Pathogens have evolved an array of strategies to establish a productive infection. The extracellular proteins secreted by pathogens are one of unique mechanisms to evade the host innate immune response. Many secretory proteins transported by the bacterial secretion systems have been widely investigated in Salmonella. Certain extracellular nucleases are essential for bacterial pathogenesis. However, there is no current data available for the enzymatic properties of the proteins secreted by Salmonella. Therefore, in the present study we have identified and characterized the nuclease activity of the extracellular proteins from Salmonella enterica serovar Typhimurium. It was demonstrated that the extracellular proteins from S. Typhimurium exhibited the deoxyribonucleases activity against λDNA by agarose gel electrophoresis and agar plate diffusion method. The activity was observed at 16 °C, 37 °C and 42 °C, and found to be highest at 42 °C and inhibited at temperatures over 60 °C. The nuclease activity was stable under alkaline conditions (pH 7-10) and the optimum pH was 9.0. The nuclease activity was promoted at high ionic strength of Ba2+, Ca2+, Mg2+, and Ni2+. Nuclease zymography analysis revealed that there were four activity bands in the extracellular proteins; followed by LC-ESI/MS/MS analysis seven proteins were identified. As demonstrated by nuclease zymography, the recombinant 5'-nucleotidase protein expressed in the prokaryotic expression system displayed the DNase activity. To our knowledge, the present findings represent the first direct and unambiguous demonstration of the nuclease activity of the extracellular proteins from S. Typhimurium, and it provides an important fundamental for further investigation of the role of the extracellular proteins in pathogenicity and immune evasion.

中文翻译：

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肠道沙门氏菌鼠伤寒血清型胞外蛋白核酸酶活性的鉴定和表征

病原体已经进化出一系列策略来建立生产性感染。病原体分泌的细胞外蛋白是逃避宿主先天免疫反应的独特机制之一。许多由细菌分泌系统转运的分泌蛋白已在沙门氏菌中得到广泛研究。某些细胞外核酸酶对于细菌的发病机制是必不可少的。然而，目前没有关于沙门氏菌分泌的蛋白质的酶学特性的数据。因此，在本研究中，我们已经鉴定并表征了来自肠道沙门氏菌鼠伤寒血清型的胞外蛋白的核酸酶活性。结果表明，来自 S. 通过琼脂糖凝胶电泳和琼脂平板扩散法，鼠伤寒杆菌表现出对λDNA 的脱氧核糖核酸酶活性。该活性在 16 °C、37 °C 和 42 °C 下观察到，发现在 42 °C 时最高，并在超过 60 °C 的温度下被抑制。核酸酶活性在碱性条件下（pH 7-10）稳定，最适pH为9.0。核酸酶活性在 Ba2+、Ca2+、Mg2+ 和 Ni2+ 的高离子强度下得到促进。核酸酶谱分析显示胞外蛋白存在4条活性条带；随后进行 LC-ESI/MS/MS 分析，鉴定出七种蛋白质。正如核酸酶酶谱所证明的，在原核表达系统中表达的重组 5'-核苷酸酶蛋白显示出 DNase 活性。据我们所知，