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 wild-type and three interposon mutants (psaE-, ndhF-, and psaE- ndhF-) of Synechoco ccus sp. PCC 7002. All three mutants grew significantly more slowly th an wild type at low light intensities, and each failed to grow photohe terotrophically in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) and a metabolizable carbon source. The kinetics of P700+ r eduction were similar in the wild-type and mutant whole cells in the a bsence of DCMU. In the presence of DCMU, the P700+ reduction rate in t he psaE mutant was significantly slower than in the wild type. In the presence of DCMU and potassium cyanide, added to inhibit the outflow o f electrons through cytochrome oxidase, P700+ reduction rates increase d for both the psaE- and ndhF- strains. The reduction rates for these two mutants were nonetheless slower than that observed for the wild-ty pe 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 psa E mutant in the absence of methyl viologen. Given the ability of methy l viologen to intercept electrons from the acceptor side of photosyste m I, this response reveals a lesion in cyclic electron flow in the psa E mutant. In the presence of DCMU, the rate of P700+ reduction in the psaE ndhF double mutant was very slow and nearly identical with that f or the wild-type strain in the presence of 2,4-dibromo-3-methyl-6-isop ropyl-p-benzoquinone, a condition under which physiological electron d onation 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 t hree 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 tra nsport pathway around photosystem I.