Photosystem I contains two potential electron transfer pathways between P(700) and F(X). These branches are made up of the electron transfer chain components A, A(0), and A(1). The primary electron acceptor A(0) is a chlorophyll a monomer that could be one or both of the two chlorophyll molecules, eC-A(3)/eC-B(3), identified in the 2.5 A resolution structure. The eC-A(3)/eC-B(3) chlorophylls are both coordinated by the sulfur atom of a methionine. This coordination is highly unusual, as interactions between the acid Mg(2+) and the soft base sulfur are weak. The eC-A(3)/eC-B(3) chlorophylls also are located close to one of the connecting chlorophylls that may link the antenna and the electron transfer chain chlorophylls. Due to their location in the structure, the eC-A(3)/eC-B(3) chlorophylls may play a role in both excitation energy transfer and electron transfer. To test the role of the eC-A(3)/eC-B(3) chlorophylls in electron transfer, Met-684 of PsaA and Met-664 of PsaB have been changed to His, Ser, and Leu. Replacement of either M(A684) or M(B664) results in a significant alteration in growth phenotype. The His and Leu mutants are very light sensitive in the presence of oxygen. Growth is impaired to a greater extent in the B-side mutants. However, all of the mutants are able to grow anaerobically at comparable rates. The His and Ser mutants all accumulate PSI at a level similar to that of wild type, whereas the Leu mutants have reduced amounts of PSI. Ultrafast transient absorbance measurements show that the (A(0)(-) - A(0)) difference signal accumulates in the MH(A684) and MH(B664) mutants under neutral conditions, demonstrating that electron transfer between A(0)(-) and A(1) is blocked or significantly slowed. The results show that both the A-branch and the B-branch of the ETC are active in PSI from Chlamydomonas reinhardtii.