Biology Reference
In-Depth Information
Anabaena
sp. strain PCC 7120 that involves the removal of the 55-kb DNA element, positioned on
the chromosome between the genes encoding the large and small subunits of ribulose-biphosphate
carboxylase (
rbcL
and
rbcS
) and the
nifS
gene, by a site-specifi c recombinase encoded by
xisF
. Under
microaerobic heterocyst-inducing conditions, the excision of 55-kb element takes place while the
excision of 11-kb element excision does not occur. Of the cyanobacteria examined for the presence of
the 55-kb element,
A
.
variabilis
ATCC 29413 (Herrero
et al
., 2001),
N
.
azollae
0708 (Ran
et al
., 2010) and
C
.
raciborskii
CS-505 (Stucken
et al
., 2010) lacked it while
Anabaena
sp. strain PCC 7120,
A
.
cylindrica
and
Nostoc
strain
MAC possess the 55 kb-element in their genomes (Meeks
et al
., 1988; Brusca
et al.
,
1989; Haselkorn, 1992). The requirement of two more genes that are present downstream of
xisF
,
xisH
and
xisI
for the excision of the 55-kb element has been shown by Ramaswamy
et al
. (1997).
Deletion of the 3.2 kb region consisting of
xisH
and
xisI
blocked the excision of the
fdxN
element. The
presence of
xisH
and
xisI
in extracopies on a replicating plasmid in
Anabaena
sp. PCC 7120 resulted
in independent excision of the 55-kb element in vegetative cells itself. In view of these observations,
these two genes are suggested to regulate the
xisF
function. Further, the inactivation of
xisA
gene by
transposon mutagenesis containing a Sp- and Sm-resistance gene cassette inhibited the excision of
11 kb element as well as nitrogen fi xation in
Anabaena
sp. strain PCC 7120. However, other geneome
rearrangements involving the excision of the 55 kb element from the
fdxN
gene continued uninhibited.
Thus these two excision events appear to be not linked to one another and occur independently of
one another (Golden and Wiest, 1988; Carrasco
et al
., 1994).
Besides the just described three gene rearrangements, i.e. excision of
nifD
,
nifS
and
fdxN
elements to give rise to continuity of gene sequences and expression of these genes in the heterocyst
for nitrogen fi xation to continue therein, another gene rearrangement involving the excision of a
10.5 kb DNA element from
hupL
gene (encoding a large subunit of uptake hydrogenase) has been
described by Carrasco
et al
. (1995). The
hupL
element (9,435 bp) is characteristic in possessing 16-bp
direct repeats (CACAGCAGTTATATGG) at each of its ends and a site-specifi c recombinase gene
xisC
towards one of its ends. The XisC of
Anabaena
sp. strain PCC 7120 is homologous to the XisA
site-specifi c recombinase of the same organism and both XisA and XisC bear no sequence similarity
to any other site-specifi c recombinase known so far. The excision of
hupL
element, located >700
kb from
nifD
and
fdxN
elements, takes place exactly coinciding with the excision of
nifD
and
fdxN
elements at about 18 h during, heterocyst differentiation. A
xisC
(
alr0677
) mutant of
Anabaena
sp.
strain PCC 7120 exhibited no excision of
hupL
element and the presence of extra copies of
xisC
in wild-type on a replicating plasmid (overexpression) caused excision of the
hupL
element even
under nitrogen-enriched conditions. Functionally, XisC has been shown to be similar to the phage
integrases (Carrasco
et al
., 2005).
ii) Requirements of nitrogen fixation
:
Either for aerobic or anaerobic nitrogen fixation the
requirements are the expression of
nif
genes, a reductant and a source of ATP. The
nifHDK
operon
becomes functional when
nifD
element is excised and the operon continuity is maintained during
heterocyst differentiation. The products of
nifHDK
operon collectively are designated as the enzyme
nitrogenase which consists of dintrogenase (Mo-Fe protein or protein I) and dinitrogenase reductase
(Fe-protein or protein II). Dinitrogenase (220 to 240 kDa) is a heterotetramer of (α
2
β
2
) polypeptides
and the α, β subunits are encoded by
nifD
(Lammers and Haselkorn, 1983; Golden
et al
., 1985) and
nifK
(Mazur and Chui, 1982), respectively. Dinitrogenase reductase (60 to 70 kDa), a homodimer is
the gene product of
nifH
,
supplies electrons to dintrogenase. Dinitrogenase possesses four 4Fe-4S
centres arranged in two clusters and two copies of iron-molybdenum co-factor. On the other hand,
dinitrogenase reductase has a single 4Fe-4S cluster. The O
2
-sensitivity of the enzyme nitrogenase is
due to the irreversible oxidation of 4Fe-4S clusters and the co-factors. Cyanobacterial nitrogenases