A strain of Synechocystis sp PCC 6803 without photosynthetic oxygen evolution and respiratory oxygen consumption: implications for the study of cyclic photosynthetic electron transport

Cyclic electron transport around photosystem (PS) I is believed to play a r ole in generation of ATP required for adaptation to stress in cyanobacteria and plants. However, elucidation of the pathway(s) of cyclic electron flow is difficult because Of low rates OF this electron flow relative to those or linear photosynthetic and respirators electron transport. We have Constr ucted a strain of Synechoystis sp. PCC 6803 that lacks both PSII and respir atory oxidases and that. consequently, neither evokes nor consumes oxygen. However, this strain is still capable of cyclic electron flow around PSI. T he photoheterotrophic growth rate of this strain increased with light inten sity up to an intensity of about 25 mu mol photons m (2) s (1). Supporting the notion that cyclic electron flow contributes to ATP generation in this strain. Indeed. the ATP-generating ability of PSI is demonstrated rated by the fact that the PSII-less oxidase-less strain is able to grow at Much hig her salt concentrations than I strain lacking PSI. A quinone electrode was used to measure the redox state of the plastoquinone pool in vivo in the va rious strains,, used in this Study. In contrast to what is observed in chlo roplasts. the plastoquinone pool was rather reduced in darkness and was oxi dized in the light. This is in line with significant electron donation by r espiratory pathways (NADPH dehydrogenase and particularly succinate dehydro genase) in darkness. In the light. the pool becomes oxidized Clue to the Pr esence Or Much more PSI than PSII In the oxidase-less strains, the plastoqu inone pool was very Much reduced in darkness and was oxidized in the light by PSI. Photosystem II activity did not greatly alter the redox State of th e plastoquinone pool. The results suggest cyclic electron flow around PSI c an contribute to generation of ATP, and a strain deficient in linear electr on transport pathways provides an excellent model for further investigation s of cyclic electron flow.