Nitrogenase switch-off by ADP ribosylation in some diazotrophs is due to the inactivation of the Fe-protein (Lowery 1986) by the transfer of an ADP-ribosyl residue to one of the subunits, catalyzed by dinitrogenase reductase ADP-ribosyl transferase (DRAT), and the activity is recovered by dinitrogenase reductase-activating glycohydrolase (DRAG) (Lowery 1986). ntrC mutants of A. brasilense were impaired in nitrogenase switch-off (Pedrosa, Yates 1984) suggesting that NtrC may participate in a signaling circuit including DRAT/DRAG, involving a signal protein. The PII protein and its paralog, GlnZ in A. brasilense are candidates for such signal molecules. Here we show that the nitrogenase activity of a glnZ mutant is switch-on deficient and that GlnZ could be involved in the control of the DRAT/DRAG system.

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