Expression of Klebsiella pneumoniae nif genes in Proteus mirabilis.

Self-transmissible plasmids carrying his and nif genes from Klebsiella pneumoniae have been introduced into three his mutants of Proteus mirabilis: strains 5006-1, WR19 and WR20. Expression of his by the transconjugants was unequivocal, if slightly temperature-sensitive, but none was Nif+ when tested for acetylene reduction in anaerobic glucose medium using inocula from rich or glucose-minimal aerobic agar cultures. Succinate or pyruvate in place of glucose, low glucose, lower temperature or elevated Na2MoO4 did not allow nif expression and no nitrogenase MoFe-protein peptide was detected immunologically after exposure to conditions in which diazotrophic enterobacteria, normal or genetically constructed, derepress nif. One strain, P. mirabilis WR19, carrying the his nif Kmr plasmid pMF250 was examined in detail. The nif activator gene nifA was introduced on the plasmid pCK1. Such derivatives remained Nif- when tested, after aerobic growth on rich agar media, with normal or low glucose, with succinate or with elevated Mo. However, pre-conditioning by aerobic growth on glucose-minimal agar led to subsequent anaerobic expression of nif in glucose medium from pMF250 in WR19 carrying pCK1. NH+4 or proline could serve as N-source in the glucose-minimal agar. Maximum activity was about 5% of that of K. pneumoniae in our assay conditions. Material cross-reacting with anti-serum to the nitrogenase MoFe protein was formed. Nitrogenase activity was not 'switched off' by NH+4. P. mirabilis WR19 (pCK1) showed NH+4-constitutive temperature-sensitive kanamycin resistance (a nif-related phenotype of this plasmid) in aerobic glucose minimal medium.(ABSTRACT TRUNCATED AT 250 WORDS)