Species-dependent alteration of electron transfer in the early stages of charge stabilization in Photosystem i

Electron transfer (ET) in Photosystem I (PS I) is bidirectional, occurring in two pseudosymmetric branches of cofactors. The relative use of two branches in the green alga Chlamydomonas reinhardtii and the cyanobacterium Synechocystis sp. PCC 6803 has been studied by changing the Met axial ligands of the chlorophyll a acceptor molecules, A_0A and A_0B, to His. The nature of the effect on the ET is found to be species dependent. In C. reinhardtii, transient absorption and transient EPR data show that in the M688H_PsaA variant, forward ET from A_0A to the quinone, A_1A, is blocked in 100% of the PS I complexes. In contrast, in Synechocystis sp. PCC 6803, forward ET from A_0A to A_1A is blocked in only 50% of the PS I complexes, but in those PS I complexes in which electrons reach A_1A, further transfer to the iron-sulfur cluster F_X is blocked. Similar species differences are found for the corresponding B-branch variants. One possible explanation of this behavior is that it is the result of two conformers in which an H-bond between the His side chain and the O1 carbonyl group of A₁ is either present or absent. The spectroscopic data suggest that the two conformers are present in nearly equal amounts in the Synechocystis sp. PCC 6803 variants, while only the conformer without the H-bond is present in the same variants of C. reinhardtii.