We sequenced the nitrogen fixation regulatory gene nfrX from Azotobacter vinelandi, mutations in which cause a Nif-phenotype, and found that it encodes a 105-kDa protein (NfrX), the N terminus of which is highly homologous to that of the uridylyltransferase-uridylyl-removing enzyme encoded by ginD in Escherichia coli.In Enteric bacteria modulate their pattern of gene expression and the activities of certain enzymes in response to the quality and quantity of their nitrogen source. The availability of fixed nitrogen is sensed by the glnD product, a uridylyltransferase-uridylyl-removing enzyme (UT-UR), which responds to the 2-ketoglutarate/glutamine ratio in the cell. In conditions of N deficiency, UT-UR uridylylates the next component of the regulatory pathway, the tetrameric protein PI, encoded by glnB, and conversely removes the modifying groups in N sufficiency. The unmodified, active form of PI, signals N sufficiency to the ntrBC system, which regulates gene expression. It also stimulates adenylylation and consequent inactivation of glutamine synthetase (GS) by an adenylyltransferase (42).In N deficiency, when PII is uridylylated, NtrB (also called NRII) phosphorylates the transcriptional activator protein NtrC (also called NRI) (27,38). The phosphorylated form of NtrC, NtrC-P, directly or indirectly activates the expression of a variety of genes, including those for utilization of histidine (hut) or arginine (aut), the structuiral gene for glutamine synthetase (glnA), and, in Klebsiella pneumoniae, the nitrogen fixation (nil) genes (3, 34, 38). In N excess, P11UMP becomes deuridylylated and unmodified PII stimulates dephosphorylation and consequent inactivation of NtrC by NtrB.In K. pneumoniae, NtrC-P activates the niTLA promoter. NifA is a transcriptional activator that is both structurally and functionally similar to NtrC but is specific for activation * Corresponding author. t Present address: