Plastid terminal oxidase 2 (PTOX2) is the major oxidase involved in chlororespiration in Chlamydomonas

Abstract

By homology with the unique plastid terminal oxidase (PTOX) found in plants, two genes encoding oxidases have been found in the Chlamydomonas genome, PTOX1 and PTOX2. Here we report the identification of a knockout mutant of PTOX2. Its molecular and functional characterization demonstrates that it encodes the oxidase most predominantly involved in chlororespiration in this algal species. In this mutant, the plastoquinone pool is constitutively reduced under dark-aerobic conditions, resulting in the mobile lightharvesting complexes being mainly, but reversibly, associated with photosystem I. Accordingly, the ptox2 mutant shows lower fitness than wild type when grown under phototrophic conditions. Single and double mutants devoid of the cytochrome b 6 f complex and PTOX2 were used to measure the oxidation rates of plastoquinols via PTOX1 and PTOX2. Those lacking both the cytochrome b 6 f complex and PTOX2 were more sensitive to light than the single mutants lacking either the cytochrome b 6 f complex or PTOX2, which discloses the role of PTOX2 under extreme conditions where the plastoquinone pool is overreduced. A model for chlororespiration is proposed to relate the electron flow rate through these alternative pathways and the redox state of plastoquinones in the dark. This model suggests that, in green algae and plants, the redox poise results from the balanced accumulation of PTOX and NADPH dehydrogenase.Chlamydomonas reinhardtii | state transitions | chloroplast | photosynthesis

Proton Gradient Regulation 5-Mediated Cyclic Electron Flow under ATP- or Redox-Limited Conditions: A Study of ƊATPase pgr5 and ƊrbcL pgr5 Mutants in the Green Alga Chlamydomonas reinhardtii

“…Proteins were transferred onto nitrocellulose filters (BioTrace NT; Pall Corporation) using a semidry apparatus (BIORAD). Primary antibodies were sourced from Michael Hippler (PSAD, LHCSR3, PGRL1, and ANR1; Naumann et al, 2005Naumann et al, , 2007Petroutsos et al, 2009;Terashima et al, 2012), Gilles Peltier (PGRL1; Tolleter et al, 2011), Pierre Cardol (NDA2; Jans et al, 2008), Agrisera (LSU, COXIIb, Psba D1, CF1, and AOX1), and Francis-Andre Wollman (PTOX2 and cyt f; Vallon et al, 1991;Houille-Vernes et al, 2011) and decorated by incubation in Tris-buffered saline. Secondary antibody was anti-rabbit (Invitrogen).…”

“…Most of these alternative pathways use molecular oxygen (O 2 ) as a terminal electron acceptor, and their prevalence is even more species dependent than CEF. The plastoquinol terminal oxidase (PTOX) is active in chlororespiration to balance dark chloroplast redox poise and can also work as a valve under specific conditions to oxidize plastoquinols while reducing oxygen when the PQ pool is overreduced (Bailey et al, 2008;Houille-Vernes et al, 2011). The Mehler reaction describes a water-water cycle in which electrons generated by the splitting of water at the donor side of PSII are used to reduce O 2 at the acceptor side of PSI.…”

The Regulation of Photosynthetic Structure and Function during Nitrogen Deprivation in Chlamydomonas reinhardtii

“…The alternative oxidases, plastid plastoquinol terminal oxidases PTOX1 and PTOX2, are thought to be involved in the chlororespiration in C. reinhardtii (Rumeau et al, 2007;Bailleul et al, 2010;Houille-Vernes et al, 2011), and changes in protein and transcript abundance have been found to be associated with nutrient deprivation in algae, particularly iron deprivation as well as light stress (Houille-Vernes et al, 2011). PTOX1 and PTOX2 transcripts decreased in abundance 1.5-fold within the first half hour but then increased through 24 h to slightly less or greater than N-replete conditions.…”

Thylakoid Terminal Oxidases Are Essential for the CyanobacteriumSynechocystissp. PCC 6803 to Survive Rapidly Changing Light Intensities

“…In several plant species, up-regulation of PTOX has been observed in response to adverse conditions, such as exposure to high-light stress in low or elevated temperature environments (Streb et al, 2005) or under salt stress (Stepien and Johnson, 2009). Deletion of the PTOX2 gene in Chlamydomonas reinhardtii, encoding PTOX, confirmed a role in preventing overreduction of the plastoquinone pool in the light (Houille-Vernes et al, 2011). The mutant demonstrated reduced fitness compared with the wild-type under illumination, although growth was not examined under varying environmental conditions.…”