Fig. 1 |. Overview of the procedure.

a, The diazotroph Azotobacter vinelandii was genomically edited to contain Fe protein C-terminally tagged with mPlum. Within the reducing environment of the A. vinelandii cytoplasm, the mPlum chromophore does not mature (immature mPlum). Once the protein is purified, if it is exposed to oxygen the mPlum chromophore will mature (mature mPlum) and can be visualized by eye if the aliquot turns purple, or through fluorescence at 649 nm. b, Procedure part 1, schematic workflow of anaerobic purification of the nitrogenase proteins following a two-step procedure of anion exchange followed by size exclusion chromatography. c, Procedure part 2, anaerobic preparation of samples for electron microscopy entirely within a vinyl anaerobic chamber or ‘tent’. QHP column, Q Sepharose high performance column.