Electron transfer from A0̄ to A₁ in Photosystem i from Chlamydomonas reinhardtii occurs in both the A and B branch with 25-30-ps lifetime

We have recorded transient absorption kinetics at 390 nm with picosecond resolution in order to observe electron transfer from the reduced primary acceptor, A-0, to the secondary acceptor, A₁, in wild type and mutated Photosystem I from Chlamydomonas reinhardtii. In the mutants, the methionine axial ligand to the primary electron acceptor in either the A- or B-branch of electron transfer cofactors, was replaced with histidine. Both of the mutations reduced the formation of a positive signal at 390 nm, characteristic of A-1 to a level approximately half of that observed in wild type Photosystem I. It is concluded that in the mutated branch of Photosystem I, electron transfer from A-0 to A₁ does not occur. The absorption kinetics resulting from subtraction of either of the mutants' traces from that of wild type is interpreted to reflect the kinetics of A- or B-side electron transfer from A-0 to A₁ in the the wild type Photosystem I. Each of these traces could be fitted with a monoexpoenential decay characterized by the same amplitude and 25-30-ps lifetime. The almost identical effect of both mutations on A-1 formation confirm a similar engagement of both the A- ad B-branches in electron transfer to A₁ in Photosystem I from C. reinhardtii. This observation is in contrast to the unidirectional electron transfer concluded from the studies on similar mutants of cyanobacterial Photosystem I.¹ Thus, this contribution provides further evidence for functional differences between these two model Photosystems.