Modification of electron transfer from the quinone electron carrier, A(1),of Photosystem 1 in a site directed mutant D576 double right arrow L within the Fe-S-x 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 Chlamydomona s reinhardtii has been described previously. [Hallahan et al. (1995) Photos ynth Res 46: 257-264]. The mutation, PsaA: D576L, changes the conserved asp artate residue adjacent to one of the cysteine ligands binding the Fe-S-x c enter to PsaA. The mutation, which prevents photosynthetic growth, was obse rved to change the EPR spectrum of the Fe-S-A/B centers bound to the PsaC s ubunit. We suggested that changes in binding of PsaC to the PsaA/PsaB react ion center prevented efficient electron transfer. Second site suppressors o f the mutation have now been isolated which have recovered the ability to g row photosynthetically. DNA analysis of four suppressor strains showed the original D576L mutation is intact, and that no mutations are present elsewh ere within the Fe-S-x binding region of either PsaA or PsaB, nor within Psa C or PsaJ. Subsequent genetic analysis has indicated that the suppressor mu tation(s) is nuclear encoded. The suppressors retain the altered binding of PsaC, indicating that this change is not the cause of failure to grow phot osynthetically. 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 (b y a factor of approximately two) and restored to wild type rates in the sup pressors. ENDOR spectra of A(1)(.-) in wild-type and mutant preparations ar e identical, indicating that the electronic structure of the phyllosemiquin one is not changed. The results suggest that the quinone to Fe-S-x center e lectron transfer is sensitive to the structure of the iron-sulfur center, a nd may be a critical step in the energy conversion process. They also indic ate that the structure of the reaction center may be modified as a result o f changes in proteins outside the core of the reaction center.