Fluorescence life-times and aggregation states of the core light harvesting complex B875 from Rubrivivax gelatinosus

The core light-harvesting complex B875 isolated from the purple bacterium Rubrivivax gelatinosus and its different spectral forms B820 and B840, which are depleted of carotenoid, were investigated by steady-state and time-resolved fluorescence, and by electron microscopy. Images of B875 have been shown to contain cyclic oligomers with a diameter of 150-200 Å and with a central hole of 25 Å [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 clumps without any visible substructure. At the same time the absorption maximum of such aggregates shifted to 840 nm. Fluorescence emission and life times were analyzed by single photon counting. In B875 samples the major component emitted at 892 nm with a life time of 0.64 ns. B820 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 concentration. This latter component is attributed to the fluorescence quenching occuring within the non-native aggregates of B820 formed in the absence of carotenoid. When the B875 antenna was reconstituted from B820 subunit and hydroxyspheroidene, it presented an emission spectrum and a fluorescence decay identical to those observed in the native core complex, pointing to the structural role of the carotenoid for the proper architecture of this antenna.