Sequence and analysis of chromosome 2 of Dictyostelium discoideum

The genome of the lower eukaryote Dictyostelium discoideum comprises six chromosomes. Here we report the sequence of the largest, chromosome 2, which at 8 megabases (Mb) represents about 25% of the genome. Despite an A + T content of nearly 80%, the chromosome codes for 2,799 predicted protein coding genes and 73 transfer RNA genes. This gene density, about 1 gene per 2.6 kilobases (kb), is surpassed only by Saccharomyces cerevisiae (one per 2 kb) and is similar to that of Schizosaccharomyces pombe (one per 2.5 kb). If we assume that the other chromosomes have a similar gene density, we can expect around 11,000 genes in the D. discoideum genome. A significant number of the genes show higher similarities to genes of vertebrates than to those of other fully sequenced eukaryotes. This analysis strengthens the view that the evolutionary position of D. discoideum is located before the branching of metazoa and fungi but after the divergence of the plant kingdom, placing it close to the base of metazoan evolution.     

A young source for the Hawaiian plume

Recycling of oceanic crust through subduction, mantle upwelling, and remelting in mantle plumes is a widely accepted mechanism to explain ocean island volcanism. The timescale of this recycling is important to our understanding of mantle circulation rates. Correlations of uranogenic lead isotopes in lavas from ocean islands such as Hawaii or Iceland, when interpreted as model isochrons, have yielded source differentiation ages between 1 and 2.5?billion years (Gyr). However, if such correlations are produced by mixing of unrelated mantle components they will have no direct age significance. Re-Os decay model ages take into account the mixing of sources with different histories, but they depend on the assumed initial Re/Os ratio of the subducted crust, which is poorly constrained because of the high mobility of rhenium during subduction. Here we report the first data on (87)Sr/(86)Sr ratios for 138 melt inclusions in olivine phenocrysts from lavas of Mauna Loa shield volcano, Hawaii, indicating enormous mantle source heterogeneity. We show that highly radiogenic strontium in severely rubidium-depleted melt inclusions matches the isotopic composition of 200-650-Myr-old sea water. We infer that such sea water must have contaminated the Mauna Loa source rock, before subduction, imparting a unique 'time stamp' on this source. Small amounts of seawater-derived strontium in plume sources may be common but can be identified clearly only in ultra-depleted melts originating from generally highly (incompatible-element) depleted source components. The presence of 200-650-Myr-old oceanic crust in the source of Hawaiian lavas implies a timescale of general mantle circulation with an average rate of about 2 (±1)?cm?yr(-1), much faster than previously thought.     

Metabolites of diethylstilboestrol induce sister chromatid exchange in human cultured fibroblasts.

Diethylstilboesterol (DES) is one of the few substances for which a clear association with carcinogenicity has been established in man. Nevertheless, it is still widely used, mainly as a cheap oestrogen to increase the slaughter weight of beef, but in spite of this it is not known if residues in the meat or metabolites excreted by the cattle are hazardous to man. It is also unknown whether there is a threshold dose below which DES is harmless. A threshold might be expected if a hormonal mechanism of carcinogensis rather than metabolic activation to an electrophically reactive species operats. This possibility was supported by the observations that DES, in contrast to most other carcinogens, failed to induce mutations in the Salmonella/microsome test or malignant transformations of eukaryotic cells in culture. It is also disturbing that DES, one of the few known human carcinogens was negative in these two most widely used short-term tests introduced as fast early-earning system for potential carcinogens. We now report that DES is positive in sister chromatid exchange (SCE) induction, a short-term test for which a high correlation with the carcinogenicity of the compounds tested has been observed. Moreover, we show that metabolic activation was involved. Two different pathways leading to metabolites much more active in SCE induction than DES itself ('proximate agents') were established.     

Deep carbon export from a Southern Ocean iron-fertilized diatom bloom

Fertilization of the ocean by adding iron compounds has induced diatom-dominated phytoplankton blooms accompanied by considerable carbon dioxide drawdown in the ocean surface layer. However, because the fate of bloom biomass could not be adequately resolved in these experiments, the timescales of carbon sequestration from the atmosphere are uncertain. Here we report the results of a five-week experiment carried out in the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current, during which we tracked sinking particles from the surface to the deep-sea floor. A large diatom bloom peaked in the fourth week after fertilization. This was followed by mass mortality of several diatom species that formed rapidly sinking, mucilaginous aggregates of entangled cells and chains. Taken together, multiple lines of evidence-although each with important uncertainties-lead us to conclude that at least half the bloom biomass sank far below a depth of 1,000 metres and that a substantial portion is likely to have reached the sea floor. Thus, iron-fertilized diatom blooms may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments.     

Divergent effects of the major mast cell products histamine, tryptase and TNF-alpha on human fibroblast behaviour

Fibroblast proliferation is a key process in tissue remodeling and mast cells (MCs) are thought to play a crucial role. Having established that the three major MC products, tryptase, histamine and TNF-alpha (TNF) are normally present in human skin MCs, which are in close proximity to dermal fibroblasts, we studied their individual effects on cell cycle-controlled human dermal fibroblasts (HFFF2). These cells express receptors (H1, PAR2, TNFR1/2) for the major MC mediators, but only tryptase or a PAR2 agonist peptide stimulated proliferation and gene expression. TNF was antimitotic, and histamine, while elevating intracellular Ca²⁺ levels at high concentrations, did not affect proliferation. We conclude that MC products but also composition and numbers of respective receptors on fibroblasts are crucially responsible for fibroproliferative events. Received: 28 June 2005; received after revision 28 September 2005; accepted 6 October 2005     

The effects of vitamin A nutritional status on microsomal lipid peroxidation and α-tocopherol level in rat liver

Summary In vitamin A-deficient rats, liver glutathione peroxidase activity was decreased, -tocopherol content was strongly enhanced, but microsomal liquid peroxidation remained unchanged.     

钢桥焊接构件疲劳寿命预测

分析了影响焊接接头疲劳寿命的各种因素,在Dugdale模型基础上构造裂纹闭合模型,并将其应用到焊接接头的表面半椭圆形裂纹,其中考虑了影响区的材性变化、焊接残余应力以及焊缝几何带来的应力集中,通过与两个试验结果的比较表明：该法能估计裂纹扩展阶段的疲劳寿命,可用于焊接钢桥抗疲劳设计。