Specific mutation near the primary donor in photosystem I from Chlamydomonas reinhardtii alters the trapping time and spectroscopic properties of P₇₀₀

Time-resolved absorption and fluorescence spectroscopy were used to investigate 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 P₇₀₀/P₇₀₀⁺ midpoint potential [Webber, A. N., Su Hui, Bingham, S. E., Kass, H., Krabben, L., Kuhn, M., Jordan, R., Schlodder, E., and Lubitz, W. (1996) Biochemistry 35, 12857-12863]. There is no indication that the HN(B656) mutation affects the spectral distribution of the antenna pigments. However, the lifetime of the trapping process measured independently by transient absorption and fluorescence spectroscopy in the mutant PSI core antenna is increased by a factor of approximately 2 (~65 ps compared to ~30 ps in the wild-type PSI). This implies that the trapping process in the PSI antenna is limited by the process where the primary donor molecule directly participates. The HN(B656) mutation results in the appearance of a new bleaching band at 670 nm in the spectrum which is due to formation of P₇₀₀⁺ upon photooxidation. The difference spectrum of the photoreduction of the possible primary acceptor, A₀ in the mutant PSI is very similar to wild type, indicating that it is unaffected by the HN(B656) mutation. Possible mechanisms for slowing of the trapping process and the appearance of a new band in the P₇₀₀ - P₇₀₀⁺ difference spectrum of the HN(B656) PSI are discussed.