Fig. S2.

LC-MS of FNR. (A) Isolated FNR (prereconstitution), FNR after reconstitution of the [4Fe-4S] cluster, and reconstituted FNR after exposure to O₂ as indicated. Each sample was treated with 20 mM DTT before measurement to remove any persulfides or other readily reducible (and removable) species. The data show that FNR samples contain a small amount of a covalently modified protein at +40 Da and that there is no significant formation of oxygen adducts of FNR after exposure to O₂. (B) FNR protein samples purified from Escherichia coli cells grown in rich media (black spectrum) and minimal media (red spectrum). The data illustrate that the +40-Da form of the protein is significantly increased relative to the unmodified form when E. coli cells are grown under nutrient-limiting conditions. The nature of the modification is unknown.