STRESS RESPONSES - Cyanobacteria - page 621

cultures indicating that there is a constitutive expression of this gene. However, in salt-stressed cells
of Synechocystis the levels of mRNA of ggps , that of protein GGPS and the synthesized GG linearly
increased with the concentration of the salt (0, 171, 342, 513, 684, 855 and 1026 mM of NaCl). During
the fi rst 4 h of salt stress, a salt shock greater than 300 mM resulted in GG synthesis of approximately
20 µmol of GG h -1 ml -1 A750 -1 when compared to the synthesis rate of 6.5 µmol of GG h -1 ml -1 A750 -1
in low salt concentrations (171 mM NaCl) (Marin et al ., 2002).
When ggpS - mutant cells of Synechocystis sp. strain PCC 6803 were subjected to salt stress (of
450 mM NaCl) a cessation of cell division occurred suggesting that salt stress inhibited cell division.
A comparison of salt stress with hyperosmotic stress (induced by sorbitol at 900 mM which was
equivalent osmotic effect as that of 450 mM NaCl) revealed that while salt stress arrested cell division
with an increase in cell size in case of ggpS - mutant but in case of hyperosmotic stress the inhibition
of cell division was associated with decrease in cell size both in case of wild-type as well as the ggpS -
mutant. The addition of GG to the medium resulted in an uptake of GG accompanied by renewed
cell division as revealed by fl ow cytometry analysis. Although the mutant cells accumulated high
levels of sucrose, it is not suffi cient to overcome salt stress but the presence of GG is essential for
overcoming salt stress. These results thus prompted Ferjani et al . (2003) to conclude that there is a
qualitative difference in the protection given by sucrose and that of GG during salt stress.
A comparison of ionic osomotic stress caused by salt and nonionic osmotic stress induced by
the presence of sorbitol and maltose revealed a differential response of Synechocystis sp. strain PCC
6803. The activation of the GG synthesizing enzymes in presence of salt resulted in GG synthesis
and this was devoid of any cell volume changes. Although there was minor change in the cell
volume in presence of sorbitol due to lack of activation of GG synthesizing enzyme system GG
accumulation did not occur but instead sorbitol was accumulated by the cells. On the other hand,
osmotic stress induced by maltose resulted in a decrease of cell volume due to effl ux of water and
in the absence of osmolyte accumulation the cells tended to be osmosensitive. Sorbitol was found
to be toxic at a concentration of 700 mM while NaCl at a concentration of 1.2 M was toxic (that is
equivalent to 2.4 M of sorbitol). In order to differentiate the osmotic and salt stress, the expression
of the gene ggpS was followed by the cloning of this gene with luxAB as the reporter gene (RP ggpS ::
luxAB ). These genes were integrated into the Synechocystis sp. strain PCC 6803 genome along with
an antibiotic resistance gene catridge. The expression of the ggpS gene product was identifi ed by the
accumulation of LuxAB proteins. The addition of NaCl above 400 mM concentrations (500 and 600
mM) and sorbitol stress (at 600 mM) induced luxAB activity. However, in presence of maltose no
such luxAB activity was noted indicating the absence of activation of ggpS promoter by the osmotic
stress induced by maltose. In sharp contrast, the ggpS mRNA levels were found to be different. In
case of salt stress, the ggpS mRNA reached a maximum after 0.5 h reaching a steady-state level at
8 h but in case of sorbitol-stressed cells, the mRNA levels were comparatively lower than NaCl-
stressed cells (Marin et al ., 2006).
The presence of an active transport system for GG uptake was found in Synechocystis sp. strain
PCC 6803. Uptake of C 14 -labelled GG was enhanced with a gradual increase of NaCl (between 10
and 700 mM) with an optimal activation at 350 mM and a pH of 4 to 7 favoured a linear increase.
Sucrose and trehalose competed well with GG transport. The uptake of GG in a salt-sensitive mutant
conferred protection against salt stress thus enabling the mutant cells to grow in presence of salt. On
the other hand, in wild-type cells the GG transporter most likely helped in preventing the cellular GG
to leak out of the cells. A concentration-dependent inhibition in uptake of GG was noted in presence
of uncouplers of photosynthetic and oxidative electron transport systems where as inhibitors of
H + -ATPases (localized in the cytoplasmic membrane) or cytochrome oxidase had no effect on the