Biology Reference
In-Depth Information
Blanco-Rivero
et al
. (2005) isolated a transposon (Tn5-1063) generated mutant of
Anabaena
sp.
strain PCC 7120 (designated as PHB11) that exhibited Na
+
sensitivity and lost the ability to grow at
alkaline pH. The insertion of Tn5 inactivated a seven ORF operon that bears sequence resemblance
to bacterial operons governing tolerance to high salt concentration and adaptation to alkaline pH.
This operon, known as
mrp
operon (for multiple resistance and pH adaptation), comprised of
mrpA
that is responsible for Na
+
tolerance at high pH and together with
mrpB
and
mrpD
that encode
protein sequences similar to hydrophobic subunits of the proton pumping NAD(P):ubiquinone
oxidoreductase (Complex 1) found in mitochondria and eubacteria. PHB11 grew 20% less than that
of wild-type and the cells lysed after 10-11 days growth at a pH of 10.5. During a shift of pH from 7.5
to 10.5 the inhibition of photosynthesis (measured as O
2
evolution) reached to a maximum of 95%.
The
mrp
operon along with Tn5 sequence was recovered from PHB11 and used in the generation of
single and double recombinant strains. The expression of this gene in these strains was monitored
by the luminescence of
luxA
and
luxB
genes (from
Vibrio fi scheri
as Tn5-1063 can make transcriptional
fusions between these genes). It appeared to be similar in both the strains as the induction of
mrpA
was dependent on the concentration of Na
+
reaching its highest at 100 mM Na
+
in the medium.
However, the response of the two strains to the combined effects of Na
+
concentration and elevated
pH (10.5) was found to be different. In the double recombinant strain (which has lost wild copy of
the
mrp
operon) though the induction of
mrpA
took place at relatively low concentration of Na
+
(3-25
mM), at elevated pH and high Na
+
concentrations the viability of this strain was affected similar to
that of PHB11. In contrast, in the single recombinant strain the induction of
mrpA
was maximum at
increasing concentration of Na
+
and alkaline pH suggesting that
mrpA
and for that matter the whole
mrp
operon confers tolerance to Na
+
at elevated pH.
Billini
et al
. (2008) showed that of the seven Na
+
/H
+
antiporters of
S
.
elongatus
PCC 7942, Nha3
possessed the Na
+
/H
+
antiporter activity because this antiporter complemented the salt-sensitive
phenotype of
E
.
coli
TO114 strain that is is defi cient in Na
+
/H
+
transporter activity. The rest of the
Na
+
/H
+
antiporter proteins Nha1, Nha4, Nha6 and Nha7 exhibited low Na
+
/H
+
antiporter activity
and were not able to complement salt-sensitive phenotype of
E
.
coli
TO114 cells. The expression
of the
nha
genes was monitored under salt stress and alkaline stress conditions (imposed by high
pH) using real time reverse-transcription PCR. The expression of all the seven genes of the Na
+
/
H
+
antiporters under the stress conditions examined signifi es that all these act synergistically to
overcome salt and pH tolerance.
iv) Gene level responses
:
A fi ve-fold increase in copy number of an endogenous plasmid (pSY10) of
Synechococcus
sp. NKBG 042902 has been reported in response to salt stress, a phenomenon that was
specifi c to NaCl but not to either to KCl or sorbitol (Takeyama
et al
., 1991). By employing random
catridge mutagenesis, Hagemann and Zuther (1992) isolated three salt-sensitive mutants (143, 406
and 549) of
Synechocystis
sp. strain PCC 6803. The characterization of mutant 549 showed a deletion
of 1.8 kb portion in the genome affecting four ORFs (a glycoprotease
gcp
homologue, the
psaFJ
gene and an unknown gene). The construction of single mutations and a study of their properties
showed that only
gcp
gene affected the salt tolerance resulting in a salt-sensitive phenotype. Nitrogen
starvation of the cells followed by changes in pigmentation (a decrease of phycobiliproteins and
increase of carotenoids), and ultrastucture (higher accumulation of cyanphycin granules) are other
properties observed in this mutant. The gene product of
gcp
, a glycoprotease seems to be related
to cyanophycin degradation. Tha salt tolerance levels of wild-type, mutant 549 and the
gcp
single
mutant varied. The former two tolerated up to 684 mM NaCl while the third showed reduced
tolerance (550 mM NaCl). DNA:DNA hybridization studies indicated that recombination occurred
