Negatively charged residues in the H loop of PsaB subunit in Photosystem Ifrom Synechocystis sp PCC 6803 appear to be responsible for electrostatic repulsions with plastocyanin

Wild-type plastocyanin from the cyanobacterium Synechocystis sp. PCC 6803 d oes not form any kinetically detectable transient complex with Photosystem I (PS I) during electron transfer, but the D44R/D47R double mutant of coppe r protein does [De la Cerda et al. (1997) Biochemistry 36: 10125-10130]. To identify the PS I component that is involved in the complex formation with the D44R/D47R plastocyanin, the kinetic efficiency of several PS I mutants , including a PsaF-PsaJ-less PS I and deletion mutants in the lumenal H and J loops of PsaB, were analyzed by laser flash absorption spectroscopy. The experimental data herein suggest that some of the negative charges at the H loop of PsaB are involved in electrostatic repulsions with mutant plastoc yanin. Mutations in the J loop demonstrate that this region of PsaB is also critical. The interaction site of PS I is thus not as defined as first exp ected but much broader, thereby revealing how complex the evolution of inte rmolecular electron transfer mechanisms in photosynthesis has been.