Nucleotidyltransferase (NT) domain of Escherichia coli adenylyltransferase (GlnE), Escherichia coli uridylyl transferase (GlnD), and similar proteins.
Escherichia coli GlnD and -E participate in the Glutamine synthetase (GS)/Glutamate synthase (GOGAT) pathway for the assimilation of ammonium nitrogen. In nitrogen sufficiency, GlnE adenylates GS, reducing GS activity; when nitrogen is limiting, GlnE deadenylates GS-AMP, restoring GS activity. When nitrogen is limiting, GlnD uridylylates the nitrogen regulatory protein PII to PII-UTP, and in nitrogen sufficiency, it removes the modifying groups. The activity of Escherichia coli GlnE is modulated by PII-proteins. PII-UMP promotes GlnE deadenylation activity, and PII promotes GlnE adenylation activity. Escherichia coli GlnE has two separate NT domains. The N-terminal NT domain catalyzes the deadenylylation of GS, and the C-terminal NT domain the adenylylation reaction. The majority of proteins in this family contain a C-terminal NT domain which is associated with a cystathionine beta-synthase (CBS) domain pair and a CAP_ED (cAMP receptor protein effector ) domain. This family belongs to the Pol beta-like NT superfamily. In the majority of enzymes in this superfamily, two carboxylates, Dx[D/E], together with a third more distal carboxylate, coordinate two divalent metal cations involved in a two-metal ion mechanism of nucleotide addition. For the majority of proteins in this family, these carboxylate residues are conserved.