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to the loss of the N-terminal helix (that is associated with the C-terminal
domain in the intact structure) and the entire C-terminal domain. Now
with the availability of genomic sequence data, this can be confirmed to be
a proteolytic fragment of the OCP because the
Arthrospira
genome does not
contain genes for N-terminal paralogues. Whether proteolytic fragments of
the OCP have a functional role (for example, in mediating photoprotec-
tion or in proteolysis-based signalling;
Turk, Turk, et al., 2012
), or are merely
degradation products is an open question. Likewise, how the protein and the
carotenoid interact in the proteolytic RCP is not at all understood, it is likely
that a structural rearrangement occurs to shield the carotenoid from solvent.
A gene for a second type of OCP fragment, missing the first 50 amino
acids, had been identified in the genome of
Gloeobacter violaceus
, in addition to
one for a full-length OCP. Recently, both forms were shown to be expressed
and the fluorescence quenching was attributed to the smaller form of the OCP
(
Bernát, Schreiber, et al.
). However, careful analysis of the
G. violaceus
genome
in the region of the glr3935 indicates that there is a more plausible start codon
for the gene 150 bp upstream of the one proposed in the databases, suggesting
that the gene product of glr3935 is a full-length OCP of 324 amino acids. Both
a full length (34 kDa) and a more abundant, lower molecular weight OCP
(∼30 kDa) were detected in conditions that induced fluorescence quenching;
the latter was may also be an indication of proteolytic processing of the OCP.
6.2. Distribution of Genes Encoding the FRP
Interestingly, genes for the FRP are not found in all OCP-containing cya-
nobacteria (see, for example, many of the organisms in clade B1 of
Fig. 1.3
).
Organisms that lack FRP tend to be those enriched in isolated copies of the
N-terminal domain (e.g. subclade B1,
Fig. 1.3
). Organisms with 2-3 copies of
the full-length OCP may or may not have a second FRP gene and no organ-
ism contains three copies of FRP genes. Homologues to the FRP are almost
exclusively found in cyanobacteria, however, credible candidates (similar sized
proteins with 35-40% identity) are found in a few bacterial genomes such as
that of
Mesorhizobium ciceri
bv biserrulae WSM1271. This gene (Mesci_2682)
is part of a gene cluster encoding several transporter homologues.
7. A MODEL FOR THE OCP-MEDIATED
PHOTOPROTECTIVE MECHANISM
Our current understanding of the OCP-mediated photoprotection
is outlined in
Fig. 1.4
. Blue-green light triggers the activation of the OCP
inducing conformational changes in the carotenoid and the protein that
