Constitutive expression of nitrogen fixation (nif) genes of Klebsiella pneumoniae due to a DNA duplication.

A spontaneous mutant of Klebsiella pneumoniae exhibiting nitrogen fixing activity in the presence of ammonia was isolated from a nifL ::Mu mutant. The main features of the nif constitutive mutation, designated nif‐8388, were as follows: (i) neither ammonia nor bases repressed, but amino acids partially repressed, nitrogen fixation; (ii) the mutation caused an escape from the regulatory effect of glnA and glnG mutations of K. pneumoniae but not that of a glnF mutation; (iii) it enabled the activation of the nifH ‐lac fusion in the presence of oxygen with or without ammonia and a nifL ‐lac fusion in the presence of ammonia without oxygen; (iv) the mutation allowed nitrogen fixation at 37 degrees C when plasmid‐borne. Restriction analysis and Southern hybridization using Mu DNA and the 8.1‐kb nifQBALF EcoRI fragment as probes demonstrated that the nif‐8388 mutation was a tandem duplication of 10 kb in the nifL region in which no Mu DNA was present. This duplication led to an operon fusion between nifLA and his since Nifc expression was shown to be increased with a specific inducer of the his operon. These results provide further evidence that the nifA product is a nif‐specific activator, and that the nifL product is involved in oxygen repression and temperature control. In addition, they suggest that there is an autoactivation of nifLA transcription by the nifA product and that glnF could act in nif regulation by a mechanism other than the glnG‐mediated control of nifLA transcription.