Langley D, Guest J R
J Gen Microbiol. 1978 May;106(1):103-17. doi: 10.1099/00221287-106-1-103.
Twenty-eight spontaneous auxotrophic aroP mutants with deletions in the azi--nadC--aroP--aceE--aceF--lpd region of the Escherichia coli K12 chromosome were characterized genetically with respect to various azi, nadC, ace and lpd markers by P1-mediated transduction. One mutant (Kdelta18; aroP--lpddelta) had a deletion which extended through the aceE and aceF genes to end within the lpd gene. The polarity of the ace operon (aceE to aceF) was confirmed. It was concluded that 10 out of 15 deletions generating a strict requirement for acetate terminated in the aceE gene. Of the ten, three mutants (Kdelta22, Cdelta41 and Cdelta41) synthesized detectable dihydrolipoamide acetyltransferase (the aceF gene product) and seven were assumed to possess deletions generating polar effects on aceF gene expression. Five deletions appeared to extend into the aceF gene. A further five deletions, which limited the expression of the ace operon without generating an Ace- phenotype or a complete Ace- phenotype, ended closest to the aroP-proximal aceE markers. The opposite ends of all these deletions appeared to terminate before (10), within (2) or extend beyond (9) the nadC gene. There was no obvious correlation between the deletion end-points and the corresponding lipoamide dehydrogenase activities, which ranged from 30 to 95% of parental levels in different deletion strains. The remaining seven deletions simply extended between the aroP and nadC genes (nad--aroPdelta) without affecting expression of the ace operon. Regulation of the synthesis of the pyruvate and alpha-ketoglutarate dehydrogenase complexes was investigated in some of the parental and deletion strains under different physiological conditions including thiamin-deprivation. The results indicate that the syntheses of the two dehydrogenase complexes are independently regulated. Expression of the lpd gene appears to be coupled to complex synthesis but can be dissociated under some conditions. Mechanisms for regulating lpd gene expression are discussed and an autogenous mechanism involving uncomplexed lipoamide dehydrogenase functioning as a negatively acting repressor at the operator site of an independent lpd gene is proposed as the simplest mechanism which is consistent with all available information.
通过P1介导的转导，对28个自发营养缺陷型aroP突变体进行了遗传学特征分析，这些突变体在大肠杆菌K12染色体的azi--nadC--aroP--aceE--aceF--lpd区域存在缺失。其中一个突变体（Kdelta18；aroP--lpddelta）的缺失延伸至aceE和aceF基因，并在lpd基因内终止。证实了ace操纵子（从aceE到aceF）的极性。得出结论，15个产生对乙酸盐严格需求的缺失中有10个在aceE基因处终止。在这10个突变体中，3个突变体（Kdelta22、Cdelta41和Cdelta41）合成了可检测到的二氢硫辛酰胺乙酰转移酶（aceF基因产物），7个被认为具有对aceF基因表达产生极性效应的缺失。5个缺失似乎延伸到了aceF基因中。另外5个缺失限制了ace操纵子的表达，但未产生Ace-表型或完全的Ace-表型，它们最接近aroP近端的aceE标记处终止。所有这些缺失区域的另一端似乎在nadC基因之前（10个）、之内（2个）或延伸超过（9个）nadC基因处终止。缺失端点与相应的硫辛酰胺脱氢酶活性之间没有明显的相关性，不同缺失菌株中的硫辛酰胺脱氢酶活性为亲本水平的30%至95%。其余7个缺失仅在aroP和nadC基因之间延伸（nad--aroPdelta），而不影响ace操纵子的表达。在包括硫胺素缺乏在内的不同生理条件下，对一些亲本菌株和缺失菌株中丙酮酸和α-酮戊二酸脱氢酶复合物的合成调控进行了研究。结果表明，这两种脱氢酶复合物的合成是独立调控的。lpd基因的表达似乎与复合物的合成相关，但在某些条件下可以解离。讨论了调控lpd基因表达的机制，并提出了一种最简单的机制，即一种涉及未复合的硫辛酰胺脱氢酶在独立的lpd基因的操纵位点作为负向作用阻遏物发挥作用的自体机制，该机制与所有现有信息一致。
