Biology Reference
In-Depth Information
petF
gene. The presence of
fdxH
gene in a non-heteocystous cyanobacterium is distinctive and the
recombinant protein produced in
E
.
coli
has a molecular mass of 10.9 kDa with 98 amino acid residues.
The presence of Lys10 and Lys11 that interact with dinitrogenase reductase has already been reported
by Schmitz
et al
. (1993). Masephol
et al
. (1997) constructed three mutant strains of
Anabaena
sp. strain
PCC 7120 for the
fdxH
gene. The performance of the mutants showed that though FdxH is very
necessary for optimum diazotrophic growth but it is not essential for nitrogen fi xation.
iv) Assimilation
:
Isotopic labelling studies (Ohmori and Hattori, 1971; Wolk
et al
., 1974; Thomas
et al
.,
1977) and enzymatic composition of heterocysts and vegetative cells (Fleming and Haselkorn, 1973;
Thomas
et al
., 1977; Peterson and Wolk, 1978; Haselkorn, 1978; Murry
et al
., 1984; Wolk, 1982, 1994)
are unequivocal in suggesting the operation of GS-GOGAT pathway in cyanobacteria for assimilation
of ammonium generated through nitrogen fi xation or taken up exogenously. The nitrogen fi xed in
the heterocysts in the form of glutamine is transported into the adjacent vegetative cells where it is
converted to two molecules of glutamate by the activity of GOGAT activity. The possible exchanges
of metabolites from vegetative cells and heterocysts and vice-versa have been depicted in the model
presented in Fig. 10 of Chapter 7. It has been proposed that one molecule of glutamate is retained by
the vegetative cells and the second molecule of glutamate is transported back to the heterocyst for
further amination (Haselkorn, 1978). This is dependent mostly on the GS and GOGAT levels. The
presence of GS activity in higher levels under nitrogen-limited conditions in several cyanobacteria
established that it is the primary ammonia assimilating enzyme (Dharmawardene
et al
., 1973; Stewart
and Rowell, 1975; Wolk
et al
., 1976; Meeks
et al
., 1977, 1978; Rowell
et al
., 1977; Thomas
et al
., 1977).
The occurrence of almost equal levels of GS activity in both vegetative cells and heterocysts of
Nostoc
sp. symbiotic to
Nephroma
(Bergman and Rai, 1989) and
Anthoceros
(Rai
et al
., 1989) and in
A
.
cylindrica
(Renström-Kellner
et al
., 1990) has been reported. Due to the absence of GOGAT activity
in the heterocysts of
Anabaena
, Thomas
et al
. (1977) suggested that the vegetative cells support this
function since they contained higher GOGAT activity. There are contradicting reports on the presence
of GOGAT activity in the heterocyst preparations. In case of
Anabaena
sp. strain PCC 7120, Gupta
and Carr (1981a) reported GOGAT activity from the heterocyst preparations where as the GOGAT
activity observed in the heterocyst extracts of
A
.
variabilis
ATCC 29413 was ascribed to glutaminase
activity (Rai
et al
., 1982). Wolk
et al
. (1994) agreed with the fi ndings of Rai
et al
. (1982).
Studies on the regulation of GS activity in cyanobacteria and gene expression studies during
heterocyst differentiation after nitrogen step-down have brought in new information in our
understanding of the above issues. Three types of GSs are known. GSI, encoded by
glnA
gene,
is present in most of the enterobacteria and in cyanobacteria. Rhizobiaceae, Frankiaceae and
Streptomycetaceae possess GSI as well as GSII. GSIII, a gene product of
glnN
, has been detected in
Bacteroides fragilis
and
Butyrivibrio fi brisolvens
(two obligate anaerobes present in mammal intestines).
GSIII has been fi rst described from
Synechocystis
sp. strain PCC 6803 by Reyes
et al
. (1994) in addition
to GSI but
glnN
sequence has not been observed from the fi lamentous heterocystous cyanobacterial
species examined (
A. variabilis
ATCC 29413,
Anabaena
sp. strain PCC 7120,
Nostoc
sp. strains PCC
6720, PCC 7413, PCC 6705,
Calothrix
sp. strain PCC 7601 and
Fischerella
sp. strain UTEX 1829). While
the transcription of
glnA
is regulated by NtcA, the transcription of
glnN
gene in
Synechocystis
sp.
strain PCC 6803 is under the regulation of some other unidentifi ed additional factors (Reyes
et al
.,
1997). The
glnN
gene product GSIII protein helps
S. elongatus
PCC 7942 to overcome from prolonged
nitrogen chlorosis (Sauer
et al
., 2000).
Pseudoanabaena
sp. PCC 6903 is an exception in cyanobacteria
in possessing only
glnN
gene to meet its requirements (Crespo
et al
., 1998). The properties of the
three GS types are found to be different. Most importantly, GSI is a dodecameric protein with 12