THE GENES ENCODING THE DELTA SUBUNITS OF DINITROGENASE-2 AND DINITROGENASE-3 ARE REQUIRED FOR MO-INDEPENDENT DIAZOTROPHIC GROWTH BY AZOTOBACTER-VINELANDII

vnfG and anfG encode the delta subunits of alternative nitrogenases 2 and 3 in Azotobacter vinelandii, respectively. As a first step towards elucidating the role of these subunits, diazotrophic growth and acety lene reduction studies were conducted on mutants containing alteration s in the genes encoding these subunits. Mutants containing a stop codo n (C36stop) or an in-frame deletion in anfG were unable to grow in N-f ree, Mo-deficient medium (Anf(-)). Mutants in which cysteine 36 of Anf G (a residue conserved between VnfG and AnfG) was changed to Ala or Se r were Anf(+). Thus, this conserved cysteine is not essential for the function of AnfG in dinitrogenase 3. A mutant with a stop codon in vnf G (C17stop) grew after a lag of 25 h in N-free, Mo-deficient medium co ntaining V2O5. However, a Nif(-) Anf(-) strain with this mutation was unable to grow under these conditions. This shows that the vnfG gene p roduct is required for nitrogenase 2-dependent growth. Strains with mu tations in vnfG and anfG reduced acetylene to different degrees. This indicates that the delta subunits are not required for acetylene reduc tion by nitrogenases 2 and 3.