Occhino D A, Wyckoff E E, Henderson D P, Wrona T J, Payne S M
Department of Microbiology and Institute for Cellular and Molecular Biology, University of Texas, Austin 78712, USA.
Mol Microbiol. 1998 Sep;29(6):1493-507. doi: 10.1046/j.1365-2958.1998.01034.x.
Vibrio cholerae was found to have two sets of genes encoding TonB, ExbB and ExbD proteins. The first set (tonB1, exbB1, exbD1) was obtained by complementation of a V. cholerae tonB mutant. In the mutant, a plasmid containing these genes permitted transport via the known V. cholerae high-affinity iron transport systems, including uptake of haem, vibriobactin and ferrichrome. When chromosomal mutations in exbB1 or exbD1 were introduced into a wild-type V. cholerae background, no defect in iron transport was noted, indicating the existence of additional genes that can complement the defect in the wild-type background. Another region of the V. cholerae chromosome was cloned that encoded a second functional TonB/Exb system (tonB2, exbB2, exbD2). A chromosomal mutation in exbB2 also failed to exhibit a defect in iron transport, but a V. cholerae strain that had chromosomal mutations in both the exbB1 and exbB2 genes displayed a mutant phenotype similar to that of an Escherichia coli tonB mutant. The genes encoding TonB1, ExbB1, ExbD1 were part of an operon that included three haem transport genes (hutBCD), and all six genes appeared to be expressed from a single Fur-regulated promoter upstream of tonB1. A plasmid containing all six genes permitted utilization of haem by an E. coli strain expressing the V. cholerae haem receptor, HutA. Analysis of the hut genes indicated that hutBCD, which are predicted to encode a periplasmic binding protein (HutB) and cytoplasmic membrane permease (HutC and HutD), were required to reconstitute the V. cholerae haem transport system in E. coli. In V. cholerae, the presence of hutBCD stimulated growth when haemin was the iron source, but these genes were not essential for haemin utilization in V. cholerae.
霍乱弧菌被发现有两组编码托恩B（TonB）、外膜蛋白B（ExbB）和外膜蛋白D（ExbD）的基因。第一组（tonB1、exbB1、exbD1）是通过对霍乱弧菌tonB突变体进行互补获得的。在该突变体中，含有这些基因的质粒允许通过已知的霍乱弧菌高亲和力铁转运系统进行转运，包括血红素、弧菌素和铁色素的摄取。当将exbB1或exbD1中的染色体突变引入野生型霍乱弧菌背景时，未观察到铁转运缺陷，这表明存在可以在野生型背景中弥补缺陷的其他基因。霍乱弧菌染色体的另一个区域被克隆出来，它编码第二个功能性托恩B/外膜蛋白系统（tonB2、exbB2、exbD2）。exbB2中的染色体突变也未表现出铁转运缺陷，但在exbB1和exbB2基因中都有染色体突变的霍乱弧菌菌株表现出与大肠杆菌tonB突变体相似的突变表型。编码托恩B1、外膜蛋白B1、外膜蛋白D1的基因是一个操纵子的一部分，该操纵子包括三个血红素转运基因（hutBCD），所有六个基因似乎都从tonB1上游的单个铁调节蛋白（Fur）调控启动子表达。含有所有六个基因的质粒允许表达霍乱弧菌血红素受体HutA的大肠杆菌菌株利用血红素。对hut基因的分析表明，预计编码周质结合蛋白（HutB）和细胞质膜通透酶（HutC和HutD）的hutBCD是在大肠杆菌中重建霍乱弧菌血红素转运系统所必需的。在霍乱弧菌中，当血红素作为铁源时，hutBCD的存在刺激了生长，但这些基因对于霍乱弧菌利用血红素并非必不可少。
