Linear dichroism and fluorescence emission of antenna complexes during photosynthetic unit assembly in Rhodopseudomonas sphaeroides

Abstract

Photosynthetic membrane formation was induced in a nonpigmented, aerobic culture of Rhodopseudomonas sphaeroides. During the course of the experiment, the bacteriochlorophyll content of the cells increased approx. 40-fold [1]. Membranes prepared from pigmenting cells were analysed by fluorescence emission and excitation spectroscopy at 4 K; at all stages efficient energy transfer was observed between B850 and B875 bacteriochlorophylls within the light-harvesting system [2]. A light-harvesting domain containing several interconnected traps (reaction centres) and about 200 B875 bacteriochlorophylls was present in membranes from cells grown under high aeration. Over the course of the experiment the size of this domain increases 5-fold in intracytoplasmic membranes, with the result that the total light-harvesting network is composed of 3000 bacteriochlorophyll molecules; these surround and interconnect approx. 30 reaction centres. We estimate the average reaction centre-reaction centre distance in mature intracystoplasmic membranes to be about 15 nm and the average bacteriochlorophyll-bacteriochlorophyll distance to be about 1.5 nm [3]. An examination of the linear dichroism properties of stretched pigmenting membranes demonstrated that all orientations of pigments are maintained during development, the orientation of the B875 absorbance band being strongly parallel to the stretch axis. The expansion of the photosynthetic unit during development appears to involve the addition of B800–850 antenna to form increasingly random aggregates around highly ordered B875 antenna complexes.