Modifies, by uridylylation and deuridylylation, the PII regulatory protein (GlnB), in response to the nitrogen status of the cell that GlnD senses through the glutamine level. Under low glutamine levels, catalyzes the conversion of the PII protein and UTP to PII-UMP and PPi, while under higher glutamine levels, GlnD hydrolyzes PII-UMP to PII and UMP (deuridylylation). Thus, controls uridylylation state and activity of the PII protein, and plays an important role in the regulation of nitrogen assimilation and metabolism (Probable). [protein-PII]-L-tyrosine + UTP = [protein-PII]-uridylyl-L-tyrosine + diphosphate [protein-PII]-uridylyl-L-tyrosine + H2O = [protein-PII]-L-tyrosine + H(+) + UMP Uridylyltransferase (UTase) activity is inhibited by glutamine, while glutamine activates uridylyl-removing (UR) activity. Slightly up-regulated in a low ammonia medium. Has four distinct domains: an N-terminal nucleotidyltransferase (NT) domain responsible for UTase activity, a central HD domain that encodes UR activity, and two C-terminal ACT domains that seem to have a role in glutamine sensing. Cells lacking this gene have no growth defect in media containing different nitrogen sources. Total glutamine synthetase (GS) activity in culture filtrates is markedly reduced in the mutant, although activity in cell-free extracts remains normal. Virulence is unaffected in both in vitro and in vivo model systems of infection. Belongs to the GlnD family.