Flexibility in photosynthetic electron transport: a newly identified chloroplast oxidase involved in chlororespiration

Besides electron transfer reactions involved in the 'Z' scheme of photosynt hesis, alternative electron transfer pathways have been characterized in ch loroplasts. These include cyclic electron flow around photosystem I (PS I) or a respiratory chain called chlororespiration. Recent work has supplied n ew information concerning the molecular nature of the electron carriers inv olved in the non-photochemical reduction of the plastoquinone (PQ) pool. Ho wever, until now little is known concerning the nature of the electron carr iers involved in PQ oxidation. By using mass spectrometric measurement of o xygen exchange performed in the presence of O-18-enriched O-2 and Chlamydom onas mutants deficient in PS I, we show that electrons can be directed to a quinol oxidase sensitive to propyl gallate but insensitive to salicyl hydr oxamic acid. This oxidase has immunological and pharmacological similaritie s with a plastid protein involved in carotenoid biosynthesis.