Bidirectional electron transfer in photosystem I: electron transfer on the PsaA side is not essential for phototrophic growth in Chlamydomonas.

We have used pulsed electron paramagnetic resonance (EPR) measurements of the electron spin polarised (ESP) signals arising from the geminate radical pair P700(z.rad;+)/A(1)(z.rad;-) to detect electron transfer on both the PsaA and PsaB branches of redox cofactors in the photosystem I (PSI) reaction centre of Chlamydomonas reinhardtii. We have also used electron nuclear double resonance (ENDOR) spectroscopy to monitor the electronic structure of the bound phyllosemiquinones on both the PsaA and PsaB polypeptides. Both these spectroscopic assays have been used to analyse the effects of site-directed mutations to the axial ligands of the primary chlorophyll electron acceptor(s) A(0) and the conserved tryptophan in the PsaB phylloquinone (A(1)) binding pocket. Substitution of histidine for the axial ligand methionine on the PsaA branch (PsaA-M684H) blocks electron transfer to the PsaA-branch phylloquinone, and blocks photoaccumulation of the PsaA-branch phyllosemiquinone. However, this does not prevent photoautotrophic growth, indicating that electron transfer via the PsaB branch must take place and is alone sufficient to support growth. The corresponding substitution on the PsaB branch (PsaB-M664H) blocks kinetic electron transfer to the PsaB phylloquinone at 100 K, but does not block the photoaccumulation of the phyllosemiquinone. This transformant is unable to grow photoautotrophically although PsaA-branch electron transfer to and from the phyllosemiquinone is functional, indicating that the B branch of electron transfer may be essential for photoautotrophic growth. Mutation of the conserved tryptophan PsaB-W673 to leucine affects the electronic structure of the PsaB phyllosemiquinone, and also prevents photoautotrophic growth.