STRESS RESPONSES - Cyanobacteria - page 649

of hik33 gene in desA - /desD - double mutant. Microarray analysis of desA - /desD - /hik33 - conducted after
a cold shock revealed that the mutation of hik33 abolished or reduced the cold-inducibility of 10 of
the 17 genes in the second group (the expression of Hik33 regulated genes hliA , hliB and sigD was
no longer inducible by cold) and 7 of the 25 genes in the third group. However, the inactivation of
hik33 did not affect signifi cantly the cold-inducibility of 15 genes in the fi rst group except for hspA
and cysA . These results point out that most of the genes included in the group 2 and 3 are under
the regulation of hik33 and that very few genes of the fi rst group are under the control of hik33 . Los
and Murata (2004) concluded that Hik33 perceives a decrease in membrane fl uidity as the primary
signal of cold stress. Since the transmembrane domains of Hik33 are associated with the lipid phase
of the cytoplasmic membrane, it is likely that these two domains sense changes in the cytoplasmic
membrane rigidity (Los et al ., 2008; Los and Zinchenko, 2009). To identify the corresponding Rre for
Hik33, a Rre knock-out library (that contains all the Rre genes of Synechocystis sp. strain PCC 6803
in a disrupted state) was screened by RNA slot-blot hybridization. The cold-inducible genes whose
expression is governed by Hik33 were chosen as the probes. In this process, Rre26 emerged as the
candidate for the corresponding Rre for Hik33 and this constitutes the two-component (Hik33-
Rre26) system for perceiving and transducing the cold stress signal (Murata and Los, 2006). The
observation that Rre26 binds to the promoter region of hliB gene suggests that it may be involved
in the transduction of the low-temperature signal (Kappel and van Waasbergen, 2007).
Studies conducted on gram-positive and gram-negative bacteria showed that the status of
supercoiling of genomic DNA plays an important role in the regulation of gene expression in response
to environmental stresses (Higgins, 1988; Wang and Lynch, 1993; Dorman, 2006). In cyanobacteria
the temperature-dependent alterations in DNA supercoiling have been recognized as an important
signal for gene expression in response to cold stress (Los, 2004; Prakash et al. , 2009). Inhibitors of
DNA gyrase such as novobiocin have been used to study the changes in negative supercoiling as
well as genome-wide expression of genes in Synechocystis sp. strain PCC 6803 during cold stress.
An increase in the negative supercoiling of the promoter region of the the desB gene during cold
stress controlled its expression at low temperatures. Since novobiocin inhibits the stress-induced
changes in DNA supercoiling, there is a lack of transcription of many genes involved in cold stress.
The expression of genes for Hik33, Hik34, crhR and rbpA1 which are known to be obligatory for
tolerating cold stress could be prevented by the inhibition of DNA gyrase (Prakash et al ., 2009).
iv) Cold shock proteins : The induction of a separate group of proteins known as Csps occurs during
cold stress (Jones et al ., 1992, 1996; Von Bogelen and Neidhart, 1990). E . coli is the best example
to understand the role of Csps. CspA is a small 5-kDa protein that is exclusively synthesized at
low temperature. It binds to single-stranded DNA and is thought to function as either a general
transcriptional activator of the cold shock regulon in E. coli (La Teana et al ., 1991) or an RNA
chaperone (Jones and Inouye, 1994). Homologues of CspA have now been identifi ed in many
different eubacteria (Av-Gay et al ., 1992; Ray et al ., 1994; Willimsky et al ., 1992). Of the nine-member
E . coli Csp family, CspA constitutes 10% of the total proteins during a cold shock (Jiang et al ., 1997).
The three-dimensional structure of CspA reveals a fi ve-stranded β-barrel structure (Newkirk et al .,
1994; Schindelin et al ., 1994) with two consensus RNA-binding motifs (RNAP1 and RNAP2). These
help in the recognition and binding to RNA (Schroder et al ., 1995). Jiang et al . (1997) suggested that
CspA prevents RNA secondary structure formation that helps in enhancing the translation at low
temperatures. Nucleic acid-binding cold shock domain proteins (CSDs) are of wide spread occurrence
in plants (Karlson and Imai, 2003). The CSDs encompass bacterial Csps and bind to single-stranded
DNA/RNA and double-stranded DNA (Graumann and Marahiel, 1996). Graumann and Marahiel