Metal Regulation of Nitrogenases and Molybdenum Transport in Azotobacter Vinelandii

Abstract

In the 1930s H. Bortels first established the importance of molybdenum and vanadium for nitrogen fixation, and also noted, albeit only in a footnote, that Azotobacter vinelandii could grow in the absence of both Mo and V (Bortels 1930; 1936). For 50 years these results were misinterpreted as showing that nitrogenase has an absolute requirement for Mo, when iron alone is the only transition metal needed; though the efficiency of fixation is significantly higher with V or Mo (Chisnell et al. 1988; Pau et al. 1989). The involvement of metals other than Mo in nitrogenase catalysis was only accepted as a result of P. Bishop’s discovery of the alternative nitrogenases (Bishop et al. 1980). A. vinelandii synthesizes three distinct nitrogenases, Mo-nitrogenase, V-nitrogenase, and nitrogenase-3, which is most probably an Fe-nitrogenase (Robson et al. 1986; Chisnell et al. 1988; Pau et al. 1989; Joerger et al. 1990). The paramount current interest of the alternative nitrogenases, now that the structure of Mo-nitrogenase has been determined, is that the cofactor of each nitrogenase differs by the presence or absence of a single atom, Mo, V, and presumably Fe (Kim, Rees 1992). The alternative nitrogenases also introduced a new subject of study, the mechanism metal regulation of nitrogenase, and it is now possible to describe in outline the regulatory pathways involved. Mo and V regulate expression of the three nitrogenases in a hierarchical manner at the level of transcription (Jacobson et al. 1986; Luque, Pau 1991; Jacobitz, Bishop 1992). Mo is essential for expression of Mo-nitrogenase and represses synthesis of both alternative nitrogenases. In the absence of Mo, V regulates the synthesis of alternative nitrogenases.