Dooley J S, Engelhardt H, Baumeister W, Kay W W, Trust T J
Department of Biochemistry and Microbiology, University of Victoria, British Columbia, Canada.
J Bacteriol. 1989 Jan;171(1):190-7. doi: 10.1128/jb.171.1.190-197.1989.
Cell-free culture supernatants of a lipopolysaccharide (LPS) O-polysaccharide-deficient, single-insertion transposon mutant of the tetragonal surface protein array (S layer)-containing fish pathogen Aeromonas salmonicida were examined by electron microscopy. Negative staining showed that the S layer was released as sheets of tetragonal material, indicating that although surface retention of assembled S layer requires the presence of wild-type LPS oligosaccharides, initial assembly of S-layer subunits into sheets does not require the presence of O-polysaccharide chains. The three-dimensional structure of the S layer was reconstructed from tilted micrographs of the released sheets. Horizontal sections through this reconstruction showed that the released sheets were composed of two identical S layers that were perfectly in register. The reconstructed layer had a lattice constant of 12.5 nm. At a resolution of 1.6 nm, the layer consisted of a major tetragon at one fourfold axis of symmetry and a minor tetragon at the second fourfold axis of symmetry. The core, composed of four of the major domains, contained a large depression and was located toward the inside of the layer. The minor tetragon provided connectivity within the layer and was located toward the outer surface of the layer. Projections through the double layer gave a type I (closed) pattern (M. Stewart, T. J. Beveridge, and T. J. Trust, J. Bacteriol. 166:120-127, 1986), yet projections through the single layer indicated that the type II (open) pattern was present. This open pattern was indistinguishable from that seen in S layer released from the surfaces of wild-type cells.
对含有四方表面蛋白阵列（S层）的鱼类病原菌杀鲑气单胞菌的脂多糖（LPS）O-多糖缺陷型单插入转座子突变体的无细胞培养上清液进行了电子显微镜检查。负染色显示S层以四方材料片的形式释放，这表明尽管组装好的S层在表面的保留需要野生型LPS寡糖的存在，但S层亚基最初组装成片并不需要O-多糖链的存在。从释放片层的倾斜显微照片重建了S层的三维结构。通过该重建的水平截面显示，释放的片层由两个完全对齐的相同S层组成。重建层的晶格常数为12.5 nm。在1.6 nm的分辨率下，该层在一个四重对称轴处由一个主要四方结构和在第二个四重对称轴处由一个次要四方结构组成。由四个主要结构域组成的核心包含一个大凹陷，位于层的内部。次要四方结构在层内提供连接，位于层的外表面。通过双层的投影给出I型（封闭）模式（M. Stewart、T. J. Beveridge和T. J. Trust，《细菌学杂志》166:120 - 127，1986），然而通过单层的投影表明存在II型（开放）模式。这种开放模式与从野生型细胞表面释放的S层中看到的模式无法区分。
