The Role of Tyrosine M210 in the Initial Charge Separation in the Reaction Center of Rhodobacter sphaeroides

Abstract

Site-directed mutants of tyrosine M210 (YM210) in the reaction center (RC) of Rhodobacter sphaeroides have been constructed and characterized to determine the effect of the changes on both the structure of the RC and its electron transfer kinetics. YM210 has been replaced by phenylalanine (FM210) and leucine (LM210). Both mutants are able to grow photosynthetically under high light but under low light the LM210 mutant is photosynthetic minus. Both mutant strains contain equal amounts of photobleachable RC in the intracytoplasmic membrane (normalized to total bacteriochlorophyll) as compared to wild type. Photobleaching spectra of mutant membranes are basically indistinguishable from wild type. Absorption spectra of purified mutant RCs also are basically the same as the wild type with small changes observed in the position and intensity of the Qy transition of the monomer bacteriochlorophylls. Picosecond kinetic analysis shows that both the lifetime of the excited state of the primary electron donor (P*) and the rise time of the first reduction step are longer in both mutants. These processes occur in 3.5 ps in the wild type RC while the corresponding time constants in the mutants are 16 ± 6 ps in FM210 and 22 ± 8 ps in LM210.