In land plants and cyanobacteria, co-translational association of chlorophyll (Chl) to the nascent D1 polypeptide, a reaction center protein of photosystem II (PSII), requires a Chl binding complex consisting of a short-chain dehydrogenase (HIGH CHLOROPHYLL FLUORESCENCE 244 [HCF244]/Uncharacterized Protein 39 [Ycf39]) and ONE-HELIX PROTEINS (OHP1 and OHP2 in chloroplasts) of the light-harvesting antenna complex (LHC) superfamily. Here, we show that an ohp2 mutant of the green alga Chlamydomonas (Chlamydomonas reinhardtii) fails to accumulate core PSII subunits, in particular D1 (encoded by the psbA mRNA). Extragenic suppressors arose at high frequency, suggesting the existence of another route for Chl association to PSII. The ohp2 mutant was complemented by the Arabidopsis (Arabidopsis thaliana) ortholog. In contrast to land plants, where psbA translation is prevented in the absence of OHP2, ribosome profiling experiments showed that the Chlamydomonas mutant translates the psbA transcript over its full length. Pulse labelling suggested that D1 is degraded during or immediately after translation. The translation of other PSII subunits was affected by assembly-controlled translational regulation. Proteomics showed that HCF244, a translation factor which associates with and is stabilized by OHP2 in land plants, still partly accumulates in the Chlamydomonas ohp2 mutant, explaining the persistence of psbA translation. Several Chl biosynthesis enzymes overaccumulate in the mutant membranes. Partial inactivation of a D1-degrading protease restored a low level of PSII activity in an ohp2 background, but not photoautotrophy. Taken together, our data suggest that OHP2 is not required for psbA translation in Chlamydomonas, but is necessary for D1 stabilization.

No abstract

In land plants and cyanobacteria, co-translational association of chlorophyll (Chl) to the nascent D1 polypeptide, a reaction center protein of photosystem II (PSII), requires a Chl binding complex consisting of a short-chain dehydrogenase (HCF244/Ycf39) and One-Helix Proteins of the LHC superfamily (OHP1 and OHP2 in chloroplasts). Here, we show that an ohp2 mutant of the green alga Chlamydomonas reinhardtii fails to accumulate core PSII subunits, in particular D1. Extragenic suppressors arise at high frequency, suggesting the existence of another route for Chl association to PSII. The ohp2 mutant can be complemented by the Arabidopsis ortholog. In contrast to land plants, where psbA translation is prevented in the absence of OHP2, ribosome profiling experiments show that the Chlamydomonas mutant translates the psbA transcript over its full length. Pulse labelling suggests that D1 is degraded during or immediately after translation. The translation of other PSII subunits is affected by assembly-controlled translational regulation (the CES process). Proteomics show that HCF244, a translation factor which associates with and is stabilized by OHP2 in land plants, still partly accumulates in the Chlamydomonas ohp2 mutant, explaining the persistence of psbA translation. Several Chl biosynthesis enzymes overaccumulate in the mutant membranes. Partial inactivation of the D1-degrading FtsH protease restores a low level of PSII activity in an ohp2 background, but not photoautotrophy. Taken together, our data suggest that OHP2 is not required for psbAD1 translation in Chlamydomonas, but necessary for its stabilization.