Fossilized nuclei and germination structures identify Ediacaran "animal embryos" as encysting protists   总被引：1，自引：0，他引：1  

Huldtgren T  Cunningham JA  Yin C  Stampanoni M  Marone F  Donoghue PC  Bengtson S 《Science (New York, N.Y.)》2011,334(6063):1696-1699

Globular fossils showing palintomic cell cleavage in the Ediacaran Doushantuo Formation, China, are widely regarded as embryos of early metazoans, although metazoan synapomorphies, tissue differentiation, and associated juveniles or adults are lacking. We demonstrate using synchrotron-based x-ray tomographic microscopy that the fossils have features incompatible with multicellular metazoan embryos. The developmental pattern is comparable with nonmetazoan holozoans, including germination stages that preclude postcleavage embryology characteristic of metazoans. We conclude that these fossils are neither animals nor embryos. They belong outside crown-group Metazoa, within total-group Holozoa (the sister clade to Fungi that includes Metazoa, Choanoflagellata, and Mesomycetozoea) or perhaps on even more distant branches in the eukaryote tree. They represent an evolutionary grade in which palintomic cleavage served the function of producing propagules for dispersion.  相似文献   

Microbial control of silicate weathering in organic-rich ground water     

Hiebert FK  Bennett PC 《Science (New York, N.Y.)》1992,258(5080):278-281

An in situ microcosm study of the influence of surface-adhering bacteria on silicate diagenesis in a shallow petroleum-contaminated aquifer showed that minerals were colonized by indigenous bacteria and chemically weathered at a rate faster than theoretically predicted. Feldspar and quartz fragments were placed in anoxic, organic-rich ground water, left for 14 months, recovered, and compared to unreacted controls with scanning electron microscopy. Ground-water geochemistry was characterized before and after the experiment. Localized mineral etching probably occurred in a reaction zone at the bacteria-mineral interface where high concentrations of organic acids, formed by bacteria during metabolism of hydrocarbon, selectively mobilized silica and aluminum from the mineral surface.  相似文献   

A conserved family of enzymes that phosphorylate inositol hexakisphosphate     

Mulugu S  Bai W  Fridy PC  Bastidas RJ  Otto JC  Dollins DE  Haystead TA  Ribeiro AA  York JD 《Science (New York, N.Y.)》2007,316(5821):106-109

Inositol pyrophosphates are a diverse group of high-energy signaling molecules whose cellular roles remain an active area of study. We report a previously uncharacterized class of inositol pyrophosphate synthase and find it is identical to yeast Vip1 and Asp1 proteins, regulators of actin-related protein-2/3 (ARP 2/3) complexes. Vip1 and Asp1 acted as enzymes that encode inositol hexakisphosphate (IP6) and inositol heptakisphosphate (IP7) kinase activities. Alterations in kinase activity led to defects in cell growth, morphology, and interactions with ARP complex members. The functionality of Asp1 and Vip1 may provide cells with increased signaling capacity through metabolism of IP6.  相似文献   

Trans-Arctic invasion in modern times     

Reid PC  Edwards M  Johns DG 《Science (New York, N.Y.)》2008,322(5901):528

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Atom collision-induced resistivity of carbon nanotubes     

Romero HE  Bolton K  Rosén A  Eklund PC 《Science (New York, N.Y.)》2005,307(5706):89-93

We report the observation of unusually strong and systematic changes in the electron transport in metallic single-walled carbon nanotubes that are undergoing collisions with inert gas atoms or small molecules. At fixed gas temperature and pressure, changes in the resistance and thermopower of thin films are observed that scale as roughly M(1/3), where M is the mass of the colliding gas species (He, Ar, Ne, Kr, Xe, CH4, and N2). Results of molecular dynamics simulations are also presented that show that the maximum deformation of the tube wall upon collision and the total energy transfer between the colliding atom and the nanotube also exhibit a roughly M(1/3) dependence. It appears that the transient deformation (or dent) in the tube wall may provide a previously unknown scattering mechanism needed to explain the atom collision-induced changes in the electrical transport.  相似文献   

Depleting neuronal PrP in prion infection prevents disease and reverses spongiosis   总被引：1，自引：0，他引：1  

Mallucci G  Dickinson A  Linehan J  Klöhn PC  Brandner S  Collinge J 《Science (New York, N.Y.)》2003,302(5646):871-874

The mechanisms involved in prion neurotoxicity are unclear, and therapies preventing accumulation of PrPSc, the disease-associated form of prion protein (PrP), do not significantly prolong survival in mice with central nervous system prion infection. We found that depleting endogenous neuronal PrPc in mice with established neuroinvasive prion infection reversed early spongiform change and prevented neuronal loss and progression to clinical disease. This occurred despite the accumulation of extraneuronal PrPSc to levels seen in terminally ill wild-type animals. Thus, the propagation of nonneuronal PrPSc is not pathogenic, but arresting the continued conversion of PrPc to PrPSc within neurons during scrapie infection prevents prion neurotoxicity.  相似文献   

Single-molecule kinetics of lambda exonuclease reveal base dependence and dynamic disorder     

van Oijen AM  Blainey PC  Crampton DJ  Richardson CC  Ellenberger T  Xie XS 《Science (New York, N.Y.)》2003,301(5637):1235-1238

We used a multiplexed approach based on flow-stretched DNA to monitor the enzymatic digestion of lambda-phage DNA by individual bacteriophage lambda exonuclease molecules. Statistical analyses of multiple single-molecule trajectories observed simultaneously reveal that the catalytic rate is dependent on the local base content of the substrate DNA. By relating single-molecule kinetics to the free energies of hydrogen bonding and base stacking, we establish that the melting of a base from the DNA is the rate-limiting step in the catalytic cycle. The catalytic rate also exhibits large fluctuations independent of the sequence, which we attribute to conformational changes of the enzyme-DNA complex.  相似文献   

Amalthea's density is less than that of water     

Anderson JD  Johnson TV  Schubert G  Asmar S  Jacobson RA  Johnston D  Lau EL  Lewis G  Moore WB  Taylor A  Thomas PC  Weinwurm G 《Science (New York, N.Y.)》2005,308(5726):1291-1293

Radio Doppler data from the Galileo spacecraft's encounter with Amalthea, one of Jupiter's small inner moons, on 5 November 2002 yield a mass of (2.08 +/- 0.15) x 10(18) kilograms. Images of Amalthea from two Voyager spacecraft in 1979 and Galileo imaging between November 1996 and June 1997 yield a volume of (2.43 +/- 0.22) x 10(6) cubic kilometers. The satellite thus has a density of 857 +/- 99 kilograms per cubic meter. We suggest that Amalthea is porous and composed of water ice, as well as rocky material, and thus formed in a cold region of the solar system, possibly not at its present location near Jupiter.  相似文献   

Recent changes in atmospheric carbon monoxide     

Novelli PC  Masarie KA  Tans PP  Lang PM 《Science (New York, N.Y.)》1994,263(5153):1587-1590

Measurements of carbon monoxide (CO) in air samples collected from 27 locations between 71 degrees N and 41 degrees S show that atmospheric levels of this gas have decreased worldwide over the past 2 to 5 years. During this period, CO decreased at nearly a constant rate in the high northern latitudes. In contrast, in the tropics an abrupt decrease occurred beginning at the end of 1991. In the Northern Hemisphere, CO decreased at a spatially and temporally averaged rate of 7.3 (+/-0.9) parts per billion per year (6.1 percent per year) from June 1990 to June 1993, whereas in the Southern Hemisphere, CO decreased 4.2 (+/-0.5) parts per billion per year (7.0 percent per year). This recent change is opposite a long-term trend of a 1 to 2 percent per year increase inferred from measurements made in the Northern Hemisphere during the past 30 years.  相似文献   

Use of stimulatory agents to enhance the production of bioactive exopolysaccharide from pleurotus tuber-regium by submerged fermentation     

Zhang BB  Cheung PC 《Journal of agricultural and food chemistry》2011,59(4):1210-1216

Fatty acids, organic solvents and surfactants were investigated for their stimulatory effects on the growth of fungal mycelium and production of exopolysaccharide (EPS) by submerged fermentation of an edible mushroom Pleurotus tuber-regium. Addition of 3.0 g/L Tween 80 at the late stage of exponential growth phase provided the best stimulatory effect on mycelial biomass and fungal EPS production with a 51.3 and 41.8% increase, respectively. The chemical structure of the EPS produced with addition of Tween 80 was found to be a glucomannan with similar monosaccharide composition and glycosidic linkages but a significantly lower molecular weight (3.18 ± 0.09 × 10(6) Da), compared to the control (4.30 ± 0.12 × 10(6) Da). These two EPS could significantly inhibit the growth of chronic myelogenous leukemia K562 cells in a dose dependent manner, with an estimated IC(50) value of 43.7 and 47.6 μg/mL, respectively. The use of stimulatory agents to enhance production of bioactive fungal EPS can be applied in other fungal fermentation processes for enhancing production of useful metabolites.  相似文献