Liberton Michelle, Howard Berg R, Heuser John, Roth Robin, Pakrasi Himadri B
Department of Biology, Washington University, St. Louis, Missouri 63130, USA.
Protoplasma. 2006 May;227(2-4):129-38. doi: 10.1007/s00709-006-0145-7. Epub 2006 May 30.
Among prokaryotes, cyanobacteria are unique in having highly differentiated internal membrane systems. Like other Gram-negative bacteria, cyanobacteria such as Synechocystis sp. strain PCC 6803 have a cell envelope consisting of a plasma membrane, peptidoglycan layer, and outer membrane. In addition, these organisms have an internal system of thylakoid membranes where the electron transfer reactions of photosynthesis and respiration occur. A long-standing controversy concerning the cellular ultrastructures of these organisms has been whether the thylakoid membranes exist inside the cell as separate compartments, or if they have physical continuity with the plasma membrane. Advances in cellular preservation protocols as well as in image acquisition and manipulation techniques have facilitated a new examination of this topic. We have used a combination of electron microscopy techniques, including freeze-etched as well as freeze-substituted preparations, in conjunction with computer-aided image processing to generate highly detailed images of the membrane systems in Synechocystis cells. We show that the thylakoid membranes are in fact physically discontinuous from the plasma membrane in this cyanobacterium. Thylakoid membranes in Synechocystis sp. strain PCC 6803 thus represent bona fide intracellular organelles, the first example of such compartments in prokaryotic cells.
在原核生物中，蓝细菌具有高度分化的内膜系统，这一点独一无二。与其他革兰氏阴性细菌一样，诸如集胞藻属PCC 6803菌株之类的蓝细菌具有由质膜、肽聚糖层和外膜组成的细胞包膜。此外，这些生物体还有一个类囊体膜内部系统，光合作用和呼吸作用的电子传递反应在此发生。关于这些生物体细胞超微结构的一个长期争议在于，类囊体膜是以独立的区室形式存在于细胞内部，还是与质膜存在物理连续性。细胞保存方案以及图像采集和处理技术的进步推动了对这一主题的重新审视。我们结合使用了多种电子显微镜技术，包括冷冻蚀刻以及冷冻替代制剂，并结合计算机辅助图像处理，以生成集胞藻细胞内膜系统的高分辨率图像。我们发现，在这种蓝细菌中，类囊体膜实际上与质膜在物理上是不连续的。因此，集胞藻属PCC 6803菌株中的类囊体膜代表了真正的细胞内细胞器，这是原核细胞中此类区室的首个例子。
