The subunit composition of the Fe protein of nitrogenase from Rhodospirillum rubrum during activation and inactivation was investigated. It was found that the upper subunit (on gel electrophoresis) of the two-subunit Fe protein was converted into the lower subunit during activation in vitro. When the Fe protein was inactivated in vivo by the addition of NH4Cl and alpha-oxoglutarate to the cells, a phosphate-labelled upper band appeared. During activation in vitro by the activating enzyme, some of the phosphate of the upper band remained with the protein and appeared in the lower band. Activations in vitro were performed on inactive Fe protein obtained from cells grown with glutamate as the nitrogen source. Both native and oxygen-denatured Fe protein exhibited the loss of upper band during treatment with activating enzyme.
