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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
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