The kinetics of electron transfer from phyllosemiquinone (PhQ(center dot-)) to the iron sulfur cluster F-X in Photosystem I (PS I) are described by lifetimes of similar to 20 and similar to 250 ns. These two rates are attributed to reactions involving the quinones bound primarily by the PsaB (PhQ(B)) and PsaA (PhQ(A)) subunits, respectively. The factors leading to a similar to 10-fold difference between the observed lifetimes are not yet clear. The peptide nitrogen of conserved residues PsaA-Leu722 and PsaB-Leu706 is involved in asymmetric hydrogen-bonding to PhQ(A) and PhQ(B), respectively. Upon mutation of these residues in PS I of the green alga, Chlamydomonas reinhardtii, we observe an acceleration of the oxidation kinetics of the PhQ(center dot-) interacting with the targeted residue: from similar to 255 to similar to 180 ns in PsaA-L722Y/T and from similar to 24 to similar to 10 ns in PsaB-L706Y. The acceleration of the kinetics in the mutants is consistent with a perturbation of the H-bond, destabilizing the PhQ(center dot-) state, and increasing the driving force of its oxidation. Surprisingly, the relative amplitudes of the phases reflecting PhQ(A)(center dot-) and PhQ(B)(center dot-) oxidation were also affected by these mutations: the apparent PhQ(A)(center dot-)/PhQ(B)(center dot-) ratio is shifted from 0.65:0.35 in wild-type reaction centers to 0.5:0.5 in PsaA-L722Y/T and to 0.8:0.2 in PsaB-L706Y. The most consistent account for all these observations involves considering reversibility of oxidation of PhQA(center dot-) and PhQ(B)(center dot-) by F-X, and asymmetry in the driving forces for these electron transfer reactions, which in turn leads to F-x-mediated interquinone electron transfer.