Sll1717 Affects the Redox State of the Plastoquinone Pool by Modulating Quinol Oxidase Activity in Thylakoids

Abstract

A Synechocystis sp. strain PCC 6803 mutant lacking CtaI, a main subunit of cytochrome c oxidase, is not capable of growing at light intensities below 5 mol photons m ؊2 s ؊1 , presumably due to an overreduced plastoquinone pool in the thylakoid membrane. Upon selection for growth at light intensities below 5 mol photons m ؊2 s ؊1 , a secondary mutant was generated that retained the CtaI deletion and had fully assembled photosystem II complexes; in this secondary mutant (pseudorevertant), oxygen evolution and respiratory activities were similar to those in the wild type. Functional complementation of the original CtaI-less strain to low-light tolerance by transformation with restriction fragments of genomic DNA of the pseudorevertant and subsequent mapping of the pseudoreversion site showed that the point mutation led to a Ser186Cys substitution in Sll1717, a protein of as-yet-unknown function and with a predicted ATP/GTP-binding domain. This mutation caused a decrease in the plastoquinone pool reduction level of thylakoids compared to that observed for the wild type. Based on a variety of experimental evidence, the most plausible mechanism to cause this effect is an activation of plastoquinol oxidation in thylakoids by the quinol oxidase CydAB that occurs without upregulation of the corresponding gene and that may be caused by an increased CydAB activity in thylakoids, conceivably due to altered CydAB sorting between cytoplasmic and thylakoid membranes. Sll1717 appears to be unique to Synechocystis sp. strain PCC 6803 and has a close homologue encoded in the genome of this organism. The transcript level of sll1717 is low, which suggests that the corresponding protein is regulatory rather than structural.

Regulatory electron transport pathways of photosynthesis in cyanobacteria and microalgae: Recent advances and biotechnological prospects

“…Although Cox primarily functions in the dark, it assists in the regulation of electron flow to PSI by transferring electrons from PC/Cyt c 6 to O 2 in light (Ermakova et al, 2016; Schmetterer, 2016). Cox is indispensable under low light (Kufryk & Vermaas, 2006) and contributes to photosynthetic control by likely pumping protons at a 4H + /2e − ratio. Mutant strains deficient in both Cox and Cyd cannot survive in a square‐wave diurnal light regime (sharp alteration of 12 h high light and 12 h dark periods), but interestingly, are able to grow under sinusoidal diurnal regime (gradual changes in light intensity), under shorter square‐wave light regime (5 min dark/5 min high light), or under constant illumination (Ermakova et al, 2016; Lea‐Smith et al, 2013).…”

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

“…At low concentrations, DBMIB inhibits electron transport on the reducing side of the PQ, but at higher concentrations excess DBMIB will inhibit the Q B site of PSII, located on the oxidising side of the PQ (Moreland 1980;Rich et al 1991). To prevent fluorescence quenching from oxidised DBMIB, it can be used in conjunction with an excess of sodium ascorbate (Kufryk and Vermaas 2006). For every Cytochrome b 6 f complex, one molecule of DBMIB red is needed for complete inhibition of electron transfer through the Cytochrome b 6 f complex (Rich et al 1991).…”

Distinguishing the roles of thylakoid respiratory terminal oxidases in the cyanobacterium Synechocystis sp. PCC 6803

“…RTOs are not essential in Synechocystis when cells are subjected to continuous moderate or high light (Howitt and Vermaas, 1998;Pils and Schmetterer, 2001;Lea-Smith et al, 2013) or 12-h-dark/12-h-moderate light (40 mmol photons m 22 s 21 ) cycle regimes (LeaSmith et al, 2013). However, the presence of Cox is essential for viability under low light (Kufryk and Vermaas, 2006), and the presence of at least one Studies of RTO mutants by gas exchange under light are complicated in oxygenic photosynthetic organisms due to the oxygen-evolving activity of PSII and the existence of other processes capable of oxygen photoreduction. The Flavodiiron proteins Flv1 and Flv3 are responsible for the majority of oxygen uptake in the light in cyanobacteria (Helman et al, 2003(Helman et al, , 2005Allahverdiyeva et al, 2011;Ermakova et al, 2014).…”