It has been established for many years that NifL and NifA form an atypical two-component sensor-response regulator system. Ray Dixon reported refined information on the mechanisms of NifL modulation of NifA activity in Azotobacter vinelandii (Little et al. 2000; Reyes-Ramirez et al. 2001). NifLs N-terminal part contains a PAS domain typical of redox sensing proteins. Oxidation of the FAD moiety converts NifL into an inhibitory form. The C-terminal domain of NifL is responsible for sensing the nitrogen status through the binding of the PII (GlnK) protein in its non-uridylylated form. 2-Ketoglutarate was found to prevent the inhibition of NifA by NifL in response to ADP. The NifA N-terminal part itself carries a GAF-regulatory domain that is present in signaling molecules, which bind small cofactors. This domain is necessary to relieve inhibition when 2-ketoglutarate is present. Thus, the GAF domain of NifA can be considered as the sensor domain for the carbon status of the cell. This is also likely to be the case in different bacteria that do not contain a nifL gene. Indeed, it was previously reported that the N-terminal domain of NifA is inhibitory in Azospirillum and in Herbaspirillum and that PII plays a role in modulating NifA activity (Arsène 1996, 1999; Souza et al. 1999). Further documentation was reported at this meeting. In particular, using the yeast two-hybrid technique, PII was found to bind to the N-terminal domain of NifA in Azospirillum (Chen et al. this volume).
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