The maltodextrin system of Escherichia coli:: Metabolism and transport
The maltodextrin system of Escherichia coli:: Metabolism and transport
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DOI：
10.1128/jb.187.24.8322-8331.2005
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发表时间：
2005-12-01
影响因子：
3.2
通讯作者：
Boos, W
中科院分区：
文献类型：
--
作者：
Dippel, R;Boos, W
大肠杆菌的麦芽糖/麦芽糖糊精调节子由10个编码结合蛋白依赖性ABC转运蛋白的基因和4个作用于麦芽糖糊精的酶组成。所有的mal基因都由MalT控制，MalT是一种只被麦芽三糖激活的转录激活因子。通过淀粉麦芽糖酶的作用，我们从麦芽糖到麦芽七糖制备了均匀标记的[C-14]麦芽糖糊精，其葡萄糖基残基具有相同的特定放射性，这使得可以定量地跟踪每种麦芽糖糊精的运输速率。构建缺乏麦芽糖糊精磷酸化酶（MalP）或麦芽糖糊精葡糖苷酶（MalZ）或两者的同基因malQ突变体。通过分析积累的[C-14]麦芽糖糊精来确定麦芽糖糊精代谢的体内模式。MalP（-）MalZ（+）菌株将所有糊精降解为麦芽糖，而MalP（+）MalZ（-）菌株将它们降解为麦芽三糖。标记的糊精用于测量在不存在细胞质代谢的情况下的转运速率。不考虑糊精的长度，在亚微摩尔浓度下的运输速率是相似的麦芽糖糊精时，每葡糖基残基计算的速率，这表明一种新的模式为底物易位。测试缺乏MalQ和麦芽糖转乙酰酶的菌株积累麦芽糖的能力。在1.8 nM外部麦芽糖时，平衡条件下内部与外部麦芽糖浓度之比达到10（6）比1，但在较高的外部麦芽糖浓度下下降。在增加外部麦芽糖浓度的麦芽糖的最大内部水平为100 mM左右。一个菌株缺乏malQ，malP，malZ以及糖原合成，其中麦芽糖糊精没有化学改变可以诱导外部麦芽糖以及所有其他麦芽糖糊精，证明了运输本身的作用，诱导。
The maltose/maltodextrin regulon of Escherichia coli consists of 10 genes which encode a binding protein-dependent ABC transporter and four enzymes acting on maltodextrins. All mal genes are controlled by MalT, a transcriptional activator that is exclusively activated by maltotriose. By the action of amylomaltase, we prepared uniformly labeled [C-14] maltodextrins from maltose up to maltoheptaose with identical specific radioactivities with respect to their glucosyl residues, which made it possible to quantitatively follow the rate of transport for each maltodextrin. Isogenic malQ mutants lacking maltodextrin phosphorylase (MalP) or maltodextrin glucosidase (MalZ) or both were constructed. The resulting in vivo pattern of maltodextrin metabolism was determined by analyzing accumulated [C-14] maltodextrins. MalP(-) MalZ(+) strains degraded all dextrins to maltose, whereas MalP(+) MalZ(-) strains degraded them to maltotriose. The labeled dextrins were used to measure the rate of transport in the absence of cytoplasmic metabolism. Irrespective of the length of the dextrin, the rates of transport at a submicromolar concentration were similar for the maltodextrins when the rate was calculated per glucosyl residue, suggesting a novel mode for substrate translocation. Strains lacking MalQ and maltose transacetylase were tested for their ability to accumulate maltose. At 1.8 nM external maltose, the ratio of internal to external maltose concentration under equilibrium conditions reached 10(6) to I but declined at higher external maltose concentrations. The maximal internal level of maltose at increasing external maltose concentrations was around 100 mM. A strain lacking malQ, malP, and malZ as well as glycogen synthesis and in which maltodextrins are not chemically altered could be induced by external maltose as well as by all other maltodextrins, demonstrating the role of transport per se for induction.