Changes in the native carotenoid of bacterial pigment-protein complexes

Abstract

Changes in the carotenoid pool of the photosynthetic membrane of Rhodospirillum rubrum caused parallel changes in the carotenoids of light-harvesting and reaction-center complexes. Thus, whereas complexes purified from exponentially growing cultures contained spirilloxanthin and several of its colored metabolic precursors at comparable levels, only spirilloxanthin was detected in preparations derived from stationary-phase cultures, and in both cases the carotenoid composition of the isolated complexes was similar or identical to that of the intact membrane. The differences between the antenna pigments of stationary and exponential cultures were reflected in the circular dichroism spectrum and in the bacteriochlorophyll fluorescence excitation spectrum of the intact membrane, since both optical activity and singlet-singlet energy transfer to bacteriochlorophyll are specific for complex-bound carotenoids. The changes in carotenoid composition that were induced by oxygen in cultures of Rhodobacter capsulatus were accompanied also by parallel changes in the pigments of both the core and the peripheral antenna complexes of this microorganism. In both species of purple bacteria, the variations of pigment pattern did not appear to influence to any significant extent the protective function of carotenoids, as assayed by following the photodestruction of antenna bacteriochlorophyll in aerobic suspensions of isolated membrane vesicles. The similar carotenoid composition of the intact membrane and the antenna may be simply explained by the limiting size of the pigment pool (all pigmented carotenoids that are present become complex-bound). However, to account for the carotenoid variability of the R. rubrum reaction center, which is less abundant in the membrane than some carotenoids, it seems necessary to assume that there are no significant differences among the binding affinities of the complex for the available pigments.