STRESS RESPONSES - Cyanobacteria - page 654

turnover in response to light intensity most probably by the recruitment of RNA-binding proteins
required for translation or degradation. Since the electron transport system for photosynthesis and
respiration have a common point at the plastoquinone pool for the entry of electrons from PSII and
NAD(P)H-dehydrogenase, the redox status of plastoquinone pool depends on both photosynthetic
light as well as on the metabolism of endogenous respiratory substrates. It is now known that the
redox status of electron transport chain carriers regulates the expression of certain genes such as
glnA (glutamine synthetase; Reyes and Florencio, 1995), glnB (for PII protein; García-Domínguez
and Florencio, 1997), cpcBA (for α- and β-subunits of phycocyanin; Li and Sherman, 2000), proteins
of the photosynthetic electron transport chain (Alfonso et al. , 2000; Li and Sherman, 2000; El Bissati
and Kirilovsky, 2001) and ntcA (a transcriptional regulator; Alfonso et al ., 2001). Synechocystis sp.
strain PCC 6803 has a RNA helicase designated as CrhR (cyanobacterial RNA helicase redox)
with a specifi c role in salt/cold shock (Vinnemeir and Hagemann, 1999). The role of CrhR in light
stress specially in reactions of bidirectional RNA winding, RNA annealing, RNA strand exchange,
redox-regulated expression has been defi ned (Kujat and Owittrim, 2000). The expression of CrhR in
Synechocystis sp. strain PCC 6803 is regulated by light-driven changes in redox status of the electron
transport chain between Q A in PSII and Q o in cytochrome b6/f. Kujat and Owittrim (2000) concluded
that a redox-responsive RNA helicase may provide the photosynthetic cyanobacteria with an ability
to regulate expression of redox-responsive genes at the transcriptional level. It is suggested that
cyanobacteria may not sense light directly but rather they may perceive the light driven changes in
redox potential of electeron carriers in the electron transport pathway. Chamot et al . (2005) studied
the biochemical properties of CrhR and showed that like other RNA helicases, CrhR possesses
RNA-stimulated ATPase and bidirectional ATP-stimulated RNA helicase activity. The unique feature
of CrhR is the catalytic activity for RNA annealing and the occurrence of helicase and annealing
reactions concurrently to promote RNA strand exchange through a branch migration mechanism.
So the reactions catalyzed by CrhR are more diverse and RNA annealing at least is not a general
property of the RNA helicases. It is interesting to note that CrhR comes closer to other proteins like
RepA, DnaB and replicative DNA helicases and RecA in its biochemical properties. The role of RNA
helicases in abiotic stress has been reviewed (Owttrim, 2006). The expression of crhR gene is shown
to be regulated by the presence of LexA-related protein that acts as a repressor when CrhR is not
required, i.e. under conditions which oxidize the electron transport chain. Patterson-Fortin et al . (2006)
identifi ed that the binding site of LexA-related protein is located downstream of the transcription
start site of CrhR. This has been suggested to be consistent with the regulatory protein binding sites
localized in other genes known to be regulated by either light or redox signals.
The functional role of CrhR in the acclimatization of Synechocystis sp. strain PCC 6803 to low
temperature has been identifi ed by the isolation of a mutant in which crhR gene has been interrupted
with a spectinomycin-resistance gene cassette. DNA microarray analysis of genome-wide gene
expression in wild-type and crhR mutant along with Northern and Western blotting analyses revealed
that in the mutant the expression of genes encoding the heat shock proteins GroEL1 and GroEL2 was
no longer inducible at the low temperature as supported by both transcript and protein levels. CrhR
helps in the stabilization of the transcripts of the genes groEL1 and groEL2 during the fi rst 5 to 60 min
cold acclimatization and also helps in the enhancement of transcription of these genes during the
later (3-5 h) part of the acclimatization process. CrhR thus regulates the low-temperature inducible
expression of the above heat-shock proteins which in turn may be required for low-temperature
acclimatization process (Prakash et al ., 2010).