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Azotobacter
Are free living bacteria which grow well on a nitrogen free medium. These bacteria
utilize atmospheric nitrogen gas for their cell protein synthesis. This cell protein is
then mineralised in soil after the death of Azotobacter cells thereby contributing
towards the nitrogen availability of the crop plants.
Azotobacter is an aerobic soil-dwelling organism with a wide variety
of metabolic capabilities that fixes the nitrogen from the atmosphere
and does not enter into symbioses with plants.
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.
Azotobacter vinelandii is a micro organism
that can fix atmospheric nitrogen into the soil by converting it to ammonia.
Micro biology of Azotobacter in the nitrogen fixation process.
It is capable of synthesising the molybdenum-containing nitrogenase enzyme that typifies most diazotrophs
(Like Klebsiella pneumoniae and Rhizobium
leguminosarum). Also process two additional 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.
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