A Rubredoxin-Like Protein Plays an Essential Role in Assembly of the F_A, F_B & F_X Iron-Sulfur Clusters in Photosystem I

Abstract

The photosystem I (PS I) reaction center is a large multisubunit complex composed of at least eleven polypeptides in cyanobacteria and chloroplasts of higher plants [1]. Six redox centers are involved in light-induced electron transfer from plastocyanin (or cytochrome c₆) to ferredoxin (or flavodoxin) in PS I. The PsaA/PsaB heterodimer harbors the primary electron carriers P700 (a chlorophyll (Chl) a dimer), A₀ (a monomeric Chl a), A₁ (a phylloquinone) and F_X (an interpolypeptide [4Fe–4S] cluster). The terminal electron acceptors F_A and F_B are [4Fe–4S] clusters which are located on the extrinsic PsaC protein. Although significant progress has been achieved in elucidating the structure [2, 3] and function of the PS I, questions still remain concerning the biogenesis, assembly, and regulation of PS I in the membrane. The biogenesis of photosynthetic complexes in cyanobacteria and higher-plant chloroplasts is probably a complex, multi-step process, which is likely to be highly regulated at the post-translation level, especially for cofactor-binding polypeptides. Here we report preliminary results from studies of a gene encoding a novel rubredoxin-like protein with an essential function in the assembly of the F_A, F_B and F_X iron-sulfur clusters in PS I reaction center.