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apparatus very much depended on the growth temperature of Synechococcus sp. strain PCC 7002, a
mesophilic cyanobacterium. The temperatures required for 50% inactivation of PSII activity were
found to be 44°C and 48°C for cells grown at 25°C and 40°C, respectively. PSII activity thus appears
to be most sensitive to heat and can adapt to high temperature. PSII-mediated electron transport
showed greater thermal stability in thylakoid membranes isolated from cells which had adapted to
high temperature than those from non-adapted cells. When the thylakoid membranes were isolated in
presence of glycine betaine, such membranes were 40% more active than those prepared in its absence.
Subsequent studies of these workers identifi ed cytochrome c-550 and PsbU (a constitutive extrinsic
protein of PSII complex) as essential components of the PSII complex for protecting Synechococcus sp.
strain PCC 7002 against heat-induced inactivation. These two protein molecules together provided
higher thermal stability than either of these individually (Nishiyama et al ., 1994, 1997). Targeted
mutagenesis of Synechococcus sp. strain PCC 7002 for psbU gene yielded mutants (PsbU - ) that were
no longer able to adjust to a rise in growth temperature and lost the ability of thermal adaptation.
The levels of heat shock proteins (Hsp70, Hsp60 and Hsp17) in wild-type and PsbU - cells remained
the same suggesting that mutation in psbU did not affect the general heat shock response but it is
the thermal acclimatization process that is affected. PsbU - cells grown at 25°C when exposed to 49°C
for 10 min did not survive but when grown at 38°C only less than 10% of the PsbU - cells survived,
in contrast to 80% survival exhibited by wild-type (Nishiyama et al ., 1999).
Light-mediated expression of heat shock genes ( htpG , groESL1 , groEL2 and hspA ) in Synechocystis
sp. strain PCC 6803 has been studied by measuring the corresponding transcripts and protein levels
in cells pre-treated (grown at 30°C in presence of 35 µE m 2 s -1 ) at 42°C for 1 hr in presence or absence
of light and given a fi nal shock at 48°C for 1 hr in presence (35 µE m 2 s -1 ) or absence of light. Light
accelerated the expression of the genes whereas in dark except for hspA the expression of other
genes was not as intense as in light. The kinetics of induction was different for the Hsp mRNAs.
As for example, htpG gene showed the greatest level within 10-20 min after a temperature shift-
up whereas hspA and groEL2 mRNAs reached after 20 to 60 min and that of groESL1 was slowest
(Asaldulghani et al ., 2003).
Various proposals have been put forward to explain the synthesis of Hsps. One of the earliest
models was based on the accumulation of denatured proteins inside cells that provides a suffi cient
signal for Hsp synthesis (Craig and Gross, 1991; Bukau, 1993). The second is the ribosome sensor
hypothesis that transduces the signal of heat stress at the level of translational process to increase
the production of Hsps. The third one was based on the extent of accumulation of ROS that could
trigger the production of Hsps. Experimental evidences in support of any one of these is lacking. The
fi rst attempt in this direction was made by Horváth et al . (1998) who discovered a “fl uidity” gene
that encodes a small Hsp (sHsp), Hsp17. This protein (17 kDa), mostly associated with thylakoid
membranes, acts as sensor for detecting thermal fl uctuations and induced the synthesis of Hsps in
Synechocystis sp. strain PCC 6803. Treatment of cells with benzyl alcohol (BA) caused a fl uidizing effect
on the membranes resulting in a reduction of thermotolerance. BA treated cells when subjected to
heat stress the thermotolerance further decreased suggesting that both of these acted synergistically.
This was substantiated by data on O 2 evolution and chlorophyll a fluorescence. Selective
hydrogenation of cytoplasmic membrane without changing the fluidity of their thyakoid
membranes was also chosen as a parameter to study the induction of Hsps. A Northern blot
analysis of RNAs from cells exposed to heat stress (after acclimatizing to contrasting temperatures
22°C vs 36°C and treated with BA or catalytically saturated selectively) revealed that induction
of dnaK , groESL , cpn60 and hsp17 genes occurred at signifi cantly higher temperatures for cells
grown at 36°C than those grown at 22°C. The threshold temperature was found to be 44°C
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