Our results suggest that while NifL senses both the redox and fixed nitrogen status, the NifA protein also plays a major role in signal perception through the response to 2-oxoglutarate. Although 2-oxoglutarate can be considered as a key physiological signal of the carbon status, the level of this metabolite is also influenced by the nitrogen status. In terms of signal integration, under oxidizing conditions we propose that signal transduction is dominated by the "ON" status of the flavin in the PAS domain of NifL, which promotes formation of the inhibitory binary complex, whereas under reducing conditions, signaling is dominated by the uridylylation state of Av GlnK and the concentration of 2-oxoglutarate. When Av GlnK is in its non-modified form our model predicts that the interaction with NifL will favor the formation of a GlnK-NifL-NifA ternary complex. Several years ago, it was predicted from studies with K. pneumoniae that "NifL may sense one signal and the N-terminal domain of NifA the other, both proteins having to be in their derepressing state to prevent inhibitory binding" (Drummond et al. 1990). In the light of our finding that the GAF domain of NifA is required for the response to 2-oxoglutarate, this was a very visionary hypothesis.
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