Mutation of the Chlamydomonas reinhardtii analogue of residue M210 of the Rhodobacter sphaeroides reaction center slows primary electron transfer in Photosystem II

Primary charge separation within Photosystem II (PS II) is much slower (time constant similar to 21 ps) than the equivalent step in the related reaction center (RC) found in purple bacteria (similar to 3 ps). In the case of the bacterial RC, replacement of a specific tyrosine residue within the M subunit (at position 210 in Rhodobacter sphaeroides), by a leucine residue slows down charge separation to similar to 20 ps. Significantly the analogous residue in PS II, within the D2 polypeptide, is a leucine not a tyrosine (at position D2-205, Chlamydomonas reinhardtii numbering). Consequently, it has been postulated [Hastings et al. (1992) Biochemistry 31: 7638-7647] that the rate of electron transfer could be increased in PS II by replacing this leucine residue with tyrosine. We have tested this hypothesis by constructing the D2-Leu205Tyr mutant in the green alga, Chlamydomonas reinhardtii, through transformation of the chloroplast genome. Primary charge separation was examined in isolated PS II RCs by time-resolved optical spectroscopy and was found to occur with a time constant of 40 ps. We conclude that mutation of D2-Leu205 to Tyr does not increase the rate of charge separation in PS II. The slower kinetics of primary charge separation in wild type PS II are probably not due to a specific difference in primary structure compared with the bacterial RC but rather a consequence of the P680 singlet excited state being a shallower trap for excitation energy within the reaction center.