The regulation of glutamine synthetase activity involves two nucleotidylation/denucleotidylation cycles, namely, the adenylylation of glutamine synthetase and the uridylylation of P_{Ⅱ} which are catalyzed by two singular converter enzymes, adenylyltransferase and uridylyltransferase, respectively. These converter enzymes sense the fluctuation in the availability of nitrogen and accordingly regulate the activity of glutamine synthetase. On the other hand, the metabolic signals perceived by these cascade control systems give the effect on the biosynthesis of glutamine synthetase through PII status, which behaves as a repressor in free form, while it loses such a function if uridylylated by uridylyl-transferase. Uridylyltransferase, the ultimate component of the complex bicyclic cascade, is observed to be nitrogen-regulatable with a repressor-mode of control. These regulatory units show to play the major and important role in the general nitrogen metabolism of the organism, ranging from nitrogen fixation to nitrogen utilization on two different levels of regulation, namely the posttranslational level and transcriptional level.