Under nitrogen excess conditions, high levels of glutamine stimulate de-uridylylation of PII, resulting in the formation of a ternary PII–NifL–NifA complex that inhibits NifA activity (left-hand side). Upon a shift to low nitrogen, where glutamine levels are low and 2-OG levels (red circles) are high, the UTase activity of GlnD is stimulated, resulting in uridylylation of PII saturated with 2-OG (red circles). Under these conditions, PII-UMP is no longer able to interact with NifL. The complex between NifL and NifA dissociates to release NifA activity, provided that the regulatory GAF domain of NifA is saturated with 2-OG (red circle, right-hand side). Under conditions of carbon limitation, NifL can still form a complex with NifA and inhibit its activity, since binding of 2-OG to the GAF domain of NifA is required to bypass inhibition by NifL, even if PII is uridylylated.