EXCITATION-ENERGY TRANSFER FROM PHYCOBILISOMES TO PHOTOSYSTEM-I IN A CYANOBACTERIAL MUTANT LACKING PHOTOSYSTEM-II

Photosynthetic light-harvesting has been investigated in two strains of the cyanobacterium Synechocystis 6803: the wild-type and a mutant lacking Photosystem II. The deletion mutant lacks the genes for the PS II proteins D2 and CP43. D1 and CP47 are also destabilised in the mutant and are not present at detectable levels in the thylakoid. Excitation energy transfer from phycobilisomes to Photosystem I was detected by low-temperature fluorescence emission spectroscopy and by measurement of action spectra for flash-induced P700 photooxidation. In the wild-type, the quantum efficiency of energy transfer from phycobilisomes to Photosystem I is determined by the physiological adaptation of the cells and is maximally about 40%. In the mutant, phycobilisomes transfer energy to Photosystem I with about 80% efficiency. It is concluded that 'spillover' from Photosystem II is not required for efficient energy transfer from phycobilisomes to Photosystem I. These results suggest the existence of a specific phycobilisome-Photosystem I complex.