Physiological significance of overproduced carotenoids in transformants ofthe cyanobacterium Synechococcus PCC7942

The functional location of carotenoids in the photosynthetic apparatus of - crtB and -pys transformants of the cyanobacterium Synechococcus PCC7942 was studied and compared with a control strain -pFP1-3. These transformants ov erproduce carotenoids due to the insertion of an additional foreign phytoen e synthase gene. A higher carotenoid content was found for -crtB and -pys t ransformants both in whole cells and isolated membranes; the crtB transform ant was also enriched with chlorophyll. 77-K fluorescence emission and exci tation spectra of the phycobilin-free membranes were examined for a possibl e location of overproduced carotenoids in pigment-protein complexes in situ . A similar ratio of the amplitudes of fluorescence bands at 716 and 695 nm emitted by photosystems I and II, found for the three strains, indicates t hat the stoichiometry between photosystems of the transformants was not cha nged. Overproduced carotenoids are not located in the core antenna of photo system I, since 77-K fluorescence excitation spectra for photosystem I of i solated membranes from the studied strains do not differ in the region of c arotenoid absorption. When illuminated with light of the same intensity but different quality, absorbed preferentially by either carotenoids, chloroph ylls or phycobilins, respectively, oxygen evolution was found always higher in the transformants -crtB and -pys than in pFP1-3 control cells. Identica l kinetics of fluorescence induction of all strains under carotenoid excita tion did not reveal a higher activity of photosystem II in cells enriched w ith carotenoids. It is suggested that overproduced carotenoids of the trans formants are not involved in photosynthetic light-harvesting; rather they m ay serve to protect the cells and its membranes against photodestruction.