Azotobacter vinelandii is an aerobic soil-dwelling organism with a wide variety of metabolic capabilities which include the ability to fix atmospheric nitrogen by converting it to ammonia. Like Klebsiella pneumoniae it fixes nitrogen in the free-living state and does not enter into symbioses with plants; a process typified by the symbiosis between members of the genus Rhizobium and a variety of leguminous plants. Two features of the biology of Azotobacter make it of particular interest to scientists studying the nitrogen fixation process.
Firstly, Azotobacter vinelandii is capable of synthesising not only the molybdenum-containing nitrogenase enzyme that typifies most diazotrophs including Klebsiella pneumoniae and Rhizobium leguminosarum, but also two alternative nitrogenases; one in which vanadium replaces molybdenum and a second which contains neither transition metal but only iron. This ability to carry out the chemistry of nitrogen reduction at sites that do not contain molybdenum is of particular importance to chemists and biochemists investigating the mechanism of biological nitrogen fixation. The alternative nitrogenases are encoded by distinct structural genes, vnfHDGK and anfHDGK: the vnfG and anfG genes encoding an extra small subunit not found in molybdenum nitrogenase. However many of the same ancillary genes e.g. nifUSVWZ and nifM are used in biosynthesis of all three enzymes.
Synthesis of the alternative nitrogenases is regulated by availability of the appropriate metals i.e. molybdenum or vanadium, and expression of each set of genes is controlled by a specific regulatory protein, the products of the nifA, vnfA and anfA genes. Interest in this regulation has focussed research on the mechanisms whereby Azotobacter transports molybdate into the cell and distinguishes it from similar molecules such as sulphate. This has led to the dissection of the molybdate transport genes, modEABC and modG of Azotobacter that have homologues in many other bacteria.
Secondly, Azotobacter has evolved a number of physiological mechanisms to allow it to fix nitrogen aerobically despite the inherent oxygen-sensitivity of nitrogenase. It has uniquely high rates of respiration coupled with specific cytochromes to ensure that nitrogenase experiences an essentially anoxic environment despite the fact that energy is being derived from aerobic metabolism. It can also synthesise a protective 2Fe-2S protein which can bind to nitrogenase in conditions of oxygen stress to form an oxygen-stable complex that is inactive but protected from damage.
Current studies are focussed on the transcriptional regulatory proteins
NifA and NifL (Ray Dixon),
the complex mechanisms underlying the regulation of the three different
nitrogenase systems (Martin
Drummond), and the molybdenum transport proteins of Azotobacter.
More details of current research projects in this area can be obtained
by reference to individual researchers who work with Azotobacter.