Is there leaderless protein secretion in plants?

The sugar alcohol mannitol and it’s catabolic enzyme mannitol dehydrogenase (MTD), in addition to welldocumented roles in metabolism and osmoprotection, may play roles in hostpathogen interactions. Research suggests that in response to the mannitol that pathogenic fungi secrete to suppress reactive oxygen-mediated host defenses, plants make MTD to catabolize fungal mannitol. Yet previous work suggested that pathogen-secreted mannitol is extracellular, while in healthy plants MTD is cytoplasmic. We have presented results showing that the normally cytoplasmic MTD is exported into the cell wall or extracellular space in response to the endogenous inducer of plant defense responses salicylic acid (SA). This SA-induced secretion is insensitive to brefeldin A, an inhibitor of Golgimediated protein transport. Together with the absence of MTD in Golgi stacks and the lack of a documented extracellular targeting sequence in the MTD protein, this suggests MTD is secreted by a non-Golgi, pathogen-activated secretion mechanism in plants. Here we discuss the potential significance of non-Golgi secretion in response to stress.Key words: protein secretion, mannitol metabolism, plant-pathogen interaction, extracellular space, apoplast     

Breakage-reunion domain of Streptococcus pneumoniae topoisomerase IV: crystal structure of a gram-positive quinolone target

The 2.7 A crystal structure of the 55-kDa N-terminal breakage-reunion domain of topoisomerase (topo) IV subunit A (ParC) from Streptococcus pneumoniae, the first for the quinolone targets from a gram-positive bacterium, has been solved and reveals a 'closed' dimer similar in fold to Escherichia coli DNA gyrase subunit A (GyrA), but distinct from the 'open' gate structure of Escherichia coli ParC. Unlike GyrA whose DNA binding groove is largely positively charged, the DNA binding site of ParC exhibits a distinct pattern of alternating positively and negatively charged regions coincident with the predicted positions of the grooves and phosphate backbone of DNA. Based on the ParC structure, a new induced-fit model for sequence-specific recognition of the gate (G) segment by ParC has been proposed. These features may account for the unique DNA recognition and quinolone targeting properties of pneumococcal type II topoisomerases compared to their gram-negative counterparts.     

Phase-resetting curve determines how BK currents affect neuronal firing

BK channels are large conductance potassium channels gated by calcium and voltage. Paradoxically, blocking these channels has been shown experimentally to increase or decrease the firing rate of neurons, depending on the neural subtype and brain region. The mechanism for how this current can alter the firing rates of different neurons remains poorly understood. Using phase-resetting curve (PRC) theory, we determine when BK channels increase or decrease the firing rates in neural models. The addition of BK currents always decreases the firing rate when the PRC has only a positive region. When the PRC has a negative region (type II), BK currents can increase the firing rate. The influence of BK channels on firing rate in the presence of other conductances, such as I _m and I _h , as well as with different amplitudes of depolarizing input, were also investigated. These results provide a formal explanation for the apparently contradictory effects of BK channel antagonists on firing rates.     

A Durable Alternative for Proton‐Exchange Membranes: Sulfonated Poly(Benzoxazole Thioether Sulfone)s

To develop a durable proton‐exchange membrane (PEM) for fuel‐cell applications, a series of sulfonated poly(benzoxazole thioether sulfone)s ( SPTESBOs) are designed and synthesized, with anticipated good dimensional stability (via acid–base cross linking), improved oxidative stability against free radicals (via incorporation of thioether groups), and enhanced inherent stability (via elimination of unstable end groups) of the backbone. The structures and the degree of sulfonation of the copolymers are characterized using Fourier‐transform infrared spectroscopy, and nuclear magnetic resonance spectroscopy (¹H NMR and ¹⁹F NMR). The electrochemical stabilities of the monomers are examined using cyclic voltammetry in a typical three‐electrode cell configuration. The physicochemical properties of the membranes vital to fuel‐cell performance are also carefully evaluated under conditions relevant to fuel‐cell operation, including chemical and thermal stability, proton conductivity, solubility in different solvents, water uptake, and swelling ratio. The new membranes exhibit low dimensional change at 25°C to 90°C and excellent thermal stability up to 250°C. Upon elimination of unstable end groups, the co‐polymers display enhanced chemical resistance and oxidative stability in Fenton's test. Further, the SPTESBO‐HFB‐60 (HFB‐60=hexafluorobenzene, 60 mol% sulfone) membrane displays comparable fuel‐cell performance to that of an NRE 212 membrane at 80°C under fully humidified condition, suggesting that the new membranes have the potential to be more durable but less expensive for fuel‐cell applications.     

炭疽芽胞杆菌(Bacillus anthracis)检测质粒的构建及其应用

根据炭疽芽胞杆菌(Bacillus anthracis)毒性质粒pX01和pX02上的2个毒力相关基因cya和capA的序列特点,以pIJ2925为出发载体,采用一步重叠延伸PCR技术(One-step Overlap Extension PCR,简称OOE-PCR)构建了包含cya基因和capA基因保守区DNA片段的炭疽检测质粒pBIB2006。采用复合PCR对模拟炭疽危险品进行分析,结果表明pBIB2006可以为炭疽芽胞杆菌的检测提供准确、安全和方便的阳性参照品,从而为检测炭疽芽胞杆菌和炭疽芽胞杆菌灭活疫苗提供了便利。     

中国新归化种广布苋的分布及其与腋花苋的关系

本文报道了中国苋科苋属的一新记录归化植物——广布苋(Amaranthus graecizans L．)。该种植株常匍匐,叶片狭长椭圆形至线状披针形,有时线形或菱形卵形,长至少为宽的2.5倍,花被片3枚,近等长,与中国有分布的本属其他物种有所区别。该种原产于欧洲地中海地区、非洲北部至亚洲西部,归化于欧洲其他地区、东亚、澳大利亚、北美洲以及我国的河南、河北和山东等地区。该种易与苋属其他种类相混淆,因此其在我国的分布区可能被低估。此外,本文还对该种的种下等级以及该种的相似种进行了区分,并对该种下亚种Amaranthus graecizans subsp. thellungianus (Nevski) Gusev与国内文献记载的腋花苋(Amaranthus roxburghianus Kung)之间的关系作了说明与澄清。