Specific mutations of the PsaB methionine axial ligand to chlorophyll (A0) of PSI in Chlamydomonas reinhardtii

Abstract

The primary electron acceptor in photosystem I is a chlorophyll a monomer termed A0. Because of its location in the structure, A0 may play a role both in excitation energy transfer and electron transfer. Methionine residues in the helix X domain of PsaA and PsaB are positioned close to A0 and may provide axial ligands, or occlude axial ligation by water. Met-664 of PsaB has been changed to His and Ser. Both the MH(B664) and MS (B664) mutants accumulate similar levels of PSI to wild type and can grow heterotrophically when acetate is supplied. Phototrophic growth is severely impaired in both mutants: MH(B664) will not grow under high light and grows only slowly under low light, whereas MS(B664) can grow slowly under high light. Femtosecond transient absorbance measurements of both mutants show a significantly increased contribution of long lived components (>>100 ps) compared to WT control. This indicates a significant decrease in the coupling between the antenna and electron transfer core, suggesting a role for A0 in excitation energy transfer. No significant change was found in the excitation trapping time, suggesting that the rate of primary charge separation was unchanged. Similar mutants of PsaA are currently under investigation.