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by three αAPC-βAPC heterodimers and have a maximum fluorescence
emission at 660 nm (APC
660
). Each basal cylinder contains two APC
660
and
two APC
680
trimers. In one APC
680
trimer, one αAPC is replaced by ApcD
and in the other one, one βAPC is replaced by ApcF and one αAPC by the
N-terminal domain of ApcE (or Lcm). Each subunit contains a phycocya-
nobilin chromophore. The APC
680
trimers transfer the energy absorbed by
the phycobilisomes to the chlorophyll antenna and reaction centres. Results
described in three recent articles strongly suggest that OCP
r
interacts exclu-
sively with one of the APC
660
trimers (
Jallet, Gwizdala, et al., 2012
;
Tian,
Gwizdala, et al., 2012
;
Tian, van Stokkum, et al., 2011
). Another publication
suggested that, in addition, the OCP can interact with APC680 trimers
(
Kuzminov, Karapetyan, et al., 2012
), whereas another proposed that the
OCP interacts only with ApcE (Lcm) (
Stadnichuk, Yanyushin, et al., 2012
).
Strong blue green light is able to induce fluorescence quenching in
Syn-
echocystis
-mutant cells lacking ApcD or ApcF or both (
Jallet, Gwizdala, et al.,
2012
;
Stadnichuk, Yanyushin, et al., 2012
). Moreover,
in vitro
reconstitution
experiments using phycobilisomes isolated from these mutants and from
WT cells showed similar amplitude and kinetics of fluorescence quenching
and recovery under different OCP and phosphate concentrations (
Jallet,
Gwizdala, et al., 2012
). Finally, in
Anabaena
PCC7120, the absence of ApcD
did not affect the strong blue-green-light-induced fluorescence quench-
ing (
Dong, Tang, et al., 2009
). Thus, three different research groups have
shown that ApcD and ApcF are not required for OCP-related fluorescence
quenching.
Because the absence of ApcE leads to the lack of phycobilisomes, the
role of the chromophore was probed by mutation; Cys 190, which binds
the phycocyanobilin, was changed to a Ser (
Jallet, Gwizdala, et al., 2012
).
The C190S-mutant ApcE binds a chromophore that emits at 710 nm, most
probably a (3Z)-phycocyanobilin (
Gindt, Zhou, et al., 1994
). In isolated
phycobilisomes, this chromophore is easily lost during illumination with
strong white light (
Jallet, Gwizdala, et al., 2012
). When the chromophore
(emitting at 710 nm)-lacking phycobilisome was illuminated in the presence
of the OCP, a large fluorescence quenching was observed. No differences
were detected in the fluorescence quenching between the ApcE mutant
and WT cells (
Jallet, Gwizdala, et al., 2012
). Thus, it was concluded that
ApcE is also not needed for fluorescence quenching and that, most prob-
ably, the OCP binds to an APC
660
trimer. However,
Stadnichuk, Yanyushin,
et al. (2012)
presented results that can suggest that OCP interacts with ApcE.
They isolated ApcE and illuminated the protein in the presence of increasing
