Time-resolved dynamics in N2O4 probed using high harmonic generation

The attosecond time-scale electron-recollision process that underlies high harmonic generation has uncovered extremely rapid electronic dynamics in atoms and diatomics. We showed that high harmonic generation can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude vibrations in dinitrogen tetraoxide, we showed that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration but populates the first excited state at the inner turning point. This state-switching mechanism is manifested as bursts of high harmonic light that is emitted mostly at the outer turning point. Theoretical calculations attribute the large modulation to suppressed emission from the first excited state of the ion. More broadly, these results show that high harmonic generation and strong-field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals.     

农业土壤N_2O产生途径及其影响因素研究进展

基于目前国内外对农业土壤N_2O产生与排放过程的研究成果,分析了N_2O的产生途径及其研究方法、影响农业N_2O产生的主要因素。农业土壤N_2O产生的主要过程有硝化作用(自养硝化作用和异养硝化作用)、反硝化作用和硝化微生物反硝化作用。目前研究硝化和反硝化作用的研究方法主要包括15N示踪法和气体抑制剂抑制法;影响土壤硝化—反硝化作用及N_2O产生的因素主要包括土壤基质、土壤物理性质、土壤化学性质、生物因素以及人类活动等。在此基础上探讨了目前研究中存在的主要问题,并对今后研究提出展望与建议。     

Structural basis of Wnt recognition by Frizzled

Wnts are lipid-modified morphogens that play critical roles in development principally through engagement of Frizzled receptors. The 3.25 angstrom structure of Xenopus Wnt8 (XWnt8) in complex with mouse Frizzled-8 (Fz8) cysteine-rich domain (CRD) reveals an unusual two-domain Wnt structure, not obviously related to known protein folds, resembling a "hand" with "thumb" and "index" fingers extended to grasp the Fz8-CRD at two distinct binding sites. One site is dominated by a palmitoleic acid lipid group projecting from serine 187 at the tip of Wnt's thumb into a deep groove in the Fz8-CRD. In the second binding site, the conserved tip of Wnt's "index finger" forms hydrophobic amino acid contacts with a depression on the opposite side of the Fz8-CRD. The conservation of amino acids in both interfaces appears to facilitate ligand-receptor cross-reactivity, which has important implications for understanding Wnt's functional pleiotropy and for developing Wnt-based drugs for cancer and regenerative medicine.     

Structural basis of Smad2 recognition by the Smad anchor for receptor activation

The Smad proteins mediate transforming growth factor-beta (TGFbeta) signaling from the transmembrane serine-threonine receptor kinases to the nucleus. The Smad anchor for receptor activation (SARA) recruits Smad2 to the TGFbeta receptors for phosphorylation. The crystal structure of a Smad2 MH2 domain in complex with the Smad-binding domain (SBD) of SARA has been determined at 2.2 angstrom resolution. SARA SBD, in an extended conformation comprising a rigid coil, an alpha helix, and a beta strand, interacts with the beta sheet and the three-helix bundle of Smad2. Recognition between the SARA rigid coil and the Smad2 beta sheet is essential for specificity, whereas interactions between the SARA beta strand and the Smad2 three-helix bundle contribute significantly to binding affinity. Comparison of the structures between Smad2 and a comediator Smad suggests a model for how receptor-regulated Smads are recognized by the type I receptors.     

过氧化氢诱导蚕豆气孔保卫细胞一氧化氮的产生

过氧化氢(H2O2)和一氧化氮(NO)已被证明是植物中的信号分子,生物和非生物胁迫都可以导致H2O2和NO的产生.以蚕豆下表皮为材料,通过表皮条生物分析方法和激光扫描共聚显微镜显微技术,检测了H2O2诱导蚕豆气孔关闭过程中NO的产生.H2O2以浓度依赖的方式诱导气孔关闭,NO也有类似的作用.100 μmol/L H2O2诱导的气孔关闭作用可明显地被25 μmol/L一氧化氮合酶(L-NAME)专一性抑制剂或200 μmol/L一氧化氮清除剂(cPTIO)逆转.但L-NAME和cPTIO单独处理时对气孔开度几乎无影响.表皮条经100 μmol/L H2O2处理,再用NO分子探针乙酰乙酸盐-4,5-二氨基荧光黄(DAF-2 DA)孵育后,与对照相比,保卫细胞胞质中荧光强度明显增加.结果表明NO可能参与H2O2诱导的气孔关闭,保卫细胞中H2O2可能是通过NOS途径诱导NO的产生,而且H2O2可能作为上游信号分子诱导NO的产生.     

Structural basis of mitochondrial tethering by mitofusin complexes

Vesicle fusion involves vesicle tethering, docking, and membrane merger. We show that mitofusin, an integral mitochondrial membrane protein, is required on adjacent mitochondria to mediate fusion, which indicates that mitofusin complexes act in trans (that is, between adjacent mitochondria). A heptad repeat region (HR2) mediates mitofusin oligomerization by assembling a dimeric, antiparallel coiled coil. The transmembrane segments are located at opposite ends of the 95 angstrom coiled coil and provide a mechanism for organelle tethering. Consistent with this proposal, truncated mitofusin, in an HR2-dependent manner, causes mitochondria to become apposed with a uniform gap. Our results suggest that HR2 functions as a mitochondrial tether before fusion.     

土壤pH影响氧化亚氮(N2O)排放的研究进展

氧化亚氮(N_2O)是第三大温室气体,也是21世纪内平流层臭氧(O_3)的首要分解者。在过去约150年间,大气中N_2O的浓度持续增加,其主要原因在于化肥和有机肥料刺激下土壤中N_2O的大量排放。因此,理解土壤中N_2O的排放机制与影响因素,已经成为估算N_2O排放清单和制定N_2O减排政策的关键科学问题。土壤pH是影响N_2O排放的重要环境因子之一,但目前对其相对重要性和影响机制尚不明确。该文基于已有文献的梳理,总结了原位观测和室内培养研究中土壤pH与N_2O排放之间的统计结果,发现多数研究中N_2O排放与土壤pH呈显著负相关关系;并且从生物硝化、生物反硝化和非生物过程3个方面探讨了土壤pH影响N_2O排放的微观机制。在此基础上,本文对今后的研究工作提出展望,以期为后续的研究提供参考和依据。     

Structural basis for double-stranded RNA processing by Dicer

The specialized ribonuclease Dicer initiates RNA interference by cleaving double-stranded RNA (dsRNA) substrates into small fragments about 25 nucleotides in length. In the crystal structure of an intact Dicer enzyme, the PAZ domain, a module that binds the end of dsRNA, is separated from the two catalytic ribonuclease III (RNase III) domains by a flat, positively charged surface. The 65 angstrom distance between the PAZ and RNase III domains matches the length spanned by 25 base pairs of RNA. Thus, Dicer itself is a molecular ruler that recognizes dsRNA and cleaves a specified distance from the helical end.     

华北平原不同作物－潮土系统中N2O 排放量的测定

在中国科学院封丘农业生态试验站应用原状土柱培养法测定了华北平原主要农作物－潮土系统中N     

High harmonic generation from multiple orbitals in N2

Molecular electronic states energetically below the highest occupied molecular orbital (HOMO) should contribute to laser-driven high harmonic generation (HHG), but this behavior has not been observed previously. Our measurements of the HHG spectrum of N2 molecules aligned perpendicular to the laser polarization showed a maximum at the rotational half-revival. This feature indicates the influence of electrons occupying the orbital just below the N2 HOMO, referred to as the HOMO-1. Such observations of lower-lying orbitals are essential to understanding subfemtosecond/subangstrom electronic motion in laser-excited molecules.     

Isotopic signature of N(2)O produced by marine ammonia-oxidizing archaea

The ocean is an important global source of nitrous oxide (N(2)O), a greenhouse gas that contributes to stratospheric ozone destruction. Bacterial nitrification and denitrification are thought to be the primary sources of marine N(2)O, but the isotopic signatures of N(2)O produced by these processes are not consistent with the marine contribution to the global N(2)O budget. Based on enrichment cultures, we report that archaeal ammonia oxidation also produces N(2)O. Natural-abundance stable isotope measurements indicate that the produced N(2)O had bulk δ(15)N and δ(18)O values higher than observed for ammonia-oxidizing bacteria but similar to the δ(15)N and δ(18)O values attributed to the oceanic N(2)O source to the atmosphere. Our results suggest that ammonia-oxidizing archaea may be largely responsible for the oceanic N(2)O source.     

Structural basis for allosteric regulation of GPCRs by sodium ions

Pharmacological responses of G protein-coupled receptors (GPCRs) can be fine-tuned by allosteric modulators. Structural studies of such effects have been limited due to the medium resolution of GPCR structures. We reengineered the human A(2A) adenosine receptor by replacing its third intracellular loop with apocytochrome b(562)RIL and solved the structure at 1.8 angstrom resolution. The high-resolution structure allowed us to identify 57 ordered water molecules inside the receptor comprising three major clusters. The central cluster harbors a putative sodium ion bound to the highly conserved aspartate residue Asp(2.50). Additionally, two cholesterols stabilize the conformation of helix VI, and one of 23 ordered lipids intercalates inside the ligand-binding pocket. These high-resolution details shed light on the potential role of structured water molecules, sodium ions, and lipids/cholesterol in GPCR stabilization and function.     

Structural basis of glmS ribozyme activation by glucosamine-6-phosphate

The glmS ribozyme is the only natural catalytic RNA known to require a small-molecule activator for catalysis. This catalytic RNA functions as a riboswitch, with activator-dependent RNA cleavage regulating glmS messenger RNA expression. We report crystal structures of the glmS ribozyme in precleavage states that are unliganded or bound to the competitive inhibitor glucose-6-phosphate and in the postcleavage state. All structures superimpose closely, revealing a remarkably rigid RNA that contains a preformed active and coenzyme-binding site. Unlike other riboswitches, the glmS ribozyme binds its activator in an open, solvent-accessible pocket. Our structures suggest that the amine group of the glmS ribozyme-bound coenzyme performs general acid-base and electrostatic catalysis.     

Structural basis of gating by the outer membrane transporter FecA

Siderophore-mediated acquisition systems facilitate iron uptake. We present the crystallographic structure of the integral outer membrane receptor FecA from Escherichia coli with and without ferric citrate at 2.5 and 2.0 angstrom resolution. FecA is composed of three distinct domains: the barrel, plug, and NH2-terminal extension. Binding of ferric citrate triggers a conformational change of the extracellular loops that close the external pocket of FecA. Ligand-induced allosteric transitions are propagated through the outer membrane by the plug domain, signaling the occupancy of the receptor in the periplasm. These data establish the structural basis of gating for receptors dependent on the cytoplasmic membrane protein TonB. By compiling available data for this family of receptors, we propose a mechanism for the energy-dependent transport of siderophores.     

根际反硝化作用与N2O释放

对根际反硝化作用及N2O的释放进行了综述。反硝化过程与N2O的产生及释放有密切的关系。反硝化作用产生的N2O量不仅取决于反硝化速率，而且也取决于那些影响反硝化产物N2O／N2比值的参数。在植物根际这一特殊土壤区域中，反硝化作用受NO3^-,C及O2的综合影响，豆科作物根瘤是一个特殊的根际系统，根瘤菌的反硝化作用及N2O释放应引起更大的关注。植物根际不仅对反硝化作用产生影响，而且对N2O释放也起了重要的通道作用，特别是对渍水土壤有重要的意义。文章最后指出了一些尚需深入研究的问题。     

Structural basis for methyl transfer by a radical SAM enzyme

The radical S-adenosyl-L-methionine (SAM) enzymes RlmN and Cfr methylate 23S ribosomal RNA, modifying the C2 or C8 position of adenosine 2503. The methyl groups are installed by a two-step sequence involving initial methylation of a conserved Cys residue (RlmN Cys(355)) by SAM. Methyl transfer to the substrate requires reductive cleavage of a second equivalent of SAM. Crystal structures of RlmN and RlmN with SAM show that a single molecule of SAM coordinates the [4Fe-4S] cluster. Residue Cys(355) is S-methylated and located proximal to the SAM methyl group, suggesting the SAM that is involved in the initial methyl transfer binds at the same site. Thus, RlmN accomplishes its complex reaction with structural economy, harnessing the two most important reactivities of SAM within a single site.     

Structural basis of DNA replication origin recognition by an ORC protein

DNA replication in archaea and in eukaryotes share many similarities. We report the structure of an archaeal origin recognition complex protein, ORC1, bound to an origin recognition box, a DNA sequence that is found in multiple copies at replication origins. DNA binding is mediated principally by a C-terminal winged helix domain that inserts deeply into the major and minor grooves, widening them both. However, additional DNA contacts are made with the N-terminal AAA+ domain, which inserts into the minor groove at a characteristic G-rich sequence, inducing a 35 degrees bend in the duplex and providing directionality to the binding site. Both contact regions also induce substantial unwinding of the DNA. The structure provides insight into the initial step in assembly of a replication origin and recruitment of minichromosome maintenance (MCM) helicase to that origin.     

巢湖流域河流沉积物N2O释放对水体溶存N2O贡献研究

2010年11月在巢湖支流丰乐河与杭埠河采用现场培养和实测方法研究了沉积物N2O释放对水体（水柱）溶存N2O的影响,并采用NO-3削减法估算了沉积物的反硝化速率。研究结果表明,丰乐河与杭埠河N2O平均溶存浓度分别为0.26±0.10（SD）μg N-N2O·L-1和0.18±0.04（SD）μg N-N2O·L-1,饱和度分别为186%和151%,表明两条河流为大气N2O的潜在释放源。丰乐河与杭埠河沉积物-水界面N2O平均释放通量分别为0.39±0.44、0.15±0.16 μg N-N2O·m-2·h-1,由此可分别贡献水体中约89% 和45%的溶存N2O。对河流沉积物的反硝化速率估算结果表明,丰乐河与杭埠河沉积物反硝化速率分别为0.12±0.07、0.10±0.05 mg N·m-2·h-1,与已有的沉积物反硝化速率报道相比较低。     

Inducible nitric oxide synthase binds, S-nitrosylates, and activates cyclooxygenase-2

Cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) are two major inflammatory mediators. Here we show that iNOS specifically binds to COX-2 and S-nitrosylates it, enhancing COX-2 catalytic activity. Selectively disrupting iNOS-COX-2 binding prevented NO-mediated activation of COX-2. This synergistic molecular interaction between two inflammatory systems may inform the development of anti-inflammatory drugs.