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1992). The presence of ntcA in a number of nitrogen-fi xing as well as non-nitrogen-fi xing cyanobacteria
has been demonstrated and NtcA from Anabaena sp. strain PCC 7120 shows 77% identity with that
of Synechococcus elongatus PCC 7942 NtcA and the helix-turn-helix of the two species is identical
(Frías et al ., 1993). Simultaneously, Wei et al . (1993) identifi ed bifA gene from Anabaena sp. strain PCC
7120 that encodes a sequence-specifi c DNA-binding protein. The VF1 (Chastain et al ., 1990), BifA
(Wei et al ., 1993) and NtcA (Frías et al ., 1993) are identical. The requirement of NtcA for the expression
of nitrogen assimilation and heterocyst development genes in Anabaena sp. strain PCC 7120 was
demonstrated by Frías et al . (1994). An ntcA disruptant mutant required ammonium for growth and
was unable to grow in nitrate-enriched and nitrogen-defi cient media because of lack of synthesis of
enzymes of nitrate metabolism and nitrogen fi xation, respectively. The genes of nitrogen assimilation
( nifHDK encoding nitrogenase, nir for nitrite reductase and glnA ) and heterocyst development ( hetR ,
a regulatory gene) are not expressed in the ntcA mutant after nitrogen step-down. Wei et al . (1994)
cloned ntcA ( bifA ) gene of Anabaena sp. strain PCC 7120, inactivated it by the insertion of a Ω
spectinomycin (Sp) and streptomycin (Sm) resistance cassette, cloned it into a suicide plasmid
(pAM1320) and introduced it into Anabaena sp. strain PCC 7120 by conjugation. Single recombinants
carried the ntcA disrupted gene integrated into the chromosome as well as the wild-type ntcA gene
whereas the double recombinants (AMC236) possessed only the disrupted gene. Complementation
of AMC236 with wild-type gene on a shuttle vector restored the wild-type character (AMC273 and
AMC274). AMC236 grew only on ammonium medium and was unable to grow and differentiate
heterocysts in a nitrogen-free medium while the wild-type and AMC273 and AMC274 grew in all
nitrogen-enriched as well as nitrogen-defi cient media. AMC236 did not show rearrangement of nifD
or fdxN elements after nitrogen step-down. The molecular mechanism by which NtcA exerts such
a regulation is explained by the presence of a consensus NtcA-binding sequence in the promoter
regions of genes under its control. The binding of BifA (NtcA) to the promoter regions of xisA (that
encodes a site-specific recombinase), glnA and rbcL and nifH genes has been reported by
Ramasubramanian et al . (1994b). The presence of NtcA-binding sites upstream of the promoter
regions in case of glnA (between -125 and -148 bp) and rbcL (between -12 and +12 bp and -43 to -54
bp) have been recognized. Luque et al . (1994) reported the presence of a palindromic DNA sequence
GTAN 8 TAC as the target site for NtcA-binding in the promoter regions of nitrogen-regulated genes
of S. elongatus PCC 7942. Frías et al . (1997) identifi ed and cloned an operon of nitrite and nitrate
assimilation genes consisting of nir - nrtABC (elements of nitrate permease system)- narB (nitrate
reductase structural gene). In presence of ammonium, the repression of this operon takes place by
the interaction of NtcA with 460 bp region upstream of the start of the nir gene and in presence of
nitrate the induction of this operon takes place. So in this respect, NtcA acts as a repressor as well
as an activator of enzymes of nitrogen assimilation. The NtcA-dependent promoters thus represent
a different class of promoters that possess a -10 box (TAN 3 T) and an NtcA-binding site characterized
by the presence of the signature sequence GTAN 8 TAC in lieu of a -35 box. The regulation of NtcA
of Anabaena sp. strain PCC 7120 by a redox-dependent mechanism has been reported by Jiang et al .
(1997) in which the cysteine residues of NtcA are expected to play a key role. This is based on the
interaction of NtcA with the promoter region of gor gene that encodes glutathione reductase. Further,
the binding of NtcA with the promoter regions of not only the genes involved in nitrogen assimilation
but also of carbon fi xation ( rbcLS ), xisA (that encodes a site-specifi c recombinase expressed during
heterocyst differentiation) and ntcA itself assumes signifi cance and underlines the role of NtcA as
a global regulatory protein. Further evidences that cellular redox status infl uences the expression
of ntcA gene expression in Synechocystis sp. strain PCC 6803 were presented by Alfonso et al . (2001).
Two transcripts of ntcA , one shorter (0.8 kb) and the other longer (1.2 kb) have been detected. The
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