Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge

The origin of the isotopic signature of Indian mid-ocean ridge basalts has remained enigmatic, because the geochemical composition of these basalts is consistent either with pollution from recycled, ancient altered oceanic crust and sediments, or with ancient continental crust or lithosphere. The radiogenic isotopic signature may therefore be the result of contamination of the upper mantle by plumes containing recycled altered ancient oceanic crust and sediments, detachment and dispersal of continental material into the shallow mantle during rifting and breakup of Gondwana, or contamination of the upper mantle by ancient subduction processes. The identification of a process operating on a scale large enough to affect major portions of the Indian mid-ocean ridge basalt source region has been a long-standing problem. Here we present hafnium and lead isotope data from across the Indian-Pacific mantle boundary at the Australian-Antarctic discordance region of the Southeast Indian Ridge, which demonstrate that the Pacific and Indian upper mantle basalt source domains were each affected by different mechanisms. We infer that the Indian upper-mantle isotope signature in this region is affected mainly by lower continental crust entrained during Gondwana rifting, whereas the isotope signature of the Pacific upper mantle is influenced predominantly by ocean floor subduction-related processes.     

喀斯特自然土壤中AM真菌对先锋植物根系的影响

【目的】研究丛枝菌根(Arbuscular mycorrhiza，AM)真菌在喀斯特自然土壤条件下对喀斯特先锋草本植物根系的影响。【方法】通过自然土接种AM真菌(N)、灭菌土接种AM真菌(M)及灭菌土壤对照(S)共3种土壤处理，种植喀斯特先锋植物狗尾草(Setaria viridis)、荩草(Arthraxon hispidus)、鬼针草(Bidens pilosa)及狼杷草(Bidens tripartita)，并测定它们的根系生物量、根长、根表面积、根体积、根平均直径、根尖数及根分枝数。【结果】荩草、鬼针草及狼杷草在N处理及M处理下具有较高的菌根侵染率，狗尾草的菌根侵染率较低。与S处理相比，M处理下AM真菌明显提高了荩草、鬼针草及狼杷草的根系生物量、根长、根表面积、根体积、根尖数、根分枝数及组织密度，降低了根平均直径、比根长、比根面积及比根体积；与M处理相比，N处理明显降低了荩草、鬼针草及狼杷草的根系生物量、根长、根表面积、根体积、根尖数、根分枝数及组织密度，提高了比根长、比根面积及比根体积，但对根平均直径无明显影响。【结论】荩草、鬼针草及狼杷草具有较高菌根侵染率，能与AM真菌共生获得更加发达的根系，而自然土壤削弱了AM真菌对荩草、鬼针草及狼杷草根系生长的促进作用。     

RNA translocation and unwinding mechanism of HCV NS3 helicase and its coordination by ATP

Helicases are a ubiquitous class of enzymes involved in nearly all aspects of DNA and RNA metabolism. Despite recent progress in understanding their mechanism of action, limited resolution has left inaccessible the detailed mechanisms by which these enzymes couple the rearrangement of nucleic acid structures to the binding and hydrolysis of ATP. Observing individual mechanistic cycles of these motor proteins is central to understanding their cellular functions. Here we follow in real time, at a resolution of two base pairs and 20 ms, the RNA translocation and unwinding cycles of a hepatitis C virus helicase (NS3) monomer. NS3 is a representative superfamily-2 helicase essential for viral replication, and therefore a potentially important drug target. We show that the cyclic movement of NS3 is coordinated by ATP in discrete steps of 11 +/- 3 base pairs, and that actual unwinding occurs in rapid smaller substeps of 3.6 +/- 1.3 base pairs, also triggered by ATP binding, indicating that NS3 might move like an inchworm. This ATP-coupling mechanism is likely to be applicable to other non-hexameric helicases involved in many essential cellular functions. The assay developed here should be useful in investigating a broad range of nucleic acid translocation motors.     

Lymphocyte activation by HIV-1 envelope glycoprotein

Cell activation by phytohaemagglutinin, phorbol ester and by the supernatant of phytohaemagglutinin-stimulated peripheral blood mononuclear cells induces the expression and cytopathic effects of latent human immunodeficiency virus type-1 (HIV-1) in vitro. The lymphocyte surface protein CD4 has been identified as a receptor for HIV-1 and binds the viral envelope glycoprotein (gp120). In the light of evidence indicating that one natural function of CD4 is as a growth factor receptor, we examined the ability of native gp120 to activate resting CD4-bearing lymphocytes. Our results indicate that gp120 has innate biological activity as a result of a specific interaction with CD4, inducing increases in intracellular levels of inositol trisphosphate and of calcium, and in interleukin-2 receptor expression and cell motility.     

Expanded niche for white sharks.

Until the advent of electronic tagging technology, the inherent difficulty of studying swift and powerful marine animals made ecological information about sharks of the family Lamnidae difficult to obtain. Here we report the tracking of movements of white sharks by using pop-up satellite archival tags, which reveal that their migratory movements, depth and ambient thermal ranges are wider than was previously thought.     

Nesting ecology of waterbirds at Grays Lake, Idaho

Montane wetlands provide valuable habitat for nesting waterfowl and other waterbirds in the western United States, but relatively little information is available about the nesting ecology of their waterbird communities. We describe the general nesting ecology of breeding waterbirds at a large, shallow, montane wetland in southeastern Idaho during 1997-2000. Habitats include upland grasslands and intermittently to semipermanently flooded wetland habitats. We located a total of 1207 nests of 23 bird species: Eared Grebe ( Podiceps nigricollis ), Canada Goose ( Branta canadensis ), Mallard ( Anas platyrhynchos ), Gadwall ( A. strepera ), American Wigeon ( A. americana ), Green-winged Teal ( A. crecca ), Blue-winged Teal ( A. discors ), Cinnamon Teal ( A. cyanoptera ), Northern Shoveler ( A. clypeata ), Northern Pintail ( A. acuta ), Redhead ( Aythya americana ), Canvasback ( A. valisineria ), Lesser Scaup ( A. affinis ), Ruddy Duck ( Oxyura jamaicensis ), Northern Harrier ( Circus cyaneus ), American Coot ( Fulica americana ), Virginia Rail ( Rallus limicola ), Greater Sandhill Crane ( Grus canadensis tabida ), American Avocet ( Recurvirostra americana ), Long-billed Curlew ( Numenius americanus ), Wilson's Snipe ( Gallinago delicata ), Wilson's Phalarope ( Phalaropus tricolor ), and Short-eared Owl ( Asio flammeus ). Most nests were initiated in May-early June and were terminated (hatched or destroyed) by the 3rd week of June. Mean daily survival rate (DSR) for Canada Goose nests was 0.954 ± 0.005 ( s ￣x ; n = 127 nests), equivalent to Mayfield nest success of 21%. Mean DSR for dabbling duck nests over all 4 years was 0.938 ± 0.006 ( n = 41), equivalent to Mayfield nest success of 11%. For all other species where we found > 10 nests each year (Eared Grebe, Redhead, Canvasback, Coot, Sandhill Crane, American Avocet, and Wilson's Snipe), > 50% of nests found hatched at least 1 young. Success rates for geese, cranes, and ducks were lower than reported for Grays Lake during 1949-1951 and lower than most other wetlands in the region.     

Single-molecule analysis of Mss116-mediated group II intron folding

DEAD-box helicases are conserved enzymes involved in nearly all aspects of RNA metabolism, but their mechanisms of action remain unclear. Here, we investigated the mechanism of the DEAD-box protein Mss116 on its natural substrate, the group II intron ai5γ. Group II introns are structurally complex catalytic RNAs considered evolutionarily related to the eukaryotic spliceosome, and an interesting paradigm for large RNA folding. We used single-molecule fluorescence to monitor the effect of Mss116 on folding dynamics of a minimal active construct, ai5γ-D135. The data show that Mss116 stimulates dynamic sampling between states along the folding pathway, an effect previously observed only with high Mg(2+) concentrations. Furthermore, the data indicate that Mss116 promotes folding through discrete ATP-independent and ATP-dependent steps. We propose that Mss116 stimulates group II intron folding through a multi-step process that involves electrostatic stabilization of early intermediates and ATP hydrolysis during the final stages of native state assembly.     

RNA substrate binding site in the catalytic core of the Tetrahymena ribozyme.

In catalysis by group I introns, the helix (P1) containing the RNA cleavage site must be positioned next to the guanosine binding site. We have identified a conserved adenine in the catalytic core that contributes to the stability of this arrangement and propose that it accepts a hydrogen bond from a specific 2'-OH in P1. Such base-backbone tertiary interactions may be generally important to the organization of RNA tertiary structure.     

A tertiary interaction that links active-site domains to the 5' splice site of a group II intron

Group II introns are self-splicing RNAs that are commonly found in the genes of plants, fungi, yeast and bacteria. Little is known about the tertiary structure of group II introns, which are among the largest natural ribozymes. The most conserved region of the intron is domain 5 (D5), which, together with domain 1 (D1), is required for all reactions catalysed by the intron. Despite the importance of D5, its spatial relationship and tertiary contacts to other active-site constituents have remained obscure. Furthermore, D5 has never been placed directly at a site of catalysis by the intron. Here we show that a set of tertiary interactions (lambda-lambda') links catalytically essential regions of D5 and D1, creating the framework for an active-site and anchoring it at the 5' splice site. Highly conserved elements similar to components of the lambda-lambda' interaction are found in the eukaryotic spliceosome.