STRESS RESPONSES - Cyanobacteria - page 625

condition (20 mM NaCl) but under salt stress (220 mM NaCl) there was signifi cant inhibition in the
synthesis of D1. Since it is already known that S. elongatus PCC 7942 genome consists of three psbA
genes ( psbAI , psbAII and psbAIII ) and the expression of these genes is dependent on the intensity of
light (the transcription of psbAI occurring under normal light conditions 70 µmol photons m -2 s -1 and
the other two being expressed under strong illumination of 250 µmol photons m -2 s -1 ; Golden et al .,
1986; Kulkarni et al ., 1992), Ohnishi and Murata (2006) observed the effects of salt stress and betaine
on the level of psbA transcripts by Northern blotting. Due to the occurrence of such changes in the
transcripts of psbA genes under all the conditions examined it was suggested that the inhibition
and enhancement in the synthesis of D1 protein was neither due to salt stress nor betaine synthesis
on transcription of psbA genes. Although the molecular mechanism responsible for the inhibition
of degradation of D1 by salt stress is not known at present, it is suggested that salt stress might
inhibit the interaction between the FtsH and DegP2 proteases and D1 or alternatively salt stress
might inactivate these proteases.
The differential role of compatible solutes in countering the external salinity was examined
by performing drying experiments with liposomes in presence of compatible solutes. Hincha and
Hagemann (2004) observed that maximum protection was given by sucrose, trehalose and sorbitol
from leakage of soluble marker from liposomes whereas GG and glycine betaine exerted hardly
any effect. However, the performance of GG and glycine betaine was improved due to the presence
of sucrose with them. By employing Fourier-transform infrared spectroscopy, phase transitions
determined in the membranes due to the presence of solutes point out that all solutes decreased
the phase transition temperatures corresponding closely with their ability to protect liposomes
against leakage. GG protected the membranes under stress. This is akin to the property of glycerol
on conferring membrane stability. Reports exist in literature on the effects of glycerol as a good
protectant for biological (Lovelock, 1953; Hincha et al ., 1985; Santarius, 1992) and model membranes
during freezing and thawing but not during drying and rehydration (Crowe et al ., 1990).
c) Trehalose : A natural constituent of common food-stuffs (such as bread, wine, beer, vinegar and
honey), trehalose is a multifunctional molecule (Elbein et al ., 2003). It is a non-reducing disaccharide
that consists of two glucose units connected by 1α,-1α glycosidic linkage. Synthesized during osmotic
stress as a compatible solute, trehalose is used as a carbon source by certain bacteria (Strǿm et al. , 1998;
Horlacher and Boss, 1997) and also constitutes the structural component of cell wall glycolipids (of
Mycobacteria , Nocardia , Rhodococcus and Corynebacterium ; Argülles, 2000; De Smet et al ., 2000; Richards
et al ., 2002). In yeast, it is acculumated as a reserve compound and its synthesis during various abiotic
stresses is considered as an adaptive response (Elbein, 1974; Thevelein, 1984; De Virgilio et al ., 1994;
Hottiger et al ., 1994; Singer and Lindquist 1998a; Hounse et al ., 1998). The accumulation of trehalose
in response to many abiotic stresses is known in various groups of organisms from bacteria to higher
plants. Avonce et al . (2006) summarized fi ve different pathways of trehalose biosynthesis in different
groups of organisms (Fig. 5). The fi rst one is a two-step biosynthetic pathway mediated by trehalose-
6-phosphate synthase (TPS) and trehalose-6-phosphate phosphatase (TPP) and is known as TPS/
TPP pathway. It occurs in eubacteria, archaea, fungi, insects and plants. A single step conversion of
maltose mediated by trehalose synthase (TS) isomerizes the α-1- α-4 bond of maltose to α-1- α-1 bond
forming trehalose. This is the second one known as TS pathway and has been reported in Pimelobacter
sp. The third pathway is a two-step process that involves the conversion of maltooligosaccharides,
glycogen or starch to trehalose. The fi rst reaction is catalyzed by maltooligosyl trehalose synthase
(TreY) that triggers the transglycosylation of the last glucose molecule of the maltooligosaccharides,
glycogen or starch at their reducing end. This results in the conversion of α-1- α-4 bond to α-1- α-1