Transcriptional activator VnfA is required for the expression of a second nitrogenase system encoded in thevnfHandvnfDGKoperons inAzotobac-ter vinelandii. In the present study, we have purified full-length VnfA pro-duced inE. colias recombinant proteins (Strep-tag attached and tag-less proteins), enabling detailed characterization of VnfA for the first time.