Biology Reference
In-Depth Information
Non akinete cultures
(aerated)
Akinete forming cultures
(unaerated)
carbon assimilation
respiration and nitrogen assimilation
carbon:nitrogen ratio
Figure 9:
Interaction between carbon and nitrogen metabolism in
Anabaena torulosa
during akinete formation under unaerated
and aerated cultures (From Ahuja
et al
., 2008).
and
G
.
ghosei
(Singh
et al.
, 1972b). The second type of mutants differentiated akinetes with a lower
frequency (oligosporogenic) as in
A
.
doliolum
(Dikshit
et al
., 1981).
vii) Genes involved in akinete differentiation
:
Early studies relate to the identifi cation of common
genes to heterocysts and akinetes that regulate synthesis of envelope polysaccharides. The
hepA
gene
that encodes a polysaccharide layer in heterocysts (Wolk
et al
., 1994) was also shown to be required
for development of envelopes of akinetes in
A
.
variabilis
(Leganes, 1994). DevR is a response regulator
component of a two-component regulatory system involved in heterocyst polysaccharide synthesis.
This has caused increased akinete induction in
N
.
punctiforme
ATCC 29133 (Campbell
et al
., 1996).
Introduction of
devR
gene into cells of
Nostoc
sp. strain 29133 resulted in a stimulation of akinete
formation in both ammonium-supplemented and N
2
-fi xing cultures. This prompted Campbell
et al
.
(1996) to conclude that akinete and heterocyst differentiation is infl uenced by similar phosphorelay
systems and there might be some interchange of information between the two cell types. Leganés
et al
. (1994) showed that mutations in two genes
hetR
and
hetP
that affect heterocyst differentiation
have different effects on akinete differentiation in
N
.
ellipsopsorum
. In the
hetR
interrupted mutant
neither heterocysts nor akinetes were produced there by showing that the transcriptional regulator
protein HetR was essential for both heterocyst and akinete formation. In contrast, the
hetP
mutant
formed akinetes but could not differentiate heterocysts. The observations of Wong and Meeks (2002)
merit mention here who showed that a
hetR
mutant strain of
N
.
punctiforme
ATCC 29133 was able
to form cold-resistant akinete-like structures devoid of typical granulation of akinetes.
Zhou and Wolk (2002) utilized
avak
gene of
A
.
variabilis
along with its promoter and fused
it with GFP (green fl uorescent protein) transcriptional reporter and introduced this construct in
A
.
cylindrica
. The expression of this gene in high levels in akinetes of
A
.
cylindrica
when compared
to heterocysts and vegetative cells confi rmed that this gene as an akinete marker gene. Likewise,
a similar sequence to
avak
gene in
N
.
punctiforme
ATCC 29133 is also similarly regulated (Argueta
et al
., 2004). But these studies have not been able to identify the function of these gene products
during akinete differentiation. DNA microarray analysis of global gene expression profi les of
N
.
punctiforme
ATCC 29133 revealed transcription patterns of 6,893 genes. Of these, 497 genes were
equally up-regulated and down-regulated during the time the cells entered into akinete phase. The
down-regulated genes belonged to core functions that were associated with a non-growth state.
There seems to be very little requirement of an adaptation to enter into akinete phase. A comparison
of gene expression profi les (including those of
hetR
) of N
2
-grown cultures and cells entering into
