Electronic spectra of PSI mutants: The peripheral subunits do not bind redchlorophylls in Synechocystis sp PCC 6803

Steady-state fluorescence and absorption spectra have been obtained in the Q(y) spectral region (690-780 nm and 600-750 nm, respectively) for several subunit-deficient photosystem I mutants from the cyanobacterium Synechocyst is sp. PCC 6803. The 77 K fluorescence spectra of the wild-type and subunit -deficient mutant photosystem I particles are all very similar, peaking at similar to 720 nm with essentially the same excitation spectrum. Because em ission from far-red chlorophylls absorbing near 708 nm dominates low-temper ature fluorescence in Synechocystis sp., these pigments are not coordinated to any the subunits PsaF, Psa I, PsaJ, PsaK, PsaL, or psaM. The room tempe rature (wild-type-mutant) absorption difference spectra for trimeric mutant s lacking the PsaF/J, PsaK, and PsaM subunits suggest that these mutants ar e deficient in core antenna chlorophylls (Chls) absorbing near 685, 670, 67 5, and 700 nm, respectively. The absorption difference spectrum for the Psa F/J/I/L-deficient photosystem I complexes at 5 K reveals considerably more structure than the room-temperature spectrum. The integrated absorbance dif ference spectra (when normalized to the total PS I Q(y) spectral area) are comparable to the fractions of Chls bound by the respective (groups of) sub units, according to the 4-Angstrom density map of PS I from Synechococcus e longatus. The spectrum of the monomeric PsaL-deficient mutant suggests that this subunit may bind pigments absorbing near 700 nm.