GENETIC-ANALYSIS OF COMPONENTS INVOLVED IN VITAMIN-B12 UPTAKE IN ESCHERICHIA-COLI
GENETIC-ANALYSIS OF COMPONENTS INVOLVED IN VITAMIN-B12 UPTAKE IN ESCHERICHIA-COLI
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DOI：
10.1128/jb.132.3.796-805.1977
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发表时间：
1977-01-01
影响因子：
3.2
通讯作者：
KADNER, RJ
中科院分区：
文献类型：
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作者：
BASSFORD, PJ;KADNER, RJ
3 个基因的产物参与大肠杆菌对氰钴胺 (B12) 的摄取。基因 btuB（以前称为 bfe）位于大肠杆菌连锁图上的第 88 分钟，编码外膜的蛋白质成分，该蛋白质成分充当 B12、大肠杆菌素和噬菌体 BF23 的受体。根据互补和回复分析，四种表型类型的突变体对这些试剂的反应有所不同，这些突变存在于单个 btuB 顺反子中。在 1 类突变体中，配体与受体的结合似乎正常，但随后的 B12 摄取有缺陷。受体水平和摄取率对btuB基因剂量有反应。先前的研究表明，tonB 产物对于能量依赖性 B12 吸收是必需的，但对于其结合却不是必需的。除了 tonB 中的突变外，没有导致对 B 族大肠菌素不敏感的突变影响 B12 的利用。对btuB和tonB产品的要求可以通过升高水平的B12(>1μM)或通过损害作为渗透性屏障的外膜的完整性的突变来绕过。缺乏 btuB-tonB 摄取系统的菌株中升高的 B12 浓度的利用取决于 btuC 产品的功能。该基因位于连锁图上的37.7分钟处，顺序为pps-btuC-pheS。 btuC 发生改变但具有完整 btuB-tonB 系统的菌株在维生素 B12 的利用方面仅略有受损，其积累存在缺陷。这种缺陷表现为无法保留 B12，因此细胞内标记几乎完全因交换或流出而丢失。据推测，btuC 编码周质中 B12 的运输系统。
The products of 3 genes are involved in cyanocobalamin (B12) uptake in E. coli. Gene btuB (formerly bfe), located at min 88 on the E. coli linkage map, codes for a protein component of the outer membrane which serves as receptor for B12, the E colicins and bacteriophage BF23. Four phenotypic classes of mutants varying in response to these agents carried mutations that, based on complementation and reversion analyses, reside in the single btuB cistron. In 1 mutant class, ligand binding to the receptor appeared to be normal, but subsequent B12 uptake was defective. The level of receptor and rate of uptake were responsive to btuB gene dosage. Previous studies showed that the tonB product was necessary for energy-dependent B12 uptake but not for its binding. Other than those in tonB, no mutations that conferred insensitivity to group B colicins affected B12 utilization. The requirement for the btuB and tonB products could be bypassed by elevated levels of B12 (> 1 .mu.M) or by mutations compromising the integrity of the outer membrane as a permeability barrier. Utilization of elevated B12 concentrations in strains lacking the btuB-tonB uptake system was dependent on the function of the btuC product. This gene was located at 37.7 min on the linkage map, with the order pps-btuC-pheS. Strains altered in btuC but with an intact btuB-tonB system were only slightly impaired in B12 utilization, being defective in its accumulation. This defect was manifested as inability to retain B12, such that intracellular label was almost completely lost by exchange or efflux. It is proposed that btuC encodes a transport system for B12 in the periplasm.