Transcript Nitrogen Fixing

Applications of
Biotechnological Processes
Enhancing Nitrogen Fixing
Nitrogen fixation Revision
 Nitrogen is an element required for the amino groups of
proteins.
 Nitrogen is taken up by plants in the form of ammonium
or nitrate. These supplies of nitrogen are usually limiting.
 Nitrogen fixing bacteria convert atmospheric nitrogen
into ammonia.
 Nitrogen fixing bacteria can be free living e.g.
Azotobacter or be in symbiosis with plants (root nodules)
e.g. Rhizobium.
 Enzyme nitrogenase is responsible for the
conversion of nitrogen to ammonia.
 Nitrogenase operates in anaerobic conditions
but nitrogen fixation is an ATP demanding
reaction (aerobic respiration is therefore
necessary).
 Leghaemoglobin in root nodules binds to oxygen
to prevent it disabling nitrogenase activity.
Leghaemoglobin releases oxygen slowly for
aerobic respiration to take place.
Rhizobia
 Contain a set of nif genes that are responsible
for the coding of nitrogenase.
 However the expression of nif genes alone is not
sufficient for nitrogen fixation to take place.
 Scientists have attempted to transfer nif genes
to other bacteria e.g. E.coli and directly into plant
cells. However nitrogenise expression has not
been successful
Enhancing nitrogen fixing
Farmers can add Rhizobia inoculum
cultures to their legume fields (this
reduces the need for artificial fertiliser and
increases crop yield).
• Mutant Rhizobia have been engineered so
that the nif genes are never “switched off”.
The effector complex which binds to the nif
genes to prevent expression in high nitrogen
environments, is prevented from binding
 There are many species of Rhizobia. A single
species will form a symbiosis with only certain
legumes (there is a degree of specificity
between plant and bacteria).
 This specificity is due to the type of plasmid
carried by the Rhizobia. The plasmid produces
enzymes that are responsible for producing
signal molecules that are detected by the plant.
 It is possible to transfer the plasmid of one
Rhizobia species to another Rhizobia.
The plasmid from (a) is transferred to Rhizobium (b), to
create (c) that can infect pea and carry out good fixation.

(a)
(b)
(c)
Infects pea plants
Infects clover plants
Infects pea plants
Moderate growth
Good growth
Good growth
Moderate fixation
Good fixation
Good fixation
An ideal inoculum will:
Have a large number of bacteria
Have been mutated to ensure that nif genes are
permanently switched on
Have the correct plasmid for the target plant
species
Past Paper Questions
 2004 Q1(ii)