REGULATION OF NITROGEN-FIXATION IN AZOSPIRILLUM-BRASILENSE

The regulation of nitrogen fixation in Azospirillum brasilense is very complicated, and it responds to exogenous fixed nitrogen or a change of oxygen concentration. This regulation occurs at both transcriptiona l and posttranslational levels. Unlike regulation seen in Klebsiella p neumoniae, transcription of nifA does not require NTRB/NTRC in A. bras ilense and the expression of nifHDK is controlled by posttranslational regulation of NIFA activity. Addition of NH4+ or a shift from microae robic to anaerobic conditions also causes a rapid loss of nitrogenase activity in A. brasilense. This posttranslational regulation of nitrog enase activity involves the DRAT/DRAG regulatory system, which is simi lar to that of Rhodospirillum rubrum. Both DRAT and DRAG activities ar e regulated in vivo, but the mechanisms for their regulation are unkno wn.