Electron Flow between Photosystem II and Oxygen in Chloroplasts of Photosystem I-deficient Algae Is Mediated by a Quinol Oxidase Involved in Chlororespiration

Abstract

In Chlamydomonas reinhardtii mutants deficient in photosystem I because of inactivation of the chloroplast genes psaA or psaB, oxygen evolution from photosystem II occurs at significant rates and is coupled to a stimulation of oxygen uptake. Both activities can be simultaneously monitored by continuous mass spectrometry in the presence of 18 O 2 . The light-driven O 2 exchange was shown to involve the plastoquinone pool as an electron carrier, but not cytochrome b 6 f. Photosystem II-dependent O 2 production and O 2 uptake were observed in isolated chloroplast fractions. Photosystem II-dependent oxygen exchange was insensitive to a variety of inhibitors (azide, carbon monoxide, cyanide, antimycin A, and salicylhydroxamic acid) and radical scavengers. It was, however, sensitive to propyl gallate. From inhibitors effects and electronic requirements of the O 2 uptake process, we conclude that an oxidase catalyzing oxidation of plastoquinol and reduction of oxygen to water is present in thylakoid membranes. From the sensitivity of flash-induced O 2 exchange to propyl gallate, we conclude that this oxidase is involved in chlororespiration. Clues to the identity of the protein implied in this process are given by pharmacological and immunological similarities with a protein (IMMUTANS) identified in Arabidopsis chloroplasts.

The Arabidopsis immutans Mutation Affects Plastid Differentiation and the Morphogenesis of White and Green Sectors in Variegated Plants

“…This similarity to AOX suggested that the IM protein may be a component of a redox pathway that functions in the desaturation of phytoene (Beyer et al, 1989;Mayer et al, 1990Mayer et al, , 1992Schulz et al, 1993;Nievelstein et al, 1995;Norris et al, 1995). Consistent with this interpretation, IM has quinol:oxygen oxidoreductase activity when expressed in Escherichia coli (Josse et al, 2000).…”

“…Nevertheless, our data point the way toward a more global role of this protein in plastid metabolism. In agreement with this hypothesis, recent evidence in Chlamydomonas reinhardtii suggests that IM serves as a terminal oxidase in chlororespiration (Cournac et al, 2000). In the context of its importance in plastid metabolism and its ubiquitous expression in all plastid types, it is interesting that IM does not seem to be required in cyanobacteria, because BLAST searches show that IM (and AOX) are not present in this evolutionary precursor of chloroplasts.…”

“…Cloning and sequencing of IM revealed that the gene product is a chloroplast membrane protein with homology to the AOX class of inner mitochondrial membrane terminal oxidases (Carol et al, 1999;Wu et al, 1999). IM also has quinol oxidase activity when expressed in E. coli (Josse et al, 2000). Considered together, these data suggest that IM is a redox component of a phytoene desaturation pathway involving PDS, plastoquinol, and oxygen as a terminal acceptor (Beyer et al, 1989;Mayer et al, 1990Mayer et al, , 1992Schulz et al, 1993;Nievelstein et al, 1995;Norris et al, 1995).…”

Fluorescence as a Tool to Understand Changes in Photosynthetic Electron Flow Regulation

“…Published KCN concentrations vary from 0.1 mM to 3 mM (Neubauer and Yamamoto 1992, Hormann et al 1994, Singh et al 1996. Another more recently discovered pseudo-cyclic electron transport pathway which cycles around PSII via the plastoquinol terminal oxidase (PTOX), reduces molecular oxygen by utilising electrons from the PQ pool to generate H 2 O (Cournac et al 2000;Peltier and Cournac 2002;Josse et al 2003). This alternative electron flow around PSII (upstream of PSI and Cytochrome b 6 f) is believed to be advantageous in both a high-light environment and under iron limitation , as it alleviates PSII excitation pressure by transporting electrons directly to oxygen while simultaneously ensuring that the electrons bypass the iron-demanding cytochrome b 6 f and PSI complexes .…”

“…This alternative electron flow around PSII (upstream of PSI and Cytochrome b 6 f) is believed to be advantageous in both a high-light environment and under iron limitation , as it alleviates PSII excitation pressure by transporting electrons directly to oxygen while simultaneously ensuring that the electrons bypass the iron-demanding cytochrome b 6 f and PSI complexes . Propyl-gallate (PGal) is an oxidase inhibitor specific to PTOX (Cournac et al 2000;Bailey et al 2008). PGal helps determine the role PTOX plays in alternative electron flow, and establish whether or not electrons are being used to reduce oxygen through PTOX activity ).…”