NTRC IS REQUIRED FOR CONTROL OF KLEBSIELLA-PNEUMONIAE NIFL ACTIVITY

In response to molecular oxygen and/or fixed nitrogen, the product of the Klebsiella pneumoniae nitrogen fixation L (nifL) gene inhibits Nif A-mediated transcriptional activation. Nitrogen regulation of NifL fun ction occurs at two levels: transcription of the nifLA operon is regul ated by the general Ntr system, and the activity of NifL is controlled by an unknown mechanism. We have studied the regulation of NifL activ ity in Escherichia coli and Salmonella typhimurium by monitoring its i nhibition of NifA-mediated expression of a K. pneumoniae Phi(nifH'-'la cZ) fusion. The activity of the NifL protein transcribed from the tac promoter is regulated well in response to changes of oxygen and/or nit rogen status, indicating that no nif- or K. pneumoniae-specific produc t is required, Unexpectedly, strains carrying ntrC (glnG) null alleles failed to release NifL inhibition, despite the fact that synthesis of NifL was no longer under Ntr control. Additional evidence indicated t hat it is indeed the transcriptional activation capacity of NtrC, rath er than ifs repression capacity, that is needed, and hence it is a pla usible hypothesis that Ntr-C activates transcription of a gene(s) whos e product(s) in term functions to relieve NifL inhibition under nitrog en-limiting conditions.