Biology Reference
In-Depth Information
becomes more important in the presence of combined stress, especially under
stress conditions that induced decrease of PSI complexes and an imbalance
between the number of phycobilisomes and reaction centres.
The OCP-induced thermal dissipation of absorbed energy protects the
cells from photodamage. A ΔOCP
Synechocystis
mutant is more sensitive to
high light intensities than WT; exposure of mutant cells to strong irradiance
caused a more rapid decrease of PSII activity than in WT cells (
Wilson,
Ajlani, et al., 2006
). Moreover, cyanobacterial strains lacking the OCP, such
as
Thermosynechococcus elongatus
and
Synechococcus elongatus
, lost their PSII
activity faster under fluctuating high light conditions than strains containing
the OCP (
Boulay, Abasova, et al., 2008
).
Thus, cyanobacteria protect themselves from high irradiance by decreas-
ing the effective antenna size of PSII. The antenna size must be fully
restored when cells are again under low light conditions to maintain opti-
mum growth. Another protein, the fluorescence recovery protein (FRP;
discussed below), plays a critical role in this process (
Boulay, Wilson, et al.,
2010
). When 'quenched' cyanobacteria cells are to low light conditions or
are incubated in darkness, a recovery of the lost phycobilisome fluores-
cence is observed (
Boulay, Wilson, et al., 2010
;
Rakhimberdieva, Bolychevt-
seva, et al., 2007b
;
Wilson, Ajlani, et al., 2006
). The fluorescence recovery is
strongly temperature dependent (
Gorbunov, Kuzminov, et al., 2011
;
Rakh-
imberdieva, Bolychevtseva, et al., 2007b
;
Wilson, Punginelli, et al., 2008
).
Interruption of the FRP gene in
Synechocystis
largely inhibits fluorescence
recovery (
Boulay, Wilson, et al., 2010
).
3. OCP-MEDIATED PHOTOPROTECTION
FROM A STRUCTURAL PERSPECTIVE
3.1. Crystal Structures of the OCP
David Krogmann et al. first isolated the OCP in the course of purification
of c-type cytochromes. They described it as a soluble protein containing the
keto-carotenoid 3-hydroxyequinenone (hECN) (
Holt & Krogmann, 1981
)
and demonstrated that it was present in three cyanobacterial strains:
Arthro-
spira maxima
,
Microcystis aeruginosa
and
Aphanizomenon flos-aqua
. Later, Krog-
mann's group isolated the OCP from
A. maxima
and
Synechocystis
(
Wu &
Krogmann, 1997
). Using protein sequencing methods to obtain the primary
structure of the N-terminus of the OCP, they were able to deduce that it
was encoded by the
slr1963
gene in the then newly sequenced genome of
Synechocystis
PCC 6803 (
Wu & Krogmann, 1997
).
