Evidence for the operation of a cyanide-sensitive oxidase in chlororespiration in the thylakoids of the chlorophyll c-containing alga Pleurochloris meiringensis (Xanthophyceae)

Abstract

For characterisation of chlororespiration in the chlorophyll c-containing alga Pleurochloris meiringensis, we measured the flash-induced electrochromic absorbance changes between 470 and 545 nm and the redox changes of cytochrome f and cytochrome c553. Cytochrome c553 was shown to be present in high amounts (1 mol cytochrome c553 per 300 mol chlorophyll) in this alga and to function as the obligate electron donor for photosystem I instead of plastocyanin. Whereas salicylhydroxamic acid had no effect on the flash-induced absorbance transients, cyanide enhanced the slow-rising (t1/2≈10 ms) kinetic component of the electrochromic absorbance change. Cyanide also accelerated the re-reduction of the cytochrome f+/c 553 + electron pool following the photooxidation by repetitive single-turnover flashes. These data suggest that an oxidase competes with the cytochromes for electrons. The KCN concentration needed to induce these effects was 0.25 mM at half-saturation, whereas mitochondrial respiration was completely blocked at 0.1 mM. Therefore, the oxidase cannot be identical to the cytochrome aa3-oxidase of mitochondria and is most likely located in the chloroplast of P. meiringensis.