The light sensitivity of ATP synthase mutants of Chlamydomonas reinhardtii

Chlamydomonas reinhardtii mutants defective in the chloroplast ATP synthase are highly sensitive to light. The ac46 mutant is affected in the MDH1 gen e, required for production or stability of the monocistronic atpH mRNA enco ding CFO-III. In this and other ATP synthase mutants, we show that short-te rm exposure to moderate light intensities-a few minutes-induces an inhibiti on of electron transfer after the primary quinone acceptor of photosystem I I (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. Th e slow process of PSII degradation involves the participation of ClpP, a ch loroplast-encoded peptidase that is part of a major stromal protease Clp. I n the light of the above findings, we discuss the photosensitivity of ATP s ynthase mutants with respect to the regular photoinhibition process that af fects photosynthetic competent strains at much higher light intensities.