Regulation of Free-Living and Symbiotic Nitrogen Fixation in Azorhizobium caulinodans

Abstract

Azorhizobium caulinodans ORS571, isolated from stem nodules of Sesbania rostrata, can fix nitrogen in the free-living state (Nif⁺) and symbiotically (Fix⁺) (Elmerich et al. 1982; Dreyfus et al. 1988). Signals involved in the regulation of nitrogen fixation are oxygen and fixed nitrogen (Ratet et al. 1989). This regulation is mediated by two-component regulatory systems which involve FixLJ, NtrBC and NtrYX (Pawlowski et al. 1987, 1991; Kaminski et al. 1991). Nif^-, Fix^- mutant strains impaired in the synthesis of polypeptides normally induced under nitrogen free-conditions, including nitrogenase polypeptides, have been characterized (Denefle et al. 1987). This led to the discovery of two new regulatory genes. One belongs to the lysR family, the other shares similarity with the E. coli hfQ gene (Kaminski et al. 1994).