Electron flow between photosystem II and oxygen in chloroplasts of photosystem I-deficient algae is mediated by a quinol oxidase involved in chlororespiration

In Chlamydomonas reinhardtii mutants deficient in photosystem I because of inactivation of the chloroplast genes psaA or psaB, oxygen evolution from p hotosystem II occurs at significant rates and is coupled to a stimulation o f oxygen uptake. Both activities can be simultaneously monitored by continu ous mass spectrometry in the presence of O-18(2), The light-driven O-2 exch ange was shown to involve the plastoquinone pool as an electron carrier, bu t not cytochrome b(6)f, Photosystem II-dependent O-2 production and O-2 upt ake were observed in isolated chloroplast fractions. Photosystem II-depende nt oxygen exchange was insensitive to a variety of inhibitors (azide, carbo n monoxide, cyanide, antimycin A, and salicylhydroxamic acid) and radical s cavengers. It was, however, sensitive to propyl gallate, From inhibitors ef fects and electronic requirements of the O-2 uptake process, we conclude th at an oxidase catalyzing oxidation of plastoquinol and reduction of oxygen to water is present in thylakoid membranes. From the sensitivity of flash-i nduced O-2 exchange to propyl gallate, we conclude that this oxidase is inv olved in chlororespiration, Clues to the identity of the protein implied in this process are given by pharmacological and immunological similarities w ith a protein (IMMUTANS) identified in Arabidopsis chloroplasts.