The plastid terminal oxidase (PTOX) encoded by theAra-bidopsis IMMUTANSgenewas expressed inEscherichia coli cells and its quinone/oxygen oxidoreductase activity moni-tored in isolated bacterial membranes using NADH as an electron donor. Specificity for plastoquinone was observed. Neither ubiquinone, duroquinone, phylloquinone nor ben-zoquinone could substitute for plastoquinone in this assay. However, duroquinol (fully reduced chemically) was an accepted substrate. Iron is also required and cannot be substituted by Cu 2+ ,Zn 2+ or Mn 2+ . .
