riboswitch is kinetically controlled.

  13. Duplicate Gene, New Tricks

    The fact that functionally new genes appear is clear, but the evolutionary process that allows for a new gain of function is not well understood. Näsvall et al. (p. 384) present the innovation-amplification-divergence model which suggests that after gene duplication, ancestral function is maintained but that the duplicate copies can gain new function that is selected for through the accumulation of mutations or changes in expression. Experimental selection on Salmonella enterica allowed an ancestral gene to evolve new enzymatic function in fewer than 3000 generations.