Biology Reference
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under saline conditions. These organisms use ADP- and also UDP-glucose
for the GgpS reaction. Moreover, in many heterotrophic bacteria, the two
enzyme activities, GgpS and GgpP, are found in one continuous protein
(
Hagemann et al., 2008
). Such fused GG synthesis proteins have not yet
been found in any cyanobacterial genome.
Genome searches using the GgpS protein (Sll1566) from
Synechocys-
tis
6803 (
Marin et al., 1998
) identified 28 genes coding for highly similar
proteins in other cyanobacteria (
Table 2.1
). The closest homologue is the
GgpS from
Synechococcus
sp. PCC 7002, which has been functionally veri-
fied (
Engelbrecht et al., 1999
). Among beta-cyanobacteria, it is also found
in many euryhaline and marine strains such as
Arthrospira
(
Yoshikawa et al.,
2011
),
Acaryochloris
, and some but not all
Cyanothece
strains (
Table 2.1
).
However, there are also many marine strains, which do not harbour
ggpS
genes in their genomes and are accordingly not using GG as main compat-
ible solute. Interestingly, the marine mat-forming
Microcoleus chtonoplastes
contains genes for GG synthesis, whereas the terrestrial
M. vaginatus
strain
FGP-2 is only able to synthesize trehalose (
Table 2.1
). Probably, in the ter-
restrial habitat, desiccation is the main stress, which is usually tolerated by
trehalose accumulation, while the marine
Microcoleus
is faced by salt stress,
therefore preferring GG.
As previously reported (
Scanlan et al., 2009
), practically all of the marine
picoplanktonic
Synechococcus
strains (only exception is
Synechococcus
sp. WH
7805) contain
ggpS
genes, whereas the related
Prochlorococcus
strains miss
it all (
Table 2.1
). The accumulation of GG in marine
Synechococcus
and its
absence in
Prochlorococcus
strains have been recently verified (
Klähn, Steglich
et al., 2010
).
The GgpP (StpA was used synonymously) protein is a specific GG-
phosphate phosphatase. Homologues of this protein are restricted to
cyanobacteria and are characterized by a specific protein domain called
Salt_tol_Pase superfamily. The only functional characterized GgpP
enzyme is encoded by
slr
0746 in
Synechocystis
6803 (
Hagemann, Schoor,
Jeanjean, Zuther, & Joset, 1997
). Highly similar proteins are found in all
cyanobacterial genomes harbouring a
ggpS
gene. Among beta-cyanobac-
teria
ggpS
and
ggpP
never form an operon. However, in the oceanic pico-
planktonic
Synechococcus
strains, these genes coding for functional linked
proteins are often found adjacent to each other indicative for operon
structures.
In one case,
Synechococcus
sp. RCC307, not only
ggpS
and
ggpP
are linked
but also the four subunits for the putative ABC-type GG-transport system
