Does ammonium uptake influence xylem sap composition inPhaseolus vulgaris L. andGlycine max (L.) Merill?

The question of whether ammonium uptake influences the occurrence of ureides in legumes has been addressed in this study by investigating threeP. vulgaris genotypes as well as one cultivar ofGlycine max. All plants were raised in sand culture during the dry season in northern Thailand and irrigated daily with nitrogen-free nutrient solution, or the same solution containing 12 mol m^–3 nitrogen in the form of (NH₄)₂SO₄ or KNO₃, each treatment consisting of different proportions of either compound. Regression analyses of xylem sap composition relative to ammonium vs. nitrate supply of plants harvested at V4, R1 and R6 indicated close positive correlations of xylem amino nitrogen content and negative correlations with xylem nitrate content and ammonium supply. Statistically significant correlations between relative xylem ureide content and ammonium availability could be established for theP. vulgaris cultivar Brilliant up to stage R1, but not for the other plants investigated. It was concluded that at least for some genotypes of common bean a relationship exists between ureide production and ammonium uptake by the root system. Since the extent to which ureide production is stimulated remains quite small, its relevance to the xylem solute technique for measurement of N₂ fixation may be limited. Nevertheless, due to the possibility of large genotypic differences in the impact of ammonium on ureide production, this factor must be considered in calculations if N₂ fixation is to be determined in soils containing significant amounts of ammonium, e.g. in paddy fields.     

Resequencing of 200 human exomes identifies an excess of low-frequency non-synonymous coding variants

Targeted capture combined with massively parallel exome sequencing is a promising approach to identify genetic variants implicated in human traits. We report exome sequencing of 200 individuals from Denmark with targeted capture of 18,654 coding genes and sequence coverage of each individual exome at an average depth of 12-fold. On average, about 95% of the target regions were covered by at least one read. We identified 121,870 SNPs in the sample population, including 53,081 coding SNPs (cSNPs). Using a statistical method for SNP calling and an estimation of allelic frequencies based on our population data, we derived the allele frequency spectrum of cSNPs with a minor allele frequency greater than 0.02. We identified a 1.8-fold excess of deleterious, non-syonomyous cSNPs over synonymous cSNPs in the low-frequency range (minor allele frequencies between 2% and 5%). This excess was more pronounced for X-linked SNPs, suggesting that deleterious substitutions are primarily recessive.     

Placement of protein and RNA structures into a 5 A-resolution map of the 50S ribosomal subunit.

We have calculated at 5.0 A resolution an electron-density map of the large 50S ribosomal subunit from the bacterium Haloarcula marismortui by using phases derived from four heavy-atom derivatives, intercrystal density averaging and density-modification procedures. More than 300 base pairs of A-form RNA duplex have been fitted into this map, as have regions of non-A-form duplex, single-stranded segments and tetraloops. The long rods of RNA crisscrossing the subunit arise from the stacking of short, separate double helices, not all of which are A-form, and in many places proteins crosslink two or more of these rods. The polypeptide exit channel was marked by tungsten cluster compounds bound in one heavy-atom-derivatized crystal. We have determined the structure of the translation-factor-binding centre by fitting the crystal structures of the ribosomal proteins L6, L11 and L14, the sarcin-ricin loop RNA, and the RNA sequence that binds L11 into the electron density. We can position either elongation factor G or elongation factor Tu complexed with an aminoacylated transfer RNA and GTP onto the factor-binding centre in a manner that is consistent with results from biochemical and electron microscopy studies.     

Signaling mechanisms of neurite outgrowth induced by the cell adhesion molecules NCAM and N-Cadherin

Formation of appropriate neural circuits depends on a complex interplay between extracellular guiding cues and intracellular signaling events that result in alterations of cytoskeletal dynamics and a neurite growth response. Surface-expressed cell adhesion molecules (CAMs) interact with the surroundings via the extracellular domain and bind to the cytoskeleton via their intracellular domain. In addition, several CAMs induce signaling events via direct interactions with intracellular proteins or via interactions with cell surface receptors. Thus, CAMs are obvious candidates for transmitting extracellular guidance cues to intracellular events and thereby regulating neurite outgrowth. In this review, we focus on two CAMs, the neural cell adhesion molecule (NCAM) and N-cadherin, and their ability to mediate signaling associated with a neurite outgrowth response. In particular, we will focus on direct interaction between NCAM and N-cadherin with a number of intracellular partners, as well as on their interaction with the fibroblast growth factor receptor (FGFR). Received 23 May 2008; received after revision 14 July 2008; accepted 21 July 2008     

An abundance of small exoplanets around stars with a wide range of metallicities

The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a 'fossil' record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets, supporting the model that planets form by accumulation of dust and ice particles. Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets. However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA's Kepler mission, including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation.     

Weight constrained maximum split clustering

ConsiderN entities to be classified, with given weights, and a matrix of dissimilarities between pairs of them. The split of a cluster is the smallest dissimilarity between an entity in that cluster and an entity outside it. The single-linkage algorithm provides partitions intoM clusters for which the smallest split is maximum. We consider the problems of finding maximum split partitions with exactlyM clusters and with at mostM clusters subject to the additional constraint that the sum of the weights of the entities in each cluster never exceeds a given bound. These two problems are shown to be NP-hard and reducible to a sequence of bin-packing problems. A (N ²) algorithm for the particular caseM =N of the second problem is also presented. Computational experience is reported.Acknowledgments: Work of the first author was supported in part by AFOSR grants 0271 and 0066 to Rutgers University and was done in part during a visit to GERAD, Ecole Polytechnique de Montréal, whose support is gratefully acknowledged. Work of the second and third authors was supported by NSERC grant GP0036426 and by FCAR grant 89EQ4144. We are grateful to Silvano Martello and Paolo Toth for making available to us their program MTP for the bin-paking problem and to three anonymous referees for comments which helped to improve the presentation of the paper.     

三维激光扫描仪油膜粗糙度检测

【目的】观测水面油膜粗糙度,以进一步研究水面油膜对后向散射的影响,提高微波监测水面溢油的精度。【方法】采用三维激光扫描仪观测激光垂直扫描油面获得测距数据,多次测量求平均值后再求其均方根误差以实现对油膜粗糙度的观测。【结果】随着含水率的增加,原油张力和粘度逐渐增大,当温度为20~33℃时,其乳化饱和含水率不大于21%,在理想状态下的原油乳化制备过程中,油膜粗糙度与含水率基本呈余弦关系。三维激光扫描仪可在0~6m/s风速下观测油膜厚度0.005mm的油面粗糙度。【结论】采用三维激光扫描仪观测油膜粗糙度是可行的。