Copyright C) 1990, American Society for Microbiology Nucleotide Sequences and Mutational Analysis of the Structural Genes for Nitrogenase 2 of Azotobacter vinelandiit

The nucleotide sequence (6,559 base pairs) of the genomic region containing the structural genes for nitrogenase 2 (V nitrogenase) from Azotobacter vinelandii was determined. The open reading frames present in this region are organized into two transcriptional units. One contains vnJH (encoding dinitrogenase reductase 2) and a ferredoxinlike open reading frame (Fd). The second one includes vnJf) (encoding the a subunit of dinitrogenase 2), vnfG (encoding a product similar to the 8 subunit of dinitrogenase 2 from A. chroococcum), and vnfK (encoding the 3 subunit of dinitrogenase 2). The 5'-flanking regions of vnfH and vnJf) contain sequences similar to ntrA-dependent promoters. This gene arrangement allows independent expression of vn.f-Fd and vnfDGK. Mutant strains (CA80 and CA11.80) carrying an insertion in vnjf are still able to synthesize the a and 1 subunits of dinitrogenase 2 when grown in N-free, Mo-deficient, V-containing medium. A strain (RP1.1l) carrying a deletion-plus-insertion mutation in the vnfDGK region produced only dini-trogenase reductase 2. Azotobacter vinelandii can grow diazotrophically, using any of three distinct nitrogenases depending on the presence or absence of Mo or V in the growth medium. Nitrogenase 1 is synthesized in cells cultured under Mo sufficiency, nitro