Biology Reference
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induction by NtcA is independent of the expression of
hetC
. The accumulation of
hetC
transcripts in
the wild-type was evident by 6 h after nitrogen step-down which reached its peak by 24 h by which
time mature heterocysts begun to appear. Since HetC is a protein of the ABC type transporters,
it has been suggested to be involved in the export of an inhibitor of heterocyst differentiation as
originally suggested by Khudyakov and Wolk (1997). Xu and Wolk (2001) showed that
hetC
gene is
very strongly expressed in the small cells that resolve themselves into proheterocysts as refl ected
by the GFP fl uorescence when
hetC
-
gfp
was used as a reporter gene. This is akin to the expression
of
hetR
-
gfp
in the small cells. The importance of
hetC
can be assessed by the number of genes up-
regulated by it after a nitrogen step-down in
Anabaena
sp. PCC 7120. Wang and Xu (2005) constructed
vectors with the promoter regions of
hetR
,
hetC
,
hepB
,
hepK
,
devB
.
hglD
,
hglE
(
alr5351
),
hetN
,
patS
,
patA
,
patB
,
xisA
,
nifB
,
coxBII
and
rbcL
with
gfp
as the reporter gene and transformed the wild-type
and
hetC
disruptant mutant of
Anabaena
sp. strain PCC 7120. Except for
rbcL
that is down-regulated,
the rest of all other genes under study were up-regulated specially in developing proheterocysts
and heterocysts in the wild-type. The up-regulation of
hepB
,
hglD
,
xisA
,
nifB
and
patA
was reported
here for the fi rst time. In the
hetC
disruptant mutant, the up-regulation of
hepB
,
hepK
devB
,
patS
,
hetN
and
coxBII
was noted in the proheterocysts while
hglD
,
hglE
,
patB
,
nifB
and
xisA
were not expressed
in the the smaller dividing cells that tend to resolve into proheterocysts. However,
patA
is the only
gene that is down-regulated in all cells of the
hetC
disruptant mutant. There are subtle differences
in the genes expressed in the
hetC
mutant, while the expression of
hepB
and
devB
is delayed, the
expression of
hglD
,
hglE
,
patB
,
xisA
and
nifB
depended on
hetC
. In this connection, it may be noted
that the expression of
hepA
also depended on
hetC
(Xu and Wolk, 2001). Other pattern formation
or maintenance genes,
patS
and
hetN
are expressed in
hetC
mutant to the same extent as they are
expressed in the wild-type. Another signifi cant aspect of
hetC
regulation is the expression of a number
of cell division genes (
ftsE
,
alr1706
;
ftsW
,
all0154
,
ftsX
,
all1757
,
ftsY
,
all1759
,
ftsZ
,
alr3858
;
minCDE
,
alr3455
-
alr3456
-
asr3457
;
sulA
,
all2390
and two other ORFs
all2033
and
all2797
) in the wild-type after
24 h of nitrogen step-down. Using
gfp
as reporter gene, the expression of all these cell division genes
in
hetC
mutant showed that the
ftsZ
gene is up-regulated in proheterocysts that continued to divide
more actively than vegetative cells. But the expression of
sulA
appears to be not affected in the
hetC
mutant suggesting that it is not under the regulation of
hetC
. Muro-Pastor
et al
. (2009) identifi ed a
second tsp located at -293 in the promoter region of
hetC
that is NtcA activated but dependent on
HetR. The fi rst tsp located at -571 in the promoter region of
hetC
has already been described that is
NtcA activated but independent of HetR (Muro-Pastor
et al
., 1999). Similarly, the promoter region
of
devBCA
operon, expressed during maturation phase of heterocyst differentiation, also possesses
a second tsp located at -454 bp that is NtcA activated and dependent on HetR. The fi rst tsp located
at -704 bp in the promoter region of
devBCA
operon has been described earlier (Cai and Wolk, 1997;
Fiedler
et al
., 2001). Thus it is interesting to know that both
hetC
and
devBCA
operon, the former
exerting its effects during early phases and the latter exerting its effects in late phases of heterocyst
differentiation possess similar tandem promoter arrangement.
v) hetF
:
During the isolation and characterization of transposon (Tn5-1063) insertion Fox
-
mutants of
N. punctiforme
strain ATCC 29133, out of 69 short-listed mutants with a probable Fox
-
phenotype,
six have been selected for their inability to grow in a nitrogen-defi cient medium and differentiate
heterocysts. In one of these mutants (UCD415), the transposon insertion has been found in an ORF
bearing 92% similarity to
hetR
gene of
Anabaena
sp. strain PCC 7120 but in the rest fi ve of them the
transposon insertion occurred in a new gene sequence that has been named by Wong and Meeks
(2001) as
hetF
. The
hetF
gene sequence is 2.4 kbp long and the deduced amino acid sequence of the
