Effect of Ammonia on the Synthesis and Function of the N 2 -Fixing Enzyme System in Clostridium pasteurianum

Abstract

The N 2 -fixing system of Clostridium pasteurianum operates under regulatory controls; no activity is found in cultures growing on excess NH 3 . The conditions which are necessary for the synthesis and function of this system were studied in whole cells by using acetylene reduction as a sensitive assay for the presence of the N 2 -fixing system. Nitrogenase of N 2 -fixing cultures normally can fix twice as much N 2 as is needed to maintain the growth rate. When cultures that have grown for four or more generations on NH 3 exhaust NH 3 from the medium, a diauxic lag of about 90 min ensues before growth is resumed on N 2 . Neither N 2 -fixing nor acetylene reduction activity can be detected before growth is resumed on N 2 . N 2 is not a necessary requirement for this synthesis since under argon that contains less than 10 −8 m N 2 , the N 2 -fixing system is made. If NH 3 is added to N 2 -dependent cultures, synthesis of the enzyme system is abruptly stopped, but the enzyme already present remains stable and functional for at least 6 hr (over three generations). Cultures grown under argon in a chemostat controlled by limiting ammonia have derepressed nitrogenase synthesis. If the argon is removed and replaced by N 2 , partial repression of nitrogenase occurs.