Modification of the Fe protein of the nitrogenase of Gloeothece (Nägeli) sp. ATCC 27152 during growth under alternating light and darkness

SUMMARY The Fe protein of the nitrogenase of the unicellular Cyanobacterium Gloeothece (Nageli) sp. ATCC 27152 can be resolved by SDS–PAGE into two antigenically detectable components of approximate Mr 38500 and 40000 respectively. The larger form of this protein may be produced by modification of the smaller form. Modification of the Fe protein is promoted under conditions where O2 (but not O2− or H2O2) has increased access to the enzyme, but does not allow nitrogenase to function under conditions of O2 stress. During growth of Gloeothece under alternating light and darkness, antigenically detectable Fe protein is absent throughout most of the light period. The restriction of nitrogenase activity to the period of darkness is better explained in terms of regulation of nitrogenase synthesis and degradation than by reversible modification of a constant intracellular concentration of Fe protein. However, newly synthesized Fe protein always appeared initially as its larger form, which may be catalytically inactive in aerobic cultures of Gloeothece. Conversion of this form to the smaller, assumed active, form of the Fe protein may be an additional factor in explaining the increase in nitrogenase activity that occurs during the first few hours of each dark phase.