Journal
APPLIED MICROBIOLOGY AND BIOTECHNOLOGY
Volume 106, Issue 13-16, Pages 5051-5061Publisher
SPRINGER
DOI: 10.1007/s00253-022-12059-x
Keywords
Azotobacter vinelandii; Rnf1 complex; Nitrogen fixation; Fix complex; Electron source
Categories
Funding
- MnDRIVE transdisciplinary research initiative through the University of Minnesota from the state of Minnesota
- National Institute of Food and Agriculture [MIN-12-070, MIN-12-081]
- National Science Foundation [CBET-1437758]
- US Department of Agriculture [2020-67019-31148]
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This study demonstrates that the Rnf1 complex in Azotobacter vinelandii is the primary source of electrons for nitrogenase in a partially deregulated strain. When the Fix complex is unable to provide enough electrons, relocating its genes can increase ammonium accumulation.
The enzyme nitrogenase performs the process of biological nitrogen fixation (BNF), converting atmospheric dinitrogen gas into the biologically accessible ammonia, which is rapidly protonated at physiological pH to yield ammonium. The reduction of dinitrogen requires both ATP and electrons. Azotobacter vinelandii is an aerobic nitrogen-fixing microbe that is a model organism for the study of BNF. Previous reports have described strains of A. vinelandii that are partially deregulated for BNF, resulting in the release of large quantities of ammonium into the growth medium. Determining the source of the electrons required to drive BNF is complicated by the existence of several protein complexes in A. vinelandii that have been linked to BNF in other species. In this work, we used the high-ammonium-accumulating strains of A. vinelandii to probe the source of electrons to nitrogenase by disrupting the Rnf1 and Fix complexes. The results of this work demonstrate the potential of these strains to be used as a tool to investigate the contributions of other enzymes or complexes in the process of BNF. These results provide strong evidence that the Rnf1 complex of A. vinelandii is the primary source of electrons delivered to the nitrogenase enzyme in this partially deregulated strain. The Fix complex under native regulation was unable to provide sufficient electrons to accumulate extracellular ammonium in the absence of the Rnf1 complex. Increased ammonium accumulation could be attained in a strain lacking the Rnf1 complex if the genes of the Fix protein complex were relocated behind the strong promoter of the S-layer protein but still failed to achieve the levels found with just the Rnf1 complex by itself.
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