Mutations in algal and cyanobacterial Photosystem I that independently affect the yield of initial charge separation in the two electron transfer cofactor branches

In Photosystem I, light-induced electron transfer can occur in either of two symmetry-related branches of co-factors, each of which is composed of a pair of chlorophylls (ec2(A)/ec3(A) or ec2(B)/ec3(B)) and a phylloquinone (PhQ(A) or PhQ(B)). The axial ligand to the central Mg2+ of the ec2(A) and ec2(B) chlorophylls is a water molecule that is also H-bonded to a nearby Asn residue. Here, we investigate the importance of this interaction for charge separation by converting each of the Asn residues to a Leu in the green alga, Chlamydomonas reinhardtii, and the cyanobacterium, Synechocystis sp. PCC6803, and studying the energy and electron transfer using time-resolved optical and EPR spectroscopy. Nanosecond transient absorbance measurements of the PhQ to F-X electron transfer show that in both species, the PsaA-N604L mutation (near ec2(B)) results in a similar to 50% reduction in the amount of electron transfer in the B-branch, while the PsaB-N591L mutation (near ec2(A)) results in a similar to 70% reduction in the amount of electron transfer in the A-branch. A diminished quantum yield of P-700(+) PhQ(-) is also observed in ultrafast optical experiments, but the lower yield does not appear to be a consequence of charge recombination in the nanosecond or microsecond timescales. The most significant finding is that the yield of electron transfer in the unaffected branch did not increase to compensate for the lower yield in the affected branch. Hence, each branch of the reaction center appears to operate independently of the other in carrying out light-induced charge separation.