Electron Transport Routes in Whole Cells ofSynechocystissp. Strain PCC 6803:  The Role of the Cytochromebd-Type Oxidase

Abstract

The plastoquinone pool is the central switching point of both respiratory and photosynthetic electron transport in cyanobacteria. Its redox state can be monitored noninvasively in whole cells using chlorophyll fluorescence induction, avoiding possible artifacts associated with thylakoid membrane preparations. This method was applied to cells of Synechocystis sp. PCC 6803 to study respiratory reactions involving the plastoquinone pool. The role of the respiratory oxidases known from the genomic sequence of Synechocystis sp. PCC 6803 was investigated by a combined strategy using inhibitors and deletion strains that lack one or more of these oxidases. The putative quinol oxidase of the cytochrome bd-type was shown to participate in electron transport in thylakoid membranes. The activity of this enzyme in thylakoids was strongly dependent on culture conditions; it was increased under conditions where the activity of the cytochrome b(6)f complex alone may be insufficient for preventing over-reduction of the PQ pool. In contrast, no indication of quinol oxidase activity in thylakoids was found for a second alternative oxidase encoded by the ctaII genes.

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

“…While the role of terminal oxidases in preventing overreduction of either cyt b 6 f (Berry et al, 2002), in the case of the plastoquinol-accepting terminal oxidases, and PSI, in the case of COX (Howitt and Vermaas, 1998), has been indicated before, we demonstrate the conditions under which this function is critical for survival. The thylakoid-located terminal oxidases help to reduce the extent of photoinhibition initially, limit oxidative damage to the cell during dark periods, and promote the use of reserves to provide energy for repair in the dark.…”

“…Cyd has also been localized to the thylakoid membrane based on inhibitor studies. The addition of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, which inhibits cyt b 6 f and pentachlorophenol, a Cyd inhibitor, results in complete suppression of plastoquinol oxidation (Berry et al, 2002). Deletion of Cyd and addition of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone also results in the same effect.…”

“…The activity of Cyd increased when cyt b 6 f was inhibited in Synechocystis, implicating a role for Cyd in preventing overreduction of the plastoquinone pool (Berry et al, 2002). COX possibly functions as a terminal electron sink under conditions of low PSI activity or to prevent overreduction of the soluble redox carriers (Howitt and Vermaas, 1998).…”

“…PCC 6803 (hereafter referred to as Synechocystis), plastoquinone is the electron acceptor from either PSII or one of several dehydrogenase complexes, which are linked to the oxidation of NADPH, NADH, or succinate (Cooley and Vermaas, 2001;Ohkawa et al, 2001). Electrons are transferred directly from plastoquinone to a cytochrome bd-quinol oxidase complex (Cyd), encoded by cydAB (Berry et al, 2002), or via cyt b 6 f and the soluble redox carriers to either PSI or an aa 3 -type cytochrome c oxidase complex (COX), encoded by ctaCIDIEI (Howitt and Vermaas, 1998; Fig. 1).…”

PetC1 Is the Major Rieske Iron-Sulfur Protein in the Cytochrome b6f Complex of Synechocystis sp. PCC 6803

“…The yield of the variable chlorophyll fluorescence (F V ) from photosystem II depends on the redox state of Q A , the primary quinone acceptor, and, indirectly, also on the state of the plastoquinone pool, which thus can be monitored by using chlorophyll fluorescence measurements (20). Light-induced changes of chlorophyll fluorescence were measured at 686 nm in a laboratory-built set up at room temperature (23°C), using a 1 ϫ 1-cm cuvette with a 2-ml sample volume.…”