ELECTROCHROMIC BAND SHIFTS OF CAROTENOID IN A BLUE-GREEN ALGA

Abstract

— The excitation with a short flash of cells of a blue-green alga, Synechococcus sp., induced, besides photooxidation of cytochrome c-553 and P-700, small absorption changes of complex kinetics in the wavelength region between 450 and 570 nm. The absorption changes were resolved into two kinetic components different in their sensitivity to gramicidin D. The ionophore-sensitive component (Gs), which rose very rapidly on flash illumination and decayed with a half-time of 3 ms, has spectral features indicating a red shift of carotenoid absorption bands. Gs was sensitive to valonomycin but not to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). The relaxation rate of Gs was markedly slowed down in the presence of tri-n-butyltin chloride. Phenazine methosulfate induced a secondary slow rise following the initial rapid rise. A similar slow rise appeared in the dark-starved cells but disappeared on the addition of methyl viologen. It is concluded from these results that Gs is an electrochromic band shift of carotenoid responding to the electric field formed by the primary charge separation of the photosystem I reaction center and its decay is related to the proton translocation through a proton channel of the membrane. The ionophore-resistant component rose and decayed with the half-times of 0.2 and 2 ms, respectively. Its difference spectrum suggests a blue band shift of carotenoid. The ionophore-resistant component was also insensitive to DCMU. However, this component may be related in some way to flash-induced electron flow, because the photoresponse was altered by dibromothymoquinone, bathophenanthroline and 2-n-heptyl-hydroxyquinoline-N-oxide or the dark starvation of cells, which were all effective in inhibiting the cytochrome c-553 reduction.