STRESS RESPONSES - Cyanobacteria - page 652

of RRM is important in RNA-binding. The cold regulation of the rbp genes in three cyanobacteria
having different temperature optima for growth has ben compared. Anabaena sp. strain PCC 7120
(a mesophilic strain), T . elongatus BP-1 (a thermophilic strain) and Oscillatoria sp. SU1 (a psychrophilic
strain) showed increase of gene transcripts and respective proteins at low temperatures. The
enhancement in the level of gene transcripts and the Rbp proteins was found to be highest in case of
Oscillatoria sp. SU1 which suggested that the mechanism of cold regulation of rbp genes is common
in various cyanobacteria with different temperature optima (Ehira et al ., 2003). The effect of osmotic
stress on the expression of rbp genes in Anabaena sp. strain PCC 7120 was followed by comparing
the transcript levels and Rbp proteins of the genes rbpA , rbpB , rbpC and rbpD in cultures grown in
source of combined nitrogen (ammonium ions) and nitrogen-fi xing conditions. A transient increase
in the levels of all the four gene products was noted under both conditions after a temperature shift
from 30°C to 20°C. However, osmotic stress enhanced the levels of transcripts and the respective
Rbp proteins in ammonium grown cultures. In the absence of combined nitrogen, osmotic stress
repressed the short-term induction of rbp gene expression. These workers concluded that the nitrogen
nutrition modulates the stress-responsive regulation of Rbp proteins that provides a mechanism to
integrate environmental and developmental signals (Mori et al ., 2003).
b) RNA helicases : One of the most important emerging areas in nucleic acid metabolism is the
recognition of RNA helicases that help in maintaining RNA secondary structure. Majority of RNA
helicases belong to superfamily 2 (SF2) subclass of helicases and contain a conserved domain of eight
or nine amino acids (Tanner and Linder, 2001; Tanner et al ., 2003; Rocak and Linder, 2004). Three
subfamilies, i.e. DEAD, DEAH and DExH/D are recognized in SF2 on the basis of variations within
a common DEAD motif (Asp-Glu-Ala-Asp) (Tanner and Linder, 2001). In addition, there are seven
additional motifs whose sequences and spacing are highly conserved (Linder et al., 1989; Schmid
and Linder, 1992; Gorbalenya and Koonin, 1993; Pause and Sonenberg, 1993; Fuller-Pace et al., 1993;
Fuller-Pace, 1994, 2006; Tanner and Linder, 2001; Rocak and Linder, 2004). The specifi city of RNA
helicases to target RNA molecules is due to the amino acid sequences outside the conserved domain
and these also help in protein-protein interactions. RNA helicases play a major role in all cellular
processes that involve RNA maturation, functioning in ribosome biogenesis, RNA splicing, transport
and turnover, transcription, translation initiation, RNAi, RNA editing and development. Moreover,
due to the fact each RNA helicase performs a unique function their role is associated with diverse
cellular functions. Now it is being realized that besides their participation in key housekeeping
metabolic pathways, they also play a regulatory role in responding to changes in environmental
variables such as temperature, light, oxygen and osmolarity. In cyanobacteria, three RNA helicases
have been described with their specifi c role in a stress response. Anabaena sp. strain PCC 7120 genome
possesses two genes designated as crhC (cyanobacterial RNA helicase cold) and crhB that encode
CrhC and CrhB proteins, respectively. Both crhC and crhB genes belong to the DEAD box subfamily
of RNA helicases . The deduced amino acid sequence of CrhC protein confi rmed its identifi cation as
a novel RNA helicase that belongs to the DEAD box family of helicases (Gorbalenya and Koonin,
1993). These two genes are differentially expressed. The expression of crhC is limited to cold shock
conditions whereas crhB is expressed under a vriety of conditions but its expression is enhanced
in the cold (Chamot et al ., 1999). Other important features of CrhC are that (i) a FAT (Pelyalanine-
Alanine-Threonine) box is present instead of the diagnostic SAT (Serine-Alanine-Threonine) box,
(ii) the change in serine to phenylalanine might be due to a transition taking place in the TCC
Ser-codon to a TTC Phe-codon and (iii) the ATPase activity and RNA helicase activity of CrhC is
coupled by the SAT box. Studies on CrhC expression revealed that it is not a general stress protein but