STRESS RESPONSES - Cyanobacteria - page 622

transport of GG. The kinetics of GG uptake resembled the sugar uptake systems noted in several
cyanobacterial strains with a weak affi nity (k m value of 50 µM) to the substrate in contrast to high
affi nity system of glycine betaine transporter (a k m of 2 µM) of A . halophytica (Mikkat et al ., 1996).
b) Glycine betaine : In E . coli the synthesis of glycine betaine takes place through a two-step reaction.
The fi rst step is mediated by a membrane-bound oxygen-dependent choline dehydrogenase
that oxidizes choline to betaine aldehyde. In the second step, betaine aldehyde is catalyzed by
a soluble NAD-dependent betaine aldehyde dehydrogenase to glycine betaine (Landfald and
Strom, 1986; Andersen et al ., 1988). Moreover, the genes of this biosynthetic pathway and those of
the choline uptake system, transport of glycine betaine and a regulatory protein are all clustered
together (Lamark et al. , 1991). The expression of these genes is induced by the presence of choline
and by the osmotic shock (Landfald and Strom, 1986). Nomura et al . (1995) reported for the fi rst
time the protective effects and acquisition of salinity tolerance by production of glycine betaine in
S. elongatus PCC 7942 that was transformed with a shuttle plasmid carrying 9 kb fragment of E.
coli bet gene cluster (consisting of bet A , betB , betI , and betT that encode choline dehydrogenase,
betaine aldehyde dehydrogenase, a putative regulatory protein and choline transport system,
respectively). Bet- transformants of S. elongatus PCC 7942 transported choline for the synthesis of
glycine betaine under salt stress. The accumulation of glycine betaine up to 45 mM concentration
inside the bet -containing cells led to a stabilization of the activities of PSI and PSII. The enhanced
growth performance of bet -transformants when compared to wild-type suggested the gene cluster
from E. coli was expressed and synthesis of functional proteins responsible for acquisition of salt
resistance proceeded. Purifi ed betaine aldehyde dehydrogenase (about 18-fold) from A . halophytica
had a specifi c activity of 298.6 µmol min -1 mg -1 protein and the enzyme was a tetramer of 30-kDa
identical subunits showing optimal activity at pH 7.5 and 25°C. The specifi c activity of the enzyme
increased with an increase in external salinity (Incharoensakdi and Kum-Arb, 1998). Extending their
studies on the protective role of glycine betaine during salt stress, Nomura et al . (1998) demonstrated
that glycine betaine protects RuBisCO from inactivation in the bet -transformants of Synechococcus .
A comparison of the wild-type and bet -transformants of Synechococcus for the levels of mRNA,
protein and enzyme activity revealed that it is the activity of RuBisCO that is affected during salt
stress in the wild-type cells whereas in the bet -transformants the activity was not affected due to
the synthesis of glycine betaine. In vitro reactivation of RuBisCO by the presence of glycine betaine
took place both in case of enzyme extracts of salt stressed cells as well as acid denatured enzyme.
However, it is not understandable as to why in salt-stressed cells RuBisCO gets inactivated as NaCl
helps in a spontaneous refolding of the dimeric RuBisCO from Rhodospirillum rubrum (Schmidt et
al ., 1994) and prevents RuBisCO from getting dissociated into small subunit and large subunit from
A . halophytica (Incharoensakdi et al ., 1986).
Another important biosynthetic pathway of glycine betaine is through methylation of glycine.
Extremely halophilic bacteria like Actinopolyspora halophila (aerobic, heterotrophic eubacterium)
and Ectothiorhodospira halochloris (anaerobic, phototrophic sulphur bacterium) synthesize glycine
betaine from glycine through three step methylation reactions catalyzed by two methyltransferases.
In the fi rst step, glycine is methylated to sarcosine by glycine sarcosine methyltransferase (GSMT).
The second step involving the methylation of sarcosine to dimethylglycine is again catalyzed by
GSMT because of its broader substrate specifi city. In the third step, dimethylglycine is methylated to
form glycine betaine catalyzed by sarcosine dimethylglycine methyltransferase (SDMT). This path
way is linked to the central metabolism as the precursor of glycine is 3-phosphoglycerate which
is an intermediate of glycolysis. Glycine betaine accumulation was noted up to 33% of the cell dry