This Week in Science

Science  04 Feb 2011:
Vol. 331, Issue 6017, pp. 507
  1. Reconciling MicroRNA Mechanisms

      MicroRNAs (miRNAs) are a large class of small noncoding RNA genes found in most eukaryotic genomes that function to repress gene expression. Initial studies suggested that miRNA-mediated regulation is posttranscriptional, most likely through the inhibition of translation initiation. Other data have suggested that messenger RNA (mRNA) deadenylation and decay are critical factors in miRNA-mediated gene silencing. Djuranovic et al. (p. 550) review miRNA-mediated silencing and suggest that it is typically manifested at the level of both mRNA translation and decay. Most data can be reconciled by a model for miRNA-mediated silencing that begins with initiation-targeted translational repression, potentially triggered (or enhanced) by deadenylation, followed by general mRNA decay to consolidate a more transient translational repression.

    1. Toward Metal Dichalcogenide Exfoliation


        Exfoliation is used to separate layered materials to form two-dimensional sheets that can be incorporated into composites with enhanced properties, for example, the exfoliation of clays to make fire retardants. A key challenge is to find an efficient exfoliation method that is compatible with the subsequent processing steps used for making composites. Coleman et al. (p. 568) describes a technique to exfoliate a broad range of transition metal dichalcogenides, which are of interest because of their conductivity and mechanical properties.

      1. Kepler Delivers

          The Kepler telescope was designed to detect transits of planets across their stars, but can also find mutually eclipsing stars, which allow precise measurements of each star's mass and radius. Using data from Kepler, Carter et al. (p. 562, published online 11 January) describe a triply-eclipsing system, KOI-126, where a tight, low-mass binary orbits a more massive star. The binary is the lowest-mass eclipsing binary yet and, with its well-measured properties, is expected to play an important role in testing low-mass stellar models.

        1. Charge Carriers

            In light-emitting diodes, interacting charge carriers (excitons) recombine to emit light but can also convert the energy into thermal vibrations. Bolinger et al. (p. 565; see the Perspective by Bardeen) developed a technique for visualizing how these processes occur in a single conducting polymer chain, embedded in a nonconducting polymer matrix, undergoing hole injection—the addition of a positive-charge carrier. Hole injection decreased fluorescence but also shifted the emission position, which suggests that the chain is quenched preferentially. The quenching interaction occurred within about 60 nanometers, a much larger distance than that deduced for pure conducting polymer films, possibly because multistep energy funneling traps and localizes hole polarons that thermalize the excitons.

          1. Turns that Favor Glycosylation

              Adding a glycan tag to a protein at a basic residue can be useful in therapeutic applications, because such tags can increase serum half-life, decrease aggregation propensity, or shield immunogenic epitopes of the protein. However, adding such tags to proteins that are not normally glycosylated (naïve proteins) can decrease their stability. Culyba et al. (p. 571) added a glycan tag to several naïve proteins at an Asn residue in a reverse turn that has a nearby Phe group. The interaction of the aromatic side chains and the glycan allowed energetic stabilization of the proteins.

            1. The Climate of Social Change

                Numerous instances of rises and falls of pre-industrial civilizations have been associated with climate change, because variations of temperature and precipitation can affect agricultural productivity and the control of disease. Büntgen et al. (p. 578, published online 13 January) present a 2500-year-long record of European temperature and precipitation variability in order to provide a better climatic context for the interpretation of social growth and decay. Recent warming is unique, even though hydroclimatic variations in the past have exceeded modern ones in magnitude and duration. The demise of the Western Roman Empire coincided with a period of increased climate variability from ∼250 to 600 C.E.

              1. Mars Dunes in Springtime

                  Sand dunes are found all over the surface of Mars. However, it is not clear whether the dunes are active or represent relic deposits from the ancient past. Using data from the High-Resolution Imaging Science Experiment onboard the Mars Reconnaissance Orbiter, Hansen et al. (p. 575) report extensive modification of northern polar dunes within one Mars year. The changes are seasonal and are caused by sand and ice avalanching down the dunes, suggesting that sediment transport is associated with springtime CO2 sublimation.

                1. Mighty Daphnia Genome


                    The water flea, Daphnia, represents a major taxonomic lineage of aquatic arthropod diversity. Colbourne et al. (p. 555; see the Perspective by Ebert) present Daphnia's tiny genome, which nevertheless contains a large inventory of genes with many duplicated genes and expanded gene families that lack homology to genes in other sequenced genomes. Genes within the Daphnia lineage appear to have duplicated three times faster than genes within insect lineages, and are also three times more likely to be preserved, explaining the large gene number. The analysis sheds light on genome structure, the functional associations of genes, and Daphnia's response to its environmental conditions.

                  1. A Tale of Two Stomata

                      The stomata found on surfaces of leaves control the plant's gas exchange and water loss by evaporation. Management of stomatal apertures feeds in to management of the plant's hydration status and photosynthetic metabolism. Brodribb and McAdam (p. 582, published online 16 December) now show that stomata of seed-bearing plants function differently than those of spore-bearing plants. Stomata of the angiosperm Helianthus annuus and the conifer Callitris rhomboidea open and close in response to the hormone abscisic acid (ABA), whereas those of the lycophyte Lycopodium deuterodensum and the fern Pteridium esculentum instead seem to be controlled by a passive hydraulic reaction. Thus, the acquisition of sophisticated metabolic controls of stomatal function occurred after lineages for modern ferns and angiosperms diverged many millions of years ago.

                    1. Building on Biology

                        CREDIT: WÖRSDÖRFER ET AL.

                        Natural protein containers, such as viral capsids, are used in nanotechnology for applications like drug delivery and constrained synthesis. In Aquifex aeolicus, lumazine synthase, the enzyme that catalyzes the penultimate step in riboflavin synthesis, forms an icosohedral capsid. By engineering electrostatic interactions between host and guest, Wörsdörfer et al. (p. 589) constructed a variant of the lumazine synthase capsid that sequestered HIV protease—an enzyme that would be toxic in the cytoplasm. The growth advantage conferred by sequestration of the protease was used to drive directed evolution within bacterial cell cultures, resulting in capsids with 5 to 10 times higher protease loading capacities.

                      1. Doomed to Repeat?

                          Satellite DNA repeats reside within regions of chromatin traditionally thought to be inactive, but increasing evidence suggests that these repeats are transcribed into noncoding RNAs. Ting et al. (p. 593, published online 13 January) find that certain classes of these satellite transcripts are expressed in pancreatic ductal carcinomas at levels that are about 40 times higher than those in nonmalignant tissue. Overexpression of the satellite transcripts, which may reflect a global change in heterochromatin silencing in cancer, occurs in both mouse and human tumors and appears to be a feature of several different types of epithelial cancers. How the satellite transcripts arise and whether they actively contribute to tumorigenesis is unclear, but the magnitude of their overexpression in cancer suggests that they may be useful biomarkers for detecting the disease.

                        1. Dangerous and Wanted

                            In West Africa, different populations of the main malaria vector Anopheles gambiae differ in insecticide resistance, climatic tolerance, feeding behavior, and relative resistance to malaria parasites. Genetic investigations have generally been performed on mosquitoes sampled within human dwellings because it has been assumed that indoors is the main venue for malaria transmission and because it is extremely difficult to catch mosquitoes out of doors. Riehle et al. (p. 596) challenged this long-held assumption by sampling ponds around villages for mosquito larvae. Over half the larvae collected were a completely distinct genotype, whose adults had never been collected. Moreover, when grown in the laboratory, the resulting adult mosquitoes were significantly more susceptible to malaria parasite infection than those collected indoors.

                          1. Variation in Transit

                              Modulation of the action potential waveform that enhances synaptic transmission has been reported in central neurons after repetitive stimulation or depolarization of the presynaptic neuron. Sasaki et al. (p. 599) used optically targeted axons to observe that modulation of synaptic transmission in cortical neurons does not exclusively depend on release processes but is also achieved by regulation of presynaptic action potential width. Activation of periaxonal astrocytes broadened action potentials and enhanced synaptic transmission.

                            1. Time for a Pause

                                In metazoan cells, accumulation of misfolded proteins in the endoplasmic reticulum (ER) triggers the unfolded protein response (UPR) in which unconventional splicing of a precursor form of XBP1 (XBP1u) mRNA by an ER membrane protein, IRE1, results in the formation of mature XBP1s messenger RNA (mRNA). The XBP1s mRNA encodes a functional transcription factor that induces the expression of the ER resident molecular chaperones to alleviate the stressful situation. The nascent XBP1u peptide recruits the mRNA–ribosome–nascent chain (R-RNC) complex to the ER membrane. Yanagitani et al. (p. 586, published online 13 January; see the Perspective by Ron and Ito) now report that translation of XBP1u mRNA is transiently paused near the 3′ end of its open reading frame to stabilize the R-RNC complex. Mutational analysis of XBP1u revealed an evolutionarily conserved peptide module at the Cterminal region responsible for translational pausing, which was required for the efficient ER-targeting and splicing of the XBP1u mRNA. Thus, regulation of translational speed using a module embedded inside a protein promotes targeting of its mRNA for efficient splicing.