Absence of lutein, violaxanthin and neoxanthin affects the functional chlorophyll antenna size of photosystem-II but not that of photosystem-I in the green alga Chlamydomonas reinhardtii

Chlamydomonas reinhardtii double mutant npq2 lor1 lacks the beta, epsilon-carotenoids lutein and loroxanthin as well as all beta, beta-epoxycarotenoids derived from zeaxanthin (e.g. violaxanthin and neoxanthin). Thus, the only carotenoids present in the thylakoid membranes of the npq2 lor1 cells are beta-carotene and zeaxanthin. The effect of these mutations on the photochemical apparatus assembly and function was investigated. In cells of the mutant strain, the content of photosystem-2 (PS2) and photosystem-2 (PS1) was similar to that of the wild type, but npq2 lor1 had a significantly smaller PS2 light-harvesting Chl antenna size. In contrast, the Chl antenna size of PS1 was not truncated in the mutant. SDS-PAGE and Western blot analysis qualitatively revealed the presence of all LHC2 and LHC2 apoproteins in the thylakoid membrane of the mutant. The results showed that some of the LHC2 and most of the LHC1 were assembled and functionally connected with PS2 and PS1, respectively. Photon conversion efficiency measurements, based on the initial slope of the lightsaturation curve of photosynthesis and on the yield of Chl alpha fluorescence in vivo, showed similar efficiencies. However, a significantly greater light intensity was required for the saturation of photosynthesis in the mutant than in the wild type. It is concluded that zeaxanthin can successfully replace lutein and violaxanthin in most of the functional light-harvesting antenna of the npq2 lor1 mutant.