The limb bud Shh-Fgf feedback loop is terminated by expansion of former ZPA cells     

Scherz PJ  Harfe BD  McMahon AP  Tabin CJ 《Science (New York, N.Y.)》2004,305(5682):396-399

Vertebrate limb outgrowth is driven by a positive feedback loop involving Sonic Hedgehog (Shh), Gremlin, and Fgf4. By overexpressing individual components of the loop at a time after these genes are normally down-regulated in chicken embryos, we found that Shh no longer maintains Gremlin in the posterior limb. Shh-expressing cells and their descendants cannot express Gremlin. The proliferation of these descendants forms a barrier separating the Shh signal from Gremlin-expressing cells, which breaks down the Shh-Fgf4 loop and thereby affects limb size and provides a mechanism explaining regulative properties of the limb bud.  相似文献   

Charge transfer equilibria between diamond and an aqueous oxygen electrochemical redox couple     

Chakrapani V  Angus JC  Anderson AB  Wolter SD  Stoner BR  Sumanasekera GU 《Science (New York, N.Y.)》2007,318(5855):1424-1430

Undoped, high-quality diamond is, under almost all circumstances, one of the best insulators known. However, diamond covered with chemically bound hydrogen shows a pronounced conductivity when exposed to air. This conductivity arises from positive-charge carriers (holes) and is confined to a narrow near-surface region. Although several explanations have been proposed, none has received wide acceptance, and the mechanism remains controversial. Here, we report the interactions of hydrogen-terminated, macroscopic diamonds and diamond powders with aqueous solutions of controlled pH and oxygen concentration. We show that electrons transfer between the diamond and an electrochemical reduction/oxidation couple involving oxygen. This charge transfer is responsible for the surface conductivity and also influences contact angles and zeta potentials. The effect is not confined to diamond and may play a previously unrecognized role in other disparate systems.  相似文献   

Role of Polo-like kinase CDC5 in programming meiosis I chromosome segregation     

Lee BH  Amon A 《Science (New York, N.Y.)》2003,300(5618):482-486

Meiosis is a specialized cell division in which two chromosome segregation phases follow a single DNA replication phase. The budding yeast Polo-like kinase Cdc5 was found to be instrumental in establishing the meiosis I chromosome segregation program. Cdc5 was required to phosphorylate and remove meiotic cohesin from chromosomes. Furthermore, in the absence of CDC5 kinetochores were bioriented during meiosis I, and Mam1, a protein essential for coorientation, failed to associate with kinetochores. Thus, sister-kinetochore coorientation and chromosome segregation during meiosis I are coupled through their dependence on CDC5.  相似文献   

The structure of ferrihydrite, a nanocrystalline material   总被引：2，自引：0，他引：2  

Michel FM  Ehm L  Antao SM  Lee PL  Chupas PJ  Liu G  Strongin DR  Schoonen MA  Phillips BL  Parise JB 《Science (New York, N.Y.)》2007,316(5832):1726-1729

Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P6(3)mc; a = approximately 5.95 angstroms, c = approximately 9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis delta-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.  相似文献   

Soil Phosphorus and Potassium Mapping Using a Spatial Correlation Model Incorporating Terrain Slope Gradient   总被引：1，自引：0，他引：1  

Kozar  Brian  Lawrence  Rick  Long  Dan S. 《Precision Agriculture》2002,3(4):407-417

Variable-rate fertilizer application requires knowledge of the spatial distribution of soil nutrients within fields. Grid soil sampling might be used for acquiring this information, but is often too expensive for resolving spatial patterns in soil nutrients at the scale of precision fertilizer application. The objective of this study was to determine whether grid sampling efficiency can be improved using cokriging estimates with slope gradient as a secondary variable, which is easily obtained from high-resolution digital elevation models. Soils in two northern Montana wheat fields were sampled at the nodes of a 100-m diagonal grid. Soil test phosphorus and potassium maps were constructed with kriging and cokriging. Co-kriging uses the spatial correlation between two variables to predict for the less intensively sampled variable of interest, often with less estimation error than a univariate method such as kriging. The average estimation variance for cokriging compared to kriging was reduced for all values of the correlation considered. The additional complexity of cokriging might be justified provided a secondary variable exists that is spatially cross correlated with the primary variable of interest.  相似文献   

Bilayer 3He: a simple two-dimensional heavy-fermion system with quantum criticality     

Neumann M  Nyéki J  Cowan B  Saunders J 《Science (New York, N.Y.)》2007,317(5843):1356-1359

Two-dimensional helium-3 (3He) provides a simple model for the experimental investigation of the emergence of quantum complexity in a strongly correlated Fermi system. We have observed two-dimensional, two-band heavy-fermion behavior in bilayer films of 3He atoms when adsorbed on the surface of graphite preplated by a solid bilayer of 4He. Thermodynamic measurements on this system showed that the relevant control parameter is the total density of the 3He film. The 3He bilayer system can be driven toward a quantum critical point at which the effective mass appears to diverge, interband coupling vanishes, and a local-moment state appears. It opens a new testing ground for theories of quantum criticality in heavy-fermion materials.  相似文献   

Global allocation rules for patterns of biomass partitioning in seed plants   总被引：3，自引：0，他引：3  

Enquist BJ  Niklas KJ 《Science (New York, N.Y.)》2002,295(5559):1517-1520

A general allometric model has been derived to predict intraspecific and interspecific scaling relationships among seed plant leaf, stem, and root biomass. Analysis of a large compendium of standing organ biomass sampled across a broad sampling of taxa inhabiting diverse ecological habitats supports the relations predicted by the model and defines the boundary conditions for above- and below-ground biomass partitioning. These canonical biomass relations are insensitive to phyletic affiliation (conifers versus angiosperms) and variation in averaged local environmental conditions. The model thus identifies and defines the limits that have guided the diversification of seed plant biomass allocation strategies.  相似文献   

Sequence variants in SLITRK1 are associated with Tourette's syndrome   总被引：1，自引：0，他引：1  

Abelson JF  Kwan KY  O'Roak BJ  Baek DY  Stillman AA  Morgan TM  Mathews CA  Pauls DL  Rasin MR  Gunel M  Davis NR  Ercan-Sencicek AG  Guez DH  Spertus JA  Leckman JF  Dure LS  Kurlan R  Singer HS  Gilbert DL  Farhi A  Louvi A  Lifton RP  Sestan N  State MW 《Science (New York, N.Y.)》2005,310(5746):317-320

Tourette's syndrome (TS) is a genetically influenced developmental neuropsychiatric disorder characterized by chronic vocal and motor tics. We studied Slit and Trk-like 1 (SLITRK1) as a candidate gene on chromosome 13q31.1 because of its proximity to a de novo chromosomal inversion in a child with TS. Among 174 unrelated probands, we identified a frameshift mutation and two independent occurrences of the identical variant in the binding site for microRNA hsa-miR-189. These variants were absent from 3600 control chromosomes. SLITRK1 mRNA and hsa-miR-189 showed an overlapping expression pattern in brain regions previously implicated in TS. Wild-type SLITRK1, but not the frameshift mutant, enhanced dendritic growth in primary neuronal cultures. Collectively, these findings support the association of rare SLITRK1 sequence variants with TS.  相似文献   

Sirt5 is a NAD-dependent protein lysine demalonylase and desuccinylase   总被引：1，自引：0，他引：1  

Du J  Zhou Y  Su X  Yu JJ  Khan S  Jiang H  Kim J  Woo J  Kim JH  Choi BH  He B  Chen W  Zhang S  Cerione RA  Auwerx J  Hao Q  Lin H 《Science (New York, N.Y.)》2011,334(6057):806-809

Silent information regulator 2 (Sir2) proteins (sirtuins) are nicotinamide adenine dinucleotide-dependent deacetylases that regulate important biological processes. Mammals have seven sirtuins, Sirt1 to Sirt7. Four of them (Sirt4 to Sirt7) have no detectable or very weak deacetylase activity. We found that Sirt5 is an efficient protein lysine desuccinylase and demalonylase in vitro. The preference for succinyl and malonyl groups was explained by the presence of an arginine residue (Arg(105)) and tyrosine residue (Tyr(102)) in the acyl pocket of Sirt5. Several mammalian proteins were identified with mass spectrometry to have succinyl or malonyl lysine modifications. Deletion of Sirt5 in mice appeared to increase the level of succinylation on carbamoyl phosphate synthase 1, which is a known target of Sirt5. Thus, protein lysine succinylation may represent a posttranslational modification that can be reversed by Sirt5 in vivo.  相似文献   

Paleoindian seafaring, maritime technologies, and coastal foraging on California's Channel Islands     

Erlandson JM  Rick TC  Braje TJ  Casperson M  Culleton B  Fulfrost B  Garcia T  Guthrie DA  Jew N  Kennett DJ  Moss ML  Reeder L  Skinner C  Watts J  Willis L 《Science (New York, N.Y.)》2011,331(6021):1181-1185

Three archaeological sites on California's Channel Islands show that Paleoindians relied heavily on marine resources. The Paleocoastal sites, dated between ~12,200 and 11,200 years ago, contain numerous stemmed projectile points and crescents associated with a variety of marine and aquatic faunal remains. At site CA-SRI-512 on Santa Rosa Island, Paleocoastal peoples used such tools to capture geese, cormorants, and other birds, along with marine mammals and finfish. At Cardwell Bluffs on San Miguel Island, Paleocoastal peoples collected local chert cobbles, worked them into bifaces and projectile points, and discarded thousands of marine shells. With bifacial technologies similar to those seen in Western Pluvial Lakes Tradition assemblages of western North America, the sites provide evidence for seafaring and island colonization by Paleoindians with a diversified maritime economy.  相似文献