Biology Reference
In-Depth Information
silicon mould microchambers (200 x 200 x 8 µm) the growth of single fi laments of
Anabaena
sp.
strain PCC 7120 was traced in a nitrate-replete medium followed by a nitrogen step-down both
in liquid and solid media. Heterocyst differentiation started after 30 h and 10 h in liquid and solid
media, respectively. The initial distributions of
hetR
-
gfp
expression and phycobilisome fl uorescence
signals could not be correlated to the subsequent pattern of heterocyst development (Figs. 3 and
4). This is in agreement with the results of Toyoshima
et al
. (2010) who observed that about 10%
heterocysts were formed without prior division when
Anabaena
sp. strain PCC 7120 was subjected to
nitrogen step-down and the growth of individual fi laments was followed on agar plates. A
hetR-gfp
transformant showed quartlet of cells from which the selection for proheterocyst rested in 75% of
cases on either of the outer cells. According to them it is still not clear as to what are the molecular
events that distinguish the inner and outer cells of the quartlet (Toyoshima
et al
., 2010). Filamentous
mutants of
S
.
elongatus
PCC 7942 formed the starting point for recognition of cell division genes such
as
fl m
(Dolganov and Grossman, 1993),
ftn2
and
ftn6
(Koksharova and Wolk, 2002). Cells of mutants
ftn2
and
ftn6
are 100- and 20-fold longer than the wild-type, respectively. Other cell division genes
have been identifi ed on the basis of their orthologues present in
E
.
coli
and
B
.
subtilis
. Previously
identifi ed genes (
ftsZ
,
minC
,
minD
,
minE
and
sulA
) and four newly identifi ed genes
ftsE
,
ftsI
,
ftsQ
and
ftsW
from the genome of
S
.
elongatus
PCC 7942 have been characterized by comparative Tn5-692
insertional mutagenesis. Five additional cell division genes
cdv1
,
cdv2
,
cdv3
,
ftn6
and
cikK
have also
been reported (Miyagishima
et al
., 2005). Studies on cell division gene
ftsZ
in relation to heterocyst
differentiation are only available. Bacterial
ftsZ
(for fi lamenting temperature-senstive mutant Z)
encodes a protein FtsZ that possesses GTPase activity and due to this it polymerizes into a ring-like
structure at the mid-point of the cell where it recruits other proteins to form a septum. A homologue
of the
ftsZ
has been cloned and sequenced along with its fl anking regions from
Anabaena
sp. strain
PCC 7120. The deduced amino acid sequence of FtsZ from
Anabaena
sp. strain PCC 7120 (FtsZ
Ana
with 379 amino acid residues) showed an identity of 49% to
E
.
coli
FtsZ. Another ORF downstream
of
ftsZ
encodes glutathione synthetase that is transcribed in the opposite direction (Doherty and
Adams, 1995). The essential nature of
ftsZ
and the glutathione synthetase gene for cell growth in all
nitrogen sources has been established (Zhang
et al.
, 1995). FtsZ
Ana
overexpressed in
E
.
coli
has been
purifi ed (47 kDa) and rabbit polyclonal antibodies raised against this recombinant protein have been
used in an immunodetection assay. The FtsZ
Ana
with the same molecular mass has been detected
from only vegetative cell preparations and heterocyst preparations are devoid of this protein (Kuhn
et al
., 2000). The localization and formation of a ring-like structure at the site of septum formation
by FtsZ
Ana
in
Anabaena
sp. PCC 7120 has been demonstrated by the expression of
ftsZ
gene under
the infl uence of
petE
promoter fused to
gfp
(P
petE
-
ftsZ
-
gfp
). Since SulA encoded by
sulA
gene from
E
.
coli
inhibits GTPase activity of FtsZ
Ana
in vitro
, introduction of
sulA
under the infl uence of
petE
promoter
into the strain with P
petE
-
ftsZ
-
gfp
inhibited ring formation by FtsZ
Ana
and heterocyst differentiation in
presence of copper. These results suggest that (i) heterocyst differentiation and cell division are
coupled processes and (ii) the arrest of cell division has no relation to the levels of 2-OG as the
signalling molecule (Sakr
et al
., 2006).
12) PATTERN FORMATION
The reappearance of the same pattern of heterocysts simultaneously from heterocystsless cultures
subjected to nitrogen stress is explained by the existence of incipient or proheterocysts at the same
loci in the fi laments of
A
.
ambigua
and
A
.
cylindrica
(Talpasayi and Kale, 1967; Talpasayi and Bahal,
