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informative since many noncharacterized aldehyde oxidase/dehydrogenases
display a certain degree of similarity. However, those proteins are also fre-
quently found in genomes of strains, which definitely do not accumulate
glycine betaine. In the moment, it seems to be more likely that cyanobac-
teria use exclusively the methylation pathway for glycine betaine synthesis
and the oxidative pathway is not active. This view is indirectly
supported by
the results of two studies, which introduced the oxidative glycine betaine
pathway into the freshwater strain
Synechococcus
sp. PCC 6301 (
Deshnium,
Los, Hayashi, Mustardy, & Murata, 1995
;
Nomura, Ishitani, Takabe, Rai, &
Takabe, 1995
). Significant glycine betaine accumulation was only detected
when the medium was supplemented with the BetAB pathway precursor
choline, which seems to be limiting in cyanobacteria, whereas glycine as
precursor for the methylation pathway is available in high amounts.
4. REGULATION
Compatible solute synthesis needs to be regulated according to the
external salt conditions, i.e. the cellular concentration of compatible sol-
utes varies according to the amount of external total salts. There are many
examples showing that transcripts for compatible solute biosynthesis genes
increase after salt-shock treatments (e.g.
ggpS
for GG synthesis,
Marin,
Huckauf, Fulda, & Hagemann, 2002
;
sps
for sucrose biosynthesis,
Cumino
et al., 2010
). In most cases, the transcription is highly stimulated after the
salt-shock treatment, in the case of
ggpS
up to 50-fold, whereas the final
steady-state contents are only slightly elevated and depend on the external
salt concentration (
Fig. 2.2
). Interestingly, the transcript amounts of glucosyl-
glycerate biosynthesis genes showed a rather small increase after salt addition,
which became much more pronounced and extended when salt-stress treat-
ments were done under N-limiting conditions (
Klähn, Steglich et al., 2010
).
The sensing of the salt-stress signal and its transduction to the gene
expression is less good understood. Many potential signals are discussed to
inform the cell about the external salinity (
Wood, 1999
). Intracellular inor-
ganic ions or turgor changes are among those possible signals. In the case of
E. coli
, a transient rise in K
+
amounts, which has been also observed in salt-
shocked cells of
Synechocystis
sp. PCC 6714 (
Reed et al., 1985
), changes the
promoter-binding specificity of the RNA polymerase towards promoters
for genes activated after salt shocks (
Gralla & Vargas, 2005
). In cyanobac-
teria, a GAF-domain-containing adenylate cyclase has been characterized
as sodium sensor (
Cann, 2007
). Moreover, knocking out the water channel
