Biology Reference
In-Depth Information
nitrogenase in the heterocysts of
Nostoc muscorum
as revealed by the isolation and separation of
nitrogenase proteins labelled with
34
SO
4
-
(Fleming and Haselkorn, 1973). At least 10 different sets of
proteins are synthesized after nitrogen step-down of
N
.
muscorum
and only some of these including
nitrogenase are allowed to continue in the proheterocysts. A rapid turnover of protein seems to
take place once the cell is committed to differentiate into a proheterocyst and the protein profi le of
proheterocysts differ greatly from those of vegetative cells (Fleming and Haselkorn, 1974). Since
incorporation of
15
N-label by the heterocysts is no greater than vegetative cells (Ohmori and Hattori,
1971), Wolk
et al
. (1974) incubated intact fi laments of
A
.
cylindrica
in presence of
13
N gas and found
the label of fi xed
13
N in the heterocysts by autoradiography. Heterocysts contributed to about 90%
of the total nitrogenase activity of
A
.
cylindrica
grown under aerobic conditions
based on acetylene
reduction assays and possessed PSI as well as oxidative phosphorylation activities. They are unable
to carry out PSII activities and fi x CO
2
. Photosynthetic electron transport and ATP generation
supported nitrogenase to function in the absence of O
2
evolution (Tel-Or and Stewart, 1976, 1977).
In contrast, Thomas
et al
. (1977) reported 34% of the acetylene-reducing activity of whole fi laments
by the isolated heterocysts of
A
.
cylindrica
. These workers put forward direct evidences for the
reduction of dinitrogen in the heterocysts as well as the formation of glutamine in the heterocysts,
its transportation into the adjacent vegetative cells and conversion of glutamine into glutamate
supported by isotopically labelled substrates. The presence of GOGAT in the heterocysts of
Anabaena
sp. strain PCC 7120 was demonstrated by the formation of [
14
C] glutamate from [
14
C] glutamine in
presence of α-oxoglutarate (Gupta and Carr, 1981a). Isolated heterocysts of
A
.
variabilis
contributed
to 60% of the nitrogenase activity of the whole fi laments and heterocysts derived from
55
Fe enriched
media yielded dinitrogenase and dinitrogenase reductase proteins to 91% and 69%, respectively of
the whole fi laments (Peterson and Wolk, 1978).
A comparative study of the whole fi laments and isolated heterocysts of a mutant strain of
Anabaena
sp. strain CA revealed that the heterocysts possessed chlorophyll
a
to the same extent as
the vegetative cells and half the amounts of C-phycocyanin and allophycocyanin. A light-dependent
reduction of acetylene was stimulated by the presence of 5% H
2
under argon that continued for more
than 3 h (Kumar
et al
., 1982). Murry
et al
. (1984) demonstrated the presence of dinitrogenase that is
restricted to the heterocysts by immunoferritin labelling of Mo-Fe protein in cultures of
A
.
cylindrica
treated with DCMU and allowed to undergo differentiation of heterocysts in presence of argon.
Heterocysts of
N
.
muscorum
are devoid of nitrate reductase apoprotein but when nitrate reductase
apoprotein was added to heterocyst preparations, the reduction of nitrate to nitrite occurred that is
suggestive of the presence of Mo-co-factor (Kumar
et al
., 1985). Ammonium ions inhibited nitrogen
fi xation in
No
.
spumigena
strain AV1 with a corresponding suppression of
nifH
as refl ected by the
transcript and dinitrogenase reductase protein levels. But the expression of
ntcA
and
hetR
genes
resulted in the differentiation of heterocysts despite the presence of ammonium ions (Vintila and
El-Shehawy, 2007).
iii) Supply of Reductant
:
The heterocystous cyanobacteria possess two types of ferredoxins, PetF in
the vegetative cells encoded by
petF
and the second
fdxH
expressed in the heterocysts. The role of
the latter, [2Fe-2S]-type of ferredoxin as an electron carrier for supplying electrons to nitrogenase
(Bothe, 1970; Smith
et al.
, 1971), the generation of reduced ferredoxin from the electrons supplied
by NADPH (Bothe, 1970; Smith
et al
., 1971; Apte
et al
., 1978), the mediation of ferredoxin-NADP
+
reductase (FNR; EC 1.18.1.2) in the reduction of ferredoxin and the operation of pentose phosphate
pathway for the supply of NADPH in the heterocysts (Stanier and Cohen-Bazire, 1977; Haselkorn,
1978; Stewart, 1980; Bothe, 1982) have been demonstrated. Privalle and Burris (1984) reported the
