STRESS RESPONSES - Cyanobacteria - page 653

specifi cally induced by a temperature shift-down to 10°C and in this respect resembles the expression
of E . coli RNA helicase gene csdA . The crhC promoter contains a σ 70 -dependent -10 mer sequence
(TAAGAT) located 9 nucleotides upstream the transcription start site but lacks the -35 mer like region
characteristic of E . coli promoter region. An AT-rich sequence resembling an upstream enhancer
element is located at positions +87 to +97. The Anabaena cold shock box 5'-TGACAGGCCGA-3'
matches with the E . coli cold shock box 5'-TGACGTACAGA-3' at 7 of 11 positions. The regulation
of cold shock-induced RNA helicase gene expression in Anabaena sp. strain PCC 7120, the cloning
of crhC gene and its transcription analysis have been studied by Chamot and Owttrim (2000). The
transcripts for crhC gene appeared only when the organism is cultivated below 25°C or upon a cold
shock (due to a shift from 30°C to 20°C) with maximum level of transcripts appearing between 20°C
and 15°C. The transcripts do not accumulate when the organism is grown at 25°C or higher than this
(i.e. 30°C, 37°C and 43°C). Though light as such is not required for the accumulation of crhC gene
transcript accumulation, light-derived metabolism seems to be essential for the transcript to appear
during a cold shock in dark. The half-life of crhC transcript is longer (67 min) at 20°C than at 30°C (11
min). Thus mRNA stability seems to play an important role in the accumulation of crhC transcript
accumulation. A time course of cold-induced CrhC protein accumulation in the cells was noted 15
min after transfer to 20°C from 30°C as supported by Wetsern blot analysis. Yu and Owttrim (2000)
described the biochemical characteristics of CrhC of Anabaena sp. strain PCC 7120 expressed in E .
coli . According to these workers, CrhC exhibits ATP-independent RNA-binding and RNA-dependent
ATPase and (d)ATP-dependent RNA unwinding activities. The unwinding of double-stranded
RNA duplexes occurs in 5'------3'direction and this indicates that an interaction with 5' region is
required for CrhC function in vivo . The RNA-binding by CrhC exhibited non-specifi c characteristics
as it was bound to partially dsRNA substrate and ssRNA, similar to those of other helicases. Thus
CrhC may help in unwinding RNA secondary structures at the 5'-end of the target mRNAs induced
during cold stress and stabilize them at low temperature. This probably leads to an improvement in
their translational effi ciency there by removing the block in translation initiation induced by cold
shock. The cold-induced transcripts are characteristic in possessing 5'-untranslated (5'-UTR) regions
that are required for cold shock specifi c expression (Jones and Inouye, 1994). Consistent with all
cold-induced transcripts, the crhC mRNA also possesses a highly structured 5'-UTR required for
the expression of CrhC. The importance of 5'-UTR sequence of ChrC is recognized by the fact that
a replacement of the crhC promoter and 5'-UTR sequences by heterlogous sequences required for
expression in E . coli has abolished its cold-regualated expression. El-Fahmawi and Owttrim (2003)
demonstrated that the CrhC is an integral membrane protein associated with cytoplasmic face of the
cytoplasmic membrane. Immunoelectron microscopy identifi ed the CrhC protein to exist in higher
concentrations at the two poles of the cell. This polar biased localization of CrhC protein suggests
that the role of CrhC RNA helicase in cold stress is associated with the cytoplasmic membrane and
occurs at the poles of the cells.
The transcriptional response of cyanobacteria to light (Mohamed and Jansson, 1989; Golden,
1995; Reyes and Florencio, 1995; Richter et al ., 1998; Kis et al ., 1998) and a correlation of light-induced
enhancement in transcript levels to the photosynthetic electron transport (Reyes and Florencio, 1995;
Kis et al ., 1998) have been well documented. The role of light in infl uencing the gene expression
post-transcriptionally by redox-mediated effects on mRNA stability added another dimension to
understand the gene expression. In case of Synechocysti s sp. strain PCC 6803, the transcripts for psbA-2
and psbA-3 were found to have increased stability in the dark controlled by the cellular redox potential
(Mohamed et al ., 1993; Tyystjarvi et a l., 1998). Similarly, Kulkarni and Golden (1997) have noted that
in case of S. elongatus PCC 7942 the 5'-UTR regions of the three psbA transcripts affect transcript