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121.
122.
Common evolution of waprin and kunitz-like toxin families in Australian venomous snakes 总被引:1,自引:0,他引:1
St Pierre L Earl ST Filippovich I Sorokina N Masci PP De Jersey J Lavin MF 《Cellular and molecular life sciences : CMLS》2008,65(24):4039-4054
The venoms of Australian snakes contain a myriad of pharmacologically active toxin components. This study describes the identification
and comparative analysis of two distinct toxin families, the kunitztype serine protease inhibitors and waprins, and demonstrates
a previously unknown evolutionary link between the two. Multiple cDNA and full-length gene isoforms were cloned and shown
to be composed of three exons separated by two introns. A high degree of identity was observed solely within the first exon
which coded for the propeptide sequence and its cleavage site, and indicates that each toxin family has arisen from a gene
duplication event followed by diversification only within the portion of the gene coding for the functional toxin. It is proposed
that while the mechanism of toxin secretion is highly conserved, diversification of mature toxin sequences allows for the
existence of multiple protein isoforms in the venom to adapt to variations within the prey environment. 相似文献
123.
自美国富翁蒂托开创了太空旅行之路 ,人类自古以来的一个梦想已成为现实。近来 ,想去太空旅行的人纷纷造访特殊旅游机构 ,因为人类在未来将飞得更远 ,而且所花费用更少 相似文献
124.
The world's soils store more carbon than is present in biomass and in the atmosphere. Little is known, however, about the factors controlling the stability of soil organic carbon stocks and the response of the soil carbon pool to climate change remains uncertain. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6-0.8 m depth) stimulated the microbial mineralization of 2,567 +/- 226-year-old carbon. Our results support the previously suggested idea that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile could however stimulate the loss of ancient buried carbon. 相似文献
125.
Recuperation of nitrogen cycling in Amazonian forests following agricultural abandonment 总被引:4,自引:0,他引:4
Davidson EA de Carvalho CJ Figueira AM Ishida FY Ometto JP Nardoto GB Sabá RT Hayashi SN Leal EC Vieira IC Martinelli LA 《Nature》2007,447(7147):995-998
Phosphorus (P) is generally considered the most common limiting nutrient for productivity of mature tropical lowland forests growing on highly weathered soils. It is often assumed that P limitation also applies to young tropical forests, but nitrogen (N) losses during land-use change may alter the stoichiometric balance of nutrient cycling processes. In the Amazon basin, about 16% of the original forest area has been cleared, and about 30-50% of cleared land is estimated now to be in some stage of secondary forest succession following agricultural abandonment. Here we use forest age chronosequences to demonstrate that young successional forests growing after agricultural abandonment on highly weathered lowland tropical soils exhibit conservative N-cycling properties much like those of N-limited forests on younger soils in temperate latitudes. As secondary succession progresses, N-cycling properties recover and the dominance of a conservative P cycle typical of mature lowland tropical forests re-emerges. These successional shifts in N:P cycling ratios with forest age provide a mechanistic explanation for initially lower and then gradually increasing soil emissions of the greenhouse gas nitrous oxide (N(2)O). The patterns of N and P cycling during secondary forest succession, demonstrated here over decadal timescales, are similar to N- and P-cycling patterns during primary succession as soils age over thousands and millions of years, thus revealing that N availability in terrestrial ecosystems is ephemeral and can be disrupted by either natural or anthropogenic disturbances at several timescales. 相似文献
126.
Wind power production data at temporal resolutions of a few minutes exhibit successive periods with fluctuations of various dynamic nature and magnitude, which cannot be explained (so far) by the evolution of some explanatory variable. Our proposal is to capture this regime‐switching behaviour with an approach relying on Markov‐switching autoregressive (MSAR) models. An appropriate parameterization of the model coefficients is introduced, along with an adaptive estimation method allowing accommodation of long‐term variations in the process characteristics. The objective criterion to be recursively optimized is based on penalized maximum likelihood, with exponential forgetting of past observations. MSAR models are then employed for one‐step‐ahead point forecasting of 10 min resolution time series of wind power at two large offshore wind farms. They are favourably compared against persistence and autoregressive models. It is finally shown that the main interest of MSAR models lies in their ability to generate interval/density forecasts of significantly higher skill. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
127.
Systemic Practice and Action Research - 相似文献
128.
129.
Many methods and algorithms to generate random trees of many kinds have been proposed in the literature. No procedure exists
however for the generation of dendrograms with randomized fusion levels. Randomized dendrograms can be obtained by randomizing
the associated cophenetic matrix. Two algorithms are described. The first one generates completely random dendrograms, i.e.,
trees with a random topology, random fusion level values, and random assignment of the labels. The second algorithm uses a
double-permutation procedure to randomize a given dendrogram; it proceeds by randomization of the fixed fusion levels, instead
of using random fusion level values. A proof is presented that the double-permutation procedure is a Uniform Random Generation
Algorithmsensu Furnas (1984), and a complete example is given.
This work was supported by NSERC Grant No. A7738 to P. Legendre and by a NSERC scholarship to F.-J. Lapointe. 相似文献
130.
Protein coats deform flat lipid membranes into buds and capture membrane proteins to form transport vesicles. The assembly/disassembly cycle of the COPI coat on Golgi membranes is coupled to the GTP/GDP cycle of the small G protein Arf1. At the heart of this coupling is the specific interaction of membrane-bound Arf1-GTP with coatomer, a complex of seven proteins that forms the building unit of the COPI coat. Although COPI coat disassembly requires the catalysis of GTP hydrolysis in Arf1 by a specific GTPase-activating protein (ArfGAP1), the precise timing of this reaction during COPI vesicle formation is not known. Using time-resolved assays for COPI dynamics on liposomes of controlled size, we show that the rate of ArfGAP1-catalysed GTP hydrolysis in Arf1 and the rate of COPI disassembly increase over two orders of magnitude as the curvature of the lipid bilayer increases and approaches that of a typical transport vesicle. This leads to a model for COPI dynamics in which GTP hydrolysis in Arf1 is organized temporally and spatially according to the changes in lipid packing induced by the coat. 相似文献