Christopherson R I, Finch L R
Eur J Biochem. 1978 Oct;90(2):347-58. doi: 10.1111/j.1432-1033.1978.tb12611.x.
The effect of exogenous adenine or uracil upon the de novo pathway for synthesis of pyrimidine nucleotides in Escherichia coli K12 was investigated. Parameters studied were levels of the enzymes carbamoyl phosphate synthase (EC 2.7.2.9), aspartate carbamoyltransferase (EC 2.1.3.2) and orotate phosphoribosyltransferase (EC 2.4.2.10) and the intermediates carbamoyl phosphate, aspartate and orotate, together with the contributions of exogenous uracil and aspartate to intracellular pyrimidine nucleotide. Taken with earlier data [Bagnara, A.S. & Finch, L. R. (1974) Eur. J. Biochem- 41, 421--430] on contents of UTP, CTP and 5-phosphoribosyl 1-diphosphate in cultures of this strain after the addition of adenine or uracil, the results obtained provide new insights into the regulatory mechanisms operating on the pathway in vivo. These insights enable evaluation of the contributions of such factors as limitation for a substrate, feed-back allosteric control by end products and enzyme repression/depression mechanisms. The evidence presented indicates that depressed levels of orotate phosphoribosyltransferase in E. coli K12 result in the wasteful ultilization of asparatate for excess synthesis of pyrimidine nucleotide precursors during balanced growth of the strain in minimal medium. Exogenous adenine increases the excessive accumulation of these precursors by lowering the intracellular content of 5-phosphoribosyl 1-diphosphate (Bagnara and Finch, 1974). This causes a decrease in the conversion of orotate to orotidine 5'-monophosphate, thus lowering the utilization or orotate and its precursors for synthesis of pyrimidine nucleotides. Further, since the contents of these nucleotide end products are thereby decreased (Bagnara nad Finch, 1974), theri feed-back on the early steps in the pathway is diminished and the production of the precursors is increased. It is postulated that growth of E. coli K12 under these conditions is limited by a compound that is metabolically related to precursors to aspartate.
研究了外源性腺嘌呤或尿嘧啶对大肠杆菌K12中嘧啶核苷酸从头合成途径的影响。所研究的参数包括氨甲酰磷酸合酶（EC 2.7.2.9）、天冬氨酸氨甲酰转移酶（EC 2.1.3.2）和乳清酸磷酸核糖转移酶（EC 2.4.2.10）的水平，以及中间体氨甲酰磷酸、天冬氨酸和乳清酸，同时还研究了外源性尿嘧啶和天冬氨酸对细胞内嘧啶核苷酸的贡献。结合早期关于添加腺嘌呤或尿嘧啶后该菌株培养物中UTP、CTP和5-磷酸核糖-1-二磷酸含量的数据[巴尼亚拉，A.S.和芬奇，L.R.（1974年）《欧洲生物化学杂志》-41，421 - 430]，所获得的结果为体内该途径的调控机制提供了新的见解。这些见解有助于评估诸如底物限制、终产物的反馈变构控制以及酶阻遏/诱导机制等因素的作用。所提供的证据表明，在基本培养基中该菌株平衡生长期间，大肠杆菌K12中乳清酸磷酸核糖转移酶水平降低导致天冬氨酸被浪费地过度用于嘧啶核苷酸前体的过量合成。外源性腺嘌呤通过降低细胞内5-磷酸核糖-1-二磷酸的含量增加了这些前体的过度积累（巴尼亚拉和芬奇，1974年）。这导致乳清酸向乳清苷5'-单磷酸的转化减少，从而降低了乳清酸及其前体用于嘧啶核苷酸合成的利用率。此外，由于这些核苷酸终产物的含量因此降低（巴尼亚拉和芬奇，1974年），它们对该途径早期步骤的反馈作用减弱，前体的产生增加。据推测，在这些条件下大肠杆菌K12的生长受到一种与天冬氨酸前体代谢相关的化合物的限制。
