Reeves A R, Wang G R, Salyers A A
Department of Microbiology, University of Illinois, Urbana 61801, USA.
J Bacteriol. 1997 Feb;179(3):643-9. doi: 10.1128/jb.179.3.643-649.1997.
Results of earlier work had suggested that utilization of polysaccharides by Bacteroides spp. did not proceed via breakdown by extracellular polysaccharide-degrading enzymes. Rather, it appeared that the polysaccharide was first bound to a putative outer membrane receptor complex and then translocated into the periplasm, where the degradative enzymes were located. In a recent article, we reported the cloning and sequencing of susC, a gene from Bacteroides thetaiotaomicron that encoded a 115-kDa outer membrane protein. SusC protein proved to be essential for utilization not only of starch but also of intermediate-sized maltooligosaccharides (maltose to maltoheptaose). In this paper, we report the sequencing of a 7-kbp region of the B. thetaiotaomicron chromosome that lies immediately downstream of susC. We found four genes in this region (susD, susE, susF, and susG). Transcription of these genes was maltose inducible, and the genes appeared to be part of the same operon as susC. Western blot (immunoblot) analysis using antisera raised against proteins encoded by each of the four genes showed that all four were outer membrane proteins. Protein database searches revealed that SusE had limited similarity to a glucanohydrolase from Clostridium acetobutylicum and SusG had high similarity to amylases from a variety of sources. SusD and SusF had no significant similarity to any proteins in the databases. Results of 14C-starch binding assays suggested that SusD makes a major contribution to binding. SusE and SusF also appear to contribute to binding but not to the same extent as SusD. SusG is essential for growth on starch but appears to contribute little to starch binding. Our results demonstrate that the binding of starch to the B. thetaiotaomicron surface involves at least four outer membrane proteins (SusC, SusD, SusE, and SusF), which may form a surface receptor complex. The role of SusG in binding is still unclear.
早期研究结果表明，拟杆菌属对多糖的利用并非通过细胞外多糖降解酶的分解来进行。相反，多糖似乎首先与假定的外膜受体复合物结合，然后转运到周质中，降解酶位于周质中。在最近的一篇文章中，我们报道了来自多形拟杆菌的susC基因的克隆和测序，该基因编码一种115 kDa的外膜蛋白。事实证明，SusC蛋白不仅对淀粉的利用至关重要，对中等大小的麦芽寡糖（麦芽糖至麦芽七糖）的利用也必不可少。在本文中，我们报道了多形拟杆菌染色体上紧邻susC下游的一个7 kbp区域的测序结果。我们在该区域发现了四个基因（susD、susE、susF和susG）。这些基因的转录受麦芽糖诱导，并且这些基因似乎与susC属于同一个操纵子。使用针对这四个基因各自编码的蛋白质产生的抗血清进行的蛋白质印迹（免疫印迹）分析表明，这四种蛋白均为外膜蛋白。蛋白质数据库搜索显示，SusE与丙酮丁醇梭菌的一种葡聚糖水解酶有有限的相似性，SusG与多种来源的淀粉酶有高度相似性。SusD和SusF与数据库中的任何蛋白质均无显著相似性。¹⁴C淀粉结合试验结果表明，SusD对结合起主要作用。SusE和SusF似乎也对结合有贡献，但程度不如SusD。SusG对在淀粉上生长至关重要，但似乎对淀粉结合贡献不大。我们的结果表明，淀粉与多形拟杆菌表面的结合涉及至少四种外膜蛋白（SusC、SusD、SusE和SusF），它们可能形成一个表面受体复合物。SusG在结合中的作用仍不清楚。
