Regulation of Nitrogen Fixation

Abstract

Klebsiella pneumoniae is the best studied nitrogen-fixing species because of its similarity to E. coli. In K. pneumo- niae, a cluster of 17 contiguous genes, grouped into 7 or 8 operons, codes for enzymes specifically involved in nitrogen fixation (reduction of di-nitrogen to ammonia). K. pneumoniae nif genes are subject to two levels of positive regulation in response to ammonia and oxygen. The first level is nif-specific and is mediated by the products of the niflA operon. The nifA product is a transcriptional activator required for the expression of all nif operons except its own (Buchanan-Wollaston et al., Nature 294, 776-778, 1981). The nifl product appears to antagonize the action of the nifA product in the presence of oxygen (Buchanan- Wollaston and Cannon, in Advances in Nitrogen Fixation Research, Veeger and Newton, eds. The Hague, Nijhoff/ Junk, 1984, p. 732). The second level of nif regulation is mediated by the centralized ntr system, which controls the expression of a variety of nitrogen assimilatory genes in enteric bacteria. Under conditions of NH,+ starvation, the ntrC+ntrA products activate the nifL4 operon (Ow and Ausubel, Nature 307, 307-313, 1983; Merrick, EMBO J. 2, 29-44, 1983) and other operons involved in nitrogen assimilation, such as hut (histidine utilization) and put (proline utilization) (for review see Magasanik, Ann. Rev. Genet. 16, 135168, 1982).