Regulation of Amino Acid Biosynthesis

天冬氨酸激酶在细菌氨基酸的生物合成中起着核心作用。它催化天冬氨酸途径的第一步，其产物包括成年人营养所必需的八种氨基酸中的四种。天冬氨酸途径的控制在很大程度上依赖于天冬氨酸激酶同工酶的存在，这些同工酶对其合成和活性的调节是不同的。在过去的两年里，我们的主要研究成果包括在遗传、酶和生理水平上对枯草芽孢杆菌完整的天冬氨酸激酶同工酶模式的总体定义，以及对其中一种同工酶--天冬氨酸激酶II(Akii)的编码操纵子的详细表征。本研究的主要目的是研究Akii操纵子表达调控的分子机制，其反馈抑制剂L赖氨酸对Akii变构的调节方式，以及Akii的两个不同亚基的生理功能。在对Akii的研究进行期间，将开始一条平行的研究路线，重点是枯草杆菌的另一种天冬氨酸激酶同工酶，其中一种(AKI)被构成二氨基甲酸酯的细胞壁所抑制，另一种(AKIII)被苏氨酸和赖氨酸抑制。AKI主要作用于富介质中生长过程中二胺基戊二酸的合成，以及孢子形成过程中二胺基戊二酸和二氢吡啶甲酸的产生，而AkII和AKIII在生长细胞中参与赖氨酸、苏氨酸和蛋氨酸的生物合成。最初的实验将涉及编码AKI和AKIII的基因的克隆和测序，随后将研究调节这些酶的合成和活性的机制。深入研究这些重要调控酶的调控，将有助于我们从根本上理解代谢调控的原理和策略。

The enzyme aspartokinase plays a central role in bacterial amino acid biosynthesis. It catalyzes the first step in the aspartate pathway, whose products include four of the eight amino acids that are essential for the nutrition of the adult human. The control of the aspartate pathway depends largely on the presence of isozymes of aspartokinase, which differ in the regulation of their synthesis and activity. Our major research accomplishments in the last two years include the general definition at the genetic, enzymatic, and physiological level of the complete aspartokinase isozyme pattern of Bacillus subtilis and the detailed characterization of the operon that encodes one of these isozymes, aspartokinase II (AKII). The primary aim of the proposed research is to study the molecular mechanisms controlling the expression of the AKII operon, the mode of allosteric regulation of AKII by its feedback inhibitor L- lysine, and the physiological function of the two non-identical subunits of AKII. A parallel line of investigation, to be begun while the studies on AKII are in progress, will focus on the other aspartokinase isozymes of B. subtilis, of which one (AKI) is inhibited by the cell wall constitute diaminopmelate and the other (AKIII) by threonine and lysine. AKI functions primarily in the synthesis of diaminopimelate during growth in rich media and in the production of diaminopimelate and dipicolinate during sporulation, whereas AKII and AKIII serve in the biosynthesis of lysine, threonine, and methionine in growth cells. Initial experiments will involve the cloning and sequencing of the genes encoding AKI and AKIII and will be following by the study of the mechanisms that regulate the synthesis and activity of these enzymes. The in-depth study of the control of these important regulatory enzymes will contribute fundamentally to our understanding of the principles and strategies of metabolic regulation.