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salt-stress-stimulated gene expression are alternative sigma factors. A muta-
tion of the group 3 sigma factor SigF resulted in the decreased expression of
many salt-stress proteins including GgpS ( Huckauf, Nomura, Forchhammer,
& Hagemann, 2000 ; Marin et al., 2002 ) ( Fig. 2.2 ). Moreover, knocking out
genes for group 2 sigma factors (especially SigB) also lead to decreased salt
tolerance in Synechocystis 6803 ( Nikkinen et al., 2012 ). The bioinformatic
analysis of the genome sequence and the analysis of salt-regulated transcrip-
tional changes resulted in the prediction that the alternative sigma factor
σ38 seems to control the salt-stress-related gene expression pattern in the
alpha-cyanobacterium Synechococcus sp. WH 8102 ( Mao et al., 2010 ).
A transcriptional factor specifically regulating compatible solute bio-
synthesis genes or other groups of salt-induced genes is not known among
cyanobacteria. The promoter of the salt-regulated ggpS gene was mapped in
Synechocystis 6803. This study revealed that the ggpS expression is negatively
regulated. An ORF for a small protein was discovered in the ggpS promoter
region, which codes for GgpR repressing ggpS under low salt conditions
( Klähn, Höhne, Simon, & Hagemann, 2010 ). The binding of GgpR to the
ggpS promoter region is influenced by the concentration of inorganic ions
(Klähn et al., unpublished results). This finding lead to the model that the
transient accumulation of K + in salt-shocked Synechocystis 6803 cells releases
the GgpR protein from the ggpS promoter resulting in its maximal activity.
Under steady-state conditions, when the K + content decreased, GgpR is
loosely associated to the ggpS promoter and alternative sigma factors (e.g.
SigF) guarantee a stress-proportional ggpS expression ( Fig. 2.2 ). Because
ggpR mutations did not completely abolish the salt regulation of ggpS , the
involvement of additional regulatory molecules is possible (marked by ? in
Fig. 2.2 ). Interestingly, a small ORF, similar to ggpR , also exists upstream of
the ggpS gene of Synechococcus sp. PCC 7002 ( Klähn, Höhne et al., 2010 ).
Additional to transcriptional regulation, compatible solute biosynthesis
is tightly regulated on biochemical level. A reversible activation and inacti-
vation of the GgpS activity by addition of high salt and removal of inorganic
ions, respectively, has been early described ( Hagemann & Erdmann, 1994 ). It
took almost 20 years to elucidate the molecular mechanism. Novak, Stirn-
berg, Roenneke, and Marin (2011) discovered a new biochemical switch
to activate or inactivate GgpS activity. It was found that GgpS can bind
DNA or RNA in a sequence-unspecific manner, which switches off the
enzyme activity. In the presence of increasing concentrations of inorganic
ions, particularly K + , the GgpS is released and becomes increasingly active.
The model explains why GgpS is maximally active in salt-shocked cells
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