SPECIFIC MUTATION NEAR THE PRIMARY DONOR IN PHOTOSYSTEM-I FROM CHLAMYDOMONAS-REINHARDTII ALTERS THE TRAPPING TIME AND SPECTROSCOPIC PROPERTIES OF P-700

Time-resolved absorption and fluorescence spectroscopy were used to in vestigate the energy and electron transfer processes in the detergent- isolated photosystem I core particles from the site-directed mutant of Chlamydomonas reinhardtii with the histidine-656 of PsaB replaced by asparagine [HN(B656) mutation]. The specific mutation near the primary donor molecule results in a 40 mV increase in the P-700/P-700(+) midp oint potential [Webber, A. N., Su Hui, Bingham, S. E., Kass, H., Krabb en, L., Kuhn, M., Jordan, R., Schlodder, E., & Lubitz, W. (1996) Bioch emistry 35, 12857-12863]. There is no indication that the HN(B656) mut ation affects the spectral distribution of the antenna pigments. Howev er, the lifetime of the trapping process measured independently by tra nsient absorption and fluorescence spectroscopy in the mutant PSI core antenna is increased by a factor of approximately 2 (similar to 65 ps compared to similar to 30 ps in the wild-type PSI). This implies that the trapping process in the PSI antenna is limited by the process whe re the primary donor molecule directly participates. The HN(B656) muta tion results in the appearance of a new bleaching band at 670 nm in th e spectrum which is due to formation of P-700(+) upon photooxidation. The difference spectrum of the photoreduction of the possible primary acceptor, Ao in the mutant PSI is very similar to wild type, indicatin g that it is unaffected by the HN(B656) mutation. Possible mechanisms for slowing of the trapping process and the appearance of a new band i n the P-700 - P-700(+) difference spectrum of the HN(B656) PSI are dis cussed.