Physiological role for the GlnK protein of enteric bacteria: Relief of NifL inhibition under nitrogen-limiting conditions

In Klebsiella pneumoniae, NifA-dependent transcription of nitrogen fixation (nif) genes is inhibited by a flavoprotein, NifL, in the presence of molec ular oxygen and/or combined nitrogen. We recently demonstrated that the gen eral nitrogen regulator NtrC is required to relieve NifL inhibition under n itrogen (N)-limiting conditions. We provide evidence that the sole basis fo r the NtrC requirement is its role as an activator of transcription for gln K, which encodes a P-II-like allosteric effector. Relief of NifL inhibition is a unique physiological function for GlnK in that the structurally relat ed GlnB protein of enteric bacteria-apparently a paralogue of GlnK-cannot s ubstitute. Unexpectedly, although covalent modification of GlnK by uridylyl ation normally occurs under N-limiting conditions, several lines of evidenc e indicate that uridylylation is not required for relief of NifL inhibition . When GlnK was synthesized constitutively from non-NtrC-dependent promoter s, it was able to relieve NifL inhibition in the absence of uridylyltransfe rase, the product of the glnD gene, and under N excess conditions. Moreover , an altered form of GlnK, GlnK(Y51N), which cannot be uridylylated due to the absence of the requisite tyrosine,,vas still able to relieve NifL inhib ition.