Biology Reference
In-Depth Information
in many alpha-cyanobacteria, i.e.
Prochlorococcus
and
Synechococcus
spp., and
in the beta-cyanobacterium
Synechococcus
sp. PCC 7002. An interesting
observation was that the GGA amount was not only dependent from the
salinity level; its accumulation became clearly stimulated or even induced
when salt addition was combined with nitrogen limitation. Especially, the
Synechococcus
strains were virtually free of GGA under N-excess but con-
tained high internal amounts at N-limited growth (
Klähn, Steglich et al.,
2010
). This finding gave rise to the hypothesis that the charged compatible
solute GGA is replacing glutamate under N-limiting conditions and serve
as organic counterion for cations, especially K
+
, inside the salt-loaded cells
of marine cyanobacteria, which are usually faced by N-limiting conditions.
Overexpression of the
gpgS
gene from
Synechocystis
sp. PCC 7002 allowed
the purification of recombinant GpgS protein, which showed the expected
biochemical activity. Similar proteins are coded in the genomes of most alpha-
cyanobacteria (only two
Prochlorococcus
and two
Synechococcus
strains miss
gpgS
genes,
Table 2.1
), while only a few beta-cyanobacterial genomes code for this
enzyme. As in heterotrophic bacteria, in alpha-cyanobacteria, the
gpgS
genes
are usually found in an operon with two other genes coding further proteins
for GGA metabolism, i.e. GGA hydrolase and GpgP. Genes for GpgP (mostly
wrongly annotated as mannosyl-3-phosphoglycerate phosphatase in cyano-
bacterial genomes) are found in almost all cyanobacterial genomes with
gpgS
genes (only exception is
Synechococcus
sp. BL107). There are few genomes (e.g.
the two thermophilic
Synechococcus
strains from Yellowstone National Park)
carrying the
gpgP
but no
gpgS
gene, which could be taken as an indication
that the GGA biosynthesis became stepwise lost in these strains.
3.5. Glycine Betaine
Glycine betaine (
N,N,N
-trimethylglycine) is a widespread compatible
solute, which has been reported from many salt-stressed organisms (for a
review, see
Chen and Murata (2011)
). In most organisms, glycine betaine is
synthesized by a two-step oxidative pathway using choline as precursor, in
which choline dehydrogenase (CDH) encoded by
betA
and betaine alde-
hyde dehydrogenases (BADH) encoded by
betB
in
E. coli
(
Andresen, Kaasen,
Styrvold, Boulnois, & Strøm, 1988
) are cooperating.
(CDH) Choline + NAD
+
↔ betaine aldehyde + NADH
(BADH) Betaine aldehyde + NAD
+
+ H
2
O ↔ glycine betaine
+ NADH + 2H
+
