Block copolymer assembly via kinetic control     

Cui H  Chen Z  Zhong S  Wooley KL  Pochan DJ 《Science (New York, N.Y.)》2007,317(5838):647-650

Block copolymers consist of two or more chemically different polymer segments, or blocks, connected by a covalent linkage. In solution, amphiphilic blocks can self-assemble as a result of energetic repulsion effects between blocks. The degree of repulsion, the lengths of the block segments, and the selectivity of the solvent primarily control the resultant assembled morphology. In an ideal situation, one would like to be able to alter the morphology that forms without having to change the chemistry of the block copolymer. Through the kinetic manipulation of charged, amphiphilic block copolymers in solution, we are able to generate different nanoscale structures with simple block copolymer chemistry. The technique relies on divalent organic counter ions and solvent mixtures to drive the organization of the block copolymers down specific pathways into complex one-dimensional structures. Block copolymers are increasingly used as templating materials; thus, the ability to control the formation of specific patterns and structures is of growing interest and applicability.  相似文献   

Secondary siRNAs result from unprimed RNA synthesis and form a distinct class   总被引：1，自引：0，他引：1  

Sijen T  Steiner FA  Thijssen KL  Plasterk RH 《Science (New York, N.Y.)》2007,315(5809):244-247

In Caenorhabditis elegans, an effective RNA interference (RNAi) response requires the production of secondary short interfering RNAs (siRNAs) by RNA-directed RNA polymerases (RdRPs). We cloned secondary siRNAs from transgenic C. elegans lines expressing a single 22-nucleotide primary siRNA. Several secondary siRNAs start a few nucleotides downstream of the primary siRNA, indicating that non-RISC (RNA-induced silencing complex)-cleaved mRNAs are substrates for secondary siRNA production. In lines expressing primary siRNAs with single-nucleotide mismatches, secondary siRNAs do not carry the mismatch but contain the nucleotide complementary to the mRNA. We infer that RdRPs perform unprimed RNA synthesis. Secondary siRNAs are only of antisense polarity, carry 5' di- or triphosphates, and are only in the minority associated with RDE-1, the RNAi-specific Argonaute protein. Therefore, secondary siRNAs represent a distinct class of small RNAs. Their biogenesis depends on RdRPs, and we propose that each secondary siRNA is an individual RdRP product.  相似文献   

Retraction     

Davis ST  Benson BG  Bramson HN  Chapman DE  Dickerson SH  Dold KM  Eberwein DJ  Edelstein M  Frye SV  Gampe RT  Grifffen RJ  Harris PA  Hassell AM  Holmes WD  Hunter RN  Knick VB  Lackey K  Lovejoy B  Luzzio MJ  Murray D  Parker P  Rocque WJ  Shewchuk-Chapman L  Veal JM  Walker DH  Kuyper LF 《Science (New York, N.Y.)》2002,298(5602):2327

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Radioactive elements on Mercury's surface from MESSENGER: implications for the planet's formation and evolution     

Peplowski PN  Evans LG  Hauck SA  McCoy TJ  Boynton WV  Gillis-Davis JJ  Ebel DS  Goldsten JO  Hamara DK  Lawrence DJ  McNutt RL  Nittler LR  Solomon SC  Rhodes EA  Sprague AL  Starr RD  Stockstill-Cahill KR 《Science (New York, N.Y.)》2011,333(6051):1850-1852

The MESSENGER Gamma-Ray Spectrometer measured the average surface abundances of the radioactive elements potassium (K, 1150 ± 220 parts per million), thorium (Th, 220 ± 60 parts per billion), and uranium (U, 90 ± 20 parts per billion) in Mercury's northern hemisphere. The abundance of the moderately volatile element K, relative to Th and U, is inconsistent with physical models for the formation of Mercury requiring extreme heating of the planet or its precursor materials, and supports formation from volatile-containing material comparable to chondritic meteorites. Abundances of K, Th, and U indicate that internal heat production has declined substantially since Mercury's formation, consistent with widespread volcanism shortly after the end of late heavy bombardment 3.8 billion years ago and limited, isolated volcanic activity since.  相似文献   

Isotopic signature of N(2)O produced by marine ammonia-oxidizing archaea     

Santoro AE  Buchwald C  McIlvin MR  Casciotti KL 《Science (New York, N.Y.)》2011,333(6047):1282-1285

The ocean is an important global source of nitrous oxide (N(2)O), a greenhouse gas that contributes to stratospheric ozone destruction. Bacterial nitrification and denitrification are thought to be the primary sources of marine N(2)O, but the isotopic signatures of N(2)O produced by these processes are not consistent with the marine contribution to the global N(2)O budget. Based on enrichment cultures, we report that archaeal ammonia oxidation also produces N(2)O. Natural-abundance stable isotope measurements indicate that the produced N(2)O had bulk δ(15)N and δ(18)O values higher than observed for ammonia-oxidizing bacteria but similar to the δ(15)N and δ(18)O values attributed to the oceanic N(2)O source to the atmosphere. Our results suggest that ammonia-oxidizing archaea may be largely responsible for the oceanic N(2)O source.  相似文献   

Single-cell mass cytometry of differential immune and drug responses across a human hematopoietic continuum     

Bendall SC  Simonds EF  Qiu P  Amir el-AD  Krutzik PO  Finck R  Bruggner RV  Melamed R  Trejo A  Ornatsky OI  Balderas RS  Plevritis SK  Sachs K  Pe'er D  Tanner SD  Nolan GP 《Science (New York, N.Y.)》2011,332(6030):687-696

Flow cytometry is an essential tool for dissecting the functional complexity of hematopoiesis. We used single-cell "mass cytometry" to examine healthy human bone marrow, measuring 34 parameters simultaneously in single cells (binding of 31 antibodies, viability, DNA content, and relative cell size). The signaling behavior of cell subsets spanning a defined hematopoietic hierarchy was monitored with 18 simultaneous markers of functional signaling states perturbed by a set of ex vivo stimuli and inhibitors. The data set allowed for an algorithmically driven assembly of related cell types defined by surface antigen expression, providing a superimposable map of cell signaling responses in combination with drug inhibition. Visualized in this manner, the analysis revealed previously unappreciated instances of both precise signaling responses that were bounded within conventionally defined cell subsets and more continuous phosphorylation responses that crossed cell population boundaries in unexpected manners yet tracked closely with cellular phenotype. Collectively, such single-cell analyses provide system-wide views of immune signaling in healthy human hematopoiesis, against which drug action and disease can be compared for mechanistic studies and pharmacologic intervention.  相似文献   

Conserved eukaryotic fusogens can fuse viral envelopes to cells     

Avinoam O  Fridman K  Valansi C  Abutbul I  Zeev-Ben-Mordehai T  Maurer UE  Sapir A  Danino D  Grünewald K  White JM  Podbilewicz B 《Science (New York, N.Y.)》2011,332(6029):589-592

Caenorhabditis elegans proteins AFF-1 and EFF-1 [C. elegans fusion family (CeFF) proteins] are essential for developmental cell-to-cell fusion and can merge insect cells. To study the structure and function of AFF-1, we constructed vesicular stomatitis virus (VSV) displaying AFF-1 on the viral envelope, substituting the native fusogen VSV glycoprotein. Electron microscopy and tomography revealed that AFF-1 formed distinct supercomplexes resembling pentameric and hexameric "flowers" on pseudoviruses. Viruses carrying AFF-1 infected mammalian cells only when CeFFs were on the target cell surface. Furthermore, we identified fusion family (FF) proteins within and beyond nematodes, and divergent members from the human parasitic nematode Trichinella spiralis and the chordate Branchiostoma floridae could also fuse mammalian cells. Thus, FF proteins are part of an ancient family of cellular fusogens that can promote fusion when expressed on a viral particle.  相似文献   

Ecology. Better science needed for restoration in the Gulf of Mexico     

Bjorndal KA  Bowen BW  Chaloupka M  Crowder LB  Heppell SS  Jones CM  Lutcavage ME  Policansky D  Solow AR  Witherington BE 《Science (New York, N.Y.)》2011,331(6017):537-538

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Genome sequence of the PCE-dechlorinating bacterium Dehalococcoides ethenogenes     

Seshadri R  Adrian L  Fouts DE  Eisen JA  Phillippy AM  Methe BA  Ward NL  Nelson WC  Deboy RT  Khouri HM  Kolonay JF  Dodson RJ  Daugherty SC  Brinkac LM  Sullivan SA  Madupu R  Nelson KE  Kang KH  Impraim M  Tran K  Robinson JM  Forberger HA  Fraser CM  Zinder SH  Heidelberg JF 《Science (New York, N.Y.)》2005,307(5706):105-108

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Impact of land-use type and harvesting on population structure of a non-timber forest product-providing tree in a semi-arid savanna, West Africa     

Katharina Schumann  Rüdiger Wittig  Adjima Thiombiano  Ute Becker  Karen Hahn 《Biological conservation》2011,144(9):2369-2376

Non-timber forest products (NTFPs) strongly contribute to livelihood security in the semi-arid tropics. Main factors determining the populations of NTFP-providing species are human activities. This study examined the impact of land-use, combined with rates and patterns of debarking and chopping on a NTFP-providing tree (Anogeissus leiocarpa) in Burkina Faso. We compared stands in a protected area (W National Park) with those of its surrounding communal area (fallows, croplands) in order to (i) obtain an indication on the status of the population, (ii) assess its harvesting tolerance, (iii) estimate the sustainability of present management, and (iv) derive which additional management strategies may foster its conservation. Our results reveal that the stands of A. leiocarpa are in healthy states in fallows and in the park. In croplands, the absence of saplings gives evidence of a declining population. Nearly all individuals of A. leiocarpa were harvested in croplands and fallows, while the number of harvested individuals in the park was negligible. Intensity of debarking and chopping was tree size-specific. The sprouting ability significantly increased with higher chopping intensity. We conclude that despite the land-use impact and the intense harvesting, stands of A. leiocarpa are still well preserved due to the species life history (fast growing and high sprouting) and due to indirect positive influences of human activities by providing better environmental conditions for its recruitment. Thus, the population of A. leiocarpa is not at risk to over-harvesting and land-use even though it is not protected.  相似文献