Thursday, October 5, 2017, 9:00am to 10:00am
Naito Laboratory, Room 205, 12 Oxford Street
Nitrogenase is the only protein on the planet capable of fixing nitrogen, yet it is extremely sensitive to oxygen and has a very high metal requirement (38 Fe + 1 Mo/V/Fe). It is thought to have originated in the Archean or early Proterozoic, and thus its evolutionary history likely includes adjustment to at least one major change to the redox state of the Earth system. We used ancestral sequence reconstruction to understand the evolution of biological nitrogen fixation through Earth history. We reconstructed ancestral nitrogenase sequences by creating a phylogeny using 355 modern sequences, including those of 20 vanadium and 48 iron-only nitrogenases. We modeled the structures of these ancestral proteins via homology to a modern nitrogenase crystal structure. Metal cluster-coordinating residues and tertiary structure are conserved in all ancestors, while some sequences diverge by over 50%. This variation is sometimes confined to particular functional domains of the protein and thus is unlikely to reflect neutral evolution. Mutation rate was elevated for an oxygen-protection domain of the proteobacterial nitrogenase ancestor and an intracellular localization domain of the similarly-aged cyanobacterial nitrogenase ancestor, suggesting tandem response in divergent lineages to increased oxidative stress.