Rhee S G, Park S C, Koo J H
Curr Top Cell Regul. 1985;27:221-32. doi: 10.1016/b978-0-12-152827-0.50026-8.
The regulation of GS activity involves two nucleotidylation cycles, the uridylylation cycle of PII and the adenylylation cycle of GS, which are catalyzed by two converter enzymes, uridylyltransferase and adenylyltransferase, respectively. The converter enzymes sense the fluctuation in the availability of nitrogen and accordingly regulate the activity of GS. On the other hand, the posttranslational modification of GS is tightly coupled to the transcriptional regulation of the glnA gene by unmodified PII protein acting as a repressor in the GS synthesis. Therefore, metabolic signals perceived by uridylyltransferase are transmitted through PII to two different levels of the regulation, namely, the posttranslational level and transcriptional level. In order to study the converter enzymes which exist in extremely low concentration, the glnD and glnE genes were cloned into a plasmid vector carrying the strong, regulatable lambda phage promoter. In this way, uridylyltransferase and adenylyltransferase were overproduced to the levels approaching 800- and 500-fold, respectively. The recombinant DNA technology also enabled us to examine the transcriptional regulation of the glnD and glnE genes. The expression of these genes was slightly repressed under nitrogen-excess conditions and the repressions were more pronounced under excess nitrogen plus carbon-limiting conditions. It was found that variations of the concentration of uridylyltransferase and adenylyltransferase also affect the rate of GS synthesis. Studies with strains harboring a multicopy plasmid, pglnD or pglnE, indicate that the elevated synthesis of the converter enzymes causes the enhancement of GS synthesis. In addition, the absence of one of the converter enzymes reduces the expression of the glnA gene. The parallel relationship between the converter enzymes and GS seems to derive from the binding capacity of the converter enzymes for the unbound PII, which is a repressor for the glnA gene. Therefore, it is believed that the metabolic regulation of the glnD and glnE genes is ultimately linked to the expression of the glnA operon.
谷氨酰胺合成酶（GS）活性的调节涉及两个核苷酸化循环，即PII的尿苷酰化循环和GS的腺苷酰化循环，分别由两种转换酶，即尿苷酰转移酶和腺苷酰转移酶催化。这些转换酶感知氮可用性的波动，并相应地调节GS的活性。另一方面，GS的翻译后修饰与谷氨酰胺合成酶基因（glnA）的转录调节紧密相关，未修饰的PII蛋白在GS合成中作为阻遏物起作用。因此，尿苷酰转移酶感知的代谢信号通过PII传递到两个不同的调节水平，即翻译后水平和转录水平。为了研究以极低浓度存在的转换酶，将glnD和glnE基因克隆到携带强的、可调控的λ噬菌体启动子的质粒载体中。通过这种方式，尿苷酰转移酶和腺苷酰转移酶的产量分别提高到接近800倍和500倍的水平。重组DNA技术还使我们能够研究glnD和glnE基因的转录调节。这些基因的表达在氮过量条件下略有抑制，在氮过量加碳限制条件下抑制更明显。发现尿苷酰转移酶和腺苷酰转移酶浓度的变化也影响GS合成的速率。对携带多拷贝质粒pglnD或pglnE的菌株的研究表明，转换酶合成的增加导致GS合成的增强。此外，一种转换酶的缺失会降低glnA基因的表达。转换酶与GS之间的平行关系似乎源于转换酶对未结合的PII的结合能力，PII是glnA基因的阻遏物。因此，人们认为glnD和glnE基因的代谢调节最终与glnA操纵子的表达相关。
