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The effects of the helix X domain of the PsaB heterodimeric protein complex in the coordination of the reaction centers (RC) of the P700 chlorophyll a dimer were analyzed in genetically engineered H656 mutants of PsaB. The oxidation midpoint potential of P700/P700 + was increased to 40 microVolts in mutant His-656 PsaB RC proteins. The midpoint potential of P700 photosystem I was affected by hydrogen bond that was formed between the His-656 of PsaB RC protein and the carbonyl group of chlorophyll a in P700.

PsaB and PsaC proteins in the photosystem I reaction center contain conserved amino acids sequences which provide the ligands for coordination of the iron-sulfur cluster, F(sub x). Mutagenesis in the prolines P560 and P564 of PsaB formed mutants whose F(sub x) was unaffected, but the interaction of PsaC and the reaction center core was damaged while mutagenesis in the R566 and D562 residues of PsaB formed mutants whose photosystem I was unstable. In addition, the mutants were incapable of photoautotrophic growth.

Conserved histidine residues in the PsaB protein encoded by a chloroplast gene are crucial for photosystem I reaction center accumulation in thylakoid membranes, as revealed by Chlamydomonas reinhardtii chloroplast transformation and site-directed mutagenesis. The post-translational step in reaction center assembly is influenced by variations in the His residues. Mutations on this residue do not regulate the spectral characteristics of the electron transfer component P700.

The direction of electron transfer along A-and/or B-branch, by changing the methionine residue that provides a ligand to the central magnesium of eC-A(sub 3) or eC-B(sub 3) in Chlamydomonas reinhardtii, is studied. The results show that both the A-branch and B-branch of the electron transport chain are active and support the bi-directional electron transfer in Chlamydomonas photosystem I.