The Light Sensitivity of ATP Synthase Mutants ofChlamydomonas reinhardtii

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.