Life Versus the Volcanoes
Correlating a specific triggering event, such as an asteroid impact or massive volcanism, to mass extinction events is clouded by the difficulty in precisely timing their occurrence in the geologic record. Based on rock samples collected in North America and Morocco, Blackburn et al. (p. 941, published online 21 March) acquired accurate ages for events surrounding the mass extinction that occurred ∼201 million years ago, between the Triassic and Jurassic Periods. The timing of the disappearance of marine and land fossils and geochemical evidence of the sequential eruption of the Central Atlantic Magmatic Province imply a strong causal relationship.
Telling Hexanes Apart
The efficiency of modern internal combustion engines depends on the relative reactivity of the hydrocarbons that comprise the fuel. In particular, branched hydrocarbons are less likely than their linear counterparts to react prematurely—a property reflected in the fuel mixture's octane number. Herm et al. (p. 960) report a metal organic framework material with triangular pore channels that discriminate among the differently shaped isomers of hexane more finely than the commercial standard.
For Good Measure
SS Cygni is a well-studied binary star system in the northern constellation Cygnus, consisting of a white dwarf that accretes matter from its companion star. Miller-Jones et al. (p. 950; see the Perspective by Schreiber) used radio observations to derive a model-independent distance to this prototypical accreting white dwarf system. The measurement places the system significantly closer than previously determined, reconciling the observed properties of SS Cygni with our current understanding of accretion theory.
A Touchy Subject
The ability to hold a glass being filled with water without dropping it depends on our ability to touch objects and to know the correct pressure to exert. Thus, for robotics or artificial skin design, methods are needed for sensitive pressure detection. Wu et al. (p. 952, published online 25 April) designed a device based on an array of zinc oxide nanowires that generate a small voltage when flexed that could be translated into a pressure signal. The device has a pressure-sensing range of up to 30 kPa, comparable to the 10 to 40 kPa range of a human finger.
The Master Switch for Itch?
Recently, gastrin-releasing peptide (GRP) has been implicated as the primary neurotransmitter between itch-sensitive nerve fibers and downstream neurons in the spinal cord. However, Mishra and Hoon (p. 968) challenge this view, provide evidence that natriuretic polypeptide b (Nppb) is the central itch neurotransmitter, and suggest that GRP is released by second-order neurons in the spinal dorsal horn that express the Nppb receptor and are excited by Nppb.
Spleen Knockout Explained
Isolated congenital asplenia (ICA) is a rare disorder where patients are born without a spleen and are at increased risk of bacterial infection but have no other developmental abnormalities. Through sequence analysis of familial and sporadic cases, Bolze et al. (p. 976, published online 11 April) found that ICA patients carry mutations in the gene encoding ribosomal protein SA and as a result express about half the normal amount of this protein. The mechanism by which reduced expression of a housekeeping protein causes an organ-specific defect remains unclear.
Folding Too Slow, Off You Go
One of the major questions of chaperone-assisted protein-folding pathways is how substrates that fail to fold avoid futile folding cycles. Xu et al. (p. 978; see the Perspective by Kleizen and Braakman) developed a model to examine a folding-competent protein that nevertheless fails to fold within the endoplasmic reticulum. Under these circumstances, the unfolded protein was subject to an unusual glycosylation, O-mannosylation, which appeared to terminate folding of the unfinished molecules. Eliminating O-mannosylation allowed the protein to fold completely.
Malaria Cloak and Dagger
Mosquitoes have a complex immune system capable of effective antiparasite responses; however, malaria transmission is still highly efficient. Molina-Cruz et al. (p. 984, published online 9 May; see the Perspective by Philip and Waters) show that Plasmodium falciparum has a gene product, Pfs47, that makes the parasite's ookinetes “invisible” to the mosquito immune system. Disruption of this mechanism could potentially be used to block malaria transmission.
Many cellular processes are regulated by mechanical signals. Single-molecule force spectroscopy has been used to probe molecular unfolding or unbinding; however, measuring the single-molecule forces required to activate signaling remains a challenge. Wang and Ha (p. 991) describe a “Tension Gauge Tether” approach to measure the force applied on a single receptor ligand bond. By using tethers with a range of tension tolerances and monitoring activation, the force required to activate signaling could be measured.
Several populations of the German cockroach have become averse to the glucose used as bait in toxic traps, which has severely reduced the traps' effectiveness. Wada-Katsumata et al. (p. 972) show that this aversion is a result of changes in the peripheral gustatory system, whereby glucose, as well as “sweet” receptors, stimulated an aversive bitter compound receptor.
CMV Breaks All the Rules
One vaccine strategy being pursued against HIV is to generate protection that is dependent on cell-mediated, rather than humoral, immune responses. A cytomegalovirus (CMV)–vectored vaccine that expresses simian immunodeficiency virus (SIV) antigens exhibits stringent and durable viral control upon SIV challenge in approximately half of vaccinated rhesus macaques. Hansen et al. (10.1126/science.1237874, see the Perspective by Goonetilleke and McMichael) sought to determine the basis for the protection and discovered that the CD8+ T cell response in vaccinated monkeys does not target canonical SIV epitopes, which SIV is known to escape, but rather generates a broad, promiscuous response.
Quelccaya Ice Cap
Ice cores drilled in the ice sheets of Greenland and Antarctica are some of the most important sources of information about the paleoclimate of high latitudes. Comparable records from the tropics are rare, however, because there are so few locations at which long-lived, undisturbed ice can be found. Thompson et al. (p. 945, published online 4 April) report results obtained from one of the few such sites, the Quelccaya ice cap in the Peruvian Andes. The annually resolved data, extending back 1800 years, provide a detailed chronicle of changes in the isotopic composition of the oxygen in the ice, which are related to the sea surface temperature of the water's source. Analyses of a collection of major ions such as ammonium and nitrate reveal how atmospheric circulation in the region varied over that period. Finally, the radiocarbon content of ancient plants—recently exposed by the retreat of the ice sheet—reveals that Quelccaya has not been smaller for at least six thousand years.
The Strength of Impurities
The practical strength of a material (rather than its theoretical strength) is influenced by the presence of defects between crystalline domains and the inclusion of impurities. In some cases, synergistic effects may arise where the impurity atoms segregate to the domain boundaries, although kinetic barriers may limit the extent to which the impurity atoms can order. Nie et al. (p. 957) show the segregation of oversized and undersized solute atoms at coherent twin boundaries in a magnesium alloy. The minimization of strain energy drives the differently sized impurities to different twin boundaries, strengthening the material.
Hollowing Out Metal Oxide Nanoparticles
Corrosion is normally a problem, but it can be useful, for example, when you wish to create hollow metal nanoparticles, whereby the reduction of one metal species in solution drives the dissolution of the core of the particle. Oh et al. (p. 964; see the Perspective by Ibáñez and Cabot) adapted this approach to metal oxide nanoparticles by placing Mn3O4 nanocrystals in solution with Fe2+ ions, which replaces the nanocrystal exterior with γ-Fe2O3. At sufficiently high Fe2+ concentrations, hollow γ-Fe2O3 nanocages formed. These hollow structures could be used as anode materials for lithium ion batteries.
Before a cell divides, it must replicate its genome so that both daughter cells receive a copy of the parental DNA. Replication must be tightly regulated to ensure that the genome is replicated only once, because over- or underreplication could result in aberrations that cause genome instability. Thus, replication must be coordinated with other events in the cell, such as the cell cycle and DNA damage response systems. Boos et al. (p. 981) analyzed the function of the Treslin/TICRR protein, an essential DNA replication factor regulated by cyclin-dependent kinases and the DNA damage checkpoint. Treslin interacts with the Mdm Two Binding Protein (MTBP), implicated in oncogenesis. MTBP and Treslin appear to integrate signals from the cell cycle and the DNA damage–response pathway, thereby controlling the initiation of DNA replication.
Sulfamethoxazole is a widely used sulfa-drug often used at high doses in the treatment of Pneumocytis pneumonia (PCP) in immunocompromised individuals. Haruki et al. (p. 987) show that sulfamethoxazole and certain other sulfa drugs inhibit the enzyme septiapterin reductase that catalyzes the final step in the biosynthesis of tetrahydrobiopterin (BH4). BH4 is a cofactor in the biosynthesis of neurotransmitters such as serotonin and dopamine. In cell culture, sulfamethoxazole lowered neurotransmitter levels through depletion of BH4, which may explain central nervous system side effects associated with PCP treatment.