Fluorescence Induction in Chloroplasts Isolated from the Green Alga, Bryopsis maxima IV. The I-D Dip

Fluorescence induction of intact Bryopsis chloroplasts whichpreviously had been illuminated in the presence of dithionitethen kept in the dark prior to measurement showed marked quenchingfrom an intermediary peak I to a lower level D before a secondaryrise to a peak P. A small hump (H), related to the membranepotential formed across the thylakoid membranes, overlappedD. The maximum extent of quenching—the I-D dip—wasattained in chloroplasts which had been illuminated for 1 secprior to dark incubation for 1 min. This illumination causedthe complete reduction of secondary electron acceptors and thepartial reduction of Q, the primary electron acceptor of photosystemII. Chloroplasts developed the capacity for transient photooxidationof cytochrome f during subsequent dark incubation, indicatingthat there was dark oxidation of electron acceptors of photosystemI which had been reduced by the illumination. A close correlationwas found between the I-D dip and the transient photooxidationof cytochrome f with respect to the kinetics of light inducedchanges as well as dark restoration after the illumination.Inhibitor studies showed that the dip decreased when the poolsize of photosystem I acceptors was reduced. Our results showthat the I-D dip and the transient photooxidation of cytochromef depend upon a common acceptor pool of photosystem I. We concludedthat the I-D dip is due to the oxidation of Q by photosystemI with a limited electron acceptor pool. (Received September 12, 1980; Accepted November 14, 1980)     

A photoactive Photosystem-II reaction-center complex lacking a chlorophyll-binding 40 kilodalton subunit from the thermophilic cyanobacterium Synechococcus sp.

The Photosystem-II reaction-center complex of the thermophilic cyanobacterium Synechococcus sp. was resolved into two complemental chlorophyll-protein complexes, CP2b which contained a chlorophyll-binding 47 kDa polypeptide, two polypeptides in the 28–31 kDa region and a 9 kDa polypeptide, and CP2c which had only a chlorophyll-binding 40 kDa polypeptide. CP2b was found to be highly active in photoreduction of 2,6-dichlorophenolindophenol with diphenylcarbazide as an electron donor. The activity was insensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea and ioxynil but was half inactivated by the treatment of the complex at 50°C for 5 min, or on addition of 0.001% sodium dodecyl sulfate, indicating its dependence on the protein conformation. CP2c also showed a low activity of the dye photoreduction, which was insensitive to heat and enhanced at high concentrations of sodium dodecyl sulfate. The quantum yield of the photoreduction was estimated to be 0.12 for CP2b and 0.002 for CP2c. It is concluded that the 47 kDa polypeptide is the site of the Photosystem-II reaction center and the 40 kDa polypeptide is not required for the Photosystem-II-driven electron transport.