The signal transduction protein GlnK is required for NifL-dependent nitrogen control of nif gene expression in Klebsiella pneumoniae

In Klebsiella pneumoniae, transcription of the nitrogen fixation (nif) gene s is regulated in response to molecular oxygen or availability of fixed nit rogen by the coordinated activities of the nifA and nifA gene products. Nif A is a nif-specific transcriptional activator, the activity of which is inh ibited by intellection with NifL,. Nitrogen control of NifL occurs at two l evels: transcription of the nifLA operon is regulated by the global ntr sys tem, and the inhibitory activity of NifL is controlled in response to fixed nitrogen by an unknown factor. K. pneumoniae synthesizes two P-II-like sig nal transduction proteins, GlnB, which we have previously shown not to be i nvolved in the response of NifL to fixed nitrogen, and the recently identif ied protein GlnK, We have now cloned the K. pneumoniae glnK gene, studied i ts expression, and shown that a null mutation in glnK prevents NifL from re sponding to the absence of fixed nitrogen, i.e., from relieving the inhibit ion of NifA activity. Hence, GlnK appears to be involved, directly or indir ectly, in NifL-dependent regulation of nif gene expression in K. pneumoniae . Comparison of the GlnB and GlnK amino acid sequences from six species of proteobacteria identifies five residues (residues 3, 5, 52, 54, and 64) whi ch serve to distinguish the GlnB and GlnK proteins.