FLUORESCENCE LIFETIMES AND AGGREGATION STATES OF THE CORE LIGHT-HARVESTING COMPLEX B875 FROM RUBRIVIVAX-GELATINOSUS

The core light-harvesting complex B875 isolated from the purple bacter ium Rubrivivax gelatinosus and its different spectral forms B820 and B 840, which are depleted of carotenoid, were investigated by steady-sta te and time-resolved fluorescence, and by electron microscopy. Images of B875 have been shown to contain cyclic oligomers with a diameter of 150-200 Angstrom and with a central hole of 25 Angstrom [Jirsakova V, Reiss-Husson F and Ranck JL (1996) Biochim Biophys Acta 1277: 150-160 ]. Dilute B820 samples contained heterogeneous, compact particles that tend to aggregate with increasing concentration of protein, forming c lumps without any visible substructure. At the same time the absorptio n maximum of such aggregates shifted to 840 nm. Fluorescence emission and life times were analyzed by single photon counting. In B875 sample s the major component emitted at 892 nm with a life time of 0.64 ns. B 820 samples emitted at 830 nm with a life-time of 1 ns. An additional short life-time component of 0.3-0.4 ns was found in B820 and emitted at about 860 nm; its contribution increased with the B820 concentratio n. This latter component is attributed to the fluorescence quenching o ccuring within the non-native aggregates of B820 formed in the absence of carotenoid. When the B875 antenna was reconstituted from B820 subu nit and hydroxyspheroidene, it presented an emission spectrum and a fl uorescence decay identical to those observed in the native core comple x, pointing to the structural role of the carotenoid for the proper ar chitecture of this antenna.