Abstract :
[en] Historically, the photosynthetic apparatus has been extensively studied within the phylum
Archaeplastida. However, this narrow focus overlooks the broader diversity of
photosynthetic eukaryotes. Euglena gracilis, a unicellular eukaryote belonging to the
Discoba supergroup, has acquired photosynthesis by kleptoplasty from a green algal prey,
resulting in substantial reconfiguration of its photosynthetic apparatus, and possibly
concomitant loss of some photoprotection mechanisms. This entails the loss of some
canonical subunits such as CP26/Lhcb5 chlorophyll a/b-binding protein of plant
photosystem (PS) II, as well as the presence of a unique family of light-harvesting
complexes (LHCE), which mainly contains red-shifted chl a. To delve into the impact of these
changes on PSII, the structure and composition of PSII were investigated by TEM singleparticle
analysis and mass spectrometry. Despite the absence of CP26, Euglena cells
assemble a dimeric PSII-LHCII supercomplex comprising CP29/Lhcb5 and three LHCII
trimers (LHCII3) per PSII core. Using a CRISPR-Cas9 gene editing approach, CP29
knockout (KO) mutant strains were generated as well as a Chlorophyll a Oxygenase (CAO)
knockout mutant deficient in chl b synthesis. Chlorophyll fluorescence induction kinetics
suggested that the PSII antenna size was altered only in CAO-KO strain, in agreement with
the absence of chl b and the reduced amount of neoxanthin specifically in the CAO-KO.
However no PSII-LHCII supercomplexes, but only PSII cores and free LHCII3, were
identified in the CP29-KO strains after Clear-Native PAGE from total membrane protein
preparation solubilized with alpha-dodecyl-maltoside. These results suggest that the loss of
CP29 might compromise the stability of PSII-LHCII in vivo. Still, CP29-KO strains did not
show growth or photosynthesis defects in both light-limited or high-light conditions, in
contrast to the CAO-KO strain that was light-sensitive. In conclusion, our study suggests
that the E. gracilis has evolved a stable PSII-LHCII3 supercomplex in the absence of CP26,
whose assembly and photoprotection capacity is not fully compromised by further CP29
loss. These results question the mechanisms by which LHCII3 are stabilized in vivo in the
native PSII-LHCII3 supercomplex of E. gracilis. We are currently investigating the possibility
that novel monomeric LHCE proteins contribute, with CP29, to PSII-LHCII stabilization.
