The Synthesis and Assembly of Functional High and Low Light LH2 Antenna Complexes from Rhodopseudomonas palustris in Rhodobacter sphaeroides

Photosynthetic bacteria respond to lowered light intensity by increasing the level of the peripheral light-harvesting (LH2) complex. Several species possess an additional mechanism, responding to variations in light conditions by making different types of LH2 complex. However, the study of these complexes in isolation and in the native membrane environment has not been possible. Therefore two LH2 gene pairs from Rhodopseudomonas palustris, associated, respectively, with high light (pucBAa) and low light (pucBAd) growth conditions, were expressed in Rhodobacter sphaeroides. The high light LH2 complex PucBAa was synthesized at appreciable levels in R. sphaeroides, had near-infrared absorption bands at approximately 800-855 nm, and was able to transfer energy efficiently to the native LH1 complex. In contrast, the low light complex PucBAd was found at comparatively low levels, had absorption bands at approximately 797-830 nm, and did not transfer energy to the native LH1 complex efficiently. These observations are discussed in the light of site-directed studies on the R. sphaeroides LH2 complex, and the recently elucidated Rhodopseudomonas acidophila 10050 LH2 structure. Potentially important residues for energy transfer between LH2 and LH1 complexes are identified, as well as some of the factors that influence stability and assembly of LH2 complexes, such as the N-terminal sequences of their protein subunits and their carotenoid binding sites.