Site-directed mutagenesis of PsaA:M684 in Chlamydomonas reinhardtii

Abstract

The electron transfer chain of PSI comprises a primary electron donor (P700; chlorophyll a dimer), and five electron acceptors: A0 (chlorophyll a monomer); A1 (phylloquinone); FX; FA and FB (all 4Fe4S2- centres). P700 and FX exist on the apparent C2 axis of symmetry of the PsaA/PsaB heterodimeric core, and there are two symmetrical branches of the redox cofactors A0 and A1 between them. It has recently been suggested that both branches of electron transfer may be functional, with a fast branch (PsaB branch) and a slow branch (PsaA branch) [1]. We have been using a combination of site-directed mutagenesis and spectroscopy to probe the protein environment surrounding the cofactor, A0. The suggested ligand to the central magnesium of the chlorophyll A0 is residue M684 on the PsaA polypeptide of PSI. The presence of a methionine residue as a ligand to a chlorophyll is highly unusual. We have been studying the effect of mutating this residue in the unicellular green alga, Chlamydomonas reinhardtii, on the dynamics of electron transfer in PSI and the properties of A0. The analysis of the mutants (M684A, M684H and M684V) has been carried out using paramagnetic spectroscopic techniques. These techniques have enabled us to investigate whether residue PsaA:M684 is a factor controlling the redox potential of A0. Inactivation of electron transfer on the PsaA branch in some of these mutants has enabled study of the significance of bidirectional electron transfer in PSI. 1. Guergova-Kuras, M., Boudreaux, B., Joliot, A., Joliot, P., Redding, K. (2001) PNAS, in press.