Biology Reference
In-Depth Information
Figure 7:
Reaction and molecular mechanism of biological nitrogen fi xation. (A) General reaction of molecular nitrogen
fi xation B. Schematic structure and operation of the nitrogenase enzyme complex and subsequent metabolism of
nitrogen.
Electrons are transferred from reduced ferredoxin
(or fl avodoxin) via azoferredoxin to molybdoferredoxin. Each mol of fi xed
nitrogen requires 16 mol ATP hydrolyzed by the NifH protein. The NH
3
produced is utilised in the synthesis of glutamine
or glutamate, respectively, for N-metabolism. NifJ: pyruvate fl avodoxin/ferrodoxin oxidoreductase,
NifF: Flavodoxin/
Ferredoxin). With the kind permission of C. Kneip, Department of Cell Biology, Philipps-University Marburg, Marburg,
Germany & Department of Molecular Biology, Max-Planck-Institute for Infection Biology, Berlin, Germany. [Kneip
et al
.
(2007)
BMC Evolutionary Biology
7:
55 doi: 10.1186/1471-2148-7-55].
i) Genome rearrangement
:
The interrupting element in the
nifD
gene is of 11 kb and possesses 11
bp direct repeats (GGATTACTCCG) at each of its ends and contains the
xisA
gene. The second
interrupting element is of 55 kb and interrupts the
fdxN
gene in the
nifB
-
fdxN
-
nifS
-
nifU
operon and
possesses 5 bp direct repeats (TATTC) at its ends. This large interrupting element contains
xisF
gene.
During heterocyst differentiation, both these interrupting elements (11 and 55 kb) are excised at the
direct repeats by the activity of the corresponding recombinase proteins and the gene continuities
are established. The excised elements remain as circular DNA molecules inside the cytoplasm of
the differentiating heterocysts. The expression of all the genes in the
nif
gene cluster enables the
