STRESS RESPONSES - Cyanobacteria - page 636

vi) Salt intake and cell signalling : The major studies relate to the identifi cation of genes governing
histidine kinases (Hiks), the corresponding response regulators (Rres), Ser/Thr protein kinases (Spks)
and transcription factors and their expression in response to salt stress. But how these coordinate
with each other during salt-stress is yet to be studied. There appears to be a variation in the number
of genes that encode histidine kinases (Hiks) in the genome of Synechocystis . Initially 42 Hiks have
been identifi ed (Mizuno et al ., 1996) but in subsequent studies we come across 43 Hiks (Marin et al .,
2003) and 47 Hiks (Murata and Suzuki, 2006). Some of these have been implicated in the acclimation
to cold, phosphate starvation, light stress, osmotic and salt stress (Hirani et al ., 2001; Suzuki et al .,
2001, 2004; Mikami et al ., 2002; Marin et al ., 2003). To identify specifi c sensory Hiks that are involved
in the perception of salt stress in Synechocystis sp. strain PCC 6803, a library of mutants defective in
43 Hiks have been generated and compared with wild-type cultures for the function of each Hik,
the type and number of proteins synthesized during salt stress using DNA microarray analysis of
genome-wide gene expression. Four Hiks, i.e. Hik16, Hik33, Hik34 and Hik41 were responsible for
perception and transmission of signals for salt stress. Hik34 mutant cells were shown to be defi cient
in the expression of 28 genes related to salt stress and certain other general stress-related proteins
such as chaperones ( dnaJ for heat shock protein 40, hsp17 , dnaK , the subunit of aprotein clpB ) and
superoxide dismutase ( SodB ). Some other genes (not generally related to stress response) that were
not induced by salt stress and not expressed in Hik34 mutant cells were a gene for ribofl avin kinase
( ribF ), a gene for hydrogenase expression/formation factor ( hypA ), a gene for group 2 σ factor ( SigB )
and 14 genes of unknown function. It is suspected that SigB might be responsible for transduction
of salt signal perceived by Hik34 and in initiating the expression of a set of genes related to salt
stress. Hik33 mutant cells were defi cient in the expression of three high-light inducible ( hli ) genes, a
SigD gene that encodes group 2 σ factor and a protein of unkown function. Some osmotic stress and
cold-inducible genes were not expressed in Hik33 mutant cells confi rming the earlier observations
that Hik33 is a cold sensor (Suzuki et al ., 2001) and osmosensor (Mikami et al ., 2002). Under salt-
stress, hik33 -knockout mutants of Synechocystis sp. strain PCC 6803 exhibited reduced levels of
cytoplasmic membrane proteins specially those of substrate-binding proteins of ABC-transporters
and certain regulatory proteins including Rre13. Altogether 26 different proteins are differentially
expressed (Li et al ., 2012). Close homologues of the gene governing Hik33 have been detected in S .
elongatus PCC 7942, Anabaena sp. strain PCC 7120, P . marinus MED4 and Bacillus subtilis . Hik16 is
considered to be a membrane bound sensor whereas Hik41 as the probable transducer of the signal
and these two respond to only NaCl. However, it is interesting to know that the salt inducibility of
genes governing the synthesis and uptake of GG, proteins involved in photosynthesis and certain
other regulatory functions were unaffected in all the types of Hik mutants (Marin et al ., 2003). In
addition to the above four Hiks (Hik16, Kik33, Hik34 and Hik41), the identifi cation of Hik10 and the
corresponding Rres Hik33-Rre31, Hik34-Rre1 and Hik10-Rre3 as well as Hik16-Hik41-Rre17 formed a
system that regulated the transcription of a specifi c set of genes responsible for hyperosmotic stress.
Hyperosmotic stress-inducible genes have been grouped into four classes based on an induction
factor exceeding 4.0. Group I consisted of fabG and 10 other genes whose expression was considerably
reduced due to mutation of Hik33. Thus this group of genes is regulated by Hik33-Rre31 system.
There were 19 genes belonging to Group 2 that were affected in Hik34 mutant cells (compared to
28 genes in Hik34 mutant cells noted above). The genes whose expression was depressed in Hik34
mutant cells are hspA (for a small heat shock protein), ClpB1 (for ClpB protease), SodB (for superoxide
dismutase), htpG (for heat shock protein 90), dnaK2 (for heat shock protein 70), dnaJ (for heat shock
protein 40), groEL2 ( for chaperonin 60-2), groEL1 (for chaperonin 60-1), groES ( for chaperonin 10)
and other genes of known and unkown function. Thus the expression of Group 2 genes was under