Modifies, by uridylylation and deuridylylation, the PII regulatory proteins GlnB and GlnK, 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 proteins and UTP to PII-UMP and PPi, while under higher glutamine levels, GlnD likely hydrolyzes PII-UMP to PII and UMP (deuridylylation). Thus, controls uridylylation state and activity of the PII proteins, and plays an important role in the regulation of nitrogen metabolism. [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 likely activates uridylyl-removing (UR) activity. Is constitutively expressed, irrespective of the nitrogen content of the 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. Deletions in the 5'-region of this gene appear to be lethal. Insertion mutations in the central part of this gene abolishes the ability to use nitrate as a sole nitrogen source but not glutamine. In addition, neither uridylylation of PII nor induction of the ntr-regulated glnII gene (encoding glutamine synthetase II) under ammonium deficiency can be observed in mutant strains. Belongs to the GlnD family.