Linear Dichroism and Fluorescence Emission of Antenna Complexes during Photosynthetic Unit Assembly in Rhodopseudomonas Sphaeroides

Abstract

When aerobically grown, unpigmented cells of Rhodopseudomonas sphaeroides are subjected to low oxygen growth conditions, an intracytoplasmic membrane system forms which bears antenna and reaction centre pigment proteins. The inner regions of the photosynthetic units, which consist of reaction centres surrounded and interconnected by B875 antenna and some B800–850 antenna, arise in small invaginations of the cytoplasmic membrane. Much of the B800–850 is not synthesised until a later stage of invagination is reached (Niederman et al., 1976); this stepwise mode of assembly was confirmed in a fluorescence study by Pradel et al. (1978). Niederman et al. (1979) demonstrated that small and large invaginations of the cytoplasmic membrane are separable by sucrose density gradient centrifugation; these are the upper pigmented band (UPB) and intracytoplasmic membrane (ICM) fractions. The UPB is thought to be derived from new sites of membrane invagination that eventually expand and assume the sedimentation characteristics of mature ICM. Hunter et al. (1979) examined the fluorescence properties of the UPB and showed that despite its relatively low B800–850 content, the UPB membrane in photo-synthetically grown cells exhibited relatively high emission at 865nm; this was attributed to some disconnected B800–850 antenna within this fraction.