Functional Accumulation of Antenna Proteins in Chlorophyll b-Less Mutants of Chlamydomonas reinhardtii

Abstract

The green alga Chlamydomonas reinhardtii contains several light-harvesting chlorophyll a/b complexes (LHC): four major LHCIIs, two minor LHCIIs, and nine LHCIs. We characterized three chlorophyll b-less mutants to assess the effect of chlorophyll b deficiency on the function, assembly, and stability of these chlorophyll a/b binding proteins. We identified point mutations in two mutants that inactivate the CAO gene responsible for chlorophyll a to chlorophyll b conversion. All LHCIIs accumulated to wild-type levels in a CAO mutant but their light-harvesting function for photosystem II was impaired. In contrast, most LHCIs accumulated to wild-type levels in the mutant and their light-harvesting capability for photosystem I remained unaltered. Unexpectedly, LHCI accumulation and the photosystem I functional antenna size increased in the mutant compared with in the wild type when grown in dim light. When the CAO mutation was placed in a yellow-in-the-dark background (yid-BF3), in which chlorophyll a synthesis remains limited in dim light, accumulation of the major LHCIIs and of most LHCIs was markedly reduced, indicating that sustained synthesis of chlorophyll a is required to preserve the proteolytic resistance of antenna proteins. Indeed, after crossing yid-BF3 with a mutant defective for the thylakoid FtsH protease activity, yid-BF3-ftsh1 restored wild-type levels of LHCI, which defines LHCI as a new substrate for the FtsH protease.

Bilin-Dependent Photoacclimation in Chlamydomonas reinhardtii

“…These mutants are photosensitive, severely chlorotic, and/or yellow in the dark (Li and Timko, 1996;Chekounova et al, 2001;Falciatore et al, 2005;von Gromoff et al, 2008;Meinecke et al, 2010;Grovenstein et al, 2013). It was recently demonstrated that the specific loss of chlorophyll b in Chlamydomonas does not disrupt accumulation of LHCI and LHCII polypeptides as long as chlorophyll a biosynthesis remains intact (Bujaldon et al, 2017). Despite the phenotypic discrepancies among these mutants, our transcriptomic data suggest that aberrant accumulation of PSI, LHCI, and LHCII polypeptides in the hmox1 mutant is not due to inhibition of chlorophyll biosynthesis-related gene expression.…”

The soluble guanylate cyclase CYG12 is required for the acclimation to hypoxia and trophic regimes in Chlamydomonas reinhardtii

“…Defects in Chl biosynthesis or its regulation can have severe impacts on the amounts of photosystem‐ or LHC proteins as well as their structure and the resulting light energy transfer within the photosynthetic apparatus (e.g. Moseley et al ., ; Formighieri et al ., ; Bujaldon et al ., ). In strain ami CYG12 , 77K Chl fluorescence spectra showed an increased and slightly blue‐shifted emission maximum for PSI/LHCI (Figure ), which might be indicative of uncoupled LHCI antennae (Moseley et al ., ; Wlodarczyk et al ., ).…”

The enigmatic loss of light‐independent chlorophyll biosynthesis from an Antarctic green alga in a light‐limited environment

“…73.4% pairwise identity with Chlamydomonas eustigma ). We did, however, uncover two other duplicated genes: GENOMES UNCOUPLED ( GUN4 ), encoding a regulatory subunit of Mg‐chelatase that enhances chlorophyll biosynthesis and contributes to retrograde signalling (Formighieri et al ., ; Brzezowski et al ., ), and chlorophyllide a oxygenase ( CAO ), which is responsible for the production of chl b (Tanaka et al ., ; Bujaldon et al ., ). To the best of our knowledge, this is the first report of duplication of these genes in a green alga.…”