Genetic analysis of the nitrogen fixation system in Klebsiella pneumoniae

Fine structure mapping of nif mutations of Klebsiella pneumoniae was accomplished by means of Pl-transductional crosses and the plasmid R144 drd mediated conjugations. The physical distance between nif mutations based on the percentage of co-transduction with hisD of the nif mutations was estimated. The maximal distance between two mutations was calculated about 3 Kb, and the average distance between different nif mutations was about 1 to 2 Kb. So no "silent region" was shown within the nif cluster nearby the histidine operon. Several hisD-unlinked nif mutants were isolated and investigated genetically and biochemically. They all differed from the glutamineless mutants, one of these mutants was tentatively assigned as a sort of N-assimilation mutant with little activity of glutamate synthetase. It differed from the known N-assimilation mutants in its absence of nitrogenase activity. Since the wild type hisD-linked nif genes carried by the plasmid RP4 failed to complement the defects of the hisD-unlinked nif genes in the recipient cells but they were effective to facilitate E. coli in acquiring the ability to fix nitrogen, which indicates that the hisD-unlinked nif genes necessary for the functioning of the hisD-linked nif genes are present in E. coli.