This is a family of bifunctional uridylyl-removing enzymes/uridylyltransferases (UR/UTases, GlnD) that are responsible for the modification (EC:126.96.36.199) of the regulatory protein P-II, or GlnB (pfam00543). In response to nitrogen limitation, these transferases catalyze the uridylylation of the PII protein, which in turn stimulates deadenylylation of glutamine synthetase (GlnA). Deadenylylated glutamine synthetase is the more active form of the enzyme. Moreover, uridylylated PII can act together with NtrB and NtrC to increase transcription of genes in the sigma54 regulon, which include glnA and other nitrogen-level controlled genes. It has also been suggested that the product of the glnD gene is involved in other physiological functions such as control of iron metabolism in certain species. The region described in this family is found in many of its members to be C-terminal to a nucleotidyltransferase domain (pfam01909), and N-terminal to an HD domain (pfam01966) and two ACT domains (pfam01842).