Two Gram-negative insect pathogens,
Xenorhabdus nematophila
and
Photorhabdus luminescens
, produce rhabduscin, an amidoglycosyl- and vinyl-isonitrile-functionalized tyrosine derivative. Heterologous expression of the rhabduscin pathway in
Escherichia coli
, precursor-directed biosynthesis of rhabduscin analogs, biochemical assays, and visualization using both stimulated Raman scattering and confocal fluorescence microscopy established rhabduscin’s role as a potent nanomolar-level inhibitor of phenoloxidase, a key component of the insect’s innate immune system, as well as rhabduscin’s localization at the bacterial cell surface. Stimulated Raman scattering microscopy visualized rhabduscin at the periphery of wild-type
X. nematophila
cells and
E. coli
cells heterologously expressing the rhabduscin pathway. Precursor-directed biosynthesis created rhabduscin mimics in
X. nematophila
pathway mutants that could be accessed at the bacterial cell surface by an extracellular bioorthogonal probe, as judged by confocal fluorescence microscopy. Biochemical assays using both wild-type and mutant
X. nematophila
cells showed that rhabduscin was necessary and sufficient for potent inhibition (low nM) of phenoloxidases, the enzymes responsible for producing melanin (the hard black polymer insects generate to seal off microbial pathogens). These observations suggest a model in which rhabduscin’s physical association at the bacterial cell surface provides a highly effective inhibitor concentration directly at the site of phenoloxidase contact. This class of molecules is not limited to insect pathogens, as the human pathogen
Vibrio cholerae
also encodes rhabduscin’s aglycone, and bacterial cell-coated immunosuppressants could be a general strategy to combat host defenses.
两种革兰氏阴性昆虫病原菌,Xenorhabdus nematophila和Photorhabdus luminescens,产生拉布杜斯辛(rhabduscin),这是一种含有
氨基糖苷和
乙烯基异腈基化的
酪氨酸衍
生物。通过在大肠杆菌Escherichia coli中异源表达拉布杜斯辛途径,预体定向
生物合成拉布杜斯辛类似物,生化分析以及使用受激拉曼散射和共聚焦荧光显微镜进行可视化,确定了拉布杜斯辛在昆虫先天免疫系统的关键组成部分苯氧化酶中作为有效的纳摩尔级
抑制剂的作用,以及拉布杜斯辛在细菌细胞表面的定位。受激拉曼散射显微镜可视化了野生型X. nematophila细胞和异源表达拉布杜斯辛途径的E. coli细胞周边的拉布杜斯辛。预体定向
生物合成在X. nematophila途径突变体中创建了拉布杜斯辛模拟物,这些模拟物可以通过胞外
生物正交探针在细菌细胞表面访问,如共聚焦荧光显微镜所示。使用野生型和突变型X. nematophila细胞的生化分析表明,拉布杜斯辛对于产生黑色聚合物(昆虫用于封闭微
生物病原体的硬质黑色聚合物)的酶苯氧化酶的强效抑制(低纳摩尔级)是必要且充分的。这些观察结果表明,拉布杜斯辛在细菌细胞表面的物理结合提供了直接在苯氧化酶接触点处高效
抑制剂浓度的模型。这类分子不仅限于昆虫病原体,因为人类病原体Vibrio cholerae也编码了拉布杜斯辛的无糖基骨架,而且细菌细胞包覆的
免疫抑制剂可能是对抗宿主防御的一般策略。