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ggtABCD
are situated downstream in the genome (
Scanlan et al., 2009
).
Because a clear functional assignment of ABC transporters by sequence
similarity searches is difficult, the occurrence of GG-transporter genes as
well as genes for other compatible uptake systems was not analysed here.
Generally, it can be assumed that at least transporters for their own main
compatible solute are present among cyanobacteria, which prevent leakage
of compatible solutes in the medium as has been shown for
ggtA
mutant of
Synechocystis
6803 (
Hagemann, Richter, & Mikkat, 1997
;
Mikkat, Effmert,
& Hagemann, 1997
).
Surprisingly,
ggpP
genes are also present in the
Prochlorococcus
genomes,
which do not contain
ggpS
genes. Since the only known biochemical func-
tion of GgpP is dephosphorylation of the intermediate GG-phosphate, one
can speculate that the common ancestor of alpha-cyanobacteria harboured
ggpS
and
ggpP
genes (
Scanlan et al., 2009
). Early in the evolution of the
Pro-
chlorococcus
clade, the
ggpS
gene was lost and sucrose replaced GG as major
compatible solute. Why the
ggpP
gene was kept is uncertain, possibly the
GgpP is able to dephosphorylate also other sugar phosphates. An interest-
ing possibility would be that in
Prochlorococcus
strains, the GgpP may act as
sucrose-phosphate phosphatase, because separate
spp
genes are missing from
all
Prochlorococcus
genomes (
Table 2.1
).
3.4. Glucosylglycerate
Glucosylglycerate (GGA) is an uncommon compatible solute because it
carries a net charge at physiological pH. GGA has been early detected in
extracts of the cyanobacterium
Synechococcus
sp. PCC 7002 (
Agmenellum
quadruplicatum
) (
Kollman, Hanners, London, Adame, & Walker, 1979
). GGA
and its structural relative mannosylglycerate have been extensively analysed
in thermophilic, heterotrophic bacteria (
Empadinhas & da Costa, 2008
).
The identification of the structural genes for GGA synthesis revealed that
genes for similar proteins occur also in cyanobacterial genomes (
Costa et al.,
2006
). The biosynthetic pathway resembles that of GG, sucrose and tre-
halose, a GGA-phosphate synthase (GpgS) cooperates with a phosphatase
(GpgP):
(GpgS) NDP-glucose + glycerate 3-phosphate → GGA-phosphate + NDP
(GpgP) GGA-phosphate → GGA + Pi
Recently, we could confirm that GGA is used as compatible solute among
cyanobacteria (
Klähn, Steglich et al., 2010
). GGA accumulation was found
