Glutamate synthase genes of the diazotroph Azospirillum brasilense. Cloning, sequencing, and analysis of functional domains.

A 10-kilobase EcoRI fragment of Azospirillum brasilense genomic DNA was cloned in Escherichia coli. Two open reading frames of 4548 and 1446 base pairs (bp) were identified within the fragment as the structural genes for the alpha and beta subunits (gltB and gltD, respectively) of A. brasilense GltS. The organization of the gltBD region of A. brasilense differs from that of the corresponding region in E. coli: in A. brasilense, gltD is upstream relative to gltB, and its stop codon is separated by 141 bp from the first ATG of gltB. The deduced amino acid sequences reveal a high similarity with GltS from E. coli and with the ferredoxin-dependent GltS from maize. Binding domains for flavin cofactors and NADPH, a domain for glutamine binding and activation, and cysteine clusters for iron-sulfur centers formation were tentatively identified on the basis of sequence comparison with flavoproteins, pyridine nucleotide-dependent enzymes, amidotransferases, and iron-sulfur proteins.