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Summary
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://www.azotobacter.org/ava.html),
for a current compilation), the analysis of the entire genome of Azotobacter
vinelandii will boost the efforts in all areas of research described above.
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