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The e¡ects of glycerol and high temperatures on structure and function of phycobilisomes (PBSs) in vivo were investigated in a chlL deletion mutant of the cyanobacterium Synechocystis sp. PCC 6803. When the mutant was grown under light-activated heterotrophic growth conditions, it contained intact and functional PBSs, but essentially no chlorophyll and photosystems. So the structural and functional changes of the mutant PBSs in vivo can be handily detected by measurement of low temperature (77 K) £uorescence emission spectra. High concentration glycerol induced an obvious disassembly of PBSs and the dissociation of phycocyanins in the rod substructures into their oligomers and monomers. PBSs also disassembled at high temperatures and allophycocyanins were more sensitive to heat stress than phycocyanins. Our results demonstrate that the chlL 3 mutant strain is an advantageous model for studying the mechanisms of assembly and disassembly of protein complexes in vivo. ß

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SummaryInactivation of the chlL gene in Synechocystis sp. PCC 6803 resulted in negligible chlorophyll content when the mutant was grown in darkness. Upon phycocyanin excitation at 580 nm, the 77K uorescence spectrum of dark-grown cells showed three peaks at 648 nm, 665 nm, and 685 nm, this last being the largest. This re ects the functional presence of major components of phycobilisomes, including phycocyanin, allophycocyanin, and the terminal emitter, and ef cient energy transfer between these components. As expected, no uorescence emission peaks corresponding to chlorophyll in the photosystems were observed. Intact phycobilisomes could be isolated from the dark-grown chlL-deletion mutant. However, the phycobilisomes had a lower ef ciency of energy transfer than did those isolated from the light-grown mutant, probably because of a decreased phycobilisome stability in the absence of chlorophyll. Exposing the dark-grown chlL-deletion mutant to light triggered the biosynthesis of chlorophyll. For the rst 6 h in the light, upon phycocyanin excitation at 580 nm, the 77K uorescence emission spectrum of greening cells was identical to that of dark-grown cells that lacked signi cant amounts of chlorophyll. With increased chlorophyll synthesis, gradual energy transfer from phycobilisomes to the two photosystems can be demonstrated. IUBMB Life, 48: 625-630, 1999

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