Biology Reference
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white or blue-green light in the presence of an excess of the OCP, a large
phycobilisome fluorescence quenching is observed (
Gwizdala, Wilson,
et al., 2011
;
Stadnichuk, Yanyushin, et al., 2011
). An excess of the OCP
in the experimental system is necessary because a high concentration of
phosphate (0.8 M or higher) is needed to maintain the integrity of puri-
fied phycobilisomes (
Gwizdala, Wilson, et al., 2011
;
Wilson, Gwizdala, et al.,
2012
). By re-isolating phycobilisomes after incubation in light or in darkness
with WT-OCP or Tyr44-OCP (nonphotoactive), it was demonstrated that
only OCP
r
is able to bind to phycobilisomes (
Gwizdala, Wilson, et al., 2011
).
In addition, it was also shown that only one OCP
r
per phycobilisome is suf-
ficient to quench almost all of the phycobilisome fluorescence (
Gwizdala,
Wilson, et al., 2011
).
When phycobilisomes are illuminated in the presence of OCP
o
, the
rate and amplitude of quenching depends on light intensity because the
accumulation of OCP
r
depends on light intensity. In contrast, fluorescence
quenching does not depend on light intensity and occurs even in darkness
when phycobilisomes are incubated with OCP
r
, indicating that OCP
r
bind-
ing to phycobilisomes is light independent (
Gwizdala, Wilson, et al., 2011
).
This was also observed
in vivo
: in whole
Synechocystis
cells, when fluores-
cence quenching is induced by a brief period of intense light, fluorescence
quenching continues to occur in darkness for several minutes (
Gorbunov,
Kuzminov, et al., 2011
;
Rakhimberdieva, Kuzminov, et al., 2011
).
The OCP
r
is also able to quench all the fluorescence of a mutant phyco-
bilisome lacking the rods (CK phycobilisomes, containing only the core) but
it is unable to quench rod fluorescence when the core is absent (
Gwizdala,
Wilson, et al., 2011
). This confirms the hypothesis that the OCP binds
the core of the phycobilisome; this was suggested by
in vivo
experiments
with phycobilisome mutants (
Scott, McCollum, et al., 2006
;
Stadnichuk,
Lukashev, et al., 2009
;
Wilson, Ajlani, et al., 2006
). The OCP
r
-core com-
plexes are less stable than the OCP
r
-whole phycobilisome complexes, indi-
cating that rods (formed by three PC hexamers) stabilize OCP
r
binding to
phycobilisomes (
Gwizdala, Wilson, et al., 2011
). The presence of only one
PC hexamer in CB phycobilisomes (core plus rods containing one PC hex-
amer) is sufficient to stabilize the binding (
Gwizdala, Wilson, et al., 2011
).
4.2. The Site of Energy and Fluorescence Quenching
in the Phycobilisome Core
The core of
Synechocystis
phycobilisomes is composed of three cylinders,
formed by four APC trimers. The trimers of the upper cylinder are formed
