Nitrogenase synthesis in Klebsiella pneumoniae: comparison of ammonium and oxygen regulation

Abstract

Rates of nitrogenase synthesis by Klebsiella pneumoniae were measured by pulse-labelling organisms with a mixture of 14C-labelled amino acids followed by sodium dodecyl sulphate gel electrophoresis and autoradiography. Populations from an NH4+-repressed, SO42--limited chemostat (0.46 mg dry wt ml-1), when released from NH4+ repression, simultaneously synthesized detectable quantities of the three nitrogenase polypeptides 45 min before acetylene-reducing activity was observed. Exposure of populations synthesizing nitrogenase to air or NH4+ (200 microgram N ml-1) repressed synthesis of both component proteins simultaneously, the rate initially decreasing by half in 11 to 12 min; in the presence of NH4+ a second slower phase with an approximate half-life of 30 min was observed. With 5% O2 in N2 the half-lives for the decreases in the rates of synthesis were 30 min for the Fe protein and 33 min for the Mo-Fe protein. Oxygen also repressed nitrogenase in a glutamine synthetase constitutive derivative of K. pneumoniae (strain SK24) which escapes NH4+ repression. Regulation of nitrogenase by O2 may therefore be independent of glutamine synthetase.