STRESS RESPONSES - Cyanobacteria - page 656

hydrolysis. It promotes protein disaggregation by unfolding aggregated proteins and hands them
over to DnaK chaperone system for refolding (Goloubinoff et al ., 1999; Lum et al ., 2005).
The genome of Synechocystis sp. strain PCC 6803 consists of four genes for ClpB, ClpC, ClpP
and ClpX proteases. Moreover, up to four isozymes of ClpP are encoded by a multigene sub-family.
S. elongatus PCC 7942 is a thermophilic cyanobacterium particularly sensitive to low temperatures
(Siva et al. , 1977). Porankiewicz and Clarke (1997) examined the role of ClpB to understand whether
this heat shock protein is also important during cold stress by the isolation of a clpB deletion strain
(D clpB ), They demonstrated that the defi cient strain exhibited a retarded growth at 25°C with a
generation time of 54.4 ± 7.5 h in contrast to the average generation time of 15.0 ± 0.9 h for the
wild-type. However, at 37°C the generation time of the wild-type and mutant have been noted to
be 7.5 ± 0.4 h and 8.6 ± 0.7 h, respectively. The photosynthetic activity of the wild-type and mutant,
as expressed in terms of O 2 evolution, was almost the same at 37°C but when shifted to 25°C both
wild-type and mutant suffered a loss of 50% of the photosynthetic activity. But in terms of recovery,
the wild-type recovered faster by returning to 70% of control after 24 h at 25°C whereas the mutant
recovered marginally to 43% of the control. A temperature shift-down from 37°C to 25°C induced
the synthesis of ClpB protein (92 kDa) by 5-fold while the same could not be detected at the normal
growth temperature (37°C ). After 24 h acclimation at 25°C, a corresponding increase in shorter
form ClpB' (78 kDa) protein was noted that was synthesized from a second translational start site.
But further decrease in temperature to 20°C and 15°C progressively decreased the induction of
ClpB protein. These results signify that the ClpB induction contributes to the acclimation process to
permissive low temperatures. On the other hand, ClpC is constitutively expressed in the wild-type
as well as the DclpB mutant. A temperature shift from 37°C to 25°C resulted in a 3-fold increase in
ClpC protein whereas in the wild-type a marginal increase was noted.
d) Protein S21 of small subunit of ribosome : Sato (1994) identifi ed a gene designated as rpsU for
protein S21 which is a component of small subunit (SSU) of ribosomes that is located just downstream
of the rbpA1 gene in the genome of A . variabilis M3. During a temperature shift-down from 38°C to
22°C the combined transcripts of rbpA1 and rpsU increased by 10-fold within 2.5 h. However, at 38°C
the transcript of only rpsU was more abundant (Sato, 1994). Subsequently, it was demonstrated that
S21 was present at an equimolar level relative to other ribosomal proteins at 22°C but the relative level
of S21 decreased at high temperatures (Sato et al ., 1997b). However, cold-inducibility of S21 protein
in Synechocystis sp. strain PCC 6803 has been explained due to the presence of rpsU downstream of
the rRNA operon, away from the rbpA1 gene. The presence or absence of S21 protein in a ribosome
makes it translationally active or inactive. The pattern of cold-inducible accumulation of S21 is thus
required for low temperature acclimatization of cyanobacteria for the translational apparatus.
The fore-going description of cold-induced physiological changes encompass (i) a transient
cessation of growth due to an inhibition in photosynthesis and protein synthesis, (ii) a decrease in
membrane fl uidity followed by the expression of desaturase genes that bring about a qualitative
and quantitative change in unsaturated fatty acids, (iii) the two-component system of Hik33-Rre26
for perception and transduction of cold-stress signals, (iv) the induction of rbp proteins and RNA
helicases for improving the structure stabilization of mRNAs of cold-inducible gene transcripts
thereby enhancing translational effi ciency and (v) the production of ribosomes to function properly
at the low temperature. It remains to be seen as to how all these processes are integrated and function
in a co-ordinated manner to overcome the cold stress in cyanobacteria.
B) Heat shock: Generally, a short exposure of cells to heat shock reduces the synthesis of normal
cellular proteins and at the same time induces the synthesis of a new set of proteins known as the