在古尔班通古特沙漠中柽柳（Tamarix spp.）群落的侵移与更新研究

 柽柳(Tamarix spp.)在荒漠的非河岸区自然侵移现象已被发现。生长季偶发大雨引起暂时性积水,在土层结构和种源的适当配合下,有可能引起柽柳的自然发生过程。这种机遇较少出现。故实生苗群数量较少,年龄结构亦非连续性。但植丛寿命长,在漫长生活周期中,这种机遇终将出现,如无人为因素干扰,这类植丛将能持续地补充幼体而实现更新。     

Acetylation of O-specific lipopolysaccharides from Shigella flexneri 3a and 2a occurs in Escherichia coli K-12 carrying cloned S. flexneri 3a and 2a rfb genes.

Most of the Shigella flexneri O-specific serotypes result from O-acetyl and/or glucosyl groups added to a common O-repeating unit of the lipopolysaccharide (LPS) molecule. The genes involved in acetylation and/or glucosylation of S. flexneri LPS are physically located on lysogenic bacteriophages, whereas the rfb cluster contains the biosynthesis genes for the common O-repeating unit (D.A.R. Simmons and E. Romanowska, J. Med. Microbiol. 23:289-302, 1987). Using a cosmid cloning strategy, we have cloned the rfb regions from S. flexneri 3a and 2a. Escherichia coli K-12 containing plasmids pYS1-5 (derived from S. flexneri 3a) and pEY5 (derived from S. flexneri 2a) expressed O-specific LPS which reacted immunologically with S. flexneri polyvalent O antiserum. However, O-specific LPS expressed in E. coli K-12 also reacted with group 6 antiserum, indicating the presence of O-acetyl groups attached to one of the rhamnose components of the O-repeating unit. This was confirmed by measuring the amounts of acetate released from purified LPS samples and also by the chemical removal of O-acetyl groups, which abolished group 6 reactivity. The O-acetylation phenotype was absent in an E. coli strain with an sbcB-his-rfb chromosomal deletion and could be restored upon conjugation of F' 129, which carries sequences corresponding to a portion of the deleted region. Our data demonstrate that E. coli K-12 strains possess a novel locus which directs the O acetylation of LPS and is located in the sbcB-rfb region of the chromosomal map.     

Crystallization and preliminary X-ray structure analysis of pigeon egg-white lysozyme.

Calcium binding lysozyme from pigeon egg-white was crystallized by the hanging drop vapor diffusion technique using ammonium sulphate as a precipitant. The crystals belong to the orthorhombic system, space group P2(1)2(1)2(1), and have unit cell dimensions of a = 34.2 A, b = 34.8 A, and c = 99.4 A. One asymmetric unit contains one molecule of the pigeon lysozyme. The crystals diffract X-rays at least to 2.0 A resolution and are suitable for high resolution structure analysis. The diffraction data up to 3.0 A resolution were collected with a diffraction image processor, DIP100, using a Fuji imaging plate as an area detector. The structure was solved by the molecular replacement technique and refined to an R factor of 0.216. Least-squares fitting of the main-chains of pigeon egg-white lysozyme with those of chicken egg-white lysozyme and baboon alpha-lactalbumin showed that the main-chain folding of pigeon lysozyme is more similar to that of chicken lysozyme than that of alpha-lactalbumin. The largest differences between the pigeon and chicken lysozymes are in the surface loop regions.     

A soluble mitochondrial protein increases the voltage dependence of the mitochondrial channel,VDAC

A soluble protein isolated from mitochondria has been found to modulate the voltage-dependent properties of the mitochondrial outer membrane channel, VDAC. This protein, called the VDAC modulator, was first found inNeurospora crassa and then discovered in species from other eukaryotic kingdoms. The modulator-containing fraction (at a crude protein concentration of 20 µg/ml) increases the voltage dependence of VDAC channels over 2–3-fold. At higher protein concentrations (50–100 µg/ml), some channels seem to remain in a closed state or be blocked while others display the higher voltage dependence and are able to close at low membrane potentials. By increasing the steepness of the voltage-dependent properties of VDAC channels, this modulator may serve as an amplifierin vivo to increase the sensitivity of the channels in response to changes in the cell's microenvironment, and consequently, regulate the metabolic flux across the outer mitochondrial membrane by controlling the gating of VDAC channels.     

Metastable ripple phase of fully hydrated dipalmitoylphosphatidylcholine as studied by small angle x-ray scattering

Fully hydrated dipalmitoylphosphatidylcholine (DPPC) undergoes liquid crystalline to metastable P_β, phase transition in cooling. A small angle x-ray scattering study has been performed for obtaining further evidence about the structure of this phase. From a high-resolution observation of x-ray diffraction profiles, a distinct multipeak pattern has become obvious. Among them the (01) reflection in the secondary ripple structure is identified clearly. There are peaks assigned straightforwardly to (10) and (20) reflections in the primary ripple structure and peaks assigned to (10) and (20) reflections in the secondary ripple structure. Therefore the multipeak pattern is due to superposition of the reflections cause by the primary and secondary ripple structures. The lattice parameters are estimated as follows: for the primary ripple structure a = 7.09 nm, b = 13.64 nm, and γ = 95°, and for the secondary ripple structure a = 8.2 nm, b = 26.6 nm, and γ = 90°. The lattice parameters thus obtained for the secondary ripple structure are not conclusive, however. The hydrocarbon chains in the primary ripple structure have been reported as being tilted against the bilayer plane and, on the other hand, the hydrocarbon chains in the secondary ripple structure are likely to be perpendicular to the bilayer plane. This fact seems to be related to a sequential mechanism of phase transitions. On heating from the L_β, phase where the hydrocarbon chains are tilted the primary ripple structure having tilted hydrocarbon chains takes place and on cooling from the L_α phase where the hydrocarbon chains are not tilted the secondary ripple structure with untilted chains tends to be stabilized. It appears that the truly metastable ripple phase is expressed by the second ripple structure although in the course of the actual cooling transition both the secondary and primary ripple structures form and coexist.     

Early Stimulation of Phosphatidylcholine Biosynthesis During Wallerian Degeneration of Rat Sciatic Nerve

Phospholipid metabolism was studied in rat sciatic nerve during Wallerian degeneration induced by crush injury. Portions of crushed sciatic nerve, incubated with labeled substrates, showed significantly higher phosphatidylcholine synthesis than normal nerve, prior to any measurable alterations of phospholipid composition. Maximum synthesis occurred 3 days after crush injury, at which time the metabolism of other phospholipids was unchanged. After a rapid decrease in biosynthetic activity, a second phase of enhanced phosphatidylcholine synthesis occurred, beginning 6 days after crush injury. Increased incorporation of [33P]phosphate, [2-3H]glycerol, and [Me-14C]choline indicated stimulation of de novo synthesis of phosphatidylcholine 3 days after injury. Neither base exchange reactions nor sequential methylation of ethanolamine phospholipids contributed significantly to phosphatidylcholine synthesis. Assay of certain key enzymes under optimal conditions in subcellular fractions of sciatic nerve revealed higher activities of cholinephosphate cytidyltransferase, choline phosphotransferase, and acyl-CoA:lysophosphatidylcholine acyltransferase in injured nerve, while choline kinase activity remained unchanged. This indicates that stimulation of phosphatidylcholine synthesis occurs via the cytidine nucleotide pathway, as well as by increased acylation of lysophosphatidylcholine. Although the cause of stimulated phosphatidylcholine synthesis remains unexplained, it is possible that trace amounts of lysophospholipids or other metabolites produced by injury-enhanced phospholipase activity may be responsible.     

Serum lipoproteins and albumin in the lecithin: Cholesterol acyltransferase reaction

The unique features of pig ovarian follicular fluids, i.e., presence of high density lipoprotein (HDL) only and lecithin: cholesterol acyltransferase (EC 2.3.1.43; LCAT) activity, provides a good model to study the effect of serum lipoproteins and serum albumin on the LCAT reaction. $\text{In}\text{vitro}$ cholesterol esterification is enhanced when very low density lipoprotein (VLDL) and low density lipoprotein (LDL) fractions are added, but is inhibited when one or the other of these lipoproteins is absent. High concentrations of HDL₂ result in decreased activation which can be compensated for by the addition of the VLDL-LDL mixture. These findings suggest that the rate of cholesterol esterification in ovarian follicular fluid may be enhanced by providing the exogenous VLDL and LDL as the recipients of HDL-cholesteryl ester. The inhibition of LCAT activity caused by free fatty acid and lysophosphatidylcholine can be partially reversed by the addition of serum albumin, suggesting that serum albumin may regulate the LCAT reaction.     

Anthraquinones in the biosynthesis of sterigmatocystin by Aspergillus versicolor.

14C-labeled averufin, versiconal hemiacetal acetate, and versicolorin A were efficiently converted to sterigmatocystin by Aspergillus versicolor, thus providing experimental evidence that these anthraquinones are biosynthetic precursors of sterigmatocystin, a xanthone.     

缙云山4种林分土壤无机磷与活性铝的含量及分布

为探索不同林分对土壤中无机磷与活性铝的含量及分布的影响,以及无机磷与活性铝之间的相互关系,以在我国西南地区酸性土壤上的农林经营管理提供理论和实践依据。研究以缙云山广泛分布的4种林分类型：山莓、马尾松、楠竹和柑橘林土壤为对象,采用酸性土壤无机磷分级方法和铝试剂比色法测定了土壤剖面A、B和C层中各形态的无机磷与活性铝的含量,分析了不同林分土壤中无机磷和活性铝的含量和分布特征。结果表明,林分类型显著影响土壤中无机磷与活性铝的含量与分布,且铁磷(Fe-P)、交换态铝(Ex-Al)和羟基铝(Hy-Al)的含量和比例还受到土壤层次的影响。4种林分相比,山莓林能促进闭蓄态磷(O-P)的形成,而马尾松、楠竹和柑橘林则有利于Al-P、Fe-P和Ca-P的形成;柑橘林有利于低活性的腐殖酸铝(Al-AH)形成,而山莓、马尾松和楠竹林促进高活性的Ex-Al或Hy-Al溶出;Ex-Al、Al-P和Fe-P在土层的分布上有表层富集现象,而Hy-Al集中分布于B层。此外,土壤Ex-Al和Al-P与Fe-P之间,Hy-Al和Ca-P之间均呈显著(P<0.05)正相关。因此,林分类型显著影响土壤无机磷与活性铝的含...     

基于土地利用及植被覆盖变化的黄河源区生境质量时空变化特征

位于青藏高原腹地的黄河源地区生态环境脆弱,面临生物多样性锐减、生态系统退化等问题,黄河源区生态系统保护及其高质量发展已成为国家的重点战略之一。土地利用与植被覆盖是影响生境质量的重要因素,定量化土地利用方式、强度及格局和植被覆盖格局对生态质量影响的研究越来越受到关注,但其对黄河源区生态质量的耦合效应尚不明确。基于2000年和2015年黄河源区土地利用类型及生长季归一化植被指数（NDVI）,采用InVEST模型探究了不同时期黄河源区生境质量时空变化,并采用地理加权回归（GWR）模型揭示了生境质量对土地利用和植被覆盖变化的空间响应特征。结果表明,2000年与2015年土地利用类型变化主要为未利用土地向草地的转移。植被覆盖变化方面,源区生长季NDVI整体上升。从生境质量的空间分布来看,黄河源区生境质量总体呈现南高北低的空间格局,高值分布在南部及中部地区,低值分布在北部布青山、东北部高海拔区及黄河乡的黄河沿岸。相较于2000年,2015年黄河源区生境质量平均提高11.47%。草地面积和NDVI与生境质量均呈显著正相关关系,其中NDVI是提高黄河源区生境质量的重要驱动因子。研究结果突出了NDVI对提高黄河源区生境质量的主导作用,可为未来源区生态保护提供借鉴。