Mutations that alter the covalent modification of glutamine synthetase in Salmonella typhimurium

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Abstract
S. typhimunum glnD and glnE mutant strains lack 3 of the 4 activities required for reversible covalent modification of glutamine synthetase (GS; EC 6.3.1.2). The glnD strains, which are unable to deadenylylate GS and therefore accumulate the adenylylated or less active form of the enzyme, were isolated as glutamine bradytrophs. They lack the activity in PIIA uridylyl-transferase, one of the proteins required for deadenylylation in GS; they also lack PIID uridylyl-removing activity. Mutations in glnD are suppressed by 2nd-site mutations in glnE that eliminate the activity of GS adenylyltransferase (EC 2.7.7.42) and thus prevent adenylylation of GS. The glnD and glnE strains have 1/3-1/2 as much total GS as the wild-type strain when they are grown in a medium containing a high concentration of NH4+. The wild-type strain derepresses synthesis of GS 4-fold in response to N2 limitation; glnD and glnE strains derepress synthesis of the enzyme 4-fold and 7-fold, respectively. Thus, mutations that alter covalent modification of GS in Salmonella do not significantly affect derepression of its synthesis. The glnD gene lies at 7 min on the Salmonella chromosome and is 50% linked to pyrH by [phage] P22-mediated transduction.