Evidence for two active branches for electron transfer in photosystem I

All photosynthetic reaction centers share a common structural theme. Two re lated, integral membrane polypeptides sequester electron transfer cofactors into two quasi-symmetrical branches, each of which incorporates a quinone. In type II reaction centers [photosystem (PS) II and proteobacterial react ion centers], electron transfer proceeds down only one of the branches, and the mobile quinone on the other branch is used as a terminal acceptor. PS I uses iron-sulfur clusters as terminal acceptors, and the quinone serves o nly as an intermediary in electron transfer. Much effort has been devoted t o understanding the unidirectionality of electron transport in type II reac tion centers, and it was widely thought that PS I would share this feature. We have tested this idea by examining in vivo kinetics of electron transfe r from the quinone in mutant PS I reaction centers. This transfer is associ ated with two kinetic components, and we show that mutation of a residue ne ar the quinone in one branch specifically affects the faster component, whi le the corresponding mutation in the other branch specifically affects the slower component. We conclude that both electron transfer branches in PS I are active.