Dissociation Kinetics of Anabaena Phycobilisomes

The dissociation kinetics of Anabaena phycobilisomes (PBS) atlow phosphate concentrations was studied with light scatteringand sensitized fluorescence arising from allophycocyanin-I (APC-I).When curve fitting was applied to the decrement time-coursesof these two characters, both fitted the same type of decaykinetics consisting of three sequential phases of first-orderdecay, and the rate constants of these phases were practicallythe same between changes of light scattering and sensitizedfluorescence. As the light scattering reflects the particlesize, the conformity of the two kinetics indicates that theuncoupling of excitation energy transfer originated from thedissociation of PBS. The fluorescence emission spectra of dissociating PBS were resolvableinto three- component spectra, those of phycocyanin (PC) hexamer,APC trimer and APC-I. In the first and second phases of dissociation,the emission from PC increased with concomitant decrease inthe emission from APC and APC-I with the ratio of APC to APC-Iunaltered, while in the third phase the transfer from APC toAPC-I was completely disrupted. These results support the viewthat the whole PBS consists of two moieties of PC subparticleand one moiety of APC-APC-I complex. (Received March 1, 1982; Accepted May 10, 1982)