Biology Reference
In-Depth Information
development. Another two-component histidine kinase encoding gene,
alr0117
of
Anabaena
sp.
strain PCC 7120 has been identifi ed by Ning and Xu (2004a) that has been suggested to control the
formation of Hep layer. Maldener
et al
. (2003) identifi ed two ORFs
alr3698
(
hepB
) and
all4388
that
are important in the synthesis and deposition of Hep layer, respectively in
Anabaena
sp. strain PCC
7120. Mutants M22 (
alr3698
) and α21 (
all4388
) were isolated by Tn5-1063 and Tn5-1065 insertions,
respectively. The heterocysts formed in these mutants were devoid of the Hep layer though the Hgl
synthesis and deposition continued. The expression of these genes was followed by transcriptional
fusions with
luxAB
as the reporter gene.
Ehira
et al
. (2003) detected a 28 kb region, on the genome of
Anabaena
sp. strain PCC 7120 (precisely
at 3.45 Mb position), consisting of 21 ORFs that is expressed after nitrogen step-down as “HEP island”.
All these genes (ORFs
alr2823
to
alr2843
including
hepA
and
hepC
) are presumed to be important in
the carbohydrate metabolism and towards the formation of Hep layer during heterocyst maturation.
Among these, the requirement of nine genes (
alr2825
,
alr2827
,
alr2831
,
alr2832,
alr283
3,
alr2837,
alr2839
,
alr2840
and
alr2841
) has been demonstrated for the formation of Hep layer that confers the Hep
+
and
Fox
+
phenotype. Tn5-1063 transposon insertional mutagenesis in between ORFs
alr2825
to
alr2841
resulted in the generation of mutants (Hep
-
and Fox
-
) that lacked the Hep layer although the synthesis
of Hgl laminated layer continued (Huang
et al
., 2005). The existence of two genes ouside the “HEP
island” has been found to be essential for the formation of Hep layer. These are ORF
all4160
that
occurs at a position other than the “HEP island” and far from
hepB
and another ORF,
alr3699
that
occurs just downstream of
hepB
. Thus we have hep genes distributed at three places in the genome
of
Anabaena
sp. strain PCC 7120. The fi rst is the “HEP island” of genes, second is the
hepB
-
alr3699
cluster and the third
all4160
. The expression of
all4160
,
alr3699
and
hepB
occurred in proheterocysts
and heterocysts of
Anabaena
sp. strain PCC 7120 as revealed by the GFP fl uorescence when
gfp
has
been used as a reporter gene under the infl uence of the respective promoter regions. The up-regulation
of
all4160
,
alr3699
and
hepB
not only occurred in response to nitrogen deprivation but also they are
co-transcribed under the regulation of
hepK
and
hepN
as confi rmed by their down-regulation in
hepK
and
hepN
disruption mutants (Wang
et al
., 2007). Fan
et al.
(2006) described the properties of
mutants that regulate the Hep layer. Insertional mutants of
henR
(
alr1086
) are characterized by the
presence of traces of heterocyst-specifi c glycolipids and are devoid of the Hgl layer as well as an hep
layer. Because of its heterocyst envelope regulation, this gene was named as
henR
. The heterocysts
stained by Alcian Blue were not positive suggesting the complete absence of the envelope. The
constriction specifi c regulatory gene,
conR
(
all0187
) when inactivated though produced Hep
+
and
Hgl
+
phenotype these layers did not join at the narrow point of contact between the vegetative cells
and heterocysts. As the constriction did not get deepen it resulted in a loose arrangement permitting
the entry of oxygen and so a loss of nitrogen-fi xing ability. So
conR
gene is constriction-specifi c rather
than heterocyst-specifi c and it is regulatory in nature. Mutants of
hepS
(
all2760
) are Hep
-
and Hgl
+
.
It is a putative Ser/Thr kinase and the nature and type of proteins phosphorylated by it are yet to
be discovered. Mutants defective in
hepN
(
alr0177
) showed Hep
-
phenotype that lost quickly the
Hgls by fragmenting into small pieces which could not be recovered for analysis but could be only
visualized in scanning electron micrographs (Fan
et al
., 2006). Mutations in four regulary genes, i.e.
hepK
,
hepN
,
henR
and
hepS
have interrelated effects in
Anabaena
sp. strain PCC 7120 (Lechno-Yossef
et al
., 2006). Mutants FQ671 (
hepN
), FQ1487 (
hepS
), Y7 (
hepK
) and FQ1227 (
henR
) were subjected to
nitrogen step-down with wild-type
Anabaena
sp. strain PCC 7120 as control. Except
henR
mutant
that showed Hep
-
and Hgl
-
phenotype the rest three exhibited Hep
-
and Hgl
+
phenotype. At
14 h after nitrogen step-down, mRNA was isolated from the cultures of the wild-type and the four
mutants and the corresponding cDNA was synthesized. Oligonucleotide probes were designed for
