Biology Reference
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(one carrying RubisCO form I-A or RubisCO form I-B) and carboxy-
some types (
Hess et al., 2001
). Later they were named alpha-cyanobacteria
(mostly picoplanktonic oceanic
Prochlorococcus
and
Synechococcus
strains)
and beta-cyanobacteria (
Badger, Hanson, & Price, 2002
). To analyse the
molecular basis of compatible solute synthesis, mostly beta-cyanobacterial
strains have been investigated:
Nostoc
(
Anabaena
) sp. PCC 7120 (hereafter
Nostoc
7120) for group 1,
Synechocystis
sp. PCC 6803 (hereafter
Synecho-
cystis
6803) for group 2, and
A. halophytica
(hereafter
Aphanothece
) for
group 3.
3.1. Sucrose
Sucrose (α-D-glucopyranosyl-(1 → 2)-β-D-fructofuranoside) accumulation
was often found in salt-stressed cyanobacteria. Still it is possible to con-
clude that all heterocystous, N
2
-fixing cyanobacteria use sucrose as major
compatible solute at elevated salinity, while generally the preference for a
specific compatible solute is not correlated with any other specific cya-
nobacterial clade (
Fig. 2.1
). The widespread occurrence of sucrose in salt-
stressed cyanobacteria is not surprising because sucrose plays a central role
in the carbon metabolism in photoautotrophic organisms (
Kolman, Torres,
Martin, & Salerno, 2011
;
Lunn, 2002
).
It has been shown that four enzymes are crucial for sucrose accumula-
tion inside photoautotrophic cells. Sucrose-phosphate synthase (Sps) and
sucrose-phosphate phosphatase (Spp) are the main sucrose biosynthesis
enzymes:
(Sps) UDP-glucose + fructose 6-phosphate → sucrose-phosphate + UDP
(Spp) Sucrose-phosphate → sucrose + Pi
The sucrose synthase (Sus) can catalyse the following reversible reaction;
however, it is believed to predominantly degrade sucrose:
(Sus) Sucrose + UDP ↔ UDP-glucose + fructose
Additionally, invertase (sucrase) is known to irreversibly hydrolyse sucrose:
Sucrose → fructose + glucose.
Thus, the activity of sucrose synthesis enzymes relative to the two sucrose
degrading enzymes should determine the sucrose steady-state level in cya-
nobacterial cells (
Kolman et al., 2011
).
