Modulation of glutamine synthetase adenylylation and deadenylylation is mediated by metabolic transformation of the P II -regulatory protein.

Earlier studies showed that two protein components, P(I) and P(II), are concerned with the adenylylation and deadenylylation of Escherichia coli glutamine synthetase (EC 6.3.1.2). P(I) by itself catalyzes both adenylylation and deadenylylation, but its activity is modulated by the P(II)-protein and by glutamine, 2-oxoglutarate, ATP, and UTP, The P(II)-protein exists in two forms: one form, P(II)-AT, stimulates P(I)-catalyzed adenylylation activity in the absence of glutamine and makes this activity very sensitive to inhibition by 2-oxoglutarate; it does not affect deadenylylation activity. The other form, P(II)-DA, stimulates adenylylation only if glutamine is present, and also stimulates the deadenylylation activity of P(I), which is then dependent upon the presence of ATP and 2-oxoglutarate. Conversion of P(II)-AT to P(II)-DA requires the presence of UTP, ATP, and 2-oxoglutarate; it is catalyzed by an enzyme present in P(I) preparations. UTP may be directly involved in this conversion since P(II)-DA fractions reisolated by filtration through Sephadex G-100 contain small quantities of a bound uridine derivative that lacks the gamma-phosphoryl group of UTP. The activity of P(II)-DA, but not of P(II)-AT, is destroyed by treatment with snake-venom phosphodiesterase. ATP and 2-oxoglutarate apparently function as allosteric effectors for the conversion of P(II)-AT to P(I)-DA.