Biology Reference
In-Depth Information
stimulation of acetylene reduction by intermediates of glycolysis and TCA cycle. Cell-free extracts
from purifi ed heterocysts of
A
.
variabilis
ATCC 29413 have been employed for
in vitro
demonstration
of electron donation to nitrogenase in light and dark. Glycolytic cycle supported nitrogen fi xation
in dark with intermediates of glycolysis like glucose-6-phosphate (G6P), fructose-1,6-biphosphate
and dihydroxyacetone phosphate each with NAD or NADP constituting the effective co-factors.
NADH (and hydrogen) contributed electrons for nitrogenase via PSI in the light which is regulated
by the presence of NADP that competed for electrons with nitrogenase. However, NADPH served
as a source of electrons to nitrogenase in dark but this was inhibited by the presence of NADP. Since
this inhibition was not reverted with time, it was suggested that FNR mediates the electron fl ow.
In dark this enzyme directly mediates the reduction of ferredoxin that in turn supplies electrons to
nitrogenase while in light the electrons fom NADPH have to fi rst pass through PSI before reducing
ferredoxin (Schrautemeier
et al
., 1984). In addition to the intermediates of glycolysis mentioned
above and confi rming the role of NADPH and NADH in light and dark, respectively, Neur and
Bothe (1985) reported that unphosphorylated sugars like glucose, fructose and erythrose also served
as electron donors. Pyruvate with coenzyme A supported C
2
H
2
reduction by the heterocysts of
A
.
cylindrica
or
A
.
variabilis
ATCC 29413. The cell-free extracts from heterocysts of these cyanobacteria
could bring about reduction of ferredoxin in presence of pyruvate and coenzyme A in light as well
as dark suggesting the mediation of pyruvate:ferredoxin oxidoreductase (PFO) in heterocysts (Neur
and Bothe, 1985). The gene encoding PFO of
A
.
variabils
ATCC 29413 and
Anabaena
sp. PCC 7119 has
been identifi ed as
nifJ
by Schmitz
et al
. (1993).
Lyons and Thiel (1995) cloned and sequenced the operon
nifB
-
fdxN
-
nifS
and
nifU
of
A
.
variabilis
ATCC 29413. The requirement of NifB for both Mo-dependent and V-dependent nitrogenases and the
non-essential nature of gene products of both
nifS
and
nifU
were some important features of nitrogen
fi xation by this organism. The existence of a [2Fe-2S]-type of ferredoxin in the heterocysts different
from the one present in vegetative cells of
A
.
variabilis
ATCC 29413
that could utilize the reducing
power either generated by H
2
/hydrogenase (from
Clostridium pasteurianum
) or by G6P and FNR (from
A
.
variabilis
) has been demonstrated in
A
.
variabilis
ATCC 29413 by Schrautemeier and Böhme (1985).
This has further been confi rmed by a comparison of the amino acid composition, molecular weight
and the redox potential of the heterocyst and vegetative cell ferredoxins from
A
.
variabilis
ATCC
29413
(Böhme and Scrautemeier, 1987). Schrautemeier
et al
. (1995) identifi ed the two ferredoxins of
A
.
variabilis
ATCC 29413 as FdxH1 and FdxH2 encoded by the genes
fdxH1
and
fdxH2
, belonging to
two nif clusters
nif1
and
nif2
, respectively. FdxH1 resembles the [2Fe-2S] ferredoxin from
Anabaena
sp. strain PCC 7120 whereas FdxH2 resembles the FdxH from
P
.
boryanum
PCC 73110. The expression
of
fdxH2
and
nif2
gene system occurred in both
A
.
variabilis
ATCC 29413 and
P
.
boryanum
PCC 73110
afte nitrogen deprivation under anaerobic conditions whereas the expression of
fdxH1
and
nif1
gene
system occurred under aerobic conditions in
A
.
variabilis
ATCC 29413. Evolutionarily it is suggested
that the
nif2
gene system operates in the vegetative cells of non-heterocystous cyanobacteria and
nif1
gene system has branched off from
nif2
gene system for its expression exclusively in heterocysts
under aerobic conditions. Thus in
A
.
variabilis
ATCC 29413 both
nif1
and
nif2
gene systems are
operative depending on the oxygen relations of the environment.
Thiel and Pratte (2001) provided evidences in favour of independent expression profi les of
nif1
and
nif2
in
A
.
variabilis
ATCC 29413. The growth properties and expression of
nif1
and
nif2
under
aerobic and anaerobic conditions of three mutants, i.e. (i)
ntcA
mutant (MM3) which could not grow
in presence of nitrate as sole source of nitrogen, (ii)
nif1
mutant that produces heterocysts but lacks
nif1
nitrogenase (JE994), and (iii) a mutant that is Het
-
and Nif1
-
(NF76) have brought to light that
cyanobacteria sense nitrogen suffi cieny or defi ciency at the fi lament level as a whole but not at the
