A chlorosome is an antenna complex located on the cytoplasmic side of the inner membrane in green photosynthetic bacteria that contains tens of thousands of self-assembled bacteriochlorophylls (BChls). Green bacteria are known to incorporate various esterifying alcohols at the C-17 propionate position of BChls in the chlorosome. The effect of these functional substitutions on the biogenesis of the chlorosome has not yet been fully explored. In this report, we address this question by investigating various esterified bacteriochlorophyll c (BChl c) homologs in the thermophilic green non-sulfur bacterium Chloroflexus aurantiacus. Cultures were supplemented with exogenous long-chain alcohols at 52 °C (an optimal growth temperature) and 44 °C (a suboptimal growth temperature), and the morphology, optical properties and exciton transfer characteristics of chlorosomes were investigated. Our studies indicate that at 44 °C Cfl. aurantiacus synthesizes more carotenoids, incorporates more BChl c homologs with unsaturated and rigid polyisoprenoid esterifying alcohols and produces more heterogeneous BChl c homologs in chlorosomes. Substitution of phytol for stearyl alcohol of BChl c maintains similar morphology of the intact chlorosome and enhances energy transfer from the chlorosome to the membrane-bound photosynthetic apparatus. Different morphologies of the intact chlorosome versus in vitro BChl aggregates are suggested by small-angle neutron scattering. Additionally, phytol cultures and 44 °C cultures exhibit slow assembly of the chlorosome. These results suggest that the esterifying alcohol of BChl c contributes to long-range organization of BChls, and that interactions between BChls with other components are important to the assembly of the chlorosome. Possible mechanisms for how esterifying alcohols affect the biogenesis of the chlorosome are discussed.
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Acknowledgments JKT thanks PARC Scientific Exchange Program for supporting SANS measurements and Dr. Sai Venkatesh Pingali at Bio-SANS CG-3 for assisting SANS measurement and discussing SANS data. Bio-SANS CG-3 is a resource of the Center for Structural Molecular Biology at ORNL supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research Project ERKP291. ADH and DMF acknowledge support from NSF Plant Genome Research Program grant IOS-1238812. Work in the laboratory of HAF was supported by grants from the National Science Foundation (MCB-1243565) and the University of Connecticut Research Foundation. JKT is supported by start-up funds.
© 2014 Springer Science+Business Media Dordrecht.
- Energy transfer
- Long-chain alcohols