STRESS RESPONSES - Cyanobacteria - page 645

sp. strain PCC 6803 described above (Wada et al ., 1990, 1994), showed replacement of D1:1 isoform
of D1 protein by D1:2 isoform within few h of cold acclimation (Campbell et al ., 1995). Thus it can
be reasonably concluded that transformation of S. elongatus PCC 7942 with desA gene shifted the
temperature critical for the replacement of D1:1 isoform by D1:2 isoform (Sippola et al ., 1998). Since
it was not known as to whether the desaturases act on fatty acids in the glycerolipids of cytoplasmic
membrane or thylakoid membranes or both, Mustardy et al . (1996) conducted immunochemical
localization studies on acyl-lipid desaturases. A temperature shift-down resulted in the enhanced
expression of desaturase genes followed by de novo synthesis of desaturases and the desaturases
targeted to both the cytoplasmic as well as thylakoid membranes. So it means any qualitative or
quantitative change that takes place in the fatty acid composition of glycerolipids due to a cold shock
or chilling, it occurs both in the cytoplasmic as well as thylakoid membranes.
Sakamoto and Bryant (1998) demonstrated that it is not due to photoinhibition that the
cyanobacterial cells show impairment in growth processes when subjected to low temperature.
S. elongatus PCC 7002 exhibited chlorotic symptoms associated with a decrease in total amounts of
phycobiliproteins and chlorophyll a when cultured at 15°C with nitrate as sole source of nitrogen.
These symptoms were observed in cultures grown at low (50 µE m -2 s -1 ) as well as high light
(250 µE m -2 s -1 ) intensities. When the chlorotic cultures were shifted from 15 to 38°C, the cultures
quickly regained their blue-green colour. Moreover, due to the occurrence of exponential growth in
presence of urea at 15°C, it was suggested that the chlorosis observed at the low temperature was
due to a nitrogen limitation and not due to limited photosynthetic activity or photodamage caused
to the photosynthetic apparatus. Extending their studies on this aspect, the growth performance,
photosynthetic activity and nitrate assimilation of S. elongatus PCC 6301 were examined at different
temperatures (15, 20 and 30°C). Growth rate decreased with decreasing temperature but it ceased
completely at 15°C. Photosynthetic effi ciency measured in terms of O 2 evolution rapidly declined
at very high light intensity (3 mE m -2 s -1 ) but at normal light intensity (250 µE m -2 s -1 ) there was no
inhibition in O 2 evolution. However, cells could not actively take up either nitrate or nitrite at 15°C,
although nitrate and nitrite reductases were quite active inside the cells. So growth inhibition at the
low temperatures is associated with inactivation of nitrate/nitrite transporters rather than due to
photoinhibition (Sakamoto and Bryant, 1999).
Kiseleva et al . (2000) identifi ed the presence of only one fatty acid desaturase gene ( desC for
Δ 9 desaturase) in the thermophilic cyanobacterium S . vulcanus which has an optimum growth
temperature of 55°C at which the cells exhibited very low levels of mRNA of desC gene. After a
temperature shift-down to 45°C, the level of the transcript increased by 10-fold accompanied by
an increase in the enzyme. However, at 35°C the levels of both the transcript as well as the protein
decreased by 3-4 times that was found at 45°C. But instead of a 10° shift, a sudden shift-down to
35°C was not accompanied by the accumulation of desC transcript or the enzyme. These workers
suggested that the pre-existing enzyme was suffi cient to bring about the required desaturation
reaction and that the Δ 9 desaturase retains its activity at vey low temperatures. The cloning of the desC
gene from S . vulcanus and its expression studies in E . coli demonstrated that the enzyme converted
stearic acid into oleic acid. The protein has only 40-45% homology with known desC desaturases
from other mesophilic cyanobacteria.
Extensive studies on the expression of desA , desB and desD genes of Synechocystis sp. strain
PCC 6803 after a temperature shift-down from 34°C to 22°C have been conducted (Los and
Murata, 1994; Los et al ., 1997). The level of transcripts for desA gene increased by 10-fold within
30 min of transfer to the low temperature but when the cultures were shifted back to 34°C the
transcripts for desA gene disappeared rapidly. Northern and Western blotting analyses revealed