Biology Reference
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of polysaccharides but could not transport Hgls from within the cell to form the laminated layer.
Mutant P2 could not synthesize the Hgls but formed the Hep layer. The particular ORF has been
identifi ed to be
alr5357
which happens to be
hglB
(formerly
hetM
).
So far we described the importance of
hglK
,
hglB
,
hglC
,
hglD
and
hglE
in the deposition of
Hgls. A genome-wide expression of genes during nitrogen step-down of
Anabaena
sp. strain PCC
7120 revealed that many of the genes are physically clustered and expressed together as 'islands”
on the chromosome. As many as 18 ORFs (
all5343
to
alr5360
including
hglE
,
hglD
,
hglC
,
hetM
,
hetN
and
hetI
genes; the three latter genes encode a polyketide synthase, ketoacyl reductase and
phosphopantethienyl transferase, respectively) are expressed together (Ehira
et al
., 2003). Such an
expression of genes for synthesis and transport of Hgls has been confi rmed subsequently by Fan
et
al
. (2005). On the basis of a comprehensive analysis of 71 Fox
-
mutants described earlier (Wolk
et al
.,
1991), involving as many as 106 loci of insertions of transposon over 29 kb chromosomal region of
Anabaena
sp. strain PCC 7120, Fan
et al
. (2005) identifi ed a cluster of genes required for the synthesis
and deposition of Hgls. In this connection, ORFs
alr5351
to
alr5357
are shown to be required for the
biosynthesis of Hgls and ORFs
all5345
to
all5347
for their deposition. Of these ORFs, the identity of
at least four of them has already been established, i.e.
alr5351
(
hglE
),
alr5354
(
hglD
),
alr5355
(
hglC
)
and
alr5357
(
hglB
).
Apart from the participation of the above mentioned genes in the synthesis and transport of
Hgls, the requirement of two more genes seems to be essential for the formation of the laminated
layer comprising of Hgls. A protein phosphatase J encoded by
prpJ
(
all1731
) of
Anabaena
sp. strain
PCC 7120 appears to govern the synthesis of the more abundant HGL, i.e. 1-(α-D-glucopyranosyl)-
3,2,5-hexacosanediol. A mutant of
prpJ
, (S20) obtained by targeted inactivation (by the introduction of
a Sp-Sm resistance cassette, corresponding to the catalytic domain of PrpJ) could not synthesize the
more abundant Hgl resulting in the absence of laminated layer. Thus in mutant S20 the expression
of
hglE
and
nifH
are affected. PrpJ (consisting of 758 amino acid residues with a molecular mass of
83.7 kDa) possesses three domains: an N-terminal domain (of 255 amino acid residues) of unknown
function, a central domain (comprising of amino acid residues 256 to 534) of catalytic activity specifi c
for PP2C-type protein phosphatases and a C-terminal domain (constituting amino acid residues
535 to 758) of unknown function. It is important to note here that amino acid residues 583 to 605 of
the C-terminal domain are hydrophobic and could form a speculated transmembrane domain for
its being located on the plasma membrane. Thus PrpJ is another important new control point for
heterocyst maturation (Jang
et al
., 2007). Another trimeric pore-forming outer membrane β-barrel
protein encoded by
hgdD
(heterocyst glycolipid deposition protein;
alr2887
) of
Anabaena
sp. strain PCC
7120 has been reported to be essential for diazotrophic growth especially for heterocyst maturation
(Moslavac
et al
., 2007). Mutants of
hgdD
, although showed a decreased synthesis of Hgl1 and Hgl2,
could not transport the Hgls to their site of deposition. Ultrastructural studies of the developing
heterocysts of
hgdD
mutants showed the absence of laminated layer, rearrangements of thylakoids,
honey-comb like structures and polar nodules, suggesting that the heterocyst maturation has been
arrested (Moslavac
et al
., 2007). At this juncture, it is important to recall here a similar phenotype of
mutants of
devBCA
operon (Fiedler
et al
., 1998, 1999) where mutants of
devA
(
alr3712
),
devB
(
alr3710
)
or
devC
(
alr3711
) could not deposit the laminated layer despite the synthesis of Hgls by them. Stucken
et al
. (2010) identifi ed the
hgl
gene cluster comprising of
hglE
,
hglG
,
hglD
,
hglC
,
hglA
and
hetM
along
with a
devBCA
operon in the toxic
Cylindrospermopsis raciborskii
DS-505 and a
devBCA
operon in the
non-nitrogen fi xing
Raphidiopsis brookii
D9.