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Internal Structure of Chlorosomes from Brown-Colored Chlorobium Species and the Role of Carotenoids in Their Assembly

Jakub Peník ^*, Juan B. Arellano , Teemu P. Ikonen , Carles M. Borrego , Pasi A. Laurinmäki ^¶, Sarah J. Butcher ^¶, Ritva E. Serimaa  and Roman Tuma ^¶

^* Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic; Institute for Natural Resources and Agrobiology (CSIC), Salamanca, Spain; Department of Physical Sciences, University of Helsinki, Helsinki, Finland; Institute of Aquatic Ecology, University of Girona, Girona, Spain; and ^¶ Institute of Biotechnology and Department of Biological and Environmental Sciences, University of Helsinki, Helsinki, Finland

Correspondence: Address reprint requests to Roman Tuma, E-mail: roman.tuma{at}helsinki.fi.

Chlorosomes are the main light harvesting complexes of green photosynthetic bacteria. Recently, a lamellar model was proposed for the arrangement of pigment aggregates in Chlorobium tepidum chlorosomes, which contain bacteriochlorophyll (BChl) c as the main pigment. Here we demonstrate that the lamellar organization is also found in chlorosomes from two brown-colored species (Chl. phaeovibrioides and Chl. phaeobacteroides) containing BChl e as the main pigment. This suggests that the lamellar model is universal among green sulfur bacteria. In contrast to green-colored Chl. tepidum, chlorosomes from the brown-colored species often contain domains of lamellar aggregates that may help them to survive in extremely low light conditions. We suggest that carotenoids are localized between the lamellar planes and drive lamellar assembly by augmenting hydrophobic interactions. A model for chlorosome assembly, which accounts for the role of carotenoids and secondary BChl homologs, is presented.

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