SPECIFIC MUTATION NEAR THE PRIMARY DONOR IN PHOTOSYSTEM-I FROM CHLAMYDOMONAS-REINHARDTII ALTERS THE TRAPPING TIME AND SPECTROSCOPIC PROPERTIES OF P-700
An. Melkozernov et al., SPECIFIC MUTATION NEAR THE PRIMARY DONOR IN PHOTOSYSTEM-I FROM CHLAMYDOMONAS-REINHARDTII ALTERS THE TRAPPING TIME AND SPECTROSCOPIC PROPERTIES OF P-700, Biochemistry, 36(10), 1997, pp. 2898-2907
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.