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Rnf1 is the primary electron source to nitrogenase in a high-ammonium-accumulating strain of Azotobacter vinelandii

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Abstract

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.

Key points

• The Rnf1 complex is integral to ammonium accumulation in A. vinelandii.

• The Fix complex can be deleted and still achieve ammonium accumulation in A. vinelandii.

• A. vinelandii can be engineered to increase the contribution of the Fix complex to ammonium accumulation.

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Data availability

All data generated or analyzed during this study are included in this published article or are available from the corresponding author on reasonable request.

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Funding

This work was supported by grants from the MnDRIVE transdisciplinary research initiative through the University of Minnesota based on funding from the state of Minnesota, the National Institute of Food and Agriculture (Project Numbers MIN-12–070 and MIN-12–081) and award number 2020–67019-31148 through the US Department of Agriculture. M. H. P. was further supported through funding from the National Science Foundation (CBET-1437758).

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Authors and Affiliations

  1. Department of Bioproducts and Biosystems Engineering, University of Minnesota, 1390 Eckles Avenue, St. Paul, MN, 55108-6130, USA

    Brett M. Barney

  2. Biotechnology Institute, University of Minnesota, St. Paul, MN, 55108, USA

    Brett M. Barney & Mary H. Plunkett

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  1. Brett M. Barney
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  2. Mary H. Plunkett
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Contributions

B. M. B. conceived the study and designed experiments. B. M. B. and M. H. P. designed plasmids and constructed strains. M. H. P. and B. M. B. developed methods and collected data. All authors contributed to writing the manuscript and discussion.

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Correspondence to Brett M. Barney.

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Barney, B.M., Plunkett, M.H. Rnf1 is the primary electron source to nitrogenase in a high-ammonium-accumulating strain of Azotobacter vinelandii. Appl Microbiol Biotechnol 106, 5051–5061 (2022). https://doi.org/10.1007/s00253-022-12059-x

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