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Mitochondria possess oxygen-consuming respiratory electron transfer chains (RETC),and the oxygen-evolving photosynthetic electron transfer chain (PETC) resides in chloroplasts.However cyanobacteria harbor both RETC and PETC on their thylakoid membranes.It is proposed that chloroplasts could possess a RETC on the thylakoid membrane,in addition to PETC.Identification of a plastid terminal oxidase (PTOX) in the chloroplast from the Arabidopsis variegation mutant immutans(im) demonstrated the presence of a RETC in chloroplasts,and the PTOX is the committed oxidase.PTOX is distantly related to the mitochondrial alternative oxidase (AOX),which is responsible for the CN-insensitive alternative RETC.Similar to AOX as a ubiquinol oxidase,PTOX is a plastoquinol (PQH2) oxidase on the chloroplast thylakoid membrane.