STRESS RESPONSES - Cyanobacteria - page 672

type cells. Comparison of wild-type and hspA deletion mutant of Synechocystis sp. strain PCC 6803
also confi rmed the role of sHsp in stabilizing the pigment molecules and thylakoid integrity. As
Synechocystis constitutively expresses HspA protein it could overcome the oxidative stress but the
hsp - mutant showed a considerable decrease in its viability and pigment content under oxidative
stress. Nakamoto and Honma (2006) demonstrated that HspA from S . elongatus PCC 7942 interacted
directly with PCs and suppressed their inactivation inder denaturing conditions. A molecular ratio
of four HspA monomers per one PC (αβ) monomer gave maximum protection.
In order to understand the regulation of hspA gene, the hspA gene from S . vulcanus was
overexpressed in E . coli under the control of lac -promoter. The transcription of hspA occurred
constitutively at 30°C and 42°C in E . coli cells but the expression of hspA was low even in presence of
the inducer Isopropyl βD-1 thiogalactopyranoside at 30°C. Heat induction of hspA occurred despite
the replacement of native hspA promoter with lacZ promoter or by the addition of rifampin. These
results suggest that the primary form of heat regulation is at the post-transcriptional level (Kojima
and Nakamoto, 2005). Nakamoto and Vígh (2007) reviewed the current status of the sHsps in stress
management and highlighted their role in membrane quality control and in maintaining the integrity
of membranes under stress conditions.
To identify the components associated with thermal acclimatization of PSII in Synechocystis
sp. strain PCC 6803 whole-genome microarrays have been conducted (Rowland et al ., 2010). The
acclimatization of PSII to elevated temperatures lasted 480 min during which as many as 176 genes
were expressed. These can be divided into seven distinct response profi le groups of which early
transient phase and sustained phase have been found to be important. The early transient phase
is characterized by the expression of a number of stress response genes including Hsps and in the
sustained phase genes involved in phycobilisome assembly and modifi cation, photosynthesis,
respiration, lipid metabolism and motility. Specifi c gene knock-outs related to heat shock response
suggested that all mutants showed a lower PSII rates under normal growth conditions. Mutations
in clpB1 , cpcC2 , hspA , htpG and slr1674 (encoding a protein of unkown function) affected basal PSII
thermotolerance. Long-term thermal acclimatization was affected by mutations in cpcC2 , hik34 ,
hspA and hypA1 .
Apart from the induction of Hsps and their role in conferring thermotolerance in various
cyanobacterial systems, other stress-inducible genes have been shown to play an important role.
In this direction, iron-stress inducible ( isi ) genes, isiA (Laudenbach and Straus, 1988) and isiB
(Laudenbach et al ., 1988) and the corresponding proteins protect cyanobacterial cells from oxidative
and heat stresses. The isiA gene encodes IsiA or CP43', a homologue of the CP43 chlorophyll-binding
proteins (Burnap et al ., 1993) that act as antenna for photosystems and light energy dissipation in PSII
(Park et al ., 1999; Sandström et al ., 2001; Cadoret et al ., 2004; Ihalainen et al ., 2005; Havaux et al ., 2005).
Flavodoxin is the gene product of isiB and helps the organisms to substitute for ferredoxin in iron-
limited habitats (Straus, 1994). These two genes exist as an operon in cyanobacteria like Syenchocystis
sp. strain PCC 6803, S. elongatus PCC 7942 and Synechococcus sp. strain PCC 7002 (Laudenbach and
Straus, 1988; Leonhardt and Straus, 1992; Burnap et al ., 1993; Straus, 1994) whereas in others they
are either dispersed ( Anabaena sp. strain PCC 7120; Leonhardt and Straus, 1994) or only one isiA
homologue is present ( T . elongatus BP-1) or both are absent ( Synechococcus sp. strain WH8102). In
addition to their induction in iron-limited habitats, induction of isiA and isiB genes under high salt,
high temperature (Vinnemeier et al ., 1998), oxidative (Jeanjean et al ., 2003; Yousef et al ., 2003) and
high-light stresses (Havaux et al ., 2005) has also been reported. Studies on isiA interrupted and isiB
deletion mutants of Synechocystis sp. strain PCC 6803 suggested their involvement in countering
heat and oxidative stresses. The presence of another gene isiC has been demonstrated downstream