Modification of electron transfer from the quinone electron carrier, A1, of Photosystem 1 in a site directed mutant D576⇒L within the Fe-Sx binding site of PsaA and in second site suppressors of the mutation in Chlamydomonas reinhardtii

A site directed mutant of the Photosystem I reaction center of Chlamydomonas reinhardtii has been described previously. [Hallahan et al. (1995) Photosynth Res 46: 257-264]. The mutation, PsaA: D576L, changes the conserved aspartate residue adjacent to one of the cysteine ligands binding the Fe-S(x) center to PsaA. The mutation, which prevents photosynthetic growth, was observed to change the EPR spectrum of the Fe-S(A/B) centers bound to the PsaC subunit. We suggested that changes in binding of PsaC to the PsaA/PsaB reaction center prevented efficient electron transfer. Second site suppressors of the mutation have now been isolated which have recovered the ability to grow photosynthetically. DNA analysis of four suppressor strains showed the original D576L mutation is intact, and that no mutations are present elsewhere within the Fe-S(x) binding region of either PsaA or PsaB, nor within PsaC or PsaJ. Subsequent genetic analysis has indicated that the suppressor mutation(s) is nuclear encoded. The suppressors retain the altered binding of PsaC, indicating that this change is not the cause of failure to grow photosynthetically. Further analysis showed that the rate of electron transfer from the quinone electron carrier A(1) to Fe-S(x) is slowed in the mutant (by a factor of approximately two) and restored to wild type rates in the suppressors. ENDOR spectra of A(1)(.-) in wild-type and mutant preparations are identical, indicating that the electronic structure of the phyllosemiquinone is not changed. The results suggest that the quinone to Fe-S(x) center electron transfer is sensitive to the structure of the iron-sulfur center, and may be a critical step in the energy conversion process. They also indicate that the structure of the reaction center may be modified as a result of changes in proteins outside the core of the reaction center.