Conformational properties of topoisomerase II inhibitors and sequence specificity of DNA cleavage

The sequence specificity of topoisomerase-II-mediated DNA cleavage, stimulated by 2-methyl-9-hydroxy ellipticinium and 4′, 5′,7-trihydroyflavone (genistein) was investigated by sequencing analysis of DNA cleavage sites and molecular modeling techniques. The former drug exhibits a marked preference for a T base at the position immediately preceding the cleavage site (?1). The latter shares the preference for the same base, with an additional preference for a thymine at position +1. The cleavage intensity patterns for the two drugs differ considerably. From a conformational point of view, ellipticinium and genistein exhibit similar overall shape and dimensions. However, the fused ring system in the former generates a planar structure whereas the single bond, connecting the two aromatic portions in the latter, allows internal rotation. The most stable conformation of genistein corresponds to a deviation of about 40° from planarity. A computer-assisted analysis was carried out to compare the steric and electrostatic properties of the two compounds. Two types of preferred (energetically almost degenerate) alignment for the two molecules were found. One corresponds to overlapping of the 9-hydroxyl containing ring of ellipticinium with the 4′-hydroxyphenyl moiety of genistein, the other envisages the same moiety of ellipticine superimposed to the hydroxyl-benzopyrone portion of genistein. The structural similarities of the test drugs might account for the common preference for stimulation of DNA cleavage at position +1, whereas the different possible arrangements of genistein in the cleavable complex could explain both the additional +1 specificity exhibited by this compound and the differences in cleavage intensity patterns observed in comparison to ellipticinium.     

质体醌重组的D1／D2／Cytb559复合物的荧光衰减动力学和光破坏作用

光系统Ⅱ反应中心Ｄ１／Ｄ２／Ｃｙｔｂ５５９ 在分离纯化过程中失去了电子受体ＱＡ 和ＱＢ,人工合成的质体醌可以与Ｄ１／Ｄ２／Ｃｙｔｂ５５９ 复合物发生重组。癸基质体醌（ＤＰＱ）与Ｄ１／Ｄ２／Ｃｙｔｂ５９９ 复合物的重组导致该复合物的荧光强度下降及发射光谱蓝移,同时两个与光化学活性相关的长寿命（２４ ｎｓ和７３ ｎｓ）荧光衰减组分占整个荧光的百分数下降,这些结果表明ＤＰＱ作为Ｐｈｅｏ－ 的电子受体,限制了Ｐ６８０＋ ·Ｐｈｅｏ－ 的电荷重组。ＤＰＱ 的加入对Ｄ１／Ｄ２／Ｃｙｔｂ５５９复合物中Ｃｈｌａ 分子的光破坏敏感性影响不大,但β－胡萝卜素在加入ＤＰＱ 之后可以被光照破坏,这个过程可能与β－胡萝卜素的生理功能相关。     

Five-stranded β-sheet sandwiched with two α-helices: A structural link between restriction endonucleases EcoRI and EcoRV

Examination of crystal structures of restriction endonucleases EcoRI and EcoRV complexes with their cognate DNA revealed a common structural element, which forms the core of both proteins. This element consists of a five-stranded β-sheet and two α-helices packed against it and could be described as α–β sandwich in which helices and β-strands lie in two stacked layers. While the spatial structure of this α–β sandwich is conserved in both enzymes, there are no detectable similarities between amino acid sequences except of a few residues involved in active site formation. Probably, other restriction endonucleases which have similar organization of the active site might possess similar structural element regardless of DNA sequence recognized and recognition elements in the enzyme used. © 1994 Wiley-Liss, Inc.     

利用林木边行优势营造杨树速生丰产林的试验研究──林农复合生态经济系统的新模式

利用林木边行优势营造杨树速生丰产林的试验研究──林农复合生态经济系统的新模式许鸿儒,刘金声（辽宁省黑山县林业局,１２１４００）（辽宁省林业厅造林处,沈阳１１０００１）范学文（辽宁省锦西市林业局,１２１５００）ＰｌａｎｔｉｎｇｏｆＦａｓｔ－Ｇｒｏｗｉｎ...     

Cytogenetical localization of Zygotic hybrid rescue (Zhr), a Drosophila melanogaster gene that rescues interspecific hybrids from embryonic lethality

Hybrid females from crosses between Drsophila melanogaster males and females of its sibling species, D. simulans, D. mauritiana, or D. sechellia die as embryos. This lethality is believed to be caused by incompatibility between the X chromosome of D. melanogaster and the maternal cytoplasm. Zygotic hybrid rescue (Zhr) prevents this embryonic lethality and has been cytogenetically mapped to a proximal region of the X chromosome of D. melanogaster, probably in the centromeric heterochromatin. We have carried out high resolution cytological mapping of Zhr using deficiencies and duplications of the X heterochromatin. Deletions of the Zhr ⁺ gene from the hybrid genome exhibit the Zhr phenotype. On the contrary, addition of the wild-type gene to the hybrid genome causes embryonic lethality, regardless of sex. The Zhr locus has been narrowed down to the region covered by Dp(1;f)1162 but not covered Dp(1;f)1205, a chromosome carrying a duplication of heterochromatin located slightly distal to the In(1)sc ⁸ heterochromatic breakpoint.     

Inhibition of mugineic acid-ferric complex uptake in barley by copper, zinc and cobalt

The interfering effects of copper, zinc, and cobalt on the uptake of mugineic acid-ferric complex were studied in barley ( Hordeum vulgare , cv. Minorimugi) grown in nutrient solution. Short-term uptake experiments of 3 h were performed utilizing both ionic and mugineic acid-complex forms of each metal at two different concentrations. Copper was most effective in decreasing iron uptake when added in an ionic form at either concentration. The inhibition order at higher concentrations followed Cu(II) > Zn(II) ≥ Co(II), Co(III), which is consistent with the stability constants of these metal complexes with mugineic acid. The displacement of iron from its mugineic acid complex by these metals is suggested as a probable explanation for the decreased iron uptake. The inhibitory effect of metal complexes with mugineic acid on iron uptake was only found in cases with higher concentrations of Cu(II) and Zn(II) complexes. Deformation of the specific iron transport system in the plasma membrane due to their adsorption may be responsible for this effect.     

The 28 kDa apoprotein of CP 26 in PS II binds copper

Photosystem II (PS II) particles isolated from spinach in the presence of 10 M CuSO₄ contained 1.2 copper/300 Chl that was resistant to EDTA. When CuSO₄ was not added during the isolation, PS II particles contained variable amounts of copper resistant to EDTA (0.1–1.1 copper/300 Chl). No correlation was found between copper content and oxygen evolving capacity of the PS II particles. To identify the copper binding protein, we developed a fractionation procedure which included solubilisation of PS II particles followed by precipitation with polyethylene glycol. A 22-fold purification of copper with respect to protein was achieved for a 28 kDa protein. Partial amino acid sequence of a 13 kDa fragment, obtained after V8 (endo Glu-C) protease treatment, showed identity with CP 26 over a 14 amino acid stretch. EPR measurements on the purified protein suggest oxygen and/or nitrogen as ligands for copper but tend to exclude sulfur. We conclude that the 28 kDa apoprotein of CP 26 from spinach binds one copper per molecule of CP 26. A possible function for this copper protein in the xanthophyll cycle is discussed.Abbreviations CP 26 and CP 29 chlorophyll a/b protein complex 26 and 29 - LHC II light-harvesting chlorophyll a/b protein complex of Photosystem II - SB14 sulfobetaine 14 A preliminary report of these results was presented at the IX Int. Congress on Photosynthesis, Nagoya, Japan, 1992.     

Evaluation of the role of State transitions in determining the efficiency of light utilisation for CO2 assimilation in leaves

Wheat leaves were exposed to light treatments that excite preferentially Photosystem I (PS I) or Photosystem II (PS II) and induce State 1 or State 2, respectively. Simultaneous measurements of CO₂ assimilation, chlorophyll fluorescence and absorbance at 820 nm were used to estimate the quantum efficiencies of CO₂ assimilation and PS II and PS I photochemistry during State transitions. State transitions were found to be associated with changes in the efficiency with which an absorbed photon is transferred to an open PS II reaction centre, but did not correlate with changes in the quantum efficiencies of PS II photochemistry or CO₂ assimilation. Studies of the phosphorylation status of the light harvesting chlorophyll protein complex associated with PS II (LHC II) in wheat leaves and using chlorina mutants of barley which are deficient in this complex demonstrate that the changes in the effective antennae size of Photosystem II occurring during State transitions require LHC II and correlate with the phosphorylation status of LHC II. However, such correlations were not found in maize leaves. It is concluded that State transitions in C₃ leaves are associated with phosphorylation-induced modifications of the PS II antennae, but these changes do not serve to optimise the use of light absorbed by the leaf for CO₂ assimilation.Abbreviations Fm, Fo, Fv maximal, minimal and variable fluorescence yields - Fm, Fv maximal and variable fluorescence yields in a light adapted state - LHC II light harvesting chlorophyll a/b protein complex associated with PS II - q_P photochemical quenching - A₈₂₀ light-induced absorbance change at 820 nm - _{PS I}, _{PS II} relative quantum efficiencies of PS I and PS II photochemistry - _{CO 2} quantum yield of CO₂ assimilation     

Spectroscopic studies of the manganese complex of Photosystem II

Our recent EPR and EXAFS experiments investigating the structure of the oxygen-evolving complex of PS II are discussed. PS II treatments which affect the cofactors calcium and chloride have been used to poise samples in modified forms of the S-states, S₁, S₂ and S₃. X-ray absorption studies indicate a similar overall structure for the manganese complex between treated and native samples although the influence of the treatments and cofactors is observed. Manganese oxidation (or oxidation of a ligand to the manganese cluster) is indicated to occur on each of the transitions S₁ S₂ and S₂ S₃ in these modified samples. The cluster appears to contain at least two inequivalent Mn-Mn pairs. In the native samples the Mn-Mn distance is 2.7 Å, but in samples where the calcium site is affected, one of the pairs has a 3.0 Å Mn-Mn distance. The intensity of the 3.3/3.6 Å interaction is reduced on sodium chloride treatment (calcium depletion) perhaps indicating calcium binding close to the manganese cluster. From EPR data we also propose that treatments which affect calcium and chloride binding cause a modification of the native S₂ state, slow the reduction of Y_z and allow an S3 EPR signal to be observed following illumination. The origin of the S3 EPR signal, a modified S₃ or S₂ X where X is an organic radical of unknown charge, is discussed in relation to the results from the EXAFS studies.Abbreviations EPR electron paramagnetic resonance spectroscopy - EXAFS extended X-ray absorption fine structure - HTG n-heptyl -d-thioglucoside - MES 2(N-morpholino)ethanesulfonic acid - OEC oxygen evolving complex - PPBQ phenyl-1,4-benzoquinone - PS II Photosystem II - Y_z redox active tyrosine     

二硫苏糖醇对叶绿体H~＋－ATP酶复合体CF_0的修饰

ＤＴＴ（二流苏糖醇）可解除ＴＰＴ（氯化三苯锡）对叶绿体光合磷酸化的抑制、减弱ＴＰＴ对类囊体跨膜△ｐＨ的抑制和对类囊体腔内质子外流的促进。由于ＴＰＴ较专一作用于ＣＦ０,推测ＣＦ０受ＤＴＴ修饰。这从ＤＴＴ可消除ＴＰＴ对去除ＣＦ１的类囊体残缺膜△ｐＨ的重建和对残缺膜光下收缩的促进得到进一步的证明。ＤＴＴ的作用是还原二硫键成为游离的巯基,因而ＣＦ０可能含有二硫键。从光下ＤＴＴ或暗中ＤＴＴ处理残缺膜对ＴＰＴ作用的影响,可以看到ＤＴＴ对ＣＦ０的修饰是需光的,类囊体膜在光下发生的构象变化,使ＣＦ０的二硫键暴露而被ＤＴＴ修饰。ＣＦ０的二硫键较ＣＦ１γ的二硫键更快、更敏感地被ＤＴＴ所修饰。