PsaE is required for in vivo cyclic electron flow around photosystem I in the cyanobacterium Synechococcus sp. PCC 7002

Electron transfer rates to P700⁺ have been determined in wildtype and three interposon mutants (psaE^-, ndhF^-, and psaF^- ndhF^-) of Synechococcus sp. PCC 7002. All three mutants grew significantly more slowly than wild type at low light intensities, and each failed to grow photoheterotrophically in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and a metabolizable carbon source. The kinetics of P700⁺ reduction were similar in the wild-type and mutant whole cells in the absence of DCMU. In the presence of DCMU, the P700₊ reduction rate in the psaf mutant was significantly slower than in the wild type. In the presence of DCMU and potassium cyanide, added to inhibit the outflow of electrons through cytochrome oxidase, P700⁺ reduction rates increased for both the psaE^- and ndhF^- strains. The reduction rates for these two mutants were nonetheless slower than that observed for the wild-type strain. The further addition of methyl viologen caused the rate of P700⁺ reduction in the wild type to become as slow as that for the psaE mutant in the absence of methyl viologen. Given the ability of methyl viologen to intercept electrons from the acceptor side of photosystem I, this response reveals a lesion in cyclic electron flow in the psaE mutant. In the presence of DCMU, the rate of P700⁺ reduction in the psaf ndhF double mutant was very slow and nearly identical with that for the wild-type strain in the presence of 2,4-dibromo-3-methyl-6-isopropyl-p-benzoquinone, a condition under which physiological electron donation to P700⁺ should be completely inhibited. These results suggest that NdhF- and PsaE-dependent electron donation to P700⁺ occurs only via plastoquinone and/or cytochrome b₆/f and indicate that there are three major electron sources for P700⁺ reduction in this cyanobacterium. We conclude that, although PsaE is not required for linear electron flow to NADP₊, it is an essential component in the cyclic electron transport pathway around photosystem I.