THE PRODUCT OF THE NITROGEN-FIXATION REGULATORY GENE NFRX OF AZOTOBACTER-VINELANDII IS FUNCTIONALLY AND STRUCTURALLY HOMOLOGOUS TO THE URIDYLYLTRANSFERASE ENCODED BY GLND IN ENTERIC BACTERIA

We sequenced the nitrogen fixation regulatory gene nfrX from Azotobacter vinelandii, 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 uridvlyltransferase-uridylyl-removing enzyme encoded by glnD in Escherichia coli. In vivo complementation experiments demonstrate that the glnD and nfrX products are functionally interchangeable. A. vinelandii nfrX thus appears to encode a uridylyltransferase-uridylyl-removing enzyme, and in this paper we report the first sequence of such a protein. The Nif- phenotype of nfrX mutants can be suppressed by a second mutation in a recently identified nifL-like gene immediately upstream of nifA in A. vinelandii. NifL mediates nif regulation in response to the N status in A. vinelandii, presumably by inhibiting NifA activator function as occurs in Klebsiella pneumoniae; thus, one role of NfrX is to modify, either directly or indirectly, the activity of the nifL product.