Photosynthetic membrane organization and role of state transition in cyt, cpII, stt7 and npq mutants of Chlamydomonas reinhardtii

Abstract

In Chlamydomonas reinhardtii, cytochrome b₆/f and chlorophyll b binding proteins are important in energy distribution between photosystem (PS)II and PSI. In this study, we have used C. reinhardtii mutants deficient in cytochrome b₆/f complex (cyt), chlorophyll b binding protein (cpII), non-photochemical quenching (npq) and LHC II kinase (stt7) to study the importance of these proteins in electron transport, phosphorylation, and structural organization of thylakoid supercomplexes under optimum growth conditions. Fast Chl a fluorescence studies have shown that lack of CpII and Cyt b6/f caused reduced photochemical yield (F_x/F_m). The disappearance of I phase in cyt mutant showed that electron transfer from Cyt b₆/f to PSI is reduced due to un availability of Q₀ site for docking of PQH2 therefore LHC II kinase was unable to phosphorylate LHCII in cyt mutant. Further, blue native gel electrophoresis revealed the differential organization of photosynthetic membrane protein complexes in different mutants. Particularly, LHCII trimerization is more in cyt and stt7 mutants. Also, chemically induced state transition (LHCII phosphorylation) was not observed in cyt mutant, however, all other mutants were similar to that of wild type. Based on our results, we propose that the LHCII trimer accumulation and its organization with other complexes are very important in state transitions.