Biology Reference
In-Depth Information
on the emerging understanding of the OCP-mediated photoprotective
mechanism, and its structural basis within the context of new genomic
information.
2. THE OCP-BASED NPQ MECHANISM
IS PHOTOPROTECTIVE
2.1. Initial Observations
The first result that suggested the existence of a photoprotective mechanism
involving energy and fluorescence quenching at the level of the phycobili-
some was published in 2000 (
El Bissati, Delphin, et al., 2000
). The authors
showed that strong blue light induced a large fluorescence quenching that
was not related to photoinhibition or State Transitions. Several subsequent
publications proposed that this blue-light-induced quenching, which
also increased in iron starvation conditions, was related to the IsiA pro-
tein (
Bailey, Mann, et al., 2005
;
Cadoret, Demouliere, et al., 2004
;
Joshua,
Bailey, et al., 2005
), a member of the light harvesting protein family. Under
some conditions, IsiA was known to accumulate as large empty rings (with-
out PSI) that are in a strongly quenched state, suggesting that they can be
responsible of thermal dissipation of absorbed energy (
Ihalainen, D'Haene,
et al., 2005
;
Yeremenko, Kouril, et al., 2004
). The demonstration that the
blue-light-induced fluorescence quenching was related to the phycobili-
somes and the OCP (and not to IsiA) came several years later based on
the results of three different groups working with IsiA, PSI, PSII and a
phycobilisome mutants of
Synechocystis
PCC 6803 (hereafter
Synechocystis
)
(
Rakhimberdieva, Stadnichuk, et al., 2004
;
Scott, McCollum, et al., 2006
;
Wilson, Ajlani, et al., 2006
;
Wilson, Boulay, et al., 2007
). The fluorescence
quenching was induced under strong blue light in strains lacking IsiA or
PSII but it was absent in strains lacking phycobilisomes or containing only
the phycobilisome rods (
Rakhimberdieva, Stadnichuk, et al., 2004
;
Scott,
McCollum, et al., 2006
;
Wilson, Ajlani, et al., 2006
;
Wilson, Boulay, et al.,
2007
). The phycobilisome fluorescence quenching did not depend on the
redox state of the plastoquinone pool or the
trans
-thylakoid ΔpH (
El Bis-
sati, Delphin, et al., 2000
;
Wilson, Ajlani, et al., 2006
). In contrast, the action
spectrum of phycobilisome fluorescence quenching suggested that a carot-
enoid could be involved in this mechanism (
Rakhimberdieva, Stadnichuk,
et al., 2004
). Subsequently, Wilson et al. in 2006 demonstrated that a soluble
carotenoid protein binding a keto-carotenoid, the OCP, is essential for this
mechanism, now known as the OCP-related NPQ mechanism. For a detailed
