The spirilloxanthine-pathway carotenoid incorporation into light-harvesting complexes of Ectothiorhodospira haloalkaliphila in vitro.

Carotenoidless light-harvesting complexes (DPA-complexes) LH1-RC and LH2 were isolated from the purple sulfur bacterium Ectothiorhodospira haloalkaliphila in which carotenoid biosynthesis was suppressed with diphenylamine (DPA). Carotenoids of the spirilloxanthine series, which were isolated from the same bacterium, were incorporated into the DPA-complexes in vitro with an efficiency of 95-100%. The comparison of characteristics of the complexes with the incorporated carotenoids and the control complexes showed that the LH2 complexes with the incorporated carotenoids restored their absorption spectra, circular dichroism signals, and energy transfer from carotenoids to bacteriochlorophyll, which indicates that carotenoids were correctly incorporated into the structure of this complex.