STRESS RESPONSES - Cyanobacteria - page 674

HrcA with CIRCE repressed the transcription of groEL1 and groEL2 . But under heat stress conditions
the inactivation of HrcA led to an induction of groESL genes. It is interesting to know that the hrcA
mutant exhibited overexpression of GroEL that conferred improved thermotolerance associated with
a reduced photobleaching of phycocyanin under heat stress. These workers also found consensus
promoter sequences of the E . coli major sigma factor upstream of the CIRCE regions in all the groESL
operons examined suggesting that groESL operon in cyanobacteria is also recognized by a major
sigma factor that may confer a positive regulation.
Regulation of expression of groESL operon by two different small proteins in S . elongatus PCC
7942 and Synechocystis sp. strain PCC 6803 has been reported. Cloning and characterization of orf 7.5
from S. elongatus PCC 7942 encoding a sHsp was reported (Nakamoto et al ., 2001). The transcripts
of orf 7.5 were barely detectable in cells grown at 30°C but a heat shock (40°C or 45°C) caused a
transient increase in the levels of this transcript. Targeted gene disruption experiments led to the
isolation of mutants that were unable to grow at 45°C and lost the ability for acquired thermotolerance
at 50°C. The requirement of orf 7.5 gene product in conferring thermotolerance was confi rmed by
complementing a wild-type orf 7.5 . Due to a strong reduction in the levels of groESL transcript in
the orf 7.5 mutant, it was concluded that orf 7.5 controls the expression of groESL operon. Sato et al .
(2007) conducted primer extension analysis of dnaK2 promoter region of S . elongatus PCC 7942 and
recognized a 20 bp region designated as MARS (multi-stress associated regulatory sequence) that
is found to be essential for stress induction of dnaK2 gene. Although this region possesses inverse
repeats that can act as probable recognition sites for a trans-factor, the probable interactions between
these two are yet to be unfolded.
The regulation of sHsp genes in many bacterial species has been reported by the specifi c DNA
sequences and their corresponding binding repressor proteins. The ibpA and ibpB sHSP homologs
present in E . coli are regulated by σ 32 (Allen et al ., 1992). In S. albus the sHSP gene hsp18 is regulated
by the RheA repressor that interacts with the inverted repeat present around the promoter region
(Servant et al ., 1999, 2000). Another DNA element present at the 5'-UTR region of hspA genes in
Bradyrhizobium japonicum is ROSE (repression of heat shock gene expression) that folds into stem-loop
like structure in the corresponding mRNA and masks the ribosome-binding site to inhibit translation.
Under heat stress, the melting of the secondary structure brings about the translation (Nocker et
al ., 2001). In cyanobacteria also such sequences and the corresponding binding proteins in hspA
gene regulation have been reported. A novel AT-rich imperfect inverted repeat (ACAAAgcAAA-
TTTagTTGT) was detected at 5'UTR region of hspA gene sequence of S . vulcanus . The regulation of
the hspA gene activity by a putative DNA binding-protein has been envisaged. A repressor protein
from the unstressed cells of a closely related thermophilic T . elongatus BP-1 was isolated and its
DNA-binding activity to the above sequence both in vitro as well as in vivo was lost at a heat shock
temperature. The size of hspA mRNA (650 nucleotides) and the time course of its accumulation
after a heat shock were similar in both the organisms and so the regulation of hspA gene expression
appeared to be very similar (Kojima and Nakamoto, 2002).
Genome-wide expression of genes in wild-type and all hik mutants of Synechocystis sp. strain PCC
6803 (http://www.kazusa.or.jp/cyano/synechocystis/mutants; Suzuki et al ., 2000) was investigated
by using DNA microarrays covering 3,075 of the 3,267 genes. Synechocystis grown at 34°C was given
a heat shock for 20 and 60 min at 44°C and genome-wide transcription was assessed by isolating the
mRNA. The level of transcripts for 59 genes rose by >3 fold whereas the activity of certain genes was
suppressed to less than one-third (58 genes) and to less than half of their initial level (232 genes) in
many others. Specially, the transcripts of genes clpB1 , hspA , groESL , htpG , dnaJ , dnaK2 rose by 6 fold
during fi rst 20 min at 44°C but the mRNA level of hik34 gene increased by 19 fold. The level of clpB1