The Role of Adenylyltransferase and Uridylyltransferase in the Regulation of Glutamine Synthetase in Escherichia coli

Abstract

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.