A plastid terminal oxidase associated with carotenoid desaturation during chromoplast differentiation

The Arabidopsis IMMUTANS gene encodes a plastid homolog of the mitochondria l alternative oxidase, which is associated with phytoene desaturation. Upon expression in Escherichia coli, this protein confers a detectable cyanide- resistant electron transport to isolated membranes. In this assay this acti vity is sensitive to n-propyl-gallate, an inhibitor of the alternative oxid ase. This protein arrears to be a plastid terminal oxidase (PTOX) that is f unctionally equivalent to a quinol:oxygen oxidoreductase ase. This protein was immunodetected in achlorophyllous pepper (Capsicum annuum) chromoplast membranes, and a corresponding cDNA was cloned from pepper and tomato (Lyco persicum esculentum) fruits. Genomic analysis suggests the presence of a si ngle gene in these organisms, the expression of which parallels phytoene de saturase and xi-carotene desaturase gene expression during fruit ripening. Furthermore, this PTOX gene is impaired in the tomato ghost mutant, which a ccumulates phytoene in leaves and fruits. These data show that PTOX also pa rticipates in carotenoid desaturation in chromoplasts in addition to its ro le during early chloroplast development.