Functional linkage between phycobilisome and reaction center in two phycobilisome oxygen-evolving photosystem II preparations isolated from the thermophilic cyanobacterium Synechococcus sp

Abstract

Photosystem II oxygen-evolving preparations with attached phycobilisomes were isolated from the thermophilic cyanobacterium Synechococcus sp. with β-octylglucoside or digitonin. Fluorescence emission spectra of the two preparations determined at 77 K largely lacked a far red band which originates from photosystem I. The spectrum of the digitonin preparation was otherwise similar to that of intact cells, whereas the β-oc-tylglucoside preparation showed a pronounced band at 687 nm, which is considered to be emitted from phycobilisomes. The relative yield of phycobilin fluorescence was similar between the digitonin preparations and the cells but was considerably larger in the β-octylglucoside preparations at room temperature. The quantum yield of ferricyanide photoreduction determined with light which is absorbed mainly by phycobiliproteins was 0.85 for the digitonin preparation and 0.57 for the β-octylglucoside preparation. The results indicate that excitation energy is transferred from phycobilisomes to photosystem II reaction centers in the digitonin preparation as efficiently as in intact cells, while a significant portion of light energy harvested by phycobilisomes is not utilized by the primary photochemistry in the β-octylglucoside preparation. Digitonin and β-octylglucoside preparations had 65 and 48 chlorophyll a molecules per photosystem II reaction center, respectively. The β-octylglucoside preparation contained twice as much phycocyanin and allophycocyanin per photosystem II reaction center as the digitonin preparation, which has a phycobiliprotein-to-photosystem II reaction center ratio very similar to that of cells. It is concluded that whereas the β-octylglucoside preparation contains a considerable amount of free phycobilisomes, all phycobilisomes present in the digitonin preparation are physically and functionally linked to photosystem II reaction center complexes.