are an important crop throughout the world and strategies to
improve crop production by manipulating the nitrogen fixing
partner are desirable.
Nitrogen fixation requires
a large input of energy and reducing potential. As a strictly respiratory
organism B. japonicum must get that energy from the respiratory
chain by oxidizing energy rich substrates and reducing oxygen. One
strategy to improve crop production is to increase the nitrogen
fixing capacity of B. japonicum through manipulation of the
B. japonicum presents
an excellent model organism for studying respiratory enzymes. There
is a large body of information describing the respiratory chains
in B. japonicum. However, other than the terminal oxidases,
little information is available on the structure, function and regulation
of the enzymes of the B. japonicum respiratory chain. B.
japonicum can express a number of terminal oxidases. Specific
terminal oxidases are expressed depending upon oxygen availability
and/or association with the symbiotic partner. In association with
soybean B. japonicum undergoes conversion to a pleomorphic
form referred to as a bacteroid. Changes in the respiratory chain
of the bacteroid are reflected in the amounts and types of cytochromes
present. In addition to the expression of alternative terminal oxidases,
the bacteroids must also express a number of other cytochromes necessary
for utilization of the nutrients available insside the nodule. The
dicarboxylic acids succinate and malate serve as energy rich substrates
for the bacteroids. These compounds can be used to provide reducing
potential to the respiratory chain of the bacterium and the enzymes
involved in conserving this reducing potential contain cytochromes..
Part of the work in our
lab focuses on the enzymes involved in transfering electrons from
the dicarboxylic acids to the respiratory
chain. We are looking at two respiratory complexes, NADH dehydrogenase
(NADH ubiquinone oxidoreductase - Nuo) and succinate dehydrogenase
(Sdh) (referred to as complexes I and II, respectively, in mitochondria).
NADH dehydrogenase takes electrons from NADH and transfers them
to ubiquinone, a lipid soluble electron carrier. Possible major
sources of NADH in bacteroids are one of the malic enzymes and malate
dehydrogenase. Both of these ezymes get electrons from malate one
of the most abundant dicarboxylic acids in the soybean nodule. Succinate
is another dicarboxylic acid that is very abundant in nodules and
the enzyme succinate dehydrogenase takes electrons from succinate
and transfers them to ubiquinone.
An additional project
in our lab is directed at determining factors that may influence
the ability of inoculant strains to compete with native strains
for infection of the roothair. One factor would be acyl homoserine
lactone (AHL) molecules that are involved in density dependent gene
expression - also known as "quorum
sensing". Another factor would be inhibitors that would
block the growth of native strains but not inoculants. To accomplish
this, we are using an inhibitor of succinate dehydrogenase (carboxin
- Vitavax) and creating carboxin resistant inoculant strains.
Yet another project in
our lab is investigating factors that influence expression of the
respiratory enzymes. Factors that we are examining are iron, heme,
oxygen and the dicarboxylic acids. Projects include studies of the
expression of the hemA, sdhCDAB and nuo genes.
Check out my research
interests to find out more about my current research involving B. japonicum.