Genetic evidence that NarL function is not required for nitrate regulation of nitrate assimilation in Klebsiella pneumoniae M5al.

We cloned the narL gene, required for nitrate induction of respiratory nitrate reductase synthesis, from Klebsiella pneumoniae. The E. coli narL gene product shares sequence similarity with the response regulator proteins of two-component regulatory systems. We found that narL(+)-containing plasmids restored nitrate regulation of anaerobic respiratory gene expression in appropriate Escherichia coli hosts. The K. pneumoniae narL region encoded a protein whose migration in Laemmli gels was indistinguishable from that of the narL product of E. coli. We constructed a narL::Km mutant of K. pneumoniae. This mutation abolished nitrate induction of respiratory nitrate reductase synthesis but had no effect on nitrate induction of assimilatory nitrate and nitrite reductase synthesis. We conclude that K. pneumoniae has distinct nitrate-responsive regulators for controlling respiratory and assimilatory gene expression.