Control of nitrogenase inAzospirillum sp.

Many N₂-fixing organisms can turn off nitrogenase activity in the presence of NH₄ ⁺ and turn it on again when the NH₄ ⁺ is exhausted. One of the most interesting systems for accomplishing this is by covalent modification of one subunit of dinitrogenase reductase by dinitrogenase reductase ADP-ribosyltransferase (DRAT). The system can be reactivated when NH₄ ⁺ is exhausted, by dinitrogenase reductase activating glycohydrolase (DRAG) which removes the inactivating group. It is fascinating that some species of the genusAzospirillum possess the DRAT and DRAG systems (A. lipoferum andA. brasilense), whereasA. amazonense in the same genus lacks DRAT and DRAG.A. amazonense responds to NH₄ ⁺ but does not exhibit modification of dinitrogenase reductase characteristic of the action of DRAT. However, it has been possible to clone DRAT and DRAG and to introduce them intoA. amazonense, whereupon they become functional in this organism. The DRAT and DRAG system does not appear to function inAcetobacter diazotrophicus, an organism isolated from sugar cane, that fixes N₂ at a pH as low as 3.0.A. diazotrophicus does show a rather sluggish response to NH₄ ⁺. A level of about 10 M NH₄ ⁺ is required to switch off the system. The response to NH₄ ⁺ is influenced by the dissolved oxygen concentration (DOC) as has been reported forAzospirillum sp. A DOC in equilibrium with 0.1 to 0.2 kPa O₂ seems optimal for the response inA. diazotrophicus.     

Oxidation of dimethyl sulfide byPseudomonas acidovorans DMR-11 isolated from peat biofilter

Summary Pseudomonas acidovorans DMR-11, capable of oxidizing dimethyl sulfide (DMS), was isolated from peat biofilter. DMS as a sole carbon or energy source was not degraded, but it was co-degraded in the medium containing organic carbon sources. The removal rate of DMS in heat-treated glucose medium was 1.12×10^–17 mole/h cell at 30 °C. Dimethyl sulfoxide (DMSO) was the only product of DMS oxidation and was formed stoichiometrically. DMS was reversibly evolved in excess of DMSO. The cell free extract of strain DMR-11 oxidized DMS in presence of NADPH.     

Multiple alanine replacements within alpha-helix 126-134 of T4 lysozyme have independent, additive effects on both structure and stability.

In a systematic attempt to identify residues important in the folding and stability of T4 lysozyme, five amino acids within alpha-helix 126-134 were substituted by alanine, either singly or in selected combinations. Together with three alanines already present in the wild-type structure this provided a set of mutant proteins with up to eight alanines in sequence. All the variants behaved normally, suggesting that the majority of residues in the alpha-helix are nonessential for the folding of T4 lysozyme. Of the five individual alanine substitutions it is inferred that four result in slightly increased protein stability and one, the replacement of a buried leucine with alanine, substantially decreased stability. The results support the idea that alanine is a residue of high helix propensity. The change in protein stability observed for each of the multiple mutants is approximately equal to the sum of the energies associated with each of the constituent substitutions. All of the variants could be crystallized isomorphously with wild-type lysozyme, and, with one trivial exception, their structures were determined at high resolution. Substitution of the largely solvent-exposed residues Asp 127, Glu 128, and Val 131 with alanine caused essentially no change in structure except at the immediate site of replacement. Substitutions of the partially buried Asn 132 and the buried Leu 133 with alanine were associated with modest (< or = 0.4 A) structural adjustments. The structural changes seen in the multiple mutants were essentially a combination of those seen in the constituent single replacements. The different replacements therefore act essentially independently not only so far as changes in energy are concerned but also in their effect on structure. The destabilizing replacement Leu 133-->Ala made alpha-helix 126-134 somewhat less regular. Incorporation of additional alanine replacements tended to make the helix more uniform. For the penta-alanine variant a distinct change occurred in a crystal-packing contact, and the "hinge-bending angle" between the amino- and carboxy-terminal domains changed by 3.6 degrees. This tends to confirm that such hinge-bending in T4 lysozyme is a low-energy conformational change.     

Functional mapping of the surface of Escherichia coli ribose-binding protein: mutations that affect chemotaxis and transport.

Ribose-binding protein is a bifunctional soluble receptor found in the periplasm of Escherichia coli. Interaction of liganded binding protein with the ribose high affinity transport complex results in the transfer of ribose across the cytoplasmic membrane. Alternatively, interaction of liganded binding protein with a chemotactic signal transducer, Trg, initiates taxis toward ribose. We have generated a functional map of the surface of ribose-binding protein by creating and analyzing directed mutations of exposed residues. Residues in an area on the cleft side of the molecule including both domains have effects on transport. A portion of the area involved in transport is also essential to chemotactic function. On the opposite face of the protein, mutations in residues near the hinge are shown to affect chemotaxis specifically.     

不同利用方式下稻田效益的综合评价

一、引言评价是多目标系统优化和决策的基础。和任何其他农业生态系统一样,稻田生态系统具有多目标、多功能的特点,单项指标难以反映系统的优劣。目前,川东南地区稻田开发利用方式多种多样,但效益评价主要是采用简单的比较分析或粗放的定性描述,尚未形成系统的     

脂质过氧化对人红细胞膜脂流动性的影响

研究枯稀过氧化氢/高铁血红素体系所产生的烷基过氧自由基对红细胞的损伤。测定了脂质过氧化的产物——丙二脂的生成,并证明阿魏酸钠对脂质过氧化的抑制。荧光偏振的结果指出,膜脂过氧化以后降低了膜脂的流动性。人红细胞用5DSA和16DSA标记并用ESR检测膜脂流动性,结果表明,序参数S几乎没有发生变化,旋转相关时间τ值的增加证明膜脂过氧化以后,疏水尾部的物理状态发生了改变。经脂质过氧化以后,红细胞膜中的不饱和脂防酸的减少,可能是降低膜脂流动性的原因之一。     

立地性日照长短与甜橙生长发育的相关性

甜橙[Citrus sinensis(L.)Osbesk,本文中均指雪柑(Citrus sinensis var.sekkan)]对日照的需要量,比温州蜜柑(Citrus reticulata var.unshiu)等桔类要少些,即温州蜜柑比甜橙和杂柑类的耐荫性稍弱。然而,究竟可少到什么临界值呢?同一果园,同一管理水平,同一树龄,由于坡向、地形等的不同,而