Sumantran V N, Schweizer H P, Datta P
Department of Biological Chemistry, University of Michigan, Ann Arbor 48109-0606.
J Bacteriol. 1990 Aug;172(8):4288-94. doi: 10.1128/jb.172.8.4288-4294.1990.
A novel L-threonine transport system is induced in Escherichia coli cells when incubated in amino acid-rich medium under anaerobic conditions. Genetic and biochemical analyses with plasmids harboring mutations in the anaerobically expressed tdcABC operon indicated that the tdcC gene product was responsible for L-threonine uptake. Competition experiments revealed that the L-threonine transport system is also involved in L-serine uptake and is partially shared for L-leucine transport; L-alanine, L-valine, and L-isoleucine did not affect L-threonine uptake. Transport of L-threonine was inhibited by the respiratory chain inhibitors KCN and carbonyl cyanide m-chlorophenylhydrazone and was Na+ independent. These results identify for the first time an E. coli gene encoding a permease specific for L-threonine-L-serine transport that is distinct from the previously described threonine-serine transport systems. A two-dimensional topological model predicted from the amino acid composition and hydropathy plot showed that the TdcC polypeptide appears to be an integral membrane protein with several membrane-spanning domains exhibiting a striking similarity with other bacterial permeases.
当在厌氧条件下于富含氨基酸的培养基中培养时，大肠杆菌细胞中会诱导出一种新型的L-苏氨酸转运系统。对携带厌氧表达的tdcABC操纵子突变的质粒进行遗传和生化分析表明，tdcC基因产物负责L-苏氨酸的摄取。竞争实验表明，L-苏氨酸转运系统也参与L-丝氨酸的摄取，并且部分用于L-亮氨酸的转运；L-丙氨酸、L-缬氨酸和L-异亮氨酸不影响L-苏氨酸的摄取。L-苏氨酸的转运受到呼吸链抑制剂KCN和羰基氰化物间氯苯腙的抑制，且不依赖于Na⁺。这些结果首次鉴定出一个大肠杆菌基因，该基因编码一种对L-苏氨酸-L-丝氨酸转运具有特异性的通透酶，它不同于先前描述的苏氨酸-丝氨酸转运系统。根据氨基酸组成和亲水性图谱预测的二维拓扑模型表明，TdcC多肽似乎是一种整合膜蛋白，具有几个跨膜结构域，与其他细菌通透酶具有显著相似性。
