RECONSTRUCTION OF NITROGENASE PREDECESSORS SUGGESTS ORIGIN FROM MATURASE-LIKE ENZYMES

Biological nitrogen fixation, catalyzed by nitrogenase metalloenzymes, is the process by which organisms reduce bioessential nitrogen to organic compounds. Nitrogen fixation has likely shaped biological productivity for much of Earth history, and investigations into its early evolution are critical for understanding global biogeochemical cycling across geologic time. Unresolved questions center on the degree to which early marine metal availabilities have constrained the evolution of nitrogen fixation. The majority of nitrogenases use a molybdenum-containing cofactor. However, it has previously been hypothesized that earliest “proto-nitrogenases” may have been capable of reducing nitrogen with a cofactor precursor lacking molybdenum. Under this model, molybdenum use was later achieved by the subsequent evolution of maturase enzymes from within the nitrogenase clade, which incorporate molybdenum into the cofactor precursor as the final step of the assembly pathway. We tested this hypothesis by the phylogenetic reconstruction and ancestral sequence inference of nitrogenase and maturase enzymes. Our phylogenetic results confirm a shared evolutionary history reflective of an ancient gene duplication event. Contrary to previous studies, we find that nitrogenases originate from within the maturase clade. Further, sequence analyses of the reconstructed common ancestor indicate that both nitrogenases and maturases likely evolved from enzymes incapable of reducing nitrogen. This provides a phylogenetic constraint on the evolution of nitrogen fixation. The binding of a molybdenum cofactor is nevertheless a conserved trait of both nitrogenases and maturases. We therefore speculate that the common ancestor may have similarly incorporated molybdenum, and that this metal binding interaction may predate the nitrogenase enzyme. Surprisingly, such a scenario would indicate that ancient limited molybdenum availability suggested by geochemical analyses may not have been a critical constraint on nitrogenase origins. These findings illustrate the need for additional future investigations into geochemical constraints on other metalloenzyme-driven nutrient cycles across geologic time.