Biology Reference
In-Depth Information
very early phase during heterocyst differentiation (within 6 h) was supported by the increased
(3 to 5-fold) transcript levels of
hetR
when
Anabaena
sp. strain PCC 7120 was subjected to a nitrogen
step-down compared to very low levels of its expression in the organism grown in nitrogen-replete
medium. Mainly two major transcripts of
hetR
gene (of 1.4 kb and 1.9 kb long) have been detected
in the wild-type. These results emphasize that heterocyst differentiation requires a functional
hetR
gene in
Anabaena
sp. strain PCC 7120.
Cloning and sequencing of
hetR
gene from
A
.
variabilis
ATCC 29413 and the characteristics of a
hetR
disruptant mutant of this organism have been reported by Schiefer
et al
. (2002). The
hetR
mutant
of
A
.
variabilis
ATCC 29413 exhibited a Fox
-
and Het
-
phenotype but under anaerobic conditions it
showed Fix
+
and Het
-
character. The vegetative cells of
hetR
mutant showed
nif2
enzyme after 4 h of
nitrogen step-down. Though the distribution of
hetR
gene has been reported to be restricted to the
heterocystous cyanobacterial species such as
Anabaena
,
Nostoc
and
Calothrix
(Buikema and Haselkorn,
1991a), the presence of
hetR
gene sequences in non-heterocystous, fi lamentous cyanobacteria such as
Symploca
sp. PCC 8002 and
Trichodesmium
spp. (which fi x nitrogen aerobically in a light/dark cycle)
and
Leptolyngbya
sp. PCC 7310 (which fi xes nitrogen anaerobically) has been reported by Janson
et al
. (1998). But the induction of
hetR
in
Symploca
sp. PCC 8002 after nitrogen deprivation did not
occur. However, in some of the non-heterocystous, non-diazotrophic, fi lamentous forms (
Schizothrix
calcicola
,
Oscillatoria lutea
,
Lyngbya
sp.,
Phormidium
mucicola
and
A
.
platensis
) not only the occurrence
of
hetR
and
patS
gene sequences but also their functional nature has been demonstrated (Zhang
et
al
., 2009). An interesting case is represented by
Trichodesmium
which fi xes nitrogen aerobically in
the absence of heterocyst differentiation. Certain morphological and physiological changes in the
trichomes of
Trichodesmium erythraeum
IMS101 lead to the formation of groups of cells known as
diazocytes. Depending on the length of the trichome, the regions of diazocyte development range
up to a maxium of four. Such diazocyte development has been detected in natural populations of
Trichodesmium
as well. Diazocytes show decreased synthesis of cyanophycin granules and increased
synthesis of additional membranes (Fredriksson and Bergman, 1995), GS (Carpenter
et al
., 1992) and
cytochrome oxidase levels (Bergman
et
al
., 1993). The requirement of
ntcA
and
hetR
genes for the
development of diazocytes in
T
.
erythraeum
IMS101, their constitutive expression during 12h/12h
light/dark cycle and a diurnal cycle between the expression of
hetR
(peaking in dark) and
nifH
(peaking in light) have been substantiated by El-Shehawy
et al
. (2003).
Black
et al
. (1993) found expression of
hetR
gene at very low levels as refl ected by the feeble
fl uorescence of P
hetR
-
luxAB
transcriptional construct in
Anabaena
sp. strain PCC 7120 in all cells derived
from nitrogen-replete conditions and localized to proheterocysts or heterocysts in the organism
subjected to a nitrogen step-down. Induction of
hetR
begins within 2 h after nitrogen shift-down
in all the cells but it increased to 20-fold in the heterocyst-forming cells. They suggested that
since induction of
hetR
requires a functional
hetR
gene, the autoregulatory nature of this gene is
explained, meaning there by that
hetR
gene is under the control of positive feed-back. Besides its
autoregulatory nature, HetR exhibits autodegradable property as evidenced from the recombinant
HetR protein overproduced in
E
.
coli
BL21 (DE3). The presence of serine-protease inhibitors such as
phenylmethanesulphonyl fl uoride (PMSF) and Dansyl fl uoride (DnsF) prevented the protease activity
and autodegradation of HetR. That HetR is an unusual serine-type protease is also proved by the
absence of autodegradation of recombinant HetR protein of mutant 216 where Ser179, essential for
proteolytic activity of HetR, was modifi ed to Asn179, though both inhibitors PMSF and DnsF could
label the Ser179Asn-recombinant HetR (Zhou
et al
., 1998). The requirement of HetR for heterocyst
differentiation and Ser152 as the active site for this function has been reported by Dong
et al
. (2000).
Of the two mutants isolated by site-directed mutagenesis, HetR
Ser142Ala
and HetR
Ser152Ala
of
Anabaena
