ABSTRACT
Besides electron transfer reactions involved in the 'Z' scheme of photosynthesis, alternative electron transfer pathways have been characterized in chloroplasts. These include cyclic electron flow around photosystem I (PS I) or a respiratory chain called chlororespiration. Recent work has supplied new information concerning the molecular nature of the electron carriers involved in the non-photochemical reduction of the plastoquinone (PQ) pool. However, until now little is known concerning the nature of the electron carriers involved in PQ oxidation. By using mass spectrometric measurement of oxygen exchange performed in the presence of 18O-enriched O2 and Chlamydomonas mutants deficient in PS I, we show that electrons can be directed to a quinol oxidase sensitive to propyl gallate but insensitive to salicyl hydroxamic acid. This oxidase has immunological and pharmacological similarities with a plastid protein involved in carotenoid biosynthesis.
Subject(s)
Chloroplasts/enzymology , Light-Harvesting Protein Complexes , Oxidoreductases/metabolism , Photosynthesis/physiology , Photosynthetic Reaction Center Complex Proteins/metabolism , Photosystem I Protein Complex , Thylakoids/metabolism , Animals , Cell Respiration , Chlamydomonas reinhardtii/metabolism , Chloroplasts/metabolism , Cytochrome b Group/metabolism , Cytochrome b6f Complex , Electron Transport , Membrane Proteins/genetics , Membrane Proteins/physiology , Oxygen/metabolism , Photosynthetic Reaction Center Complex Proteins/genetics , Plant Proteins/genetics , Plant Proteins/physiologyABSTRACT
The Arabidopsis IMMUTANS gene encodes a plastid homolog of the mitochondrial 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 activity is sensitive to n-propyl-gallate, an inhibitor of the alternative oxidase. This protein appears to be a plastid terminal oxidase (PTOX) that is functionally equivalent to a quinol:oxygen oxidoreductase. This protein was immunodetected in achlorophyllous pepper (Capsicum annuum) chromoplast membranes, and a corresponding cDNA was cloned from pepper and tomato (Lycopersicum esculentum) fruits. Genomic analysis suggests the presence of a single gene in these organisms, the expression of which parallels phytoene desaturase and zeta-carotene desaturase gene expression during fruit ripening. Furthermore, this PTOX gene is impaired in the tomato ghost mutant, which accumulates phytoene in leaves and fruits. These data show that PTOX also participates in carotenoid desaturation in chromoplasts in addition to its role during early chloroplast development.
Subject(s)
Arabidopsis Proteins , Carotenoids/biosynthesis , Nuclear Proteins/genetics , Plants/genetics , Plastids/metabolism , Amino Acid Sequence , Arabidopsis/genetics , Arabidopsis/metabolism , Base Sequence , Blotting, Southern , Capsicum/genetics , Capsicum/metabolism , Carotenoids/metabolism , Chloroplasts/metabolism , Escherichia coli/genetics , Escherichia coli/metabolism , Immunoblotting , Solanum lycopersicum/genetics , Solanum lycopersicum/metabolism , Molecular Sequence Data , Mutation , Nuclear Proteins/metabolism , Plants/metabolism , Plants, Medicinal , RNA, Messenger/analysis , Reverse Transcriptase Polymerase Chain Reaction , Sequence Homology, Amino AcidABSTRACT
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.
