EscC is a chaperone for the Edwardsiella tarda type III secretion system putative translocon components EseB and EseD

Edwardsiella tarda is a Gram-negative enteric pathogen that causes disease in both humans and animals. Recently, a type III secretion system (T3SS) has been found to contribute to Ed. tarda pathogenesis. EseB, EseC and EseD were shown to be secreted by the T3SS and to be the major components of the extracellular proteins (ECPs). Based on sequence similarity, they have been proposed to function as the 'translocon' of the T3SS needle structure. In this study, it was shown that EseB, EseC and EseD formed a protein complex after secretion, which is consistent with their possible roles as translocon components. The secretion of EseB and EseD was dependent on EscC (previously named Orf2). EscC has the characteristics of a chaperone; it is a small protein (13 kDa), located next to the translocators in the T3SS gene cluster, and has a coiled-coil structure at the N-terminal region as predicted by COILS. An in-frame deletion of escC abolished the secretion of EseB and EseD, and complementation of Delta escC restored the export of EseB and EseD into the culture supernatant. Further studies showed that EscC is not a secreted protein and is located on the membrane and in the cytoplasm. Mutation of escC did not affect the transcription of eseB but reduced the amount of EseB as measured by using an EseB-LacZ fusion protein in Ed. tarda. Co-purification studies demonstrated that EscC formed complexes with EseB and EseD. The results suggest that EscC functions as a T3SS chaperone for the putative translocon components EseB and EseD in Ed. tarda.

迟缓爱德华氏菌III型分泌系统(T3SS)输送器蛋白及分子伴侣的研究

迟缓爱德华氏菌（Edwardsiella tarda）是重要的革兰氏阴性致病菌，可以感染包括人类在内的多种动物。由迟缓爱德华氏菌引发的爱德华氏菌病已经在许多水产养殖动物中被发现，每年给淡水和海水水产养殖业带来巨大的损失。目前为止对于迟缓爱德华氏菌病的防治以化学治疗为主，疫苗的研究还在进行中。III型分泌系统（T3SS）是E. tarda重要的致病因子，虽然T3SS基因簇的结构及部分基因的功能得到了初步研究，但其作用机制还未得到阐明。本研究着重开展了迟缓爱德华氏菌T3SS输送器蛋白EseC的分子伴侣的鉴定及功能研究，并对输送器蛋白及其免疫功能进行了初步探讨，希望进一步地了解T3SS在E. tarda致病中的功能及其在疫苗研制中的作用。 一、迟缓爱德华氏菌III型分泌系统（T3SS）输送器蛋白EseC分子伴侣的鉴定和功能研究 以前的工作表明，EseB、EseC和EseD蛋白是E. tarda T3SS输送器蛋白的组成成分，在分泌到细菌细胞外后可以组成输送器装置。分子伴侣对于输送器蛋白的稳定和分泌具有重要的作用，EscC已经被鉴定为EseB和EseD的分子伴侣，而EseC的分子伴侣还没有得到鉴定。在本实验中，我们以EseC作为研究对象，主要开展了EseC分子伴侣鉴定的研究。 生物信息学分析表明，在E. tarda T3SS基因簇上的escA基因与eseC相邻，其编码的蛋白形成一个大的螺旋结构，为分子量较小（17.5kD）的酸性蛋白（pI 4.79），并与已鉴定的分子伴侣具有序列的同源性，这些符合细菌T3SS分子伴侣的特征。研究发现，EscA蛋白分布在细菌的细胞质和细胞膜上。在escA基因缺失后，大大降低了EseC分泌到细菌细胞外的量，同时EseC蛋白在细菌细胞质中的积聚量也减少，当escA基因缺失突变株得到escA基因互补后，EseC的分泌和在细胞质内的积聚恢复到了野生型菌株水平。氯霉素阻断蛋白质合成的实验发现，当细菌不表达EscA的情况下，EseC蛋白逐渐降解，说明了EscA可以影响EseC在胞质中的稳定。蛋白体外结合试验和免疫共沉淀实验发现，EseC和EscA在体外可以结合，在细菌细胞质中也可以相互结合，表明EseC和EscA可以相互作用。上述结果表明，EscA是EseC的分子伴侣。 在确定了EscA是EseC的分子伴侣之后，我们进一步确定EscA对EseC表达的影响，以及两者相互作用的结构域。通过检测转录水平和翻译水平的EseC-LacZ融合蛋白表达情况，发现在EscA缺失的情况下，EseC的转录水平没有变化，而翻译水平下降，表明EscA对EseC的影响在转录后水平。通过构建含有部分结构域缺失的escA或eseC的体外共表达体系，并进行Western blot分析，确定了EseC的31-137氨基酸序列为与EscA结合的区域，而在EscA中并没有找到与EseC结合的区域。EseC的31-137氨基酸片段缺失后，EseC的分泌和在E. tarda细菌细胞中的积聚下降，其下降幅度与escA突变株相当，进一步表明EseC的31-137氨基酸为与EscA相互作用的区域。最后人工感染实验表明，分子伴侣EscA及其与EseC的相互作用对E. tarda的致病力有影响。 二、迟缓爱德华氏菌T3SS输送器蛋白的研究 一些研究表明，T3SS在细菌与宿主相互作用的过程中表达，在体外诱导的条件下也可表达。为了确定E. tarda T3SS体外诱导表达的条件，我们检测了不同培养温度、pH条件下，E. tarda T3SS输送器蛋白表达的情况。研究表明，37°C条件下，E. tarda生长快，T3SS的输送器蛋白表达较低；28°C条件下，T3SS的输送器蛋白表达最高，而在20°C条件下，没有检测到T3SS输送器蛋白的表达。在28°C和37°C的培养条件下，中性和碱性相对酸性来说适合细菌的生长和T3SS输送器蛋白的表达。我们分析了E. tarda野生型和输送器蛋白突变株中的输送器蛋白的细胞分布，并据此推测输送器形成的机制。单一输送器蛋白的缺失不影响其它两个输送器蛋白的积聚，而输送器蛋白的分泌之间存在一定的相互影响。 通过检测输送器蛋白突变株ΔeseB, ΔeseC, ΔeseD生长、泳动、自凝聚和溶血能力的变化，发现在输送器蛋白基因缺失后，体外培养的E. tarda的生长速度变慢，泳动、自凝聚和溶血能力也变弱，说明了输送器蛋白在细菌的生长和功能行使中的重要作用。 为了检测输送器蛋白的免疫保护效果，我们克隆了eseD基因，将其在表达菌株BL21(DE3)中进行表达，并将重组表达的EseD蛋白经Ni-NTA树脂进行纯化。以EseD纯化蛋白作为蛋白抗原对大菱鲆进行注射，EseD蛋白表现出了对鱼类的免疫原性，其抗体效价在第7周达到了最高，为1：5120。攻毒实验表明该蛋白对于保护大菱鲆免疫E. tarda的感染具有帮助作用，在105cfu攻毒浓度下大菱鲆的相对存活率(RPS)为62.5%。结果说明EseD蛋白可以作为蛋白抗原疫苗的候选，并能够在保护鱼类免疫爱德华氏菌病中发挥作用。

Synthesis of nanometric-size magnesium nitride by the nitriding of pre-activated magnesium powder

Magnesium nitride (Mg3N2) was synthesized by the reaction of magnesium in the highly reactive form (Mg*) with nitrogen at 450 degrees C under normal pressure. The effect of doping with nickel dichloride on the nitridation of Mg* was investigated. Differential thermal analysis (DTA) of Mg* systems and transmission electron microscopy (TEM) measurement of the product formed were carried out. TEM measurement showed that the particle size of the Mg3N2 synthesized was in the nanometric range. The dependence of nitridation of the NiCl2-doped Mg* on temperature was investigated at temperatures ranging from 300 to 500 degrees C. The nitridation of NiCl2-doped Mg* could occur even at temperature as low as 300 degrees C. (C) 1999 Kluwer Academic Publishers.

Simultaneous hydrodesulfurization of thiophene and hydrogenation of cyclohexene over dimolybdenum nitride catalysts

A series of unsupported dimolybdenum nitride (gamma-Mo(2)N) catalysts differing in surface area were prepared by temperature programmed reduction of MoO(3) with a mixture of NH(3):N(2) (90:10). Characterization of catalysts by BET, XRD, TPR and XPS techniques was carried out. The samples were used as catalysts in hydrotreating reactions (simultaneous hydrodesulfurization of thiophene and hydrogenation of cyclohexene). Low surface area gamma-Mo(2)N materials show much higher specific conversions than those with higher surface area. These results indicate that HDS and HYD reactions over gamma-Mo(2)N seem to be structure-sensitive. The relative exposure extent of crystalline planes (111) and (200) over the different catalysts can be associated with their hydrogen adsorption capacities and with their catalytic performances. The catalytic activities are significantly affected by the catalyst pretreatment conditions. (C) 1999 Elsevier Science B.V. All rights reserved.