Anderson J J, Oxender D L
J Bacteriol. 1978 Oct;136(1):168-74. doi: 10.1128/jb.136.1.168-174.1978.
The Escherichia coli K-12 mutant strain AE4107 (livH::Mu) is defective in the high-affinity binding protein-mediated uptake system for L-leucine, L-valine, and L-isoleucine (LIV-I). We have used this strain to produce mutations in the residual LIV-II membrane-bound branched-chain amino acid uptake system. Mutants selected for their inability to utilize exogenous L-leucine were found to be defective in the LIV-II system and fell into two classes. One class, represented by strain AE410709 (livP9), showed a complete loss of saturable uptake for L-leucine, L-valine, and L-isoleucine up to 50 muM, and a second class, represented by strain AE4017012 (liv-12), showed a residual component of saturable leucine uptake with increased Km. These mutations, livP9 and liv-12, were closely linked and mapped in the 74 to 78 min region of the E. coli genetic map. Strains constructed so that they lacked both LIV-I and LIV-II transport systems excreted leucine. Strains of the genotype livH+ livP were found to have normal high-affinity binding protein-mediated transport (LIV-I and leucine specific), whereas the low-affinity (LIV-II) transport was completely missing. We concluded from these studies that the high-affinity binding protein-mediated transport systems (LIV-I and leucine specific) can operate independently of the membrane-bound LIV-II system.
大肠杆菌K-12突变株AE4107（livH::Mu）在L-亮氨酸、L-缬氨酸和L-异亮氨酸（LIV-I）的高亲和力结合蛋白介导的摄取系统方面存在缺陷。我们利用该菌株在残余的LIV-II膜结合支链氨基酸摄取系统中产生突变。筛选出的不能利用外源L-亮氨酸的突变体在LIV-II系统中存在缺陷，并分为两类。一类以菌株AE410709（livP9）为代表，对高达50 μM的L-亮氨酸、L-缬氨酸和L-异亮氨酸的可饱和摄取完全丧失；另一类以菌株AE4017012（liv-12）为代表，显示出可饱和亮氨酸摄取的残余成分，其米氏常数（Km）增加。这些突变，即livP9和liv-12，紧密连锁并定位在大肠杆菌遗传图谱的74至78分钟区域。构建的缺乏LIV-I和LIV-II转运系统的菌株会分泌亮氨酸。发现基因型为livH+ livP的菌株具有正常的高亲和力结合蛋白介导的转运（LIV-I和亮氨酸特异性），而低亲和力（LIV-II）转运则完全缺失。我们从这些研究中得出结论，高亲和力结合蛋白介导的转运系统（LIV-I和亮氨酸特异性）可以独立于膜结合的LIV-II系统运行。
