ABSTRACT
Chlamydomonas reinhardtii mutants defective in the chloroplast ATP synthase are highly sensitive to light. The ac46 mutant is affected in the MDH1 gene, required for production or stability of the monocistronic atpH mRNA encoding CF(O)-III. In this and other ATP synthase mutants, we show that short-term exposure to moderate light intensities-a few minutes-induces an inhibition of electron transfer after the primary quinone acceptor of photosystem II (PSII), whereas longer exposure-several hours-leads to a progressive loss of PSII cores. An extensive swelling of thylakoids accompanies the initial inhibition of electron flow. Thylakoids deflate as PSII cores are lost. The slow process of PSII degradation involves the participation of ClpP, a chloroplast-encoded peptidase that is part of a major stromal protease Clp. In the light of the above findings, we discuss the photosensitivity of ATP synthase mutants with respect to the regular photoinhibition process that affects photosynthetic competent strains at much higher light intensities.
Subject(s)
Chlamydomonas reinhardtii/radiation effects , Light , Proton-Translocating ATPases/metabolism , Adenosine Triphosphatases/metabolism , Animals , Base Sequence , Chlamydomonas reinhardtii/enzymology , Chlamydomonas reinhardtii/ultrastructure , DNA Primers , Endopeptidase Clp , Fluorescence , Freeze Fracturing , Hydrolysis , Microscopy, Electron , Photosynthetic Reaction Center Complex Proteins/metabolism , Photosystem II Protein Complex , Proton-Translocating ATPases/genetics , RNA, Messenger/genetics , Serine Endopeptidases/metabolism , Thylakoids/radiation effectsABSTRACT
In the green alga Chlamydomonas reinhardtii, the ClpP protease is encoded by an essential chloroplast gene. Mutating its AUG translation initiation codon to AUU reduced ClpP accumulation to 25 to 45% of that of the wild type. Both the mature protein and the putative precursor containing its insertion sequence were present in reduced amounts. Attenuation of ClpP did not affect growth rates under normal conditions but restricted the ability of the cells to adapt to elevated CO(2) levels. It also affected the rate of degradation of the cytochrome b(6)f complex of the thylakoid membrane in two experimental situations: (1) during nitrogen starvation, and (2) in mutants deficient in the Rieske iron-sulfur protein. The ClpP level also controls the steady state accumulation of a mutated version of the Rieske protein. In contrast, attenuation of ClpP did not rescue the fully unassembled subunits in other cytochrome b(6)f mutants. We conclude that proteolytic disposal of fully or partially assembled cytochrome b(6)f is controlled by the Clp protease.
