| | Azotobacter vinelandii is a large, obligately aerobic soil bacterium which has one of the highest respiratory rates known among living organisms and is able to grow on a wide variety of carbohydrates, alcohols and organic acids, in addition to be able to fix nitrogen. A. vinelandii has been intensely studied for many years because of its ability to synthesize three different nitrogenase enzymes and to fix nitrogen in air. Nitrogenase cofactors vary with respect to metal content and may contain molybdenum, vanadium or iron. A. vinelandii can form unique cysts that survive desiccation and the organism produces polymers of carbon for storage. In addition, this organism produces 5-alkylresorcinols that are phenolic lips commonly present in plants and animals but are uncommon in bacteria. A. vinelandii is highly amenable to genetic manipulation. Its genome content can vary greatly, from a few to 50 or more copies of the chromosome per cell, depending on growth conditions. Another unusual feature is its apparent inability to transport amino acids. The genome sequence will be invaluable to the community of scientists who work on this important and unusual microbe.
The full story
Azotobacter vinelandii is a nitrogen-fixing bacterium, found in soils world-wide, with many features relevant to energy consumption and carbon sequestration. It has been studied for more than 90 years by hundreds of scientists throughout the world. Among its unique abilities are the capacity to fix, or reduce, atmospheric nitrogen gas (N2) to compounds of ammonium (NH4 +), by using one of three distinct but related nitrogenase enzymes which vary in metal content (molybdenum, Mo; vanadium, V; or iron, Fe) and which require a large amount of cellular energy for biosynthesis and activity. This organism recycles the hydrogen produced as a byproduct of nitrogen fixation thereby increasing its efficiency. A.vinelandii has intrigued scientists not only because of its remarkable capacity to fix nitrogen but also because it can do this under conditions of atmospheric oxygen supply (20%), in contrast to other diazotrophic bacteria which must fix nitrogen either anaerobically or microaerobically. Paradoxically, nitrogenase is an extremely oxygen sensitive enzyme and A.vinelandii has developed sophisticated physiological mechanisms to protect the enzyme from oxygen damage. The ability of the organism to fix nitrogen under aerobic conditions has considerable energetic benefits since the extra ATP generated via aerobic respiration can be used to support the high energy demands of nitrogenase. Metal transport and metabolism are under study in several laboratories to answer questions concerning metal cluster biosynthesis and activity; metal clusters are important components of nitrogenase and also other enzymes involved in the nitrogen cycle or in other oxidation/reduction reactions. A. vinelandii utilizes a large number of different carbon sources and also synthesizes carbon storage molecules such as alginates and poly-b-hydroxybutyric acid, both of which are of importance in the food and biodegradable plastics industries, respectively. A. vinelandii also undergoes a simple form of differentiation to form cysts which are resistant to drought and other physical and chemical agents. These cysts contain 5-alkylresorcinols that are phenolic lipids commonly present in plants and animals but are uncommon in bacteria. These compounds have importance in agriculture and medicine. Little is known about the genes in plants or animals that have a function in the biosynthesis of these compounds. Knowledge of the A. vinelandii genome will help to elucidate the biosynthetic pathway in this and other organisms.
Another feature of A. vinelandii is the ease of its genetic manipulation because of its ability to accept DNA from the same or other species of bacteria by either transformation (direct transfer and incorporation of either plasmid or linear DNA into cells) or by conjugation of plasmids from donor bacteria. A. vinelandii has a unique plasticity of genome content: when in exponential growth phases, the number of chromosomes per cell is low (2-4 per cell, as is typical for Eubacterial species); when cultures reach stationary phase, the number of chromosomes can increase to 50-100 per cell, accompanied by a large increase, as expected, in cell size. What is the genetic basis for this? What is the biological advantage for the ability of this organism to accumulate vast numbers of chromosomes?
A. vinelandii is apparently unable to efficiently transport amino acids for utilization as either C or N sources. The sequence of the genome will reveal whether or not genes encoding amino acid transporter proteins are present. If not, why not? The presence of alternative nitrogenases and possibly also nitrate reductases might indicate that A. vinelandii, and other Azotobacter species have evolved extreme mechanisms to obtain and store nitrogen, that its soil niche or habitat is one in which organic N sources are not available (or might be toxic?), and that they have evolved a genome with an unusual number of genes needed to capture inorganic nitrogen. The road-map for the comparison of this genome with that of related bacteria will be provided by the genome sequence of Pseudomonas aeruginosa. Both genera, Azotobacter and Pseudomonas, are in the domain Eubacteria, phylum Proteobacteria, Class Deltaproteobacteria, family Pseudomonodaceae. While more than 200 genes have been identified and sequenced in many different laboratories (see http://ava.biosci.arizona.edu/, for a current compilation), the analysis of the entire genome of Azotobacter vinelandii will boost the efforts in all areas of research described above. |
