The Orange Carotenoid Protein of Cyanobacteria - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 4

In contrast to our understanding of NPQ processes in plants, until recently, relatively little was
known about the mechanisms of photoprotection in cyanobacteria. Yet it is an important feature of
these organisms' lifestyles. The cyanobacteria as a group differ from the eukaryotic photosynthetic
organisms in their ability to thrive in a wide range of extreme habitats, many characterized by tem-
perature extremes, high salinity, and drought conditions that exacerbate the threat of photodamage.
Many cyanobacteria are known to be UV-B tolerant, perhaps through vestiges of molecular adap-
tations that arose during several billion years of intense UV radiation before the formation of the
earth's protective ozone layer.
There is a fundamental difference between the LHCs of the cyanobacteria and those of eukary-
otic photosynthetic organisms. In contrast to the integral membrane pigment (chlorophylls and caro-
tenoid) protein LHCs of plants, the main cyanobacterial (with the exception of the prochlorophytes)
light-harvesting antenna, the phycobilisome, has a very different architecture. Instead of trans-
membrane LHCs, the cyanobacterial phycobilisome consists of soluble phycobiliproteins and linker
proteins that form a complex (core and rods) attached to the outer surface of thylakoid membranes.
The phycobilisome is devoid of intrinsic carotenoids. The rod pigments (principally phycocyanin
and phycoerythrin) transfer the absorbed energy to the allophycocyanin core, which contains two
terminal energy acceptors, L CM and APC

B (MacColl 1998, Adir 2005). The energy is transferred
then to the chlorophylls of the inner chlorophyll antenna and to RCII. Phycobilisomes can also
transfer energy to PSI (Mullineaux 1992, Rakhimberdieva et al. 2001).
Despite their absence in phycobilisomes, carotenoids, especially the so-called secondary carote-
noids such as echinenone, were presumed to play a role in cyanobacterial photoprotection. Indeed,
classic biochemical approaches have led to several reports of cyanobacterial carotenoid-proteins
and evidence for their photoprotective function (Kerfeld et al. 2003, Kerfeld 2004b). One of these,
the water soluble orange carotenoid protein (OCP), has been structurally characterized and has
recently emerged as a key player in cyanobacterial photoprotection.
The OCP was i rst described by David Krogmann more than 25 years ago (Holt and Krogmann
1981). Highly conserved homologs of the 34 kDa OCP are found in most cyanobacteria for which
genomic data are available, as shown in Table 1.1. The genomic context of the OCP gene varies
considerably, as shown in Figure 1.1. In some of the marine Synechococcus species there is some
conservation among the putative coding sequences in the vicinity of the OCP gene; homologs of
a putative

α

-carotene ketolase l ank the OCP, followed by a homolog of a conserved hypothetical
protein ( slr1964 in Synechocystis PCC6803), which is present and adjacent to the OCP in most
cyanobacterial genomes (see Table 1.1 and Figure 1.1). This small protein (106-134 amino acids), is
of unknown function. A global yeast two-hybrid analysis in Synechocystis PCC6803 neither links
the OCP and slr1964 gene product functionally (Sato et al. 2007) nor does this screen of protein-
protein interactions offer insight into the function of the OCP. Instead, our understanding of the
function of the OCP is based on molecular, genetic, and spectroscopic approaches complemented
by structural biology.

β

1.2 RECENT STUDIES ON THE FUNCTION OF THE OCP
In contrast to the photosynthetic eukaryotes, photoprotection in cyanobacteria is not induced by the
presence of a transthylakoid

pH or the excitation pressure on PSII. Instead, intense blue-green
light (400-550 nm) induces a quenching of PSII l uorescence that is reversible in minutes even in
the presence of translation inhibitors (El Bissati et al. 2000). Fluorescence spectra measurements
and the study of the NPQ mechanism in phycobilisome- and PSII-mutants of the cyanobacterium
Synechocystis PCC6803 indicate that this mechanism involves a specii c decrease of the l uores-
cence emission of the phycobilisomes and a decrease of the energy transfer from the phycobilisomes
to the RCs (Scott et al. 2006, Wilson et al. 2006). The site of the quenching appears to be the core of
the phycobilisome (Scott et al. 2006, Wilson et al. 2006, Rakhimberdieva et al. 2007b).

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