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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