Biology Reference
In-Depth Information
In
Synechocystis
PCC 6803, Mn
2+
limitation induces changes in the
activity and organization of both photosystems, resulting in a reduction of
photochemical activity of PSII as is made evident by lower oxygen evolu-
tion rates, lower maximal photosynthesis yield of PSII values, and faster
plastoquinone reoxidation rates. On the other hand, Mn
2+
deficit leads to
loss of PSI activity as a result of loss of PSI core proteins and Mn
2+
limi-
tation-dependent dissociation of PSI trimers into monomers (
Salomon &
Keren, 2011
). Thus, since Mn
2+
is essential to the function of PSII, and even
the state of cellular Mn
2+
availability influences the rate of photochemical
activities of both photosystems, there is clearly an intricate genetic network
for controlling Mn
2+
homeostasis in cyanobacteria (
Chandler, Bartsevich, &
Pakrasi, 2003
;
Ogawa et al., 2002
;
Yamaguchi et al., 2002
).
Mn
2+
is accumulated in high concentrations in the cytoplasm of pro-
karyotes by high-affinity uptake systems. In
Synechocystis
PCC 6803, Mn
2+
acquisition takes place through several transport systems. The best known
is MntABC, an ABC-type permease that mediates high-affinity transport
under starvation conditions (
Bartsevich & Pakrasi, 1995
). A second high-
affinity transporter acting under Mn-sufficient conditions and a low-affinity
transporter indirectly observed by transport kinetics have been reported, but
they remain to be characterized (
Bartsevich & Pakrasi, 1996
). Mn
2+
uptake
appears to be dependent on active photosynthesis, leading to accumulation
in the cyanobacterial envelope layer. The Mn
2+
outer membrane pool is
used as a reservoir for intracellular Mn
2+
, which is kept constant at approxi-
mately 10
6
atoms per cell of which a large fraction is associated with PSII
(
Keren, Kidd, Penner-Hahn, & Pakrasi, 2002
;
Salomon & Keren, 2011
).
Transcription of the
mntABC
operon in
Synechocystis
sp. occurs under
Mn
2+
starvation conditions (nM levels of Mn
2+
), but not in a Mn-sufficient
environment (µM Mn
2+
). Such an inducible high-affinity Mn
2+
-transport
mechanism is controlled via a two-component signal transduction pathway
that negatively regulates the expression of the
mntABC
operon (
Ogawa
et al., 2002
;
Yamaguchi et al., 2002
). This two-component system, also
described in
Anabaena
PCC 7120 (
Huang & Wu, 2004a
,
2004b
), includes
a membrane-bound histidine kinase, ManS, which senses the extracellular
concentration on Mn
2+
ions and activates a transcriptional response regula-
tor, ManR, which specifically binds to the promoter region of
mntABC
to
repress the expression of the ABC-type transporter encoded by this operon.
Under Mn
2+
starvation conditions, ManS does not generate a signal, result-
ing in inactivation of ManR and subsequent expression of the
mntABC
operon.
