STRESS RESPONSES - Cyanobacteria - page 631

via a double cross-over event with the replacement of wild-type alleles by the mutated copies in
the gcp mutant (Zuther et al ., 1998).
Mutagenesis of Anabaena sp. strain PCC 7120 with Tn5 transposon carrying luxAB as the reporter
gene resulted in the identifi cation of a gene resembling in its sequence low-temperature inducible
( lti2 ) gene sequence that has been identifi ed to be a cold-inducible gene. This gene was inducible by
low temperature, osmotic as well as salt stress. A second round of transposon mutagenesis of this
mutant gave rise to another mutant in which the transcription of lti2 -like gene was reduced and
the insertion of the transposon was found to be in an ORF (known as orrF ) adjacent to lti2 gene that
resembled in sequence with that of a response regulator. Complementation with the wild-type DNA
fragment carrying the entire orrF sequence but not the truncated sequence restored the activity of
response regulator (Schwartz et al ., 1998)
A gene (slr0897) of Synechocystis sp. strain PCC 6803, designated as Ssglc , encodes a β-1,4
glucanase-like protein (SsGlc). The deduced amino acid sequence of this protein showed a high
degree of similarity to sequences of glycoside hydrolase (GH) family β-1,4-glucanases (cellulases)
from various sources. Cloning and expression of this gene in E . coli resulted in the production of a
protein that could hydrolyze barley β-glucan and lichenan (β-1,3,1,4-glucan) but not cellulose (β-1,4-
glucan), curdlan (β-1,3 glucan) or laminarin (β-1,3,1,6- glucan). A mutant derived by the disruption
of the Ssglc gene with kanamycin-resistance catridge gene exhibited similar growth performance as
that of wild-type in continuous light (40 µmol of photons/m 2 per s), 12 h light (40 µmol of photons/
m 2 per s) and 12 h dark cycle, cold stress (4°C), osmotic stress (100-400 mM sorbitol) and high light
stress (200 µmol of photons/m 2 per s) but under salt stress (300-450 mM NaCl) the growth of the
SsGlc mutant was signifi cantly inhibited. In view of the above, Tamoi et al . (2007) concluded that
SsGlc functions in salt stress tolerance.
v) Genome- and proteome-based studies : The potential of the techniques like reverse transcriptase-
polymerase chain reaction (RT-PCR) and DNA microarrays has been exploited for the identifi cation
of gene expression at genome-wide (global) level in cyanobacteria under various stress conditions.
With the availability of complete genome sequence of Synechocystis sp. strain PCC 6803, it has
now been possible to identify gene expression patterns of this organism under different stress
conditions. Such expression studies have also been conducted in other unicellular and fi lamentous
cyanobacteria. Proteome of an organism represents the total number of proteins encoded by its
genome. The automated identifi cation of proteins has been possible due to the MALDI-TOF analysis
along with two-dimensional electrophoresis (2-DE). Thus these studies reveal knowledge on
the level of protein expression, protein isoforms produced from each gene, the extent to which
proteins are post-translationally modifi ed and cellular and subcellular distribution of proteins
(Figs. 6 and 7; Pandhal et al ., 2008; Castielli et al ., 2009). The cellular responses to varying degrees
of salt stress are represented in Fig. 8 that incorporated the proteomic studies of Fulda et al. (1999,
2000, 2006) and Sudhir et al . (2005). The different cellular compartments that experience salt stress
are noted to be cytoplasm, plasma membrane, periplasmic space and thylakoids. In the cytoplasm
there is a general increase in salt stress-specifi c proteins with an increase in carbon metabolism. A
change in the nature of lipids in the plsama membrane is associated with an increase in phosphate,
nitrate and nitrite-binding proteins. The effect of salt stress on the periplasmic space is refl ected
at the level of synthesis of new extracellular layers. There is a structural reorganization in the
proteins of the thylakoid membrane with repression in phycobilisome synthesis (Pandhal et al .,
2008). Kanesaki et al . (2002) performed a DNA microarray analysis of proteins in salt-stressed cells
of Synechocystis sp. strain PCC 6803. Salt stress-induced proteins belonged to the class of ribosomal